WO2007055578A1

WO2007055578A1 - Cyclic antimicrobial peptides derived from lactoferrin

Info

Publication number: WO2007055578A1
Application number: PCT/NL2006/050284
Authority: WO
Inventors: Rolf Ide Johannes Feitsma; Peter Hendricus Nibbering; Jan Wouter Drijfhout
Original assignee: Leids Universitair Medisch Centrum
Priority date: 2005-11-11
Filing date: 2006-11-10
Publication date: 2007-05-18

Abstract

The present invention relates to cyclic compounds, preferably cyclic peptides, that at least comprise a stretch of positively charged amino acids, at some distance thereof an aromatic amino acid. The aromatic amino acid may be immediately followed by an amino acid with a thiol-group in the side chain. The cyclic compounds show a dramatic increase in antimicrobial activity as compared linear versions having the same amino acid sequence, in particular under physiological salt conditions. The cyclic compounds show antimicrobial activity against a broad spectrum of microbes and may be used in both prophylactic and therapeutic methods against infectious diseases.

Description

CYCLIC ANTIMICROBIAL PEPTIDES DERIVED FROM LACTOFERRIN

Field of the invention The present invention relates to the field of polypeptides having various therapeutic and prophylactic applications, including bactericidal activity. As such, the present invention broadly relates to the fields of protein and medicinal chemistry.

Background of the invention Antimicrobial peptides have received increasing attention as a new pharmaceutical substance for combating infectious diseases. Often these antimicrobial peptides display a broad spectrum of antimicrobial activities which renders potentially useful in combating multi-drug-resistant pathogenic microorganisms that have rapidly spread in recent years. Useful antimicrobial peptides have been found as endogenous peptides in mammals, birds, amphibia, insects, and plants where they appear to be components of the host defense systems. Frequently such endogenous antimicrobial peptides are cationic amphipathic molecules that contain 10 to 45 amino acid residues and an excess of lysine and arginine residues, (for a review, see Broekaert et al, Plant Physiol. 108:1353, 1995; Ganz and Lehrer, Pharmacol. Ther. 66:191, 1995; Martin et al., J Leukoc. Biol. 58:128, 1995; Hancock and Lehrer, TIBTECH 16:82, 1998). Examples of cationic peptides include rabbit de fens in, crab tachyplesin, bovine bactenecin, silk-moth cecropin A, frog magainins, bovine indolicidin. More recently, a peptide consisting of the first 11 amino acids of human lactoferrin (hLFl-11) was reported to have broad spectrum antimicrobial activity against a wide variety of bacteria, fungi, viruses and other microbes (WO 01/34641; Nibbering et al., 2001, Infect Immun. 69:1469-76; Lupetti et al., 2002, Antimicrob Agents Chemother. 46:1634-9; Lupetti et al., 2003, Antimicrob Agents Chemother. 47:262-7; Faber et al., 2005, Antimicrob Agents Chemother. 49:2438-44).

US 5,652,211 discloses several endotoxin-binding peptides, one of which has the amino acid sequence Cys-Lys-Lys-Lys-Lys-Phe-Phe-Phe-Phe-Cys.

WO 02/090503 discloses antimicrobial peptides having an amino acid sequences of alternating D- and L-α-amino acids. There is however still a need for more potent antimicrobial agents. It is thus an object of the present invention to provide for novel antimicrobial agents having improved activity under physiological conditions.

Description of the invention

In a first aspect the present invention relates to a cyclic compound of general formula (I)

Z¹ -X¹ -Z² -X²

wherein

Z¹ represents a stretch of 3 - 6 amino acids or equivalents thereof, wherein at least 3 of said amino acids are selected from the group consisting of arginine and homoarginine, and wherein no more than two of said amino acids are selected from the group consisting of lysine, homo lysine and ornithine,

X¹ represents a spacer of from 5 to 12 atoms, said spacer connecting Z¹ to Z²,

Z² represent an amino acid or equivalent thereof having an aromatic group in the side chain, and X² represents a spacer having 1 to 9 atoms connecting Z² to Z¹ or X² is absent in which case Z² is connected to Z¹,

and pharmaceutically acceptable salts thereof.

Preferably in a cyclic compound of the invention Z¹ comprises no more than one amino acid that is selected from the group consisting of lysine, homolysine and ornithine. Z¹ may comprise one or more additional amino acids other than arginine, homoarginine, lysine, homolysine and ornithine. Such additional amino acid may any amino acid other than arginine, homoarginine, lysine, homolysine and ornithine but preferably the additional amino acid is selected from the group consisting of glycine, alanine, serine, threonine, valine, leucine and isoleucine. More preferably in a cyclic compound of the invention Z¹ comprises only amino acids selected from the group consisting of arginine, homoarginine, lysine, homolysine and ornithine. The skilled person will appreciate that a Z¹ that represents a stretch of 7, 8, 9 or 10 amino acids or equivalents thereof as defined above, is equivalent to the preferred Z¹ as defined above.

In a preferred cyclic compound of the invention Z¹ is selected from the group consisting of R-R-R, R-R-R-R, K-R-R-R, R-K-R-R, R-R-K-R, R-R-R-K, K-K-R-R, R-

K-K-R, R-R-K-K, K-R-K-R, K-R-R-K, R-K-R-K, R-R-R-R-R, K-R-R-R-R, R-K-R-R-

R, R-R-K-R-R, R-R-R-K-R, R-R-R-R-K, K-K-R-R-R, K-R-K-R-R, K-R-R-K-R, K-R- R-R-K, R-K-K-R-R, R-K-R-K-R, R-K-R-R-K, R-R-K-K-R, R-R-K-R-K, R-R-R-K-K, R-R-R-R-R-R, K-R-R-R-R-R, R-K-R-R-R-R, R-R-K-R-R-R, R-R-R-K-R-R, R-R-R-R- K-R, R-R-R-R-R-K, K-K-R-R-R-R, K-R-K-R-R-R, K-R-R-K-R-R, K-R-R-R-K-R, K- R-R-R-R-K, R-K-K-R-R-R, R-K-R-K-R-R, R-K-R-R-K-R, R-K-R-R-R-K, R-R-K-K- R-R, R-R-K-R-K-R, R-R-K-R-R-K, R-R-R-K-K-R, R-R-R-K-R-K, and R-R-R-R-K-K, wherein R represents an amino acid selected from the group consisting of arginine and homoarginine, and K represents an amino acid selected from the group consisting of lysine, homolysine and ornithine, and wherein, optionally, each R or K may be preceded or followed by an additional R or an additional K or any additional other amino acid, any additional other amino acid preferably being selected from the group consisting of glycine, alanine, serine, threonine, valine, leucine and isoleucine.

