US20030180222A1 - Peptide-based multimeric targeted contrast agents - Google Patents

Peptide-based multimeric targeted contrast agents Download PDF

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US20030180222A1
US20030180222A1 US10/209,183 US20918302A US2003180222A1 US 20030180222 A1 US20030180222 A1 US 20030180222A1 US 20918302 A US20918302 A US 20918302A US 2003180222 A1 US2003180222 A1 US 2003180222A1
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group
moiety
precursor
iii
peptide
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Zhaoda Zhang
John Amedio
Peter Caravan
Stephane Dumas
Andrew Kolodziej
Thomas McMurry
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Epix Pharmaceuticals Inc
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Epix Pharmaceuticals Inc
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Priority to US10/209,183 priority Critical patent/US20030180222A1/en
Assigned to EPIX MEDICAL, INC. reassignment EPIX MEDICAL, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: DUMAS, STEPHANE, MCMURRY, THOMAS J., ZHANG, ZHAODA, AMEDIO, JOHN C., CARAVAN, PETER D., KOLODZIEJ, ANDREW
Assigned to SCHERING AKTIENGESELLSCHAFT reassignment SCHERING AKTIENGESELLSCHAFT SECURITY INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: EPIX MEDICAL, INC.
Publication of US20030180222A1 publication Critical patent/US20030180222A1/en
Priority to US10/786,791 priority patent/US7238341B2/en
Assigned to EPIX PHARMACEUTICALS, INC. reassignment EPIX PHARMACEUTICALS, INC. CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: EPIX MEDICAL, INC.
Priority to US11/098,665 priority patent/US7927581B2/en
Priority to US11/564,648 priority patent/US20070185311A1/en
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07K—PEPTIDES
    • C07K7/00—Peptides having 5 to 20 amino acids in a fully defined sequence; Derivatives thereof
    • C07K7/04—Linear peptides containing only normal peptide links
    • C07K7/08—Linear peptides containing only normal peptide links having 12 to 20 amino acids
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K47/00—Medicinal preparations characterised by the non-active ingredients used, e.g. carriers or inert additives; Targeting or modifying agents chemically bound to the active ingredient
    • A61K47/50—Medicinal preparations characterised by the non-active ingredients used, e.g. carriers or inert additives; Targeting or modifying agents chemically bound to the active ingredient the non-active ingredient being chemically bound to the active ingredient, e.g. polymer-drug conjugates
    • A61K47/51—Medicinal preparations characterised by the non-active ingredients used, e.g. carriers or inert additives; Targeting or modifying agents chemically bound to the active ingredient the non-active ingredient being chemically bound to the active ingredient, e.g. polymer-drug conjugates the non-active ingredient being a modifying agent
    • A61K47/62—Medicinal preparations characterised by the non-active ingredients used, e.g. carriers or inert additives; Targeting or modifying agents chemically bound to the active ingredient the non-active ingredient being chemically bound to the active ingredient, e.g. polymer-drug conjugates the non-active ingredient being a modifying agent the modifying agent being a protein, peptide or polyamino acid
    • A61K47/64—Drug-peptide, drug-protein or drug-polyamino acid conjugates, i.e. the modifying agent being a peptide, protein or polyamino acid which is covalently bonded or complexed to a therapeutically active agent
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A61K49/001—Preparation for luminescence or biological staining
    • A61K49/0013—Luminescence
    • A61K49/0017—Fluorescence in vivo
    • A61K49/0019—Fluorescence in vivo characterised by the fluorescent group, e.g. oligomeric, polymeric or dendritic molecules
    • A61K49/0021—Fluorescence in vivo characterised by the fluorescent group, e.g. oligomeric, polymeric or dendritic molecules the fluorescent group being a small organic molecule
    • A61K49/0039—Coumarin dyes
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A61K49/001—Preparation for luminescence or biological staining
    • A61K49/0013—Luminescence
    • A61K49/0017—Fluorescence in vivo
    • A61K49/0019—Fluorescence in vivo characterised by the fluorescent group, e.g. oligomeric, polymeric or dendritic molecules
    • A61K49/0021—Fluorescence in vivo characterised by the fluorescent group, e.g. oligomeric, polymeric or dendritic molecules the fluorescent group being a small organic molecule
    • A61K49/0041—Xanthene dyes, used in vivo, e.g. administered to a mice, e.g. rhodamines, rose Bengal
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A61K49/001—Preparation for luminescence or biological staining
    • A61K49/0013—Luminescence
    • A61K49/0017—Fluorescence in vivo
    • A61K49/0019—Fluorescence in vivo characterised by the fluorescent group, e.g. oligomeric, polymeric or dendritic molecules
    • A61K49/0021—Fluorescence in vivo characterised by the fluorescent group, e.g. oligomeric, polymeric or dendritic molecules the fluorescent group being a small organic molecule
    • A61K49/0041—Xanthene dyes, used in vivo, e.g. administered to a mice, e.g. rhodamines, rose Bengal
    • A61K49/0043—Fluorescein, used in vivo
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A61K49/001—Preparation for luminescence or biological staining
    • A61K49/0013—Luminescence
    • A61K49/0017—Fluorescence in vivo
    • A61K49/005—Fluorescence in vivo characterised by the carrier molecule carrying the fluorescent agent
    • A61K49/0056—Peptides, proteins, polyamino acids
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A61K49/06—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations
    • A61K49/08—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations characterised by the carrier
    • A61K49/085—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations characterised by the carrier conjugated systems
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A61K49/06—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations
    • A61K49/08—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations characterised by the carrier
    • A61K49/10—Organic compounds
    • A61K49/12—Macromolecular compounds
    • A61K49/122—Macromolecular compounds dimers of complexes or complex-forming compounds
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A61K49/06—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations
    • A61K49/08—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations characterised by the carrier
    • A61K49/10—Organic compounds
    • A61K49/12—Macromolecular compounds
    • A61K49/124—Macromolecular compounds dendrimers, dendrons, hyperbranched compounds
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K49/00—Preparations for testing in vivo
    • A61K49/06—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations
    • A61K49/08—Nuclear magnetic resonance [NMR] contrast preparations; Magnetic resonance imaging [MRI] contrast preparations characterised by the carrier
    • A61K49/10—Organic compounds
    • A61K49/14—Peptides, e.g. proteins
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K51/00—Preparations containing radioactive substances for use in therapy or testing in vivo
    • A61K51/02—Preparations containing radioactive substances for use in therapy or testing in vivo characterised by the carrier, i.e. characterised by the agent or material covalently linked or complexing the radioactive nucleus
    • A61K51/04—Organic compounds
    • A61K51/08—Peptides, e.g. proteins, carriers being peptides, polyamino acids, proteins
    • A61K51/088—Peptides, e.g. proteins, carriers being peptides, polyamino acids, proteins conjugates with carriers being peptides, polyamino acids or proteins
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07K—PEPTIDES
    • C07K1/00—General methods for the preparation of peptides, i.e. processes for the organic chemical preparation of peptides or proteins of any length
    • C07K1/107—General methods for the preparation of peptides, i.e. processes for the organic chemical preparation of peptides or proteins of any length by chemical modification of precursor peptides
    • C07K1/1072—General methods for the preparation of peptides, i.e. processes for the organic chemical preparation of peptides or proteins of any length by chemical modification of precursor peptides by covalent attachment of residues or functional groups
    • C07K1/1077—General methods for the preparation of peptides, i.e. processes for the organic chemical preparation of peptides or proteins of any length by chemical modification of precursor peptides by covalent attachment of residues or functional groups by covalent attachment of residues other than amino acids or peptide residues, e.g. sugars, polyols, fatty acids
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07K—PEPTIDES
    • C07K7/00—Peptides having 5 to 20 amino acids in a fully defined sequence; Derivatives thereof
    • C07K7/04—Linear peptides containing only normal peptide links
    • C07K7/06—Linear peptides containing only normal peptide links having 5 to 11 amino acids

Definitions

  • This invention relates to contrast agents for diagnostic imaging, and more particularly to peptide-targeted, multimeric contrast agents, wherein a peptide functions as a targeting group and a point of attachment for one or more chelates at both the amino and carboxy termini of the peptide.
  • Diagnostic imaging techniques such as magnetic resonance imaging (MRI), X-ray, nuclear radiopharmaceutical imaging, ultraviolet-visible-infrared light imaging, and ultrasound, have been used in medical diagnosis for a number of years. Contrast media additionally have been used to improve or increase the resolution of the image or to provide specific diagnostic information.
  • MRI magnetic resonance imaging
  • X-ray nuclear radiopharmaceutical imaging
  • ultraviolet-visible-infrared light imaging and ultrasound
  • Contrast media additionally have been used to improve or increase the resolution of the image or to provide specific diagnostic information.
  • the contrast media must interfere with the wavelength of electromagnetic radiation used in the imaging technique, alter the physical properties of tissue to yield an altered signal, or, as in the case of radiopharmaceuticals, provide the source of radiation itself.
  • MRI and optical methods are unique among imaging modalities in that they yield complex signals that are sensitive to the chemical environment. While the signal from X-ray or radionuclide agents remains the same whether agents are free in plasma, bound to proteins or other targets, or trapped inside bone, certain contrast agents for MRI and optical imaging will have different signal characteristics in differing physiological environments. It is important that the contrast agent be sufficiently sensitive and present at high enough concentration so that signal changes can be observed.
  • Gadolinium complexes increase contrast by increasing the nuclear magnetic relaxation rates of protons found in the water molecules that are accessible to the contrast agents during MRI (Caravan, P., et al., R. B. Chem. Rev. 99, 2293 (1999)).
  • the relaxation rate of the protons in these water molecules increases relative to protons in other water molecules that are not accessible to the contrast agent. This change in relaxation rate leads to improved contrast of the images.
  • this increase in relaxivity within a specific population of water molecule protons can result in an ability to collect more image data in a given amount of time. This in turn results in an improved signal to noise ratio.
  • Imaging may also be performed using light, in which case an optical dye is chosen to provide signal.
  • an optical dye is chosen to provide signal.
  • light in the 600-1300 nm (visible to near-infrared) range passes relatively easily through biological tissues and can be used for imaging purposes.
  • the light that is transmitted through, or scattered by, reflected, or re-emitted (fluorescence), is detected and an image generated.
  • Changes in the absorbance, reflectance, or fluorescence characteristics of a dye including an increase or decrease in the number of absorbance peaks or a change in their wavelength maxima, may occur upon binding to a biological target, thus providing additional tissue contrast.
  • UV or visible light may also be used.
  • the invention is based on peptides and peptide-targeted multimeric contrast agents for MR, optical, and radionuclide imaging, wherein a single peptide can function both as a targeting group and a point of attachment for one or more chelates at both the N- and C-termini, either directly or via an optional intervening linker.
  • contrast agents of the invention maintain binding affinity for biological targets such as fibrin and high relaxivity.
  • Agents of the invention have a sufficient half-life following in vivo administration such that effective imaging studies can be performed.
  • the invention features purified peptides that include the amino acid sequence: P*-Y*-X 1 *-L* (SEQ ID NO:1), wherein P* is a proline or a non-natural derivative thereof; Y* is a tyrosine or a non-natural derivative thereof; X 1 * is G or D or a non-natural derivative of G or D; L* is a leucine or a non-natural derivative thereof; and wherein at least one of P*, Y*, X 1 *, and L* is a non-natural derivative of the respective amino acid.
  • X 1 * can be G or D and L* can be leucine.
  • P* is proline or 4-hydroxyproline
  • Y* is tyrosine or a non-natural derivative of tyrosine substituted at the 3 position with a moiety selected from the group consisting of F, Cl, Br, I, and NO 2 .
  • Compounds of the invention can include such peptides linked to a thrombolytic agent.
  • the invention features purified peptides that include the amino acid sequence X 1 -X 2 -C-P*-Y*-X 3 -L-C-X 4 -X 5 -X 6 (SEQ ID NO:2), wherein: P* is a proline or a non-natural derivative thereof; Y* is a tyrosine or a non-natural derivative thereof; XI is selected from the group consisting of W, Y, F, S, Bip, Hx, Dpr, Cy, Gu, Ad, Hfe, 3-Pal, 4-Pal, DopaMe2, nTyr, dW, dF, F(3/4*), and Y(3*), wherein F(3/4*) is a phenylalanine substituted at either the 3 or the 4 position with a moiety selected from the group consisting of CH 3 , CF 3 , NH 2 , CH 2 NH 2 , CN, F, Cl, Br,
  • P* can be proline or 4-hydroxyproline
  • Y* can be tyrosine or a non-natural derivative of tyrosine substituted at the 3 position with a moiety selected from the group consisting of F, Cl, Br, I, and NO 2 .
  • the purified peptides can be capable of forming a disulfide bond under non-reducing conditions and can have specific binding affinity for fibrin.
  • the peptides include a disulfide bond.
  • Compounds of the invention can include such peptides linked to a thrombolytic agent.
  • the invention also features purified peptides having an amino acid sequence selected from the group consisting of W-dE-C-P(4-OH)-Y(3-Cl)-G-L-C-W-1-Q (SEQ ID NO:4), Y-dE-C-P(4-OH)-Y(3-Cl)-G-L-C-Y-1-Q (SEQ ID NO:5), Y-dE-C-P(4-OH)-Y(3-Cl)-G-L-C-W-1-Q (SEQ ID NO:6), W-dE-C-P(4-OH)-Y(3-Cl)-G-L-C-Y-1-Q (SEQ ID NO:7), W-dE-C-P(4-OH)-Y(3-Cl)-D-L-C-W-1-Q (SEQ ID NO:8), Y-dE-C-P(4-OH)-Y(3-Cl)-D-L-C-W
  • the peptides can be capable of forming a disulfide bond under non-reducing conditions, and in some embodiments, the peptides include a disulfide bond.
  • the peptides can have specific binding affinity for fibrin.
  • Compounds of the invention can include such peptides linked to a thrombolytic agent.
  • the invention features purified peptides that include the amino acid sequence: C-P*-Y*-X 1 -L-C (SEQ ID NO:3), wherein X 1 is G or D, P* is proline or its non-natural derivative 4-hydroxyproline; and Y* is tyrosine or a non-natural derivative of tyrosine substituted at the 3 position with a moiety selected from the group consisting of F, Cl, Br, I, and NO 2 ; provided that at least one of P* or Y* is a non-natural derivative of the respective amino acid.
  • the purified peptides can be capable of forming a disulfide bond under non-reducing conditions and can have specific binding affinity for fibrin.
  • the peptides include a disulfide bond.
  • Compounds of the invention can include such peptides linked to a thrombolytic agent.
  • the invention also features purified peptides that include the amino acid sequence: C-D-Y-Y-G-T-C-X 10 (SEQ ID. NO: 17), wherein X 10 is selected from the group consisting of n(decyl)G, n(4-PhBu)G, MeL, Bpa, Bip, Me-Bip, F(4*), F(3-Me), F(3,4-difluoro), Amh, Hfe, Y(3,5-di-iodo), Pff, INal, dINal, and MeL, wherein F(4*) is a phenylalanine substituted at the 4 position with a moiety selected from the group consisting of Et, CF 3 , I, and iPr.
  • X 10 is selected from the group consisting of n(decyl)G, n(4-PhBu)G, MeL, Bpa, Bip, Me-Bip, F(4
  • Purified peptides can include the amino acid sequence C-D-Y-Y-G-T-C-X 10 -X 11 (SEQ ID. NO: 18), wherein X 11 is selected from the group consisting of D, dD, ⁇ D, Inp, Nip, Me-D, dC, Cop, and Cmp.
  • a peptide can have the follow amino acid sequences: L-P-C-D-Y-Y-G-T-C-n(Decyl)G-dD (SEQ ID NO:19), L-P-C-D-Y-Y-G-T-C-n(Decyl)G-D (SEQ ID NO:20), L-P-C-D-Y-Y-G-T-C-Bip-D (SEQ ID NO:21), L-P-C-D-Y-Y-G-T-C-Bip-dD (SEQ ID NO:22), L-P-C-D-Y-Y-G-T-C-MeL-Inp (SEQ ID NO:23), L-P-C-D-Y-Y-G-T-C-MeL-Cmp (SEQ ID NO:24), or L-P-C-D-Y-Y-G-T-C-MeBip-D (SEQ ID NO:25).
  • the purified peptides can be capable of forming a disulfide bond under non-reducing conditions and can have specific binding affinity for fibrin.
  • the peptides include a disulfide bond.
  • Compounds of the invention can include such peptides linked to a thrombolytic agent.
  • the invention features a method of making an MR imaging agent.
  • the method includes reacting a peptide having an N-terminal amine functional group with a linker-subunit moiety to form a modified peptide having both a C-terminal amine functional group and N-terminal amine functional group; covalently attaching a linker moiety to the C-terminal amine functional group and to the N-terminal amine functional group to form a precursor MR imaging agent; and converting the precursor MR imaging agent to the MR imaging agent.
  • the linker-subunit moiety can be selected from the group consisting of:
  • the linker moiety can be selected from the group consisting of
  • n is an integer from 0 to 4
  • LG is a leaving group
  • R′ and R′′ independently are selected from the group consisting of hydrogen and a chemical protecting group.
  • linker moiety also can be selected from the group consisting of:
  • LG is a leaving group; and R 1 and R 2 independently are selected from the group consisting of hydrogen and a chemical protecting group.
  • the LG can be selected from the group consisting of —OH, activated ester, halide, and anhydride.
  • the activated ester can be selected from the group consisting of pentafluorophenol (Pfp), N-hydroxysuccinimide (NHS), N-Hydroxysulfosuccinimide Sodium Salt (NHSS), 2-Thioxothiazolidin-1yl, and hydroxybenzotriazole (OBT).
  • the halide can be selected from the group consisting of F, Cl, Br, and I.
  • the chemical protecting group can be selected from the group consisting of Boc, Fmoc, CBZ, t-butyl, benzyl, and allyl.
  • Converting the precursor MR imaging agent to the MR imaging agent can include reacting the precursor imaging agent with a precursor chelate moiety to form a covalent bond between the precursor chelate moiety and the linker moiety of the precursor MR imaging agent, the precursor chelate moiety comprising a plurality of carboxylate precursor groups, the carboxylate precursor groups capable of being transformed into carboxylate moieties; transforming a plurality of the carboxylate precursor groups of the bound precursor chelate moiety to a plurality of carboxylate moieties, the carboxylate moieties capable of complexing a paramagnetic metal ion; and complexing a paramagnetic metal ion to the plurality of carboxylate moieties to produce the MR imaging agent.
  • the precursor chelate moiety can be selected from the group consisting of:
  • Y is a synthetic moiety capable of forming a covalent bond with the attached linker moiety
  • each X independently, is an O ⁇ or an O ⁇ precursor so that X, upon conversion to O, is capable of forming a carboxylate moiety with its adjacent carbonyl
  • R 1 is an uncharged chemical moiety, an aliphatic, alkyl group, or cycloalkyl group, or uncharged substituted versions thereof.
  • the synthetic moiety can be selected from the group consisting of a carboxylic acid, activated ester, acid halide, anhydride, alkyl halide, isocyanate, and isothiocyanate, and wherein the O ⁇ precursor is selected from the group consisting of —OH, —OMe, OEt, OtBu, Obenzyl, and O-allyl.
  • the precursor chelate moiety also can be selected from the group consisting of:
  • LG is a leaving group selected from the group consisting of —OH, activated ester, halide, and anhydride
  • each R independently, is an O ⁇ or an O ⁇ precursor selected from the group consisting of OH, —O-Me, O-Et, O-tBu, O-benzyl, and O-allyl, so that R, upon conversion to O, is capable of forming a carboxylate moiety with its adjacent carbonyl.
  • the precursor chelate moiety also can be selected from the group consisting of:
  • n is an integer from 1 to 4; R is selected from the group consisting of a negative charge and a negative charge precursor capable of being transformed into a negative charge; and X is a chemical leaving group selected from the group consisting of —Cl, —Br, —I, -MsO, -TsO, and -TfO.
  • the precursor chelate moiety can be selected from the group consisting of:
  • R is selected from the group consisting of a negative charge and a negative charge precursor capable of being transformed into a negative charge
  • X is a chemical leaving group selected from the group consisting of —Cl, —Br, —I, -MsO, -TsO, and -TfO.
  • the negative charge precursor is selected from the group consisting of —H, -Me, -Et, -t-Bu, -benzyl, and -allyl.
  • the linker moiety can be covalently conjugated to a precursor chelate moiety, the covalent conjugate comprising a plurality of carboxylate precursor groups, the carboxylate precursor groups capable of being transformed into carboxylate moieties.
  • Converting the precursor MRI imaging agent to the MR imaging agent can include transforming a plurality of the covalent conjugate's carboxylate precursor groups into carboxylate moieties, the carboxylate moieties capable of complexing a paramagnetic metal ion; and complexing a paramagnetic metal ion to the plurality of carboxylate moieties to result in the MR imaging agent.
  • the paramagnetic metal ion can be selected from the group consisting of: Gd(III), Fe(III), Mn(II and III), Cr(III), Cu(II), Dy(III), Tb(III and IV), Ho(III), Er(III), Pr(III), Eu(II) and Eu(III).
  • Gd(III) is a particularly useful paramagnetic ion.
  • the covalent conjugate can be selected from the group consisting of
  • n is an integer from 1 to 4; LG is a leaving group selected from the group consisting of —OH, activated ester, halide, and anhydride; and R 1 , R 2 , R 3 , R 4 , and R 5 are independently selected from the group consisting of an acetate moiety, a -Me, -Et, or -t-Bu protected acetate moiety, an acetamide moiety, and an acetoxy moiety.
  • the covalent conjugate also can be selected from the group consisting of:
  • LG is a leaving group selected from the group consisting of —OH, activated ester, halide, and anhydride; and R 1 , R 2 , R 3 , and R 4 are selected from the group consisting of an acetate moiety, a -Me, -Et, or -t-Bu protected acetate moiety, an acetamide moiety, and an acetoxy moiety.
  • the covalent conjugate can be selected from the group consisting of:
  • the covalent conjugate also can be selected from the group consisting of:
  • R is a -tBu group
  • LG is a leaving group selected from the group consisting of —OH, activated ester, halide, and anhydride.
  • Methods of the invention further can include, before covalently attaching a linker moiety to the C- and N-terminal amine functional groups, reacting a linker-subunit with the N-terminal amine functional group of the peptide to produce a derivatized N-terminal amine functional group of the peptide.
  • the linker-subunit can be selected from the group consisting of:
  • Base is selected from the group consisting of adenosine, guanosine, thymine, and cytosine; LG is a leaving group selected from the group consisting of OH, activated ester, halide, and anhydride; and R is an aliphatic or aromatic moiety.
  • the linker-subunit also can be selected from the group consisting of:
  • n is independently an integer from 0 to 3; R is an aliphatic or aromatic group; and LG is a leaving group selected from the group consisting of: OH, activated ester, halide, and anhydride.
  • the linker-subunit also can be selected from the group consisting of:
  • n is independently 1 or 2; R is an aliphatic or aromatic group; and LG is a leaving group selected from the group consisting of: OH, activated ester, halide, and anhydride.
  • the invention features a method of making a MR imaging agent.
  • the method includes covalently binding an amino acid residue to a linker-subunit moiety to form a C-terminal end of a peptide, wherein the linker-subunit moiety is covalently attached to a resin; synthesizing a peptide on the resin from the covalently bound C-terminal end to an N-terminal residue of the peptide, the N-terminal residue comprising an N-terminal amine functional group; cleaving the peptide from the resin to produce a peptide having a C-terminal amine functional group; covalently attaching a linker moiety to the peptide's C-terminal amine functional group and N-terminal amine functional group to form a precursor MR imaging agent; and converting the precursor MR imaging agent to the MR imaging agent.
  • the method further can include before cleaving the peptide from the resin, covalently attaching a linker-subunit moiety to the N-terminal amino functional group to produce a derivatized N-terminal amine functional group.
  • the linker moiety can be covalently conjugated to a precursor chelate moiety, the covalent conjugate comprising a plurality of carboxylate precursor groups, the carboxylate precursor groups capable of being transformed into carboxylate moieties.
  • Converting the precursor MR imaging agent to the MR imaging agent can include reacting the precursor MR imaging agent with a precursor chelate moiety to form a covalent bond between the precursor chelate moiety and the linker moiety of the precursor MR imaging agent, the precursor chelate moiety comprising a plurality of carboxylate precursor groups, the carboxylate precursor groups capable of being transformed into carboxylate moieties; transforming a plurality of the carboxylate precursor groups of the bound precursor chelate moiety to a plurality of carboxylate moieties, the carboxylate moieties capable of complexing a paramagnetic metal ion; and complexing a paramagnetic metal ion to the plurality of carboxylate moieties to produce the MR imaging agent.
  • Converting the precursor MRI imaging agent to the MR imaging agent also can include transforming a plurality of the covalent conjugate's carboxylate precursor groups into carboxylate moieties, the carboxylate moieties capable of complexing a paramagnetic metal ion; and complexing a paramagnetic metal ion to the plurality of carboxylate moieties to result in the MR imaging agent.
  • the paramagnetic metal ion can be selected from the group consisting of: Gd(III), Fe(III), Mn(II and III), Cr(III), Cu(II), Dy(III), Tb(III and IV), Ho(III), Er(III), Pr(III), Eu(II) and Eu(III).
  • Gd(III) is a particularly useful paramagnetic metal ion.