More preferably in a cyclic compound of the invention Z¹ is selected from the group consisting of R-R-R, R-R-R-R, K-R-R-R, R-K-R-R, R-R-K-R, R-R-R-K, K-K- R-R, R-K-K-R, R-R-K-K, K-R-K-R, K-R-R-K, R-K-R-K, R-R-R-R-R, K-R-R-R-R, R- K-R-R-R, R-R-K-R-R, R-R-R-K-R, R-R-R-R-K, K-K-R-R-R, K-R-K-R-R, K-R-R-K- R, K-R-R-R-K, R-K-K-R-R, R-K-R-K-R, R-K-R-R-K, R-R-K-K-R, R-R-K-R-K, R-R- R-K-K, R-R-R-R-R-R, K-R-R-R-R-R, R-K-R-R-R-R, R-R-K-R-R-R, R-R-R-K-R-R, R-R-R-R-K-R, R-R-R-R-R-K, K-K-R-R-R-R, K-R-K-R-R-R, K-R-R-K-R-R, K-R-R- R-K-R, K-R-R-R-R-K, R-K-K-R-R-R, R-K-R-K-R-R, R-K-R-R-K-R, R-K-R-R-R-K, R-R-K-K-R-R, R-R-K-R-K-R, R-R-K-R-R-K, R-R-R-K-K-R, R-R-R-K-R-K, and R-R- R-R-K-K.

Still more preferably in a cyclic compound of the invention Z¹ is selected from the group consisting of R-R-R, R-R-R-R, K-R-R-R, R-K-R-R, R-R-K-R, R-R-R-K, R- R-R-R-R, K-R-R-R-R, R-K-R-R-R, R-R-K-R-R, R-R-R-K-R, R-R-R-R-K, R-R-R-R- R-R, K-R-R-R-R-R, R-K-R-R-R-R, R-R-K-R-R-R, R-R-R-K-R-R, R-R-R-R-K-R, and R-R-R-R-R-K, and wherein, optionally, each R or K may be preceded or followed by an additional R or an additional K or any additional other amino acid, any additional other amino acid preferably being selected from the group consisting of glycine, alanine, serine, threonine, valine, leucine and isoleucine.

Most preferably in a cyclic compound of the invention Z¹ is selected from the group consisting of R-R-R, R-R-R-R, K-R-R-R, R-K-R-R, R-R-K-R, R-R-R-K, R-R-R- R-R, K-R-R-R-R, R-K-R-R-R, R-R-K-R-R, R-R-R-K-R, R-R-R-R-K, R-R-R-R-R-R, K-R-R-R-R-R, R-K-R-R-R-R, R-R-K-R-R-R, R-R-R-K-R-R, R-R-R-R-K-R, and R-R- R-R-R-K.

In an alternative embodiment of the cyclic compound of the invention, Z¹ represents a stretch of 3 or 4 amino acids or equivalents thereof, wherein at least 3 of said amino acids have at physiological pH a positive charge in the side chain.

In the alternative embodiment the cyclic compound of the invention Z¹ thus preferably represents a stretch of at least 3 amino acids or equivalents thereof, wherein at least 3 of said amino acids have at physiological pH a positive charge in the side chain. The term "physiological pH" refers to the pH in a biological tissue, cell, or extracellular space. For example, the term can refer to the pH inside a cell, inside an organelle, on or inside one or more layers of an epithelial or endothelial tissue, in an organ. Physiological pH depends, of course, upon the particular tissue or organ (e.g., the gastrointestinal tract has a much lower pH than is found in the skin). However, generally the physiological pH will be below pH 8.0, more preferably between pH 6.0 and pH 8.0. An amino acid that has a positive charge in the side chain at physiological pH preferably is a basic amino acid. Basic amino acids are herein understood to mean amino acids or equivalents thereof that are hydrophilic amino acid and have a side chain pK value of greater than 7. Basic amino acids typically have positively charged side chains at physiological pH due to association with hydrogen ion (H⁺) or hydronium ion (H₃O⁺). The amino acids in Z¹ that have a positive charge in the side chain at physiological pH are preferably independently selected from the basic amino acids that include the genetically encoded amino acids arginine (Arg), lysine (Lys), and histidine (His) as well as the non-genetically encoded amino acids ornithine, 2,3- diaminopropionic acid, 2,4-diaminobutyric acid, homoarginine, homolysine and homohistidine. In a preferred alternative embodiment of the cyclic compound according to the invention, Z¹ is selected from the group consisting of Lys-Lys-Lys, Lys-Lys-Arg, Lys-Arg-Lys, Arg-Lys-Lys, Arg-Arg-Lys, Arg-Lys-Arg, Lys-Arg-Arg and Arg-Arg-Arg, wherein each Lys or Arg may be preceded or followed by an additional Lys or an additional Arg or any additional other amino acid, any additional other amino acid preferably being selected from the group consisting of glycine, alanine, serine, threonine, valine, leucine and isoleucine. A more preferred alternative embodiment of the cyclic compound according to the invention is a compound wherein Z¹ is selected from the group consisting of Lys-Lys-Lys-Lys, Lys-Lys-Lys- Arg, Lys- Lys-Arg-Lys, Lys-Arg-Lys-Lys, Arg-Lys-Lys-Lys, Lys-Lys-Arg-Arg, Lys-Arg-Lys- Arg, Lys- Arg-Arg-Lys, Arg-Lys-Lys- Arg, Arg-Lys-Arg-Lys, Arg- Arg-Lys-Lys, Lys- Arg-Arg-Arg, Arg-Lys-Arg-Arg, Arg-Arg-Lys-Arg, Arg-Arg-Arg-Lys and Arg-Arg- Arg- Arg. Although Z¹ preferably represents a stretch comprising 3 or 4 amino acids or equivalents thereof having a positive charge in the side chain at physiological pH, stretches of 5 or more such amino acids are not excluded from the invention. In the cyclic compounds of the invention, Z represent an amino acid or equivalent thereof that has an aromatic group in the side chain. Z² thus preferably represents an aromatic amino acid. An aromatic amino acid is herein understood to refer to a hydrophobic amino acid having a side chain containing at least one ring having a conjugated π-electron system (aromatic group). The aromatic group may be further substituted with groups such as alkyl, alkenyl, alkynyl, hydroxyl, sulfanyl, nitro and amino groups, as well as others. Preferably in the cyclic compound of the invention Z² is selected from the group consisting of the genetically encoded aromatic amino acids phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp), as well as the non- genetically encoded aromatic amino acids phenylglycine, 1-naphtylalanine, 2- naphtylalanine, thienylalanine, β-2-thienylalanine, cyclohexylalanine, pyrrolidinylalanine, piperidin-3-ylalanine, piperidin-4-ylalanine 1,2,3,4- tetrahydroisoquinoline-3-carboxylic acid, and p-substituted phenylalanines like A- chloro-phenylalanine, 2-fluorophenylalanine, 3-fluorophenylalanine and A- fluorophenylalanine. More preferably however, Z² is tryptophan. A cyclic compound of the invention may further comprise Z³, wherein Z³ connects Z² to X² if X² is present or to Z¹ if X² is not present. Z³ preferably represents an amino acid or equivalent thereof capable of forming a covalent linkage with a side chain of another amino acid residue, such as a disulfide linkage. Preferably therefore, Z³ represents an amino acid or equivalent thereof having a thiol (SH) group in the side chain. Z³ is preferably selected from the group consisting of cysteine (Cys), homocysteine and penicillamine. More preferably, Z³ is cysteine. Thus in one embodiment the invention relates to a cyclic compound of general formula (II)

z'-x'-z^-x²

wherein, Z¹, X¹, Z², Z³ are as defined above, wherein X² represents a spacer having 1 to 9 atoms connecting Z³ to Z¹ or X² is absent in which case Z³ is connected to Z¹, and pharmaceutically acceptable salts thereof. A cyclic compound of the general formula (II) is particularly suited for the prevention and/or treatment of infections with Gram- positive bacteria.

The term "spacer" is used herein to define a distance between Z¹ and Z², between Z³ and Z¹ or between Z² and Z¹ in terms of number of atoms. The number of atoms is counted in a straight chain from the first atom that is bonded to any Z to the last atom to which another Z is coupled. The spacer thus begins (or ends) at the atom that is connected to a backbone carbonyl-carbon atom of an amino acid Z or the atom that is equivalent to a carbonyl-carbon of Z and ends (or begins) at a backbone nitrogen atom of another Z or the atom that is equivalent to a nitrogen atom of another Z. The atoms in the spacer may be any suitable atom known to the skilled person, preferably the atoms in the spacer, which may be the same or different, are selected from C, O, S, N and P.

The spacer may be branched and/or the atoms in the spacer may be substituted. Such substituents need not be inert but may have a function. Such substituents may for example have a functionality that enhances the in vivo stability of the cyclic compound, or otherwise enhance its pharmacological properties, e.g. increase the half-life of the compound, reduce its toxicity, enhance solubility or uptake and the like, or for example to enable detection of the compound by substituted with a fluorescent label. In the cyclic compound of the invention X² represents a spacer having 1 to 9 atoms connecting Z³ to Z¹ or X² may be absent in which case Z³ is connected directly to Z¹. Preferably, however, X² comprises 1 or 2 amino acids. These 1 or 2 amino acids are preferably independently selected from the group consisting of glycine, alanine, serine, threonine, proline, aminohexanoic acid, aminopentanoic acid, aminobutanoic acid and aminopropanoic acid. More preferably X² is GIy- Ala.

In the cyclic compound of the invention X¹ represents a spacer from 5 to 12 atoms, said spacer connecting Z¹ to Z². Preferably, X¹ comprises 2 or 3 amino acids. These 2 or 3 amino acids are preferably independently selected from the group consisting of glycine, alinine, valine, serine, threonine, asparagine and glutamine. More preferably X¹ is independently selected from valine, serine and glutamine. Most preferably X¹ is Ser-Val-Gln.

In addition the amino acids comprised in X¹ and X² may include certain commonly encountered amino acids which are not genetically encoded including e.g. β- alanine (b-Ala) and other omega-amino acids such as 3-aminopropionic acid (Dap), 2,3-diaminopropionic acid (Dpr), 4-aminobutyric acid and so forth; α-aminoisobutyric acid (Aib); ε-aminohexanoic acid (Aha); δ-aminovaleric acid (Ava); N-methylglycine or sarcosine (MeGIy); ornithine (Orn); citrulline (Cit); t-butylalanine (t-BuA); t- butylglycine (t-BuG); N-methyliso leucine (Melle); phenylglycine (Phg); cyclohexylalanine (Cha); norleucine (NIe); 2-naphthylalanine (2-Nal); 4- chlorophenylalanine (Phe(4-Cl)); 2-fluorophenylalanine (Phe(2-F)); 3- fluorophenylalanine (Phe(3-F)); 4-fluorophenylalanine (Phe(4-F)); penicillamine (Pen); l,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic); β-2-thienylalanine (Thi); methionine sulfoxide (MSO); homoarginine (hArg); N-acetyl lysine (AcLys); 2,3- diaminobutyric acid (Dab); 2,3-diaminobutyric acid (Dbu); p-aminophenylalanine (Phe(pNH2)); N-methyl valine (MeVaI); homocysteine (hCys) and homoserine (hSer).

Preferably the cyclic compound of the invention comprises 7, 8, 9, 10, 11, 12, 13, 14 or 15 amino acids or equivalents thereof, more preferably the compound comprises 7-12 amino acids or equivalents thereof and most preferably the compound comprises 8-11 amino acids or equivalents thereof.