  • the invention features a method of making a MR imaging agent that includes reacting a peptide having a C-terminal carboxylate functional group with a linker-subunit moiety to form a modified peptide having both a C-terminal carboxylate functional group and an N-terminal carboxylate functional group; covalently attaching a linker moiety to both the N-terminal and C-terminal carboxylate functional groups of the modified peptide to form a precursor MR imaging agent; and converting the precursor MR imaging agent to the MR imaging agent.
  • the linker-subunit moiety can be selected from the group consisting of:
  • LG is a leaving group selected from the group consisting of OH, activated ester, halide, and anhydride; and R is an aromatic or aliphatic group.
  • the linker moiety also can be selected from the group consisting of:
  • linker moiety can be selected from the group consisting of:
  • R 1 and R 2 are selected independently from the group consisting of hydrogen and a chemical protecting group, the chemical protecting group selected from the group consisting of: Boc, Fmoc, CBZ, t-butyl, benzyl, and allyl.
  • Converting the precursor MR imaging agent to the MR imaging agent also can include reacting the precursor MR imaging agent with a precursor chelate moiety in order to form a covalent bond between the linker moiety of the precursor MR imaging agent and the precursor chelate moiety, the precursor chelate moiety comprising a plurality of carboxylate precursor groups, the carboxylate precursor groups capable of being transformed into carboxylate moieties; transforming a plurality of the carboxylate precursor groups of the bound precursor chelate moiety to a plurality of carboxylate moieties, the carboxylate moieties capable of complexing a paramagnetic metal ion; and complexing a paramagnetic metal ion to the plurality of carboxylate moieties to produce the MR imaging agent.
  • the linker moiety can be covalently conjugated to a precursor chelate moiety, the covalent conjugate comprising a plurality of carboxylate precursor groups, the carboxylate precursor groups capable of being transformed into carboxylate moieties.
  • Converting the precursor MR imaging agent to the MR imaging agent also can include transforming a plurality of the covalent conjugate's carboxylate precursor groups into carboxylate moieties, the carboxylate moieties capable of complexing a paramagnetic metal ion; and complexing a paramagnetic metal ion to the plurality of carboxylate moieties to produce the MR imaging agent.
  • a covalent conjugate can be selected from the group consisting of:
  • n is an integer from 1 to 4; and R 1 , R 2 , R 3 , R 4 , and R 5 are independently selected from the group consisting of an acetate moiety, a -Me, -Et, or -t-Bu protected acetate moiety, an acetamide moiety, and an acetoxy moiety.
  • the covalent conjugate can be:
  • Converting the precursor MR imaging agent to the MR imaging agent also can include reacting the precursor imaging agent with a chelate moiety, wherein the chelate moiety contains a paramagnetic metal ion, to form a covalent bond between the chelate moiety and the linker moiety of the precursor MR imaging agent to produce the MR imaging agent.
  • a chelate moiety contains a paramagnetic metal ion
  • the invention features a contrast agent that includes a metal chelate complex at a —CO 2 R and NHR termini of a biopolymer (e.g., a peptide), wherein R is independently selected from the group consisting of hydrogen, alkyl, aliphatic, and a leaving group.
  • the contrast agent can include two metal chelate complexes at the CO 2 R and NHR termini of the biopolymer.
  • the biopolymer can have a specific binding affinity for fibrin.
  • the peptide can be capable of forming a disulfide bond under non-reducing conditions, and in some embodiments, includes a disulfide bond.
  • a contrast agent can have the formula:
  • Chelate represents a metal chelate complex
  • Linker represents a linker moiety
  • Linker-subunit represents a linker-subunit moiety
  • m is independently an integer from 1 to 10
  • p is independently an integer from 0 to 5
  • s is independently 0 or 1
  • R 1 is an amino acid side chain or a derivative thereof
  • R 2 is independently a hydrogen or an aliphatic group.
  • a contrast agent also can have a structure of any one of structures 4-55.
  • the invention features a method for altering the stability of a peptide, the peptide having an N-terminal amine functional group.
  • the method includes reacting the peptide with a linker-subunit moiety to form a peptide having a C-terminal amine functional group; and covalently attaching a linker moiety to the peptide's C-terminal amine functional group and N-terminal amine functional group to form a modified peptide.
  • the method further can include reacting the modified peptide with a capping moiety to form a covalent bond between the capping moiety and the linker moiety of the modified peptide.
  • the method also can include reacting the modified peptide with a precursor chelate moiety to form a covalent bond between the precursor chelate moiety and the linker moiety of the modified peptide, the precursor chelate moiety comprising a plurality of carboxylate precursor groups, the carboxylate precursor groups capable of being transformed into carboxylate moieties.
  • the carboxylate moieties capable of complexing a paramagnetic metal ion; a paramagnetic metal ion can be complexed to the plurality of carboxylate moieties.
  • the method further can include assaying the stability of the modified peptide or assaying the stability of the unmodified peptide and comparing the stability of said modified peptide to the stability of the unmodified peptide.
  • Stability of the modified peptide can be improved relative to the stability of the unmodified peptide (e.g., improved 10-fold, 20-fold, or 30-fold relative to the stability of the unmodified peptide). Stability can be assayed using a rat liver homogenate assay.
  • the invention features a modified peptide having the structure:
  • Chelate precursor represents a chelate precursor moiety
  • Linker represents a linker moiety
  • Linker-subunit represents a linker-subunit moiety
  • m is independently an integer from 1 to 10
  • p is independently an integer from 0 to 5;
  • R 2 is selected from the group consisting of H and an aliphatic group.
  • the invention features a modified peptide having the structure:
  • Linker represents a linker moiety
  • Linker-subunit represents a linker-subunit moiety
  • p is independently an integer from 0 to 5
  • s is independently 0 or 1
  • R 1 is an amino acid side chain or a derivative thereof
  • R 2 is selected from the group consisting of H and an aliphatic group.
  • Method of making an MR imaging agent also are featured that include reacting a peptide having an N-terminal amine functional group with a linker-subunit moiety to form a modified peptide having an amine functional group on both its N-terminus and C-terminus, or reacting a peptide having a C-terminal carboxylate functional group with a linker-subunit moiety to form a modified peptide having a carboxylate functional group on both its C-terminus and N-terminus; and converting the modified peptide to the MR imaging agent.
  • Converting the modified peptide to the MR imaging agent can include covalently attaching a chelate moiety to the modified peptide, wherein the chelate moiety contains a paramagnetic metal ion, to produce the MR imaging agent. Converting the modified peptide to the MR imaging agent also can include covalently linking a linker moiety to a chelate moiety to form a covalent conjugate, wherein the chelate moiety contains a paramagnetic metal ion; and reacting the covalent conjugate with the modified peptide to form the MR imaging agent. Suitable paramagnetic ions are described above.
  • the invention features a method of making an MR imaging agent that includes covalently binding an amino acid residue to a linker-subunit moiety to form a C-terminal end of a peptide, wherein the linker-subunit moiety is covalently attached to a resin; synthesizing a peptide on the resin from the covalently bound C-terminal end to an N-terminal residue of the peptide, the N-terminal residue comprising an N-terminal amine functional group; cleaving the peptide from the resin to produce a C-terminal amine functional group of the modified peptide; converting the modified peptide to the MR imaging agent.
  • Converting the modified peptide to the MR imaging agent can include covalently attaching a chelate moiety to the modified peptide, wherein the chelate moiety contains a paramagnetic metal ion, to produce the MR imaging agent. Converting the modified peptide to the MR imaging agent also can include covalently linking a linker moiety to a chelate moiety to form a covalent conjugate, wherein the chelate moiety contains a paramagnetic metal ion; and reacting the covalent conjugate with the modified peptide to form the MR imaging agent. Suitable paramagnetic ions are described above.
  • FIG. 1 provides the chemical structures of non-natural amino acids.
  • FIG. 2 depicts the relaxivities per Gd at 20 MHz, 35° C. in Tris buffered saline (TBS) or 10 mg/ml fibrin in TBS.
  • FIG. 3 depicts the accumulation of a contrast agent in the thrombus.
  • FIG. 4A is an image of a thrombus.
  • FIG. 4B is an image of a thrombus with black blood.
  • the term “chemical protecting group” or “protecting group” means any chemical moiety temporarily covalently bound to a molecule throughout one or more synthetic chemistry steps in a reaction sequence to prevent undesirable reactions. Common protecting group strategies are described in “Protecting Groups in Organic Synthesis, Third Ed.” by P. Wuts and T. Greene, ⁇ 1999 John Wiley & Sons, Inc.
  • the term “leaving group” means any chemical moiety that is displaced by a nucleophile in a nucleophilic substitution or sequence of addition-elimination reactions.
  • a molecule comprising a leaving group may be isolated or it may be formed in situ as a transient intermediate in a chemical reaction.
  • aliphatic describes any acyclic or cyclic, saturated or unsaturated, branched or unbranched carbon compound, excluding aromatic compounds.
  • alkyl includes saturated aliphatic groups, including straight-chain alkyl groups (e.g., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, etc.), branched-chain alkyl groups (isopropyl, tert-butyl, isobutyl, etc.), cycloalkyl (alicyclic) groups (cyclopropyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl), alkyl substituted cycloalkyl groups, and cycloalkyl substituted alkyl groups.
  • straight-chain alkyl groups e.g., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl,
  • alkyl further includes alkyl groups, which can further include oxygen, nitrogen, sulfur or phosphorous atoms replacing one or more carbons of the hydrocarbon backbone.
  • a straight chain or branched chain alkyl has 6 or fewer carbon atoms in its backbone (e.g., C 1 -C 6 for straight chain, C 3 -C 6 for branched chain), and more preferably 4 or fewer.
  • preferred cycloalkyls have from 3-8 carbon atoms in their ring structure, and more preferably have 5 or 6 carbons in the ring structure.
  • C 1 -C 6 includes alkyl groups containing 1 to 6 carbon atoms.
  • alkyl includes both “unsubstituted alkyls” and “substituted alkyls,” the latter of which refers to alkyl moieties having substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone.
  • substituents can include, for example, alkenyl, alkynyl, halogen, hydroxyl, alkylcarbonyloxy, arylcarbonyloxy, alkoxycarbonyloxy, aryloxycarbonyloxy, carboxylate, alkylcarbonyl, arylcarbonyl, alkoxycarbonyl, aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, alkylthiocarbonyl, alkoxyl, phosphate, phosphonato, phosphinato, cyano, amino (including alkyl amino, dialkylamino, arylamino, diarylamino, and alkylarylamino), acylamino (including alkylcarbonylamino, arylcarbonylamino, carbamoyl and ureido), amidino, imino, sulfhydryl, alkylthio, arylthio, thiocarboxylate, sul
  • Cycloalkyls can be further substituted, e.g., with the substituents described above.
  • An “arylalkyl” moiety is an alkyl substituted with an aryl (e.g., phenylmethyl (benzyl)).
  • alkyl also includes the side chains of natural and unnatural amino acids.
  • n-alkyl means a straight chain (i.e., unbranched) unsubstituted alkyl group.
  • alkenyl includes aliphatic groups that may or may not be substituted, as described above for alkyls, containing at least one double bond and at least two carbon atoms.
  • alkenyl includes straight-chain alkenyl groups (e.g., ethylenyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, etc.), branched-chain alkenyl groups, cycloalkenyl (alicyclic) groups (cyclopropenyl, cyclopentenyl, cyclohexenyl, cycloheptenyl, cyclooctenyl), alkyl or alkenyl substituted cycloalkenyl groups, and cycloalkyl or cycloalkenyl substituted alkenyl groups.
  • alkenyl further includes alkenyl groups that include oxygen, nitrogen, sulfur or phosphorous atoms replacing one or more carbons of the hydrocarbon backbone.
  • a straight chain or branched chain alkenyl group has 6 or fewer carbon atoms in its backbone (e.g., C 2 -C 6 for straight chain, C 3 -C 6 for branched chain).
  • cycloalkenyl groups may have from 3-8 carbon atoms in their ring structure, and more preferably have 5 or 6 carbons in the ring structure.
  • C 2 -C 6 includes alkenyl groups containing 2 to 6 carbon atoms.
  • alkenyl includes both “unsubstituted alkenyls” and “substituted alkenyls,” the latter of which refers to alkenyl moieties having substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone.
  • substituents can include, for example, alkyl groups, alkynyl groups, halogens, hydroxyl, alkylcarbonyloxy, arylcarbonyloxy, alkoxycarbonyloxy, aryloxycarbonyloxy, carboxylate, alkylcarbonyl, arylcarbonyl, alkoxycarbonyl, aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, alkylthiocarbonyl, alkoxyl, phosphate, phosphonato, phosphinato, cyano, amino (including alkyl amino, dialkylamino, arylamino, diarylamino, and alkylarylamino), acylamino (including alkylcarbonylamino, arylcarbonylamino, carbamoyl and ureido), amidino, imino, sulfhydryl, alkylthio, arylthio, thiocarboxylate,
  • alkynyl includes unsaturated aliphatic groups analogous in length and possible substitution to the alkyls described above, but which contain at least one triple bond and two carbon atoms.