The invention additionally provides a method of preparing cyclic compounds of the invention. The method of synthesis comprises the steps of synthesizing a linear peptide of an amino acid sequence as defined herein, and cyclizing the peptide by linking the carboxyl and amino termini to form a cyclic peptide. The cyclization step can be performed as described in the Examples herein. Alternative methods for cyclizing linear peptides are known in the art and e.g. include the use of N-(3- dimethylaminopropyl)-N'-ethylcarbodiimide and N-hydroxybenzotriazole, for example,

60 equivalents and 20 equivalents, respectively, in a solvent such as dimethylsulfoxide.

An advantage of using chemical synthesis to prepare a cyclic compound of the invention is that (D)-amino acids can be substituted for (L)-amino acids, if desired. The incorporation of one or more (D)-amino acids into a cyclic compound analog can confer, for example, additional stability of the peptide in vitro or, particularly, in vivo, since endogenous endoproteases generally are ineffective against peptides containing

(D)-amino acids. Naturally occurring antimicrobial peptides that have been chemically synthesized to contain (D)-amino acids maintain their antimicrobial activity (see e.g. Wade et al, 1990, Proc. Natl. Acad. Sci. USA 87: 4761-4765). Preferably, however, the cyclic compounds of the invention comprise at least two, three, four or five consecutive (L)-amino acids. More preferably the cyclic compounds of the invention comprise less than four alternating (D)- and (L)-amino acids, or less than four alternating (D)- and (L)-α-amino acids. If desired, the reactive side group of one or more amino acids in a cyclic compound of the invention can be modified or amino acid derivatives can be incorporated into the peptide (see, for example, Protein Engineering: A practical approach (IRL Press 1992); Bodanszky, Principles of Peptide Synthesis (Springer- Verlag 1984)). Selective modification of a reactive group can impart desirable characteristics upon a cyclic compound of the invention. The choice of including such a modification is determined, in part, by the characteristics required of the peptide. Such modifications can result, for example, in cyclic compound variants having greater antimicrobial selectivity or potency.

The individual amino acids or equivalents thereof in the cyclic compounds of the inventions can be incorporated in the compound by a peptide bond or a peptide bond mimetic. A peptide bond mimetic of the invention includes peptide backbone modifications well known to those skilled in the art. Such modifications include modifications of the amide nitrogen, the α-carbon, amide carbonyl, complete replacement of the amide bond, extensions, deletions or backbone cross-links. See, generally, Spatola, Chemistry and Biochemistry of Amino Acids, Peptides and Proteins, Vol. VII (Weinstein ed., 1983). Several peptide backbone modifications are known, these include, ψ [CH₂S], ψ [CH₂NH], ψ [CSNH₂ ], ψ [NHCO], ψ [COCH₂ ] and ψ [(E) or (Z) CH=CH]. The nomenclature used above, follows that suggested by Spatola, above. In this context, ψ indicates the absence of an amide bond. The structure that replaces the amide group is specified within the brackets.

Equivalents of amino acids for incorporation into the cyclic compounds of the invention include amino acid mimetics. An "amino acid mimetic" as used here is a moiety other than a naturally occurring amino acid that conformationally and functionally serves as a substitute for an amino acid in a cyclic compound of the present invention. Such a moiety serves as a substitute for an amino acid residue if it does not interfere with the antimicrobial activity of the compound. Preferably the substitute improves the antimicrobial activity of the compound. Amino acid mimetics may include non-protein amino acids, such as β-, γ-, δ-amino acids, β-, γ-, δ~imino acids (such as piperidine-4-carboxylic acid) β2 amino acids, β3 amino acids, α,α- disubstituted amino acids, β-amino sulfonyl compounds, α-aminohydrazino acids, CC- hydroxy acids, α-amino nitriles as well as many derivatives of L-α-amino acids. Peptide mimetics suitable for peptides of the present invention are discussed by Morgan and Gainor, (1989) Ann. Repts. Med. Chem. 24:243-252. Preferably, however, a cyclic compound of the present invention comprises less than three consecutive homochiral β-amino acids, more preferably less than three homochiral β-amino acids.

An important advantage of the cyclic compounds of the invention is that they show a dramatic increase in antimicrobial activity as compared linear versions having the same amino acid sequence. Already under non-physiological low ionic strength conditions (e.g. 10 mM phosphate buffer) the cyclic compounds show antimicrobial activity at a dosage that is almost a factor 100 lower than needed for a linear compound having the same peptide sequence (see Figure 1). More importantly, however, under physiological conditions (e.g. 155 mM NaCl; phosphate buffered) 100 μg/mL of linear peptide show no detectable antimicrobial activity whereas the cyclic compounds of the invention already kill more than 99% of the microbes in the sub-μM range in vitro (see Figure 2).

A further advantage of the cyclic compounds of the invention is that they are distinguishable from linear compounds in that the cyclic compounds lack a free amino or carboxyl terminus, i.e. they lack a free back-bone amino or carboxyl group. As a result the cyclic compounds of the invention are resistant to exo-peptidases such as aminopeptidases and carboxypeptidases because there is no amino or carboxyl terminus to serve as a substrate for the exo-peptidases. The invention thus further provides a method of enhancing protease resistance of a peptide by synthesizing a peptide, wherein a peptide bond is formed between the amino -terminal and carboxyl-terminal amino acids.

A preferred cyclic compound of the invention is a cyclic peptide or peptide mimetic having an amino acid sequence as disclosed in the Examples, Tables and Sequence Listing herein.

A further aspect of the invention relates to a composition comprising a cyclic compound as defined herein above. Preferably the composition further comprises a pharmaceutically acceptable carrier. Depending on the formulation desired, the composition comprises one or more pharmaceutically-acceptable, non-toxic carriers and/or diluents, which are defined as vehicles commonly used to formulate pharmaceutical compositions for animal or human administration. The diluent is selected so as not to affect the biological activity of the combination. Examples of such diluents are distilled water, buffered water, physiological saline, PBS, Ringer's solution, dextrose solution, and Hank's solution. In addition, the pharmaceutical composition or formulation may also include other carriers, adjuvants, or non-toxic, non-therapeutic, non-immunogenic stabilizers, excipients and the like. The compositions may also include additional substances to approximate physiological conditions, such as pH adjusting and buffering agents, toxicity adjusting agents, wetting agents, detergents and the like.