  • alkynyl includes straight-chain alkynyl groups (e.g., ethynyl, propynyl, butynyl, pentynyl, hexynyl, heptynyl, octynyl, nonynyl, decynyl, etc.), branched-chain alkynyl groups, and cycloalkyl or cycloalkenyl substituted alkynyl groups.
  • alkynyl further includes alkynyl groups that include oxygen, nitrogen, sulfur or phosphorous atoms replacing one or more carbons of the hydrocarbon backbone.
  • a straight chain or branched chain alkynyl group has 6 or fewer carbon atoms in its backbone (e.g., C 2 -C 6 for straight chain, C 3 -C 6 for branched chain).
  • C 2 -C 6 includes alkynyl groups containing 2 to 6 carbon atoms.
  • aryl includes groups, including 5- and 6-membered single-ring aromatic groups that may include from zero to four heteroatoms, for example, benzene, phenyl, pyrrole, furan, thiophene, thiazole, isothiaozole, imidazole, triazole, tetrazole, pyrazole, oxazole, isooxazole, pyridine, pyrazine, pyridazine, and pyrimidine, and the like.
  • aryl includes multicyclic aryl groups, e.g., tricyclic, bicyclic, such as naphthalene, benzoxazole, benzodioxazole, benzothiazole, benzoimidazole, benzothiophene, methylenedioxyphenyl, quinoline, isoquinoline, napthridine, indole, benzofuran, purine, benzofuran, deazapurine, or indolizine.
  • aryl groups e.g., tricyclic, bicyclic, such as naphthalene, benzoxazole, benzodioxazole, benzothiazole, benzoimidazole, benzothiophene, methylenedioxyphenyl, quinoline, isoquinoline, napthridine, indole, benzofuran, purine, benzofuran, deazapurine, or indolizine.
  • aryl groups having heteroatoms in the ring structure may also be referred to as “aryl heterocycles,” “heterocycles,” “heteroaryls,” or “heteroaromatics.”
  • An aryl group may be substituted at one or more ring positions with substituents.
  • DTPA refers to a chemical compound comprising a substructure composed of diethylenetriamine, wherein the two primary amines are each covalently attached to two acetyl groups and the secondary amine has one acetyl group covalently attached according to the following formula:
  • X is a heteroatom electron-donating group capable of coordinating a metal cation, preferably O ⁇ , OH, NH 2 , OPO 3 2 ⁇ , or NHR, or OR wherein R is any aliphatic group.
  • DTPE tert-butoxy
  • DOTA refers to a chemical compound comprising a substructure composed of 1,4,7,11-tetraazacyclododecane, wherein the amines each have one acetyl group covalently attached according to the following formula:
  • NOTA refers to a chemical compound comprising a substructure composed of 1,4,7-triazacyclononane, wherein the amines each have one acetyl group covalently attached according to the following formula:
  • DO3A refers to a chemical compound comprising a substructure composed of 1,4,7,11-tetraazacyclododecane, wherein three of the four amines each have one acetyl group covalently attached and the other amine has a substituent having neutral charge according to the following formula:
  • R 1 is an uncharged chemical moiety, preferably hydrogen, any aliphatic, alkyl group, or cycloalkyl group, and uncharged derivatives thereof.
  • the carbon atoms of the indicated ethylenes may be referred to as “backbone” carbons.
  • the designation “bbDTPA” may be used to refer to the location of a chemical bond to a DTPA molecule (“bb” for “back bone”).
  • bb(CO)DTPA-Gd means a C ⁇ O moiety bound to an ethylene backbone carbon atom of DTPA.
  • chelating ligand may be used to refer to any polydentate ligand which is capable of coordinating a metal ion, including DTPA (and DTPE), DOTA, DO3A, or NOTA molecule, or any other suitable polydentate chelating ligand as is further defined herein, that is either coordinating a metal ion or is capable of doing so, either directly or after removal of protecting groups, or is a reagent, with or without suitable protecting groups, that is used in the synthesis of a contrast agent and comprises substantially all of the atoms that ultimately will coordinate the metal ion of the final metal complex.
  • chelate refers to the actual metal-ligand complex, and it is understood that the polydentate ligand will eventually be coordinated to a medically useful metal ion.
  • the specific binding affinity of a binding group for a target is expressed in terms of the equilibrium dissociation constant “Kd.”
  • open coordination site refers to a site on a metal ion that is generally occupied by a water or solvent molecule.
  • the term “purified” refers to a peptide that has been separated from either naturally occurring organic molecules with which it normally associates or, for a chemically-synthesized peptide, separated from any other organic molecules present in the chemical synthesis.
  • the polypeptide is considered “purified” when it is at least 70% (e.g., 70%, 80%, 90%, 95%, or 99%), by dry weight, free from any other proteins or organic molecules.
  • peptide refers to a chain of amino acids that is about 2 to about 75 amino acids in length (e.g., 3 to 50 amino acids).
  • biopolymer refers to a polymeric substance that is naturally formed in a biological system. Certain biopolymers can be constructed from a defined set of building subunits and with common functionalities linking the subunits, e.g., a peptide is usually constructed from a set of amino acids (both natural and non-natural) with amide bonds linking the subunits.
  • multimer for purposes herein is defined as a contrast agent or a subunit thereof comprising at least two covalently bonded chelates or synthetic precursors thereof.
  • natural amino acid refers to one of the twenty most common occurring amino acids. Natural amino acids modified to provide a label for detection purposes (e.g., radioactive labels, optical labels, or dyes) are considered to be natural amino acids. Natural amino acids are referred to by their standard one- or three-letter abbreviations.
  • non-natural amino acid refers to any derivative of a natural amino acid including D forms, and 0 and y amino acid derivatives. It is noted that certain amino acids, e.g., hydroxyproline, that are classified as a non-natural amino acid herein, may be found in nature within a certain organism or a particular protein.
  • stable refers to compounds that possess stability sufficient to allow manufacture and which maintains the integrity of the compound for a sufficient period of time to be useful and safe for the purposes detailed herein. Typically, such compounds are stable at a temperature of 40° C. or less, in the absence of moisture or other chemically reactive conditions, for at least a week. Combinations of substituents and variables envisioned by this invention are only those that result in the formation of stable compounds.
  • target binding and “binding” for purposes herein refer to non-covalent interactions of a contrast agent with a target. These non-covalent interactions are independent from one another and may be, inter alia, hydrophobic, hydrophilic, dipole-dipole, pi-stacking, hydrogen bonding, electrostatic associations, or Lewis acid-base interactions.
  • binding moiety refers to a chelate, organic dye, contrast agent, thrombolytic, or stabilizing moiety.
  • Suitable stabilizing moieties are biologically inert, i.e., does not have biological activity.
  • the present invention relates to MRI, optical, and radionuclide contrast agents that include a targeting polymer (e.g., peptide) in which both the N- and C-terminal amino acids are each conjugated, either directly or via an optional intervening linker-subunit and linker, to at least one chelate of a paramagnetic (for magnetic resonance imaging) or radioactive (for radionuclide imaging) metal ion or an optical dye (for optical imaging).
  • a targeting polymer e.g., peptide
  • the linker or linker-subunit may be branched and therefore allow for multiple chelates or dyes to be attached to each end of the peptide, i.e. a multimer.
  • the compounds of this invention may contain one or more asymmetric carbon atoms and thus may occur as racemates and racemic mixtures, single enantiomers, diastereomeric mixtures and individual diastereomers. All such isomeric forms of these compounds are expressly included in the present invention.
  • Each stereogenic carbon may be of the R or S configuration unless specifically designated otherwise.
  • the specific compounds exemplified in this application may be depicted in a particular stereochemical configuration, compounds having either the opposite stereochemistry at any given chiral center or mixtures thereof are also envisioned. It should be understood that the compounds of this invention may adopt a variety of conformational and ionic forms in solution, in pharmaceutical compositions and in vivo. Although the depictions herein of specific preferred compounds of this invention are of particular conformations and ionic forms, the disclosure of the invention is not so limited.
  • Novel peptide-based multimers of the present invention offer several advantages as targeted contrast agents.
  • the compounds can deliver two or more capping moieties (e.g., chelates, organic dyes, or thrombolytics) to the target using a single targeting peptide so that sufficient improvement in the tissue contrast will be observed in part because of a meaningful concentration of the imaging moiety around the target.
  • capping moieties e.g., chelates, organic dyes, or thrombolytics
  • the MRI contrast agents of this invention also exhibit a high relaxivity upon binding to the target due to the receptor induced magnetic enhancement (RIME) effect combined with the ability of the peptide to limit the local motion of individual chelates when bound to the target.
  • RIME receptor induced magnetic enhancement
  • the compounds have a high affinity for one or more targets.
  • the compounds of the invention can have higher in vivo stability (i.e., longer half-lives) from diminished enzyme metabolism (e.g., decreased cleavage by peptidases).
  • chelate represents a metal chelate complex
  • p is independently an integer from zero to five
  • s is independently one or zero
  • R 1 is any amino acid side chain including side chains of non-natural amino acids
  • R 2 is any aliphatic group or hydrogen
  • n is an integer from 3 to 50 inclusive.
  • R 1 and R 2 may be taken together to form a ring structure (including proline and substituted versions thereof). Linkers, if present, may be different.
  • Metal ions preferred for MRI include those with atomic numbers 21-29, 39-47, or 57-83, and, more preferably, a paramagnetic form of a metal ion with atomic numbers 21-29, 42, 44, or 57-83.
  • Particularly preferred paramagnetic metal ions are selected from the group consisting of Gd(III), Fe(III), Mn(II and III), Cr(III), Cu(II), Dy(III), Tb(III and IV), Ho(III), Er(III), Pr(III) and Eu(II and III). Gd(III) is particularly useful.
  • Gd is meant to convey the ionic form of the metal gadolinium; such an ionic form can be written as GD(III), GD3+, gado, etc., with no difference in ionic form contemplated.
  • radionuclides 90 Y, 99m Tc, 111 In, 47 Sc, 67 Ga, 51 Cr, 177m Sn, 67 Cu, 167 Tm, 97Ru, 188 Re, 177 Lu, 199 Au, 203 Pb, and 141 Ce are particularly useful.
  • Metal complexes with useful optical properties also have been described. See, Murru et al., J. Chem. Soc. Chem. Comm. 1993, 1116-1118.
  • lanthamide chelates such as La(III), Ce(III), Pr(III), Nd(III), Pn(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III) and Ln(III) are suitable. Eu(III) and Tb(III) are particularly useful.
  • Metal chelates should not dissociate to any significant degree during the imaging agent's passage through the body, including while bound to a target tissue. Significant release of free metal ions can result in toxicity, which would generally not be acceptable.
  • m is 2, n, s, R 1 , and R 2 are defined as above, and the Linker moiety comprises:
  • the “Chelate” is preferably bb(CO)DTPA.Gd.
  • m is 2, n, s, R 1 , and R 2 are defined as above, and the linker moiety comprises:
  • the “Chelate” moiety can be bb(CO)DTPA.Gd.
  • R amino acid side chains such that the peptide has affinity for a biological target
  • m metal ion (paramagnetic for MFT, radioactive for radionuclide imaging, and fluorescent, luminescent, or absorbant for optical imaging).
  • optical contrast agents contemplated by the invention may be illustrated by the formula:
  • R 1 is any amino acid side chain including non-natural amino acid side chains
  • R 2 is any aliphatic group or hydrogen.
  • R 1 and R 2 taken together form a ring structure (including Pro and derivatives thereof.
  • the linker moieties can be different.
  • the optical dye may be an organic dye or an appropriate metal chelate.
  • Organic dyes is suitable for optical imaging have been described and include, for example, fluorescent porphyrin and fluorescent phthalocyanines [see, e.g., U.S. Pat. No. 5,641,878], particulate materials [see, e.g., WO 96/23524], and polymethine dyes [see, e.g., WO 97/13490].
  • Commonly used optical organic dyes are fluorescein, rhodamine [see, e.g., Kojima H, et al., Anal. Chem. 73, 1967-1973 (2001)], tetramethylrhodamine [e.g., Anal. Biochem. 223, 39 (1994)], and Texas red [e.g., Proc. Natl. Acad. Sci. USA 85, 3546 (1988)]. Fluoroscein and luminescent lanthamide chelates are particularly useful.
  • the peptide moiety of the contrast agents of the present invention can exhibit specific binding for a biological target and function as a point of attachment for one or more chelates at each terminus.
  • biological targets are present in a low (e.g., micromolar or less) concentration and are inefficiently imaged using existing monomeric gadolinium complex MRI contrast agents.
  • the peptide-based multimeric MRI contrast agents according to the instant invention provide a much higher concentration of the agent at the target as well as high relaxivity to make imaging of these targets possible.
  • the peptide-based multimeric radionuclide contrast agents of the invention may deliver more radionuclides to targets so that imaging can be further improved.
  • targeting creates an increased concentration of the imaging agent at the site to be imaged and increases the relaxivity of MRI contrast agents in the bound state through the RIME effect and also limits local chelate motion by rigidifying the bound peptide.
  • Targets for the contrast agents can be in any body compartment, cell, organ, or tissue or component thereof.
  • Preferred targets are those that are of diagnostic and therapeutic relevance, i.e., those that are associated with disease states.
  • Particularly preferred targets are those in association with body fluids, and particularly those in association with blood, plasma, lymph and fluids of the central nervous system.
  • Other preferred targets are proteins and receptors that either exist in high concentration or have a large number of binding sites for certain ligands. Included among such target proteins are enzymes and glycoproteins.
  • HSA Human serum albumin
  • fibrin are useful targets for MRI contrast agents.
  • serum albumin is a preferred target. Since HSA is present at high concentration in serum (approximately 0.6 mM) and binds a wide array of molecules with reasonably high affinity, it is a preferred target plasma protein for blood pool contrast agents. HSA is a particularly preferred target for cardiovascular imaging; see U.S. patent application Ser. No. 08/875,365, filed Jul. 24, 1997, and WO 96/23526.
  • fibrin is a preferred target because it is present in all clots and it can be targeted without interfering with the normal thrombolytic process.
  • target binding moieties that include fibrin-binding peptides, see PCT Patent Application WO 01/09188.
  • protein targets include, but are not limited to, alpha acid glycoprotein, fibrinogen, collagen, platelet GPIlb/IIla receptor, chemotactic peptide receptor, somatostatin receptors, vasoactive intestinal peptides (VIP) receptor, bombesin/Gastrin release peptide receptor, and integrin receptors.
  • VIP vasoactive intestinal peptides
  • Suitable peptides for use in the invention include those capable of specifically binding to the targets identified above. Included among such peptides are RGD-containing peptides targeting platelet GPIIb/IIIa receptor for thrombus imaging, chemotactic peptides targeting white blood cells for infection/inflammation imaging, Octreotide and P-829 peptide targeting somastatin receptors for tumor imaging, vasoactive intestinal peptides (VIP) targeting VIP receptor for tumor imaging, bombesin analogs targeting bombesin/Gastrin release peptide receptor for tumor imaging, and RGD-containing peptides targeting the integrin ⁇ v ⁇ 3 (vitronectin receptor) for tumor imaging.
  • RGD-containing peptides targeting platelet GPIIb/IIIa receptor for thrombus imaging
  • chemotactic peptides targeting white blood cells for infection/inflammation imaging
  • Octreotide and P-829 peptide targeting somastatin receptors for tumor imaging
  • any peptide with an affinity for a biological target may be used in a contrast agent of the invention.
  • the peptide may be linear or cyclic.
  • insoluble lipophilic peptides are considered unsuitable for pharmacological use, but such peptides may be suitable according to the invention because addition of hydrophilic metal chelates to the two termini of the peptide may increase solubility.
  • the peptides have between 3 to 50 amino acids (e.g., 3 to 30, 3 to 20, 3 to 15, 5 to 30, 5 to 20, 5 to 15, 10 to 12 amino acids in length).
  • Suitable amino acids include natural and non-natural amino acids.
  • Amino acids with many different protecting groups appropriate for immediate use in the solid phase synthesis of peptides are commercially available.
  • the following non-natural amino acids or amino acid derivatives may be constituents of the peptide targeting group of the invention (common abbreviations in parentheses, see FIG.
  • (xn-alkylated amino acids may be employed, as well as amino acids having amine-containing side chains (such as Lys and Orn) in which the amine has been acylated or alkylated.
  • Peptides of the invention can include the general formula P*-Y*-X 1 *-L* (SEQ ID NO: 1), wherein P* is a proline or a non-natural derivative of proline, Y* is a tyrosine or a non-natural derivative thereof, X 1 * is glycine or aspartic acid, or a non-natural derivative of glycine or aspartic acid, and L* is leucine or a non-natural derivative thereof.
  • at least one of P*, Y*, X 1 *, or L* is a non-natural derivative of the respective amino acid.
  • X 1 * can be glycine or aspartic acid
  • L* can be leucine
  • at least one of P* or Y* can be a non-natural derivative, such as hydroxyproline or a tyrosine substituted at the 3 position with F, Cl, Br, I, or NO 2 .
  • a peptide of the invention also can include the general formula X 1 -X 2 -C-P*-Y*-X 3 -L-C-X 4 -X 5 -X 6 (SEQ ID NO:2), wherein P* is a proline or a non-natural derivative thereof; Y* is a tyrosine or a non-natural derivative thereof; X 1 is W, Y, F, S, Bip, Hx, Dpr, Cy, Gu, Ad, Hfe, 3-Pal, 4-Pal, DopaMe2, nTyr, dW, dF, F(3/4*), or Y(3*).
  • F(3/4*) can be a phenylalanine substituted at either the 3 or the 4 position with a moiety such as CH 3 , CF 3 , NH 2 , CH 2 NH 2 , CN, F, Cl, Br, I, Et, or Ome.
  • Y(3*) can be a tyrosine substituted at the 3 position with a moiety such as F, Cl, Br, I, and NO 2
  • X 2 can be E, H, dE, S, H(Bzl), 2-Pal, Dpr, or Th
  • X 3 can be G or D
  • X 4 can be H, F, Y, or W
  • X 5 can be I, L, V, N, Bpa, Bal, Hfe, Nle, Tle, Nval, Phg, Cha, Taz, Fua, Th, 4-Pal, or F(3/4*), wherein F(3/4*) is a phenylalanine substituted at either the 3 or the 4 position with a moiety such as CF 3 , Et, iPr, or OMe
  • X 6 can be N, Q, I, L, or V, or not present.
  • X 1 , X 2 , X 5 , P*, and Y* is a non-natural derivative of an amino acid.
  • P* can be proline and Y* can be a non-natural derivative of tyrosine substituted at the 3 position with a moiety such as F, Cl, Br, I, or NO 2
  • P* can be a non-natural derivative of proline such as 4-hydroxyproline and Y* can be tyrosine.
  • Such peptides can form a disulfide bond under non-reducing conditions.
  • Another example of a peptide that can bind fibrin includes the general formula C-P*-Y*-X 1 -L-C (SEQ ID NO:3), wherein X 1 is G or D, P* is proline or its non-natural derivative 4-hydroxyproline; Y* is tyrosine or a non-natural derivative of tyrosine substituted at the 3 position with a moiety such as F, Cl, Br, I, or NO 2 . Typically, at least one of P* or Y* is a non-natural derivative of the respective amino acid.
  • the peptide can have the following sequences: W-dE-C-P(4-OH)-Y(3-Cl)-G-L-C-W-1-Q (SEQ ID NO:4), Y-dE-C-P(4-OH)-Y(3-Cl)-G-L-C-Y-1-Q (SEQ ID NO:5), Y-dE-C-P(4-OH)-Y(3-Cl)-G-L-C-W-1-Q (SEQ ID NO:6), W-dE-C-P(4-OH)-Y(3-Cl)-G-L-C-Y-1-Q (SEQ ID NO:7), W-dE-C-P(4-OH)-Y(3-Cl)-D-L-C-W-1-Q (SEQ ID NO:8), Y-dE-C-P(4-OH)-Y(3-Cl)-D-L-C-Y-1-Q (SEQ ID NO:8),
  • a peptide also can have the general formula C-D-Y-Y-G-T-C-X 10 (SEQ ID. NO:17), wherein X 10 is n(decyl)G, n(4-PhBu) q MeL, Bpa, Bip, Me-Bip, F(4*), F(3-Me), F(3, 4-difluoro), Amh, Hfe, Y(3,5-di-iodo), Pff, INal, dINal, or MeL, wherein F(4*) is a phenylalanine substituted at the 4 position with a moiety such as Et, CF 3 , I, or iPr.
  • X 10 is n(decyl)G, n(4-PhBu) q MeL, Bpa, Bip, Me-Bip, F(4*), F(3-Me), F(3, 4-difluoro), Amh, Hfe,
  • a peptide can include additional residues, X 1 , P*, and/or X 11 , to provide the general formula: C-D-Y-Y-G-T-C-X 10 -X 11 (SEQ ID. NO:18) or XI-P*-C-D-Y-Y-G-T-C-X 10 -X 11 (SEQ ID. NO:26), wherein X 1 is any natural or non-natural amino acid, P* is proline or a non-natural derivative thereof, and X 11 is D, dD, OD, Inp, Nip, Me-D, Cop, or Cmp.
  • a peptide can have the sequence of L-P-C-D-Y-Y-G-T-C-n(Decyl)G-dD (SEQ ID NO:19), L-P-C-D-Y-Y-G-T-C-n(Decyl)G-D (SEQ ID NO:20), L-P-C-D-Y-Y-G-T-C-Bip-D (SEQ ID NO:21), L-P-C-D-Y-Y-G-T-C-Bip-dD (SEQ ID NO:22), L-P-C-D-Y-Y-G-T-C-MeL-Inp (SEQ ID NO:23), L-P-C-D-Y-Y-G-T-C-MeL-Cmp (SEQ ID NO:24), or L-P-C-D-Y-Y-G-T-C-MeBip-D (SEQ ID NO:25).
  • Peptides having the formula of SEQ ID NO:26 were synthesized (structure confirmed by mass spectrometry) according to standard synthesis methods, such as those disclosed in WO 01/09188 or in WO 01/08712, and assayed for affinity to the DD(E) fragment of fibrin. Each peptide was found to have a Kd ⁇ 10 ⁇ M (Table 2). TABLE 2 X 01 X 10 X 11 L n(Decyl)G dD L n(Decyl)G D L MeL Inp L Bip D L Bip dD L Me-Bip D L MeL Cmp L Bip D L L D Cha Bip D
  • the ability of the peptides to bind a target such as HSA or fibrin can be assessed by known methodology.
  • affinity of the peptide for fibrin can be assessed using the DD(E) fragment of fibrin, which contains subunits of 55 kD (Fragment E) and 190 kD (Fragment DD).
  • the DD(E) fragment can be biotinylated and immobilized via avidin to a solid substrate (e.g., a multi-well plate). Peptides can be incubated with the immobilized DD(E) fragment in a suitable buffer and binding detected using known methodology. See, for example, WO 01/09188.
  • linker-subunits and linkers are used to covalently attach capping moieties such as chelates, thrombolytics, and other groups to the two ends of a peptide.
  • a linker-subunit moiety can (i) convert the functionality of either the C-terminus carboxylate to an amine functional group or the N-terminus amine to a carboxylate functional group; or (ii) provide a spacer moiety or group between the peptide terminus and the linker, if present, or capping group.
  • a peptide can be reacted with a linker-subunit to form a modified peptide having a C-terminal amine functional group and a N-terminal amine functional group.
  • a peptide in another embodiment, can be reacted with a linker-subunit to form a modified peptide having a N-terminal carboxylate functional group and a C-terminal carboxylate functional group.
  • a peptide can be synthesized from a C-terminal linker-subunit that is bound to a resin, whereby upon cleaving the peptide from the resin, a peptide having a C-terminal amine functional group is produced.
  • a linker-subunit can be used as a spacer group and not to change the terminus functional group.
  • a linker-subunit may have multiple functional groups for attachment of linker moieties or capping moieties.
  • linker-subunit Many types of reactions can be used, including acylation, reductive amination, nucleophilic displacement reactions, urea formation, thiourea formation, and chemoselective ligation in chemically conjugating the linker-subunit to the peptide, linker, and/or capping moieties.
  • One advantage of using a linker-subunit is to create similar functional groups on the peptide, thereby facilitating subsequent synthesis.
  • the linker moiety can be used to covalently attach one or more capping moieties to the peptide terminus.
  • the linker may be branched or unbranched and may comprise multiple functional groups for precursor chelate and chelate attachment.
  • the chemical structure of the linker may affect the physical and pharmacological properties of the contrast agent, such as affinity, stability, blood half-life, relaxivity, and plasma protein binding.
  • Linkers may be substituted with alkyl, aryl, alkenyl, or alkynyl groups.
  • Linkers, if present, at each termini typically are relatively small and rigid for MRI contrast agents.
  • a linker can have a molecular weight less than about 350 (e.g., less than about 200).
  • the C-terminus carboxylate of a peptide may be converted to an amine functional group with a linker-subunit (e.g., a diamine synthon) to form a peptide having an amine functional group on each end of the peptide to which the remaining linker moiety can be attached.
  • linker-subunit e.g., a diamine synthon
  • Examples of such peptides modified to have a C-terminal amine function group are:
  • linker-subunits may be employed as spacer groups at the N-terminal amine functional group:
  • ⁇ N-alkylated amino acids may be employed, as well as amino acids having amine-containing side chains (such as Lys and Orn) in which the amine has been acylated or alkylated as in the following examples:
  • n is an integer from 0 to 3
  • R is any aliphatic or aromatic group
  • LG is a leaving group such as OH, activated ester, halide, and anhydride.