The composition may also include any of a variety of stabilizing agents, such as an antioxidant for example. Moreover, the cyclic compounds of the invention may be complexed with various well-known compounds that enhance the in vivo stability of the polypeptide, or otherwise enhance its pharmacological properties (e.g., increase the half-life of the compound, reduce its toxicity, enhance solubility or uptake). Examples of such modifications or complexing agents include the production of sulfate, gluconate, citrate, phosphate and the like. The polypeptides of the composition may also be complexed with molecules that enhance their in vivo attributes. A list of such molecules, provided by way of example and not limitation, includes carbohydrates, polyamines, amino acids, other peptides, ions (e.g., sodium, potassium, calcium, magnesium, manganese, ammonium, Al, Zn, Fe etc.), and lipids. Further guidance regarding formulations that are suitable for various types of administration can be found in Remington's Pharmaceutical Sciences, Mace Publishing Company, Philadelphia, PA, 17th ed. (1985). For a brief review of methods for drug delivery, see, Langer, Science 249:1527-1533 (1990).

The above compositions of the invention may comprise, in addition to the cyclic compound of the invention, one or more further antimicrobial agents. Preferably the further antimicrobial agent is an antibiotic, an antifungal or an antiviral agent. The combination of the cyclic compound of the invention and the further antimicrobial agent preferably produces a synergistic antimicrobial effect and may be particularly useful against drug-resistant pathogens or even multidrug resistant pathogens. Suitable antibiotics for use in combination with the cyclic compound of the invention include but are not limited to β-lactam (penicillin) antibiotics, cephalosporin antibiotics, aminoglycoside antibiotics, lincosamide antibiotics, quinolone antibiotics, macrolide antibiotics and tetracycline antibiotics. Suitable antifungals or antimycotics for use in combination with the cyclic compound of the invention include but are not limited to amphotericin B, nystatin, miconazole, fluconazole, clotrimazole, terbinafine, naftifine, butenafine, new generation azoles, e.g. voriconazole, and members of the group of echinocandins. Suitable antivirals for use in combination with the cyclic compound of the invention include but are not limited to aclycovir, ganciclovir, zidovudine (AZT), didanosine, tamifiu, and inhibitors of the HIV-protease.

In a further aspect the invention relates to a cyclic compound or a composition comprising the compound as defined herein above for use as a medicament. The cyclic compounds or the compositions comprising the compounds are preferably used in methods for preventing or treating an infectious disease in a patient. The cyclic compounds of the invention (or the combination thereof with one or more other antimicrobials) are effective in preventing or treating infectious diseases caused by a wide range of microbes including e.g. viruses, bacteria, fungi and (protozoan) parasites. The methods, compounds and compositions of the invention are e.g. effective in preventing or treating infections with Gram-negative and Gram-positive bacteria. More specifically, they are effective in preventing or treating infections caused by Gram- negative bacteria including but not limited to Escherichia, Acinetobacter, Chlamydia, Rickettsia- bacteria, Meningococci, Gonococci, Bordetella, Klebsiella, Helicobacter, Proteus, Serratia, Pseudomonas, Legionella, Salmonella, Shigella, Campylobacter, Borrelia, Yersinia as well as the bacteria causing Cholera, Plague, Leptospirosis, and Lyme's disease. Cyclic compounds of the invention of the general formula (I) are preferred for preventing or treating infections caused by Gram-negative bacteria.

The cyclic compounds of the invention are also effective in preventing or treating infections caused by Gram-positive bacteria. More specifically, they are effective in preventing or treating infections caused by Gram-positive bacteria including but not limited to Listeria, Mycobacteria, Staphylococci, Streptocci, Pneumonococci, Diphtheria, Clostridium, Bacilli, as well as the bacteria causing Tetanus, Botulism and Anthrax. Particularly preferred for preventing or treating infections caused by Gram- positive bacteria are cyclic compounds of the invention of the general formula (II), that further comprise Z³, which represents an amino acid or equivalent thereof capable of forming a covalent linkage with a side chain of another amino acid residue, such as a disulfide linkage, preferably an amino acid or equivalent thereof having a thiol (SH) group in the side chain.

The cyclic compounds of the invention, of both general formulas (I) and (II), are also effective in preventing or treating non-bacterial pathogens such as viruses, fungi and (protozoan) parasites.

In yet a separate aspect the invention relates to linear forms of the peptides as defined herein and/or disclosed herein.

Examples of viruses causing infections that may be prevented and/or treated by methods, compounds and/or compositions of the invention include but are not limited to hepatitis (K, B, or C), herpes virus (e.g., VZV, HSV-I, HAV-6, HSV-II, and CMV,

Epstein Barr virus), adenovirus, influenza virus (e.g. H5N1), flaviviruses, echovirus, rhinovirus, coxsackie virus, coronavirus, respiratory syncytial virus (RSV), mumps virus, rotavirus, measles virus, rubella virus, parvovirus, vaccinia virus, HTLV virus, dengue virus, papillomavirus, molluscum virus, poliovirus, rabies virus, JC virus, arboviral encephalitis virus, and human immunodeficiency virus (HIV virus; e.g., type

I and II).

Examples of fungi causing infections that may be prevented and/or treated by methods, compounds and/or compositions of the invention include but are not limited to Candida (e.g., albicans, krusei, glabrata, tropicalis), Cryptococcus neoformans, Aspergillus (e.g., fumigatus, niger), Genus Mucorales (Mucor, Absidia, Rhizopus), Sporothrix schenkii, Blastomyces dermatitidis, Paracoccidioides brasiliensis, Coccidioides immitis and Histoplasma capsulatum. Examples of pathogenic parasites causing infections that may be prevented and/or treated by methods, compounds and/or compositions of the invention include but are not limited to Entamoeba histolytica, Balantidium coli, Naegleria, Fowleri, Acanthamoeba sp., Giardia lambia, Cryptosporidium sp., Pneumocystis carinii, Plasmodium vivax, Babesia microti, Trypanosoma brucei, Trypanosoma cruzi, Leishmania donovani, Toxoplasma gondii, Schistosomae and Plasmodium falcipar is.

The methods, compounds and/or compositions of the invention are particularly suited for preventing and/or treating infection caused by microbes that are resistant to one or more conventional antimicrobial agents, such as e.g. multidrug resistant Staphylococcus aureus (see Examples herein) or triazole resistant Candida.