  • linker-subunits include the following:
  • n is independently 1 or 2
  • R is any aliphatic or aromatic group
  • LG is a leaving group such as OH, activated ester, halide, and anhydride.
  • linker moieties that are useful when following an amide bond construction strategy in which a peptide molecule has two terminal amine groups include the following:
  • each m is independently an integer from 1 to 4
  • n is independently an integer from 0 to 4 inclusive
  • LG is a leaving group
  • R′ or R′′ are independently hydrogen or a chemical protecting group.
  • a linker moiety also may have branch points for attachment of more than two chelates.
  • a linker that includes a carbonyl with a leaving group LG for example, a carboxylic acid or an activated ester
  • LG for example, a carboxylic acid or an activated ester
  • the following carbonyl-based linker reagents may be appropriate for introducing three or more amine functional groups:
  • LG is a leaving group (e.g., —OH, activated ester such as pentafluorophenol (Pfp), N-hydroxysuccinimide (NHS), N-hydroxysulfosuccinimide sodium salt (NHSS), 2-thioxothiazolidin-1 yl, or hydroxybenzotriazole (HBT) and R 1 and R 2 are preferably independently hydrogen or a chemical protecting group (e.g., Boc, Fmoc, CBZ, t-butyl, benzyl, or allyl).
  • Pfp pentafluorophenol
  • NHS N-hydroxysuccinimide
  • NHSS N-hydroxysulfosuccinimide sodium salt
  • HBT 2-thioxothiazolidin-1 yl
  • R 1 and R 2 are preferably independently hydrogen or a chemical protecting group (e.g., Boc, Fmoc, CBZ, t-butyl, benzyl, or allyl).
  • an amine functional group at the N-terminus of a peptide may be converted to an N-terminus carboxylate functional group by reaction with a cyclic acid anhydride (linker-subunit moiety) thereby producing a modified peptide with a N-terminal carboxylate functional group:
  • linker-subunits that can be used to convert an N-terminal amine to a carboxylate functional group include:
  • R is any aliphatic or aromatic group.
  • both terminal carboxylates in the above examples may be simultaneously reacted with an amino group on a linker moiety as shown below to form a precursor MR imaging agent.
  • the precursor MR imaging agent is a peptide molecule derivatized with linkers at both termini thru amide bonds:
  • linker moieties useful for producing precursor MR imaging agents terminating with two carboxylates are:
  • each m is independently 1 to 4 inclusive
  • R is hydrogen or an appropriate chemical protecting group, such as methyl, ethyl, benzyl, or t-butyl.
  • the protecting groups can be removed and chelating or precursor chelating moieties can be attached through standard methods, for example, amide bond formation.
  • linker reagents may be appropriate to introduce three or more amine functional groups:
  • R 1 and R 2 are independently hydrogen or a chemical protecting group such as OS, Boc, Fmoc, CBZ, tbutyl, benzyl, or allyl.
  • Linker strategies that involve formation of amide bonds are useful because they typically are compatible with the protecting groups on the peptide.
  • the peptide, linker, and linker-subunits may be covalently attached to each other by formation of other bond types (nucleophilic displacement, reductive amination and thiourea formation, for example).
  • the linkers may also have effects on the properties of the contrast agents such as affinity, pharmacokinetic properties, stability in vivo, and relaxivity.
  • a covalent conjugate that includes both a linker moiety and a chelating or chelating precursor moiety can be reacted directly with a peptide with appropriate terminal functionality.
  • a covalent conjugate capable of reacting with terminal carboxylate groups follows:
  • R 1 , R 2 , R 3 , R 4 , and R 5 are independently an acetate group, acetamide group, or an acetoxy group.
  • R 1 , R 2 , R 3 , R 4 , and R 5 are independently an acetate group, acetamide group, or an acetoxy group.
  • An example of a covalent conjugate useful for converting a modified peptide with carboxylate functional groups at the two termini to a precursor imaging agent has the following structure:
  • LG is a leaving group
  • n 1 to 4
  • R 1 , R 2 , R 3 , R 4 , and R 5 are independently an acetate group, acetamide group, or an acetoxy group
  • LG is a leaving group, wherein R 1 , R 2 , R 3 , R 4 , and R 5 are independently an acetate group, acetamide group, or an acetoxy group.
  • a particularly useful covalent conjugate for synthesizing a multimer contrast agent has the following structure, hereinafter “Synthon #1”:
  • Another embodiment of a covalent conjugate useful for synthesizing a multimer has the following structure, hereinafter “Synthon #2”:
  • chelate refers to bb-DTPA-Gd(III).
  • structures 15 and 16 are similar to 32 except for different N-terminal linker-subunits.
  • the experimental results show that a linker can affect the relaxivity, as well as other characteristics of a contrast agent of the invention.
  • Chelating moieties are chelating ligands complexed with metal ions. These chelating moieties contain a synthetic moiety capable of forming a point of attachment to the linker, linker-subunit, and/or modified peptide. One or more chelating moieties may be covalently conjugated to the functional group at each terminus of the modified peptide. In one embodiment, the chelate is attached to a linker-subunit. In another embodiment, the chelate is attached to a linker moiety. In other embodiments, the chelate may be conjugated with a linker moiety to form a covalent conjugate before attaching the covalent conjugate to the modified peptide.
  • Precursor chelating moieties are chelating ligands that have not been complexed with metal ions. Chelating ligands may have protecting groups or may be precursors to chelating ligands. Precursor chelating moieties have a synthetic moiety capable of forming a point of attachment to the linker, linker-subunit, and/or modified peptide. Precursor chelating moieties can be converted into chelating moieties by complexing with a metal ion. One or more precursor chelate moieties may be covalently conjugated to the functional group at each terminus of the modified peptide. In one embodiment, the precursor chelate can be attached to a linker-subunit. In another embodiment, the precursor chelate is attached to a linker moiety. In other embodiments, the precursor chelate may be conjugated with a linker moiety to form a covalent conjugate before attaching the covalent conjugate to the modified peptide.
  • Precursor chelate moieties and chelate moieties according to the invention can have any of the following structures:
  • X is a heteroatom electron-donating group capable of coordinating a metal cation, such as O ⁇ , OH, NH 2 , OPO 3 2 ⁇ , NHR, or OR, wherein R is any aliphatic group; R 1 is an uncharged chemical moiety, selected from hydrogen, any aliphatic, alkyl group, or cycloalkyl group, or uncharged substituted versions thereof (e.g.
  • Y is a synthetic moiety (e.g., capable of forming a point of attachment, or being the point of attachment, to the functional group of the modified peptide, linker, and/or linker-subunit either directly or with an intervening carbonyl, methylene, methylene-oxygen, thiocarbonyl).
  • Moieties with (chelate moiety) or without (precursor chelate moiety) a coordinated metal ion may be used.
  • a variety of chelating ligands may be used in contrast agents of the invention.
  • Such chelating ligands include, but are not limited to, derivatives of DTPA, DOTA, NOTA, and DO3A.
  • metal chelates such as gadolinium diethylenetriaminepentaacetate (DTPA.Gd), gadolinium tetraamine 1,4,7,10-tetraazacyclododecane-N,N′,N′′,N′′′-tetraacetate (DOTA-Gd) and gadolinium 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (DO3A.Gd) are particularly useful.
  • Particularly useful chelates include bb(CO)DTPA.Gd.
  • Other metals may be substituted for Gd(III) in MRI applications.
  • Examples of functionalized chelates that have been synthesized for the purpose of preparing multimeric chelates include pNCS-Bz-DTPA [Martin, V., et al. Bioconjugate Chem. 6,616-23, 1995] and Gd(4-NCS-phenyl)-amino-carbonylmethyl-DO3A [Ramachandran, R. et al., Invest. Rad. 1998, 33(11), 779-797].
  • a reagent that includes a chelating ligand with a backbone carbonyl group for connecting to an amine functional group or a linker-subunit or linker, wherein “LG” is a leaving group (e.g., an activated ester) and R represents a group which may be easily cleaved to form O ⁇ (—OtBu, e.g. a carboxylate ester) thereby forming a carboxylate with the neighboring carbonyl group:
  • LG is a leaving group (e.g., an activated ester)
  • R represents a group which may be easily cleaved to form O ⁇ (—OtBu, e.g. a carboxylate ester) thereby forming a carboxylate with the neighboring carbonyl group:
  • the present invention also relates to intermediates useful in the synthesis of contrast agents for MRI according to the following formulae:
  • R 1 , R 2 , R 3 , R 4 , and R 5 may be any protected or unprotected acetyl ligand suitable for forming a chelate of a paramagnetic metal with an appropriate formation constant, including the following:
  • LG is a “leaving group” and represents —OH and ester forms thereof including, NHS esters, pentafluorophenol, and other activated esters.
  • LG is —OH
  • R 1 , R 2 , R 3 , R 4 , and R 5 are CH 2 CO 2 O t Bu, hereinafter “Synthon #3”, and has the following structure:
  • the present invention also provides methods of manufacturing compounds.
  • a novel oxidation reaction permits the facile preparation of Synthon #3, a preferred embodiment of a chelating ligand.
  • the synthesis of Synthon #3 can be achieved through two different synthetic routes, both commencing with hydroxymethyl-diethylenetriamine.
  • One route involves a two synthetic step sequence (alkylation, followed by oxidation) and the other involves a six-step process (protection, oxidation, esterification, deprotection, alkylation and hydrogenolysis). Both produce Synthon #3 in high chemical and optical purity (vide Examples, below).
  • U.S. Pat. No. 5,637,759 discloses a synthesis of Synthon #1 from 2,3-diaminopropionic acid and aza-lysine by a selective hydrolysis protocol with sodium thiophenoxide. The method disclosed herein avoids the use of this toxic reagent.
  • a targeting peptide can be synthesized with or without a C-terminal linker-subunit, typically using solid phase peptide synthesis.
  • a protected linear peptide may be cyclized in solution or on the resin.
  • Unprotected peptide may also be cyclized in solution or on resin.
  • a C-terminal linker-subunit may be conveniently derived from the solid phase synthesis resin and an N-terminus linker or N-terminal linker-subunit can be coupled to the peptide during the solid phase synthesis.
  • the linker-subunit-chelate precursor moieties were coupled to the peptide.
  • Protecting groups were removed to provide the ligand precursors, and then chelates were prepared.
  • Radionuclide compounds of this invention were prepared from ligand precursors using commercially available radionuclides (for example, 99m Tc from Nycomed Amersham Boston cat. #RX-290195, 111 In from NEN Life Science Products cat. #NEZ304, or 153 Gd from NEN Life Science Products cat. #NEZ142) by reaction in aqueous media, typically at pH 4-6 for 1 hour.
  • an organic dye may be substituted for a chelate precursor.
  • Compounds of this invention can be more stable with respect to degradation by endogenous enzymes than the parent peptide (i.e., the peptide without any attached chelates), a peptide with one or more chelates attached to the N-terminus, or a peptide with one or more chelates attached to the C-terminus.
  • test compounds can be incubated with rat liver homogenates. After selected intervals, the reactions can be quenched and centrifuged, and the supernatant can be analyzed by liquid chromatography-mass spectrometry to quantitate the amount of compound remaining.
  • Compounds of the invention also can bind a target such as human serum albumin or fibrin.
  • a target such as human serum albumin or fibrin.
  • at least 10% (e.g., at least 50%, 80%, 90%, 92%, 94%, or 96%) of the contrast agent can be bound to the desired target at physiologically relevant concentrations of drug and target.
  • the extent of binding of a contrast agent to a target, such as HSA or fibrin can be assessed by a variety of equilibrium binding methods. For example, binding to HSA can be measured by ultrafiltration.
  • a fibrin clot may be formed in a well of a microtiter plate and contacted with the targeting group.
  • the supernatant is removed by aspiration (the insoluble fibrin remains bound as a gelled clot to the bottom of the well).
  • the concentration of unbound targeting group in the supernatant is then measured.
  • the concentration of bound contrast agent is determined as the difference between the total targeting group concentration initially present and the unbound targeting group concentration following the binding assay.
  • the bound fraction is the concentration of bound targeting group divided by the concentration of total targeting group.
  • Compounds of the invention can exhibit high relaxivity as a result of target binding (e.g., to fibrin), which can lead to better image resolution.
  • the increase in relaxivity upon binding is typically 1.5-fold or more (e.g., at least a 2, 3, 4, 5, 6, 7, 8, 9, or 10 fold increase in relaxivity).
  • Targeted contrast agents having 7-8 fold, 9-10 fold, or even greater than 10 fold increases in relaxivity are particularly useful.
  • relaxivity is measured using an NMR spectrometer.
  • the preferred relaxivity of an MRI contrast agent at 20 MHz and 37° C.
  • Contrast agents having a relaxivity greater than 60 mM ⁇ 1 s ⁇ 1 at 20 MHz and 37° C. are particularly useful.