Thus in a further aspect the invention relates to a method for preventing or treating infectious disease in a patient, comprising administering to the patient a prophylactic or therapeutic dose of a cyclic compound or a composition as defined herein above. The cyclic compounds of the invention may be administered orally, they may be administered parentally or they may be administered by inhalation.

For oral administration, the active compounds can be administered in solid dosage forms, such as capsules, tablets, and powders, or in liquid dosage forms, such as elixirs, syrups, and suspensions. Active compounds can be encapsulated in gelatin capsules together with inactive ingredients and powdered carriers, such as glucose, lactose, sucrose, mannitol, starch, cellulose or cellulose derivatives, magnesium stearate, stearic acid, sodium saccharin, talcum, magnesium carbonate and the like. Examples of additional inactive ingredients that may be added to provide desirable colour, taste, stability, buffering capacity, dispersion or other known desirable features are red iron oxide, silica gel, sodium lauryl sulfate, titanium dioxide, edible white ink and the like. Similar diluents can be used to make compressed tablets. Both tablets and capsules can be manufactured as sustained release products to provide for continuous release of medication over a period of hours. Compressed tablets can be sugar coated or film coated to mask any unpleasant taste and protect the tablet from the atmosphere, or enteric-coated for selective disintegration in the gastrointestinal tract. Liquid dosage forms for oral administration can contain colouring and flavouring to increase patient acceptance.

Compositions for parental administration must be sterile. Sterilisation is readily accomplished by filtration through sterile filtration membranes, prior to or following lyophilisation and reconstitution. The parental route for administration is in accord with known methods, e.g. injection or infusion by intravenous, intraperitoneal, intramuscular, intraarterial or intralesional routes. The active compounds of the invention may be administered continuously by infusion or by bolus injection. A typical composition for intravenous infusion could be made up to contain 10 to 50 ml of sterile 0.9% NaCl or 5% glucose optionally supplemented with a 20% albumin solution and 1 to 1000 μg of the cyclic compounds of the invention. A typical pharmaceutical composition for intramuscular injection would be made up to contain, for example, 1 - 10 ml of sterile buffered water and 0.1 to 200 μg of the cyclic compounds of the invention. However, as indicated above, the compositions comprising the cyclic compounds of the invention may also be administered orally and be formulated accordingly.

Alternatively the active compounds of the invention may be administered by inhalation. Formulations suitable for pulmonary administration via the buccal cavity are presented such that particles containing the active ingredient and desirably having a diameter of under 10 μm are delivered into the bronchial tree of the recipient. As one possibility such formulations are in the form of fine powders that may conveniently be presented either in an easily pierced capsule, such as a gelatin, for use in an inhalation device, or alternatively as a formulation comprising active ingredient, a suitable liquid propellant and optionally other ingredients such as surfactant and/or a solid diluent. So- called "self-propelling" formulations of this nature may also be employed wherein the active ingredient is dispensed in the form of droplets of a solution or suspension. Such formulations are analogous to those well known in the art and may be prepared by established procedures. Such formulations are advantageously presented in a container provided with either a manually-operable or automatically functioning valve having the desired spray characteristics and having some type of valve structure for delivering a metered amount or fixed volume, for example 50 to 100 microliters, upon each operation thereof. Solutions of the active compounds of the invention useful for the methods disclosed herein may also be maintained in the form of a solution for use in an atomiser or nebuliser whereby an accelerated airstream, whether by ultrasonic agitation or some other means, is employed to produce a fine droplet mist for inhalation by the patient. Compositions of the invention intended for atomization and inhalation administration thus include aqueous solutions, suspensions, and dry powders comprising a safe and effective amount of the active compound of the invention. Such compositions are typically contained in a container with attached atomizing means. Such compositions also typically include propellants such as chlorofluorocarbons 12/11 and 12/114, and more environmentally friendly fluorocarbons, or other nontoxic volatiles; solvents such as water, glycerol and ethanol, these include cosolvents as needed to solvate or suspend the active compound and optional stabilizers, preservatives, tonicity adjustors, buffers and flavoring agents.

The cyclic compounds and compositions of the invention may be administered for prophylactic treatment of individuals that have a higher than normal susceptibility to infectious diseases. Individuals having a higher than normal susceptibility to infectious diseases are individuals having a compromised or deficient immunity such as e.g. patients suffering from acquired or inborn immunodeficiency syndromes (AIDS or SCID), cancer patients undergoing chemotherapy or radiotherapy, cancer patient having a tumor of the hematopoietic system, patients treated with immunosuppressive drugs such as patients undergoing organ transplantation, in particular patients undergoing bone marrow transplantation, e.g. allogeneic bone marrow stem cell transplantation. Such patients are susceptible to opportunistic infections that may be suitably prevented and/or treated by administering an amount of the cyclic compounds and/or compositions of the invention that is sufficient to prevent, delay or reduce the severity of the infectious disease. An amount adequate to accomplish this is defined as a "therapeutically-" or "prophylactically-effective dose". Such effective dosages will depend on the severity and/or virulence of the infectious disease and on the general state of the patient's health. Given the broad spectrum of antimicrobial activity of the cyclic compounds of the invention, it is understood that they may be applied to simultaneously prevent or treat infections by more than one pathogenic microbe.

In the present methods, cyclic compounds of the invention are usually administered at a dosage of at least about 0.01 μg/kg patient body weight or more per week to a patient. Often dosages are about 10 μg/kg per week. Dosage regimes can range from about 0.01 μg/kg per week to at least 10 mg/kg per week. Typically dosage regimes are about 0.1 - 100 μg per kg/week. In preferred regimes about 0.4 - 40 μg/kg is administered once, twice or three times weekly. Treatment is typically continued for at least 4 weeks, sometimes 24 weeks, and sometimes for the life of the patient. In a further aspect particularly for jurisdictions other than the USA, the invention pertains to the use of a cyclic compound or a composition as defined herein above for the manufacture of a medicament for preventing or treating an infectious disease in accordance with any of the methods defined herein above. In this document and in its claims, the verb "to comprise" and its conjugations is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. In addition, reference to an element by the indefinite article "a" or "an" does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article "a" or "an" thus usually means "at least one".