  • targeted contrast agents can show an increase in clot uptake. Specificity of uptake of fibrin-targeted agents can be determined by comparing the uptake of the agent by blood clots to the uptake by blood. See Example 11 for more details. The specificity of fibrin-targeted contrast agents also can be demonstrated using MRI and observing enhancement of clot signal.
  • Peptides of the invention can be used to improve therapies for treating thromboembolic disease.
  • Current thrombolytic therapy has limitations, including a significant risk of bleeding, failure to restore blood flow, thrombotic reocclusion after cessation of therapy, and a lag between initiation of therapy and clot lysis.
  • An improved therapeutic index can be achieved by conjugating a fibrin targeting peptide of the invention to a thrombolytic agent (e.g., a protein thrombolytic such as plasminogen activators of human or bacterial origin).
  • a thrombolytic agent e.g., a protein thrombolytic such as plasminogen activators of human or bacterial origin.
  • Such conjugates can activate plasminogen locally or increase endogenous levels of tPA.
  • a fibrin targeting peptide can be conjugated to human plasminogen activators including recombinant tissue type plasminogen activator (tPA), prourokinase and urokinase (both single and two chain forms), bacterium derived plasminogen activator including streptokinase, staphylokinase, and animal derived plasminogen activators, including vampire bat plasminogen activator.
  • fibrin targeting peptides can be conjugated to fibrinolytics such as copperhead snake fibrolase, which exhibits direct fibrinolytic activity. Such enzymes and proteins can be obtained commercially, extracted from natural sources or tissues, or prepared recombinantly.
  • compositions of the invention can be linked or fused in known ways, using the same type of linkers discussed above with respect to constructing MRI contrast agents.
  • Conjugation to a protein can be achieved by standard chemical techniques including the formation of amide, ester, disulfide, and thioether bonds.
  • a fibrin binding peptide can be covalently linked, either directly or through a linker, to a protein by forming an amide bond between the fibrin binding peptide or the linker and the lysine residues on the surface of the protein. These surface lysine residues are usually distant from the enzyme's catalytic site. Therefore, the tethered moieties do not interfere with the enzyme's catalytic activity.
  • the ratio of the fibrin targeting peptide to the thrombolytic or fibrinolytic agent can be controlled by adjusting the stoichiometry of the ligation chemistry. Multiple ligation is particularly useful in the case of a moderately strong fibrin binding ligand because higher binding affinity can be realized through the so called “avidity” effect.
  • a coupling agent or an activated ester can be used to achieve amide bond formation between the lysine and the fibrin binding moiety or the linker.
  • the below scheme shows an example of a hybrid molecule formed by chemical ligation of urokinase to multiple fibrin binding peptides via linker moieties. The number of surface lysine residues and the number of fibrin binding molecules are illustrative.
  • the fibrin targeting peptide can be incorporated into the hybrid molecule using recombinant DNA technology.
  • peptides of the invention can be linked to a thrombolytic agent with a linker encompassing an enzymatic cleavage site, e.g., an enzymatic cleavage site normally cleaved by enzymes in the coagulation cascade, such as Factor Xa, thrombin, or plasmin cleavage sites, etc.
  • the thrombolytic agent is not activated until it is cleaved from the clot binding compositions of the invention at the site of the clot, the risk of unwanted bleeding events at sites distant from the clot would be minimized.
  • thrombolytic moieties can be linked to a peptide-targeted multimeric contrast agent such that a clot can be identified, imaged and dissolved.
  • Contrast agents prepared according to the disclosures herein may be used in the same manner as conventional MRI contrast agents and are useful for the diagnosis of deep vein thrombosis, pulmonary embolus, coronary thrombosis, carotid and intracranial thrombosis, atrial and ventricular thrombi, aortic arch thrombi, and high risk plaque.
  • certain MR techniques and pulse sequences may be preferred to enhance the contrast of the thrombus compared to the background blood and tissues. These techniques include, but are not limited to, black blood angiography sequences that seek to make blood dark, such as fast spin echo sequences and flow-spoiled gradient echo sequences.
  • These methods also include flow independent techniques that enhance the difference in contrast due to the T1 difference between contrast-enhanced thrombus and blood and tissue, such as inversion-recovery prepared or saturation-recovery prepared sequences that will increase the contrast between thrombus and background tissues. Methods of preparation for T2 techniques may also prove useful. Finally, preparations for magnetization transfer techniques may also improve contrast with agents of the invention.
  • compositions of the invention can be formulated as a pharmaceutical composition in accordance with routine procedures.
  • the compounds of the invention can include pharmaceutically acceptable derivatives thereof.
  • “Pharmaceutically acceptable” means that the compound or composition can be administered to an animal without unacceptable adverse effects.
  • a “pharmaceutically acceptable derivative” means any pharmaceutically acceptable salt, ester, salt of an ester, or other derivative of a compound of this invention that, upon administration to a recipient, is capable of providing (directly or indirectly) a compound of this invention or an active metabolite or residue thereof.
  • compositions of this invention are those that increase the bioavailability of the compounds of this invention when such compounds are administered to a mammal (e.g., by allowing an orally administered compound to be more readily absorbed into the blood) or which enhance delivery of the parent compound to a biological compartment (e.g., the brain or lymphatic system) thereby increasing the exposure relative to the parent species.
  • Pharmaceutically acceptable salts of the compounds of this invention include counter ions derived from pharmaceutically acceptable inorganic and organic acids and bases known in the art.
  • compositions of the invention can be administered by any route, including both oral and parenteral administration.
  • Parenteral administration includes, but is not limited to, subcutaneous, intravenous, intraarterial, interstitial, intrathecal, and intracavity administration.
  • pharmaceutical compositions may be given as a bolus, as two or more doses separated in time, or as a constant or non-linear flow infusion.
  • compositions of the invention can be formulated for any route of administration.
  • compositions for intravenous administration are solutions in sterile isotonic aqueous buffer.
  • the composition may also include a solubilizing agent, a stabilizing agent, and a local anesthetic such as lidocaine to ease pain at the site of the injection.
  • the ingredients will be supplied either separately, e.g. in a kit, or mixed together in a unit dosage form, for example, as a dry lyophilized powder or water free concentrate.
  • the composition may be stored in a hermetically sealed container such as an ampule or sachette indicating the quantity of active agent in activity units.
  • compositions of this invention comprise the compounds of the present invention and pharmaceutically acceptable salts thereof, with any pharmaceutically acceptable ingredient, excipient, carrier, adjuvant or vehicle.
  • a contrast agent is preferably administered to the patient in the form of an injectable composition.
  • the method of administering a contrast agent is preferably parenterally, meaning intravenously, intra-arterially, intrathecally, interstitially or intracavitarilly.
  • Pharmaceutical compositions of this invention can be administered to mammals including humans in a manner similar to other diagnostic or therapeutic agents.
  • the dosage to be administered, and the mode of administration will depend on a variety of factors including age, weight, sex, condition of the patient and genetic factors, and will ultimately be decided by medical personnel subsequent to experimental determinations of varying dosage followed by imaging as described herein.
  • dosage required for diagnostic sensitivity or therapeutic efficacy will range from about 0.001 to 50,000 ⁇ g/kg, preferably between 0.01 to 25.0 ⁇ g/kg of host body mass. The optimal dose will be determined empirically following the disclosure herein.
  • the quantity of material administered will depend on the seriousness of the thromboembolic condition and position and the size of the clot.
  • the precise dose to be employed and the mode of administration can be decided according to the circumstances by the physician supervising treatment.
  • dosages of the combined composition/thrombolytic agent conjugate will follow the dosages that are routine for the thrombolytic agent alone, although the improved affinity for fibrin/clot binding added by the compositions disclosed herein may allow a decrease in the standard thrombolytic dosage.
  • thrombolytics contemplated for use in this therapy are as follows: Streptokinase 1-3 megaunits over 30 minutes to 3 hrs Anistreplase 30 units; 2-5 minute injection tPA (wild-type) 50-150 mg; infusion over up to 6 hrs Two-chain urokinase (40-100 mg); infusion over up to 6 hrs Single-chain urokinase (scuPA) 3-12 megaunits (30-100 mg; infusion over up to 5 hrs Hybrid plasminogen activators 20-100 mg; injection or infusion and derivatives Muteins of plasminogen 10-100 mg; injection or infusion activators
  • the peptide with linker-subunit moiety bound to C-terminus (P-flinker-subunit moietyl).
  • the unprotected peptide was prepared using standard Fmoc strategy and a diaminotrityl resin. The peptide was cyclized using thallium trifluoroacetate on the resin or in solution. After being cleaved from the resin, the unprotected peptide was purified by RP-HPLC (C-18 column, H 2 O/CH 3 CN/TFA).
  • Linker Moiety To a solution of Boc-Dpr(Boc)-OH.DCHA (1 eq.) and pentafluorophenol (1.2 eq.) in dichloromethane was added PS-carbodilmide (1.2-1.5 eq.). The mixture was shaken for 3-5 h at room temperature. After LC-mass results indicated the reaction was complete, the resin was removed by filtration and the solvent was evaporated under reduced pressure to give the crude Linker Moiety (Boc-Dpr(Boc)-OPft (N- ⁇ -Boc-N- ⁇ -Boc-L-diaminopropionic acid pentafluorophenyl ester, 356-128) as a white foam.
  • Precursor MR Imaging Agent To a solution of P-[linker-subunit moiety] (1 eq.) and the Linker Moiety ⁇ Boc-Dpr(Boc)-Opft ⁇ (2.2 eq.) in DMF was added DIPEA (4-6 eq.). The mixture was stirred overnight at room temperature. After LC-mass results indicated the reaction was complete, the solvent was removed under reduced pressure. The crude product was then stirred in a mixture of TFA, water and anisole (90%/5%/5%) at room temperature for 3 h.
  • Precursor Clhelate Moiety DOTAGA-Opft. To a solution of DOTAGA-OH (1 eq.) and pentafluorophenol (1.2 eq.) in dichloromethane was added PS-carbodiimide (1.2-1.5 eq.). The mixture was shaken for 3-5 h at room temperature. After LC-mass results indicated the reaction was complete, the resin was removed by filtration and the solvent was evaporated under reduced pressure to give the crude Precursor Chelate Moiety as a white foam.
  • the crude product was reacted with GdCl 3 .H 2 O in deionized water to form the crude MR imaging agent, which was purified using RP-HPLC (C-18 column, Ethanol/50 mmol AcONH 4 ). Appropriate fractions were combined and the ethanol removed under reduced pressure, and then the combined fractions were treated with sodium acetate for salt exchange. After lyophilization the excess salts were removed using reverse-phase chromatography on a Waters Sep-Pak® C-18 cartridge with water and ethanol:water (50:50) eluants. Appropriate fractions were combined, the ethanol removed under reduced pressure, and the solution was lyophilized to give the desired peptide MR imaging agent as a white solid.
  • BOC-protected triamine (29.94 g) was dissolved in acetonitrile.
  • Phosphate buffer consisting of 21.6 g NaH 2 PO 4 , 21.6 g Na 2 HPO 4 and enough deionized water to produce a 500 mL volume, was added (300 mL), followed by 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) (0.07 equiv.). The mixture was stirred vigorously and warmed to 35° C.
  • TEMPO 2,2,6,6-tetramethylpiperidinyl-1-oxy
  • the mixture was stirred vigorously and warmed to 35° C.
  • Sodium chlorite (2.0 equiv.) was dissolved in deionized water (100 mg/ml). The sodium chlorite solution and bleach (0.02 equiv., approx.
  • 0.25% aqueous sodium hypochlorite were added while maintaining a constant temperature. After addition of oxidant, the reaction was stirred for 24 hours. Additional TEMPO (0.07 equiv.) was added and the reaction mixture was stirred for 24 hours. The reaction was cooled to room temperature. Water was added and the pH was adjusted to 8 with 2.0 N aqueous NaOH. A cold solution of aqueous sodium sulfite was added (300 mL) while maintaining a constant pH. The solution was extracted with a small volume of methyl tert-butyl ether and set aside. The aqueous layer was acidified to pH 3-4 with 2.0 N aqueous HCl and extracted with two small volumes of methyl tert-butyl ether.
  • the peptides have been prepared by solid-phase synthesis.
  • the linkers and resins are selected depending on the type of the peptides to be synthesized (e.g., the functional group required at the C-terminus, the protected or unprotected peptide) and the synthetic method to be used (e.g. Fmoc or BOC chemistry, manual or automated synthesis, the continuous flow or batch reactor).
  • the synthetic method to be used e.g. Fmoc or BOC chemistry, manual or automated synthesis, the continuous flow or batch reactor.
  • a protected-amino acid is attached to the different resins such as HMPB resins, 2-chlorotritylchloride resin and SUSRIN resin.
  • a diamine in the synthesis of a peptide with an amino group at the C-terminus, a diamine can be attached to a trityl resin.
  • the polystyrene (PS) resins can be used for batch synthesis, while polyethyleneglycol (PEG) modified resins are suitable for continuous flow and batch synthesis.
  • PEG polyethyleneglycol
  • Many trityl PS resins including 1,3-bis-(aminomethyl)-benzene trityl PS resin are commercially available. If the required trityl PS resin is not available, a similar procedure as described for PEG resins can be used to attach a diamine to a trityl PS resin.
  • trityl alcohol resin 25 g, NovaSyn TGT resin, NovaBiochem
  • trityl alcohol resin 25 g, NovaSyn TGT resin, NovaBiochem
  • a hydrogenation vessel is charged with the above compound, 10% palladium on carbon, ethyl acetate, and triethylamine. The vessel is purged with nitrogen then hydrogen. The mixture is shaken for 24 hours under a hydrogen atmosphere (50 psi). The vessel is purged with nitrogen and the mixture is filtered through Celite®, and the filtrate is concentrated under reduced pressure to give the product as an oil.
  • the resin was then treated with 1% trifluoroacetic acid in CH 2 Cl 2 and placed on a mechanical shaker for 10 minutes.
  • the mixture was filtered through the reaction vessel and the filtrate was collected.
  • the pH of the combined filtrate was adjusted to approximately 8 with triethylamine, and the solution was concentrated under vacuum to an oil.
  • the white solid was filtered and rinsed with water and diethyl ether.
  • the solid was dried by suction filtration, taken up in DMF and diluted with acetonitrile.
  • the solution was cooled in an ice bath and was treated with 1.5 g of thallium trifluoroacetate for 2.0 hours.
  • the pH of the solution was adjusted with triethylamine to pH 8 and then concentrated under vacuum.
  • Precursor MR imaging agent (1 g) was reacted with one equivalent of GdCl 3 -6H 2 O in distilled deionized water with the pH adjusted to ca. 6 by the addition of 1 M NaOH.
  • the gadolinium complex was purified by reverse phase chromatography (Waters Sep-Pak(V C-18) using distilled deionized water and 50:50 (v:v) methanol:water eluent. Appropriate fractions were combined and the methanol removed under reduced pressure at 50° C. and lyophilized to give 811 mg of the MR imaging agent.
  • the peptide may be cyclized on a resin as illustrated in the following Scheme:
  • Precursor MR imaging agent was reacted with one equivalent of GdCl 3 .6H 2 O in deionized water (pH 6, NaOH).
  • the gadolinium chelate was purified using reverse-phase chromatography on a Waters Sep-Pak® C-18 cartridge with water and methanol:water 50:50 eluant. Appropriate fractions were combined and the methanol removed under reduced pressure at 50° C. and lyophilized to give the desired MR imaging agent.
  • Table 3 provides mass spectrometry data confirming each of the compounds. See the detailed description for the structure of each of the compounds.
  • 3B is synthesized using the peptide 1 (177 mg, 0.10 mmol) and 5-carboxyfluorescein succinimidyl ester B (99.4 mg, 0.21 mmol).
  • 3C is synthesized using the peptide 1 (177 mg, 0.10 mmol) and Texas Red®-X succinimidyl ester C (172 mg, 0.21 mmol).
  • the extent of binding of a contrast agent according to the present invention to a target, such as HSA or fibrin, can be assessed by a variety of equilibrium binding methods.
  • binding to HSA can be measured by ultrafiltration.
  • the contrast agent is mixed with 4.5% weight/volume HSA in a pH 7.4 buffer.
  • the sample is loaded into a commercially available centrifugation apparatus equipped with a 30 kDa molecular weight cutoff filter (Millipore Ultrafree MC Low Binding Regenerated Cellulose 30 KDa mol. wt. cutoff catalog #UFC3LTK00), permeable to the targeting group, but not to HSA.
  • a small portion (5-10%) of the sample volume is filtered by centrifugation at 2000 ⁇ g for 20 min through the cutoff filter, and the concentration of unbound targeting group in the sample is measured in the filtrate.
  • a fibrin clot may be formed in a well of a microtiter plate and contacted with the targeting group. After an incubation time sufficient to establish equilibrium, the supernatant is removed by aspiration (the insoluble fibrin remains bound as a gelled clot to the bottom of the well). The concentration of unbound targeting group in the supernatant is then measured.
  • the concentration of bound contrast agent is determined as the difference between the total targeting group concentration initially present and the unbound targeting group concentration following the binding assay.
  • the bound fraction is the concentration of bound targeting group divided by the concentration of total targeting group.
  • rat liver homogenate which contains both intra- and extracellular enzymes and represents a particularly harsh chemical environment for peptide bonds.
  • Freshly prepared rat liver homogenate (630 ⁇ L) was placed in a glass test tube and incubated at 37° C. in a water bath for 4 minutes.
  • To the rat liver homogenate at 37° C. was added 70 ⁇ L of a 1 mM solution of test compound.
  • time points 0, 5, 15, 30, and 60 minutes a 100 ⁇ L aliquot of the reaction mixture was removed and mixed with 100 ⁇ L of methanol in a microfuge tube to quench the reaction. The quenched reaction mixture was centrifuged for 3 minutes at 10,000 rpm to pellet the precipitated protein.
  • T1/2 Half life
  • the MRI contrast agents of the present invention were evaluated for relaxivity using a Bruker NMS-120 Minispec NMR spectrometer operating at 0.47 Tesla (20 MHz H-1 Larmor frequency) and 37° C. or a Konig-Brown relaxometer (20 MHz, H-1 Larmor frequency) operating at 35° C. T1 of water protons was determined by an inversion recovery pulse sequence using the instrument's software. Relaxivity was determined by measuring the Ti of multiple solutions of the target (for example, homodisperse gels of freshly polymerized fibrinogen, 10 mg/mL) containing zero, 20, 30, and 40 ⁇ M Gd(III), respectively. The samples are incubated at 37° C.
  • the target for example, homodisperse gels of freshly polymerized fibrinogen, 10 mg/mL
  • the Gd(III) content of the samples is determined by inductively coupled plasma—mass spectrometry (ICP-MS).
  • the relaxivity (per Gd(III) ion) is determined by plotting the relaxation rate (1/T1) in s ⁇ 1 versus the Gd(III) concentration in mM. The slope of a linear fit to the data gives the relaxivity.
  • the relaxivity of the compounds in the absence of target is also determined in an analogous manner, except there is no target present.
  • Compounds of the invention show increased relaxivity upon binding to fibrin (FIG. 2) as compared with the relaxivity in the absence of biological target.
  • thrombus blood clot
  • a 600 g guinea pig Hartley male was anaesthetized. An incision was made in the abdomen and the inferior vena cava (IVC) isolated and the vessel was allowed to recover for 10 minutes. A 1 cm portion of the IVC was clamped and human thrombin (50 ⁇ L, 4 units) was injected into the vessel to promote thrombus formation. The lower clamp was opened and closed allowing partial blood flow to the segment. After 2-3 minutes the clips were removed. The thrombus was allowed to age in the animal for 30 minutes.
  • IVC inferior vena cava
  • the contrast agent compound 32 at a dose of 2 ⁇ mol/kg and trace radiolabeled with 70 ⁇ Ci of 111 In, was injected via the jugular vein.
  • a non-specific control comprising Gd(DTPA)at a dose of 2 ⁇ mol/kg mixed with 70 ⁇ Ci 99m TcDTPA was injected via the jugular vein.
  • the blood sample was weighed and counted using a Packard Cobra II gamma counter. The thrombus was also weighed and counted.
  • Radiolabeling with 111 In was performed in advance: An appropriate radiochemical amount of 111 InCl 3 (New England Nuclear) was added to the fibrin targeted contrast agent. The pH was adjusted to 4 by addition of 1 M HCl. The sample was heated at 45° C. for 1 hour. The pH was adjusted to neutral by addition of 1 M NaOH. The labeled agent compound 32 was >95% pure by ⁇ -detected HPLC.
  • Fibrin-specific agents show a marked increase in clot uptake.
  • FIG. 3 shows that an agent, compound 32, is accumulating in the thrombus. There is specific clot uptake compared to 99m TcDTPA and there is a higher concentration of the agent in the thrombus than in the surrounding blood.
  • clot uptake also can be demonstrated using MRI.
  • the procedure for in vivo imaging of a thrombus with an agent is as follows: A 600 g guinea pig (Hartley male) is anaesthetized. An incision is made in the throat and one of the jugular veins isolated. A 1 cm section of the jugular vein is isolated with vascular clamps. Freshly drawn blood from the animal (50 ⁇ L) is mixed with human thrombin (50 ⁇ L, 4 units) and is injected into the clamped segment of the vein. Four minutes after injection, the clamps are removed and the thrombus is allowed to age for 30 minutes.
  • a 2.5 kg female New Zealand White rabbit was anesthetized with a cocktail of Ketamine (50 mg/kg), Aceapromazine (2.5 mg/kg), and Rompon (5 mg/kg) and anesthesia maintained with sodium pentobarbital (approx.35 mg/kg as needed).
  • An i.v. catheter 24 g was placed into the ear vein and the ear artery. The jugular vein and carotid artery were isolated. A stenosis was created in the carotid artery by placing an 18 g needle on top of the vessel and then suturing it into place with 3-0 suture. The needle was then removed.
  • a 5 mm portion of the artery was then segmented off distally to the stenosis with microvascular clips.
  • the artery was then crushed twice along the 5 mm section.
  • the proximal vascular clip was released to allow blood flow into the section for ca. 3 sec.
  • the clip was reapplied and artery was crushed twice again along the 5 mm section. After 4 minutes, the clips were removed.
  • a 5 mm segment of the jugular vein was isolated with microvascular clips.
  • a thrombus was created by injecting 100 PL of a 3.7 units of thrombin, 0.06 M CaCl 2 , rabbit whole blood mixture. After 4 minutes, the clips were removed.
  • thrombi were allowed to age for 50 minutes.
  • a 1.0 mL solution of the thrombus targeted agent (Structure III, 5 mM, 2 ⁇ mol/kg) was administered via the ear vein.
  • SPGR 3D RF spoiled gradient echo sequence
  • a second MRI data set was acquired using the same parameters with the addition of 40 mm spatial inferior and superior saturation bands to generate a “black blood” image.
  • FIG. 4A shows the maximum intensity projection (MIP) of the first data set.
  • the blood vessels are partially enhanced from time of flight effects.
  • FIG. 4B shows the MIP of the second data set where the signal from in-flowing blood was suppressed (black blood) by the use of superior and inferior saturation bands.
  • the identification of the stationary target (a thrombus) through use of the targeted contrast agent is clearly facilitated.
  • the resin was washed thoroughly with NMP/ether/NMP, and was treated with a solution of fluorascein-5-isothiocyanate (23.4 mg, 60 ⁇ mol) and diisopropylethylamine (11.6 mg, 15.7 ⁇ L, 90 ⁇ mol) in DMF (1.5 mL) for 12 hours.
  • the resin was washed thoroughly (NMP/ether/NMP), and treated with a solution of Tl(TFA) 3 (18.7 mg, 34.5 ⁇ mol) in DMF (1.5 mL) at 4° C. for three hours.
  • the resin was washed after this treatment, and treated with a cocktail of TFA/TIS/water (95/2.5/2.5, 2.0 mL) for two hours.
  • N-terminal labeling of the peptides with optical probes can modulate the binding affinity of the optical contrast agents.
  • Kd's were observed (Table 6): TABLE 6 Compound Fluorophore Kd J Fluorescein 0.06 L Tetramethylrhodamine 2.0 N 4-Methoxycoumarin 0.2
  • Fibrin targeted urokinase is prepared according to the following procedure.
  • a fibrin binding peptide with a Gly-Gly dipeptide linker is prepared according to solid phase procedures.
  • the N-terminus of the peptide is blocked with an acetyl group, and the C-terminal carboxylic acid is converted to a succinamidal active ester.
  • Direct chemical ligation is achieved by mixing urokinase and the activated peptide in appropriate proportions in an aqueous buffer and gently agitating the solution for 30 minutes.
  • Fibrin targeted urokinase can be purified by HPLC. Binding to fibrin can be assessed. Compound I binds fibrin selectively versus fibrinogen.
  • the rabbit jugular vein model of Collen et al. ( J. Clin. Invest. 1983, 71, 368-376) is used for thrombolysis assays.
  • Compound (2 mg/kg) is administered by infusion of a bolus (consisting of 20% of the total dose) over 1 min, along with a heparin bolus (300 units/kg) over 1 min. The remainder of the dose is continuously infused over the next 60 min, and heparin (60 units/kg/hr) is continuously infused over the next 180 min.
  • the animals are sacrificed, and clots analyzed.
  • Compound 1 is more potent in clot lysis than scuPA alone.
  • At 3 hr with 2 mg/kg of compound 1, there is less consumption of fibrinogen and ⁇ 2-antiplasmin, relative to equivalent doses of scuPA alone, demonstrating that compound I was more fibrin specific than scuPA alone.

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