Description of the figures

Figure 1 shows a comparison of the antimicrobial activities of the linear versus the cyclic hLFl-11 peptide against a multi drug resistant S. aureus under low salt conditions.

Figure 2 shows a comparison of the antimicrobial activities of the linear versus the cyclic hLFl-11 peptide against a multi drug resistant S. aureus under physiological salt conditions.

Examples Example 1 : Synthesis of cyclic peptides

All cyclic peptides were generated by solid phase peptide synthesis. For synthesis Fmoc amino acids of the L-configuration carrying acid labile side chain protecting groups when needed were used (tBu for S and E, Pmc for R; Trt for C, Q, N, penicillamine and homoCys; Boc for W and K). N-methylpyrrolidone (NMP) was used as a solvent for washings and couplings, Fmoc-removal was performed with 20 % (v/v) piperidine in NMP. All acylations were performed using a solution of a sixfold molar excess (compared to the loading of the resin) of Fmoc amino acid, a sixfold molar excess (compared to the loading of the resin) of benzotriazole-1-yl-oxy-tris- pyrrolidino-phosphonium hexafluorophosphate (PyBOP) and a twelvefold (compared to the loading of the resin) of N-methylmorpholin (NMM) in NMP. In order to be able to prepare "head-to -tail" cyclic peptides anchoring to the resin was performed via the side chain of Q. Thus, N-α-Fmoc-L-glutamic acid α-4-{N-[l-(4,4-dimethyl-2,6- dioxocyclohexylidene)-3-methylbutyl]-amino} benzylester (Fmoc-Glu-ODmab) was coupled to TentagelS-AM resin. Repeated coupling cycles, consisting of Fmoc removal and coupling of the new amino acid, were performed until the required peptide length was reached. After the last Fmoc removal had been completed the peptidylresin was treated with 2% hydrazine in NMP to remove the Dmab protection on the C-terminus of the peptide. By now the N-terminus and the C-terminus of the peptide are both free and are coupled to each other by addition of a solution of a tenfold excess (compared to the loading of the resin) of O-(7-azabenzotriazol-l-yl)-l,l,3,3-tetramethyluronium hexafluorophosphate (HATU) and a tenfold excess (compared to the loading of the resin) of N-methylmorpholin (NMM) in NMP. Overnight cyclisation yielded the resin bound cyclic peptides. Cleavage of the peptide from the resin and side chain deprotection using trifluoroacetic acid containing a scavenger cocktail and subsequent ether precipitation of the peptide from the reaction mixture yielded the cyclic peptides. Cyclic peptides were analysed by RP-HPLC and Maldi-Tof mass spectrometry indicating the expected molecular masses. Table 1 lists the cyclic and linear peptides that were synthesized and tested. Further cyclic and linear peptides that were synthesized and tested are listed in Tables 6 - 11. Example 2: In vitro antimicrobial efficacy of cyclic peptides 2.1 Multidrug resistant Staphylococcus aureus

The methicillin (multidrug) resistant Staphylococcus aureus (MRSA) is a clinical isolate (Dept. of Infectious Diseases, LUMC, Leiden, The Netherlands). Overnight cultures of bacteria were prepared in brain heart infusion broth (Oxoid, Basingstoke, UK) in an incubator at 37⁰C. Virulent bacteria were maintained in mice. Briefly, about IxIO⁷ colony- forming units (CFU) in 0.1 ml of the micro-organisms were injected into a tail vein of mice and 24 hrs thereafter the mice were sacrificed. The spleen was aseptically removed, homogenized and appropriate dilutions of the homogenate were plated onto blood agar plates. After 24 h of incubation at 37⁰C a single CFU was transferred into 25 mL of the appropriate broth and incubated for 24 h at 37⁰C and 1 mL aliquots of these suspensions containing about 5x10⁸ virulent bacteria per mL were stored at -2O⁰C. 2.2 Assay for in vitro bactericidal activity of the antimicrobial peptides against various multidrug resistant bacteria

An in vitro killing assay was used to assess the bactericidal activity of antimicrobial peptides towards MRSA (Nibbering P.H. et al., Infect. Immun. 69, 1469- 1476, 2001). The bacteria were washed twice either in 10 mM sodium phosphate buffer pH 7.4 (NaPB) or in PBS. Next, they were diluted to about lxlO⁶/mL in NaPB or PBS. Micro-organisms were exposed to various amounts (0-100 μg/mL) of the various peptides for 1 h at 37⁰C and thereafter, the number of viable micro-organisms was assessed microbiologically using blood agar plates. All negative cultures were assigned the value of 15 CFU/mL, being the detection limit. Data are expressed as the effective dose to kill 90%, 99%, 99.9% and 99.99% of the initial number of viable micro-organisms (ED90%, ED99%, ED99.9% and ED99.99%). Results are presented in Tables 1 and 2.

Using essentially the same procedures the bactericidal activity of a selection of cyclic and linear peptides was tested towards 3 further bacterial strains: 1) a multidrug resistant Listeria monocytogenes, nr LUH 2035, clinical isolate of patient from LUMC (results are shown in Table 3); 2) a multidrug resistant Acinetobacter baumannii, nr LUH 6034, clinical isolate of patient from LUMC (results are shown in Table 4); and 3) a multidrug resistant Pseudomonas aeruginosa, nr. LUH 7552, clinical isolate of a cystic fibrosis patient of the Leyenburg hospital in The Hague (results are shown in Table 5).

Using again essentially the same procedures the antimicrobial activity of the further cyclic and linear peptides as listed in Tables 6 - 11 was tested towards the microbial strains as indicated in the corresponding Tables 6 - 11.

In Tables 6 - 9 the length of the spacer X¹ is further varied between 6 - 12 and inclusion of a proline residue (inducing a bend in the peptide chain) is tested. All cyclic variants tested in Tables 6 - 9 are active towards the various tested microorganisms.

In Table 10 the length of Z¹ is extended to up to six positively charged amino acids and a length of spacer X² of 3 or 6 atoms is tested. All cyclic variants tested in Tables 10 are active towards the various tested microorganisms.

In Table 11 we demonstrate that none of the possible linearized variants of the cyclic peptide has the antimicrobial activity of the cyclic peptide and that therefore the cyclic nature of the peptide is required for its activity.

Table 1 Concentrations of various peptides (effective dose [ED%] in μM) leading to 90%,

J = 2-naphtylalanine, U = 2-amiπobutyric acid, j = homocysteiπ, u = penicillamine

Table 2

Concentrations of various peptides (effective dose [ED%] in μM) leading to

Table 3

Concentrations of various peptides (effective dose [ED%] in μM) leading

Table 4

Concentrations of various peptides (effective dose [ED%] in μM) leading

Table 5

Concentrations of various peptides (effective dose [ED%] in μM) leading

Table 6. Concentrations of various peptides (effective dose [ED%] in μM) leading to 90%, 99%, 99.9% and 99.99% killing of drug resistent S. aureus in physiological salt concentration (155 mM NaCI; 10 mM phosphate buffered). All peptides are cyclic except for 0624-19.

spacer lenght in atoms between the R-cluster and the W; spacer containing prolines, bent spacer;

U = β-alanine (4 chain atoms); O = γ-aminobutyric acid (5 chain atoms); B = 5-aminopentanoic acid (6 chain atoms). Table 7. Concentrations of various peptides (effective dose [ED%] in μM) leading to 90%, 99%, 99.9% and 99.99% killing of drug resistent L. monocytogenes in physiological salt concentration (155 mM NaCI; 10 mM phosphate buffered). All peptides are cyclic except for 0624-19.

U = β-alanine (4 chain atoms); O = γ-aminobutyric acid (5 chain atoms); B = 5-aminopentanoic acid (6 chain atoms).

Table 8. Concentrations of various peptides (effective dose [ED%] in μM) leading to 90%, 99%, 99.9% and 99.99% killing of drug resistent A. baumannii in physiological salt concentration (155 mM NaCI; 10 mM phosphate buffered). All peptides are cyclic except for 0624-19.

U = β-alanine (4 chain atoms); O = γ-aminobutyric acid (5 chain atoms); B = 5-aminopentanoic acid (6 chain atoms). Table 9. Concentrations of various peptides (effective dose [ED%] in μM) leading to 90%, 99%, 99.9% and 99.99% killing of drug resistent P. aerigunosa in physiological salt concentration (155 mM NaCI; 10 mM phosphate buffered). All peptides are cyclic except for 0624-19.

Tabel 10. Concentrations of various peptides (effective dose [E D%] in μM) leading to 90%, 99%, 99.9% and 99.99% killing of the indicated micro-organisms in physiological salt concentration (155 mM NaCI; 10 mM phosphate buffered).

SA = S. aureus; LM = L. monocytogenes; AB = A. baumannii

Table 11. Concentrations of various peptides (effective dose [E D%] in μM) leading to 90%, 99%, 99.9% and 99.99% killing of drug resistent L. monocytogenes in physiological salt concentration (155 mM NaCI; 10 mM phosphate buffered). All peptides are linear except for 0604-24.

n.t. = not tested; n.k. = no killing

Claims

Claims 1. A cyclic compound of general formula

z^l-x^l-z²-x²

wherein

Z¹ represents a stretch of 3 - 6 amino acids or equivalents thereof, wherein at least 3 of said amino acids are selected from the group consisting of arginine and homoarginine, and wherein no more than one/two of said amino acids is selected from the group consisting of lysine, homo lysine and ornithine,

Z² represent an amino acid or equivalent thereof having an aromatic group in the side chain,

X² represents a spacer having 1 to 9 atoms connecting Z² to Z¹ or X² is absent in which case Z² is connected to Z¹, and wherein the compound comprises at least two consecutive (L)-amino acids,

and pharmaceutically acceptable salts thereof.

2. The cyclic compound according to claim 1, wherein Z¹ is selected from the group consisting of R-R-R, R-R-R-R, K-R-R-R, R-K-R-R, R-R-K-R, R-R-R-K, K-K-R-R, R-

K-K-R, R-R-K-K, K-R-K-R, K-R-R-K, R-K-R-K, R-R-R-R-R, K-R-R-R-R, R-K-R-R-

R, R-R-K-R-R, R-R-R-K-R, R-R-R-R-K, K-K-R-R-R, K-R-K-R-R, K-R-R-K-R, K-R-

R-R-K, R-K-K-R-R, R-K-R-K-R, R-K-R-R-K, R-R-K-K-R, R-R-K-R-K, R-R-R-K-K,

R-R-R-R-R-R, K-R-R-R-R-R, R-K-R-R-R-R, R-R-K-R-R-R, R-R-R-K-R-R, R-R-R-R- K-R, R-R-R-R-R-K, K-K-R-R-R-R, K-R-K-R-R-R, K-R-R-K-R-R, K-R-R-R-K-R, K-

R-R-R-R-K, R-K-K-R-R-R, R-K-R-K-R-R, R-K-R-R-K-R, R-K-R-R-R-K, R-R-K-K-

R-R, R-R-K-R-K-R, R-R-K-R-R-K, R-R-R-K-K-R, R-R-R-K-R-K, and R-R-R-R-K-K, wherein R represents an amino acid selected from the group consisting of arginine and homoarginine, and K represents an amino acid selected from the group consisting of lysine, homo lysine and ornithine, and wherein each R or K may be preceded or followed by an additional R or an additional K or any additional other amino acid, any additional other amino acid preferably being selected from the group consisting of glycine, alanine, serine, threonine, valine, leucine and isoleucine.

3. The cyclic compound according to claim 1 or 2, wherein Z¹ is selected from the group consisting of R-R-R, R-R-R-R, K-R-R-R, R-K-R-R, R-R-K-R, R-R-R-K, K-K- R-R, R-K-K-R, R-R-K-K, K-R-K-R, K-R-R-K, R-K-R-K, R-R-R-R-R, K-R-R-R-R, R- K-R-R-R, R-R-K-R-R, R-R-R-K-R, R-R-R-R-K, K-K-R-R-R, K-R-K-R-R, K-R-R-K- R, K-R-R-R-K, R-K-K-R-R, R-K-R-K-R, R-K-R-R-K, R-R-K-K-R, R-R-K-R-K, R-R- R-K-K, R-R-R-R-R-R, K-R-R-R-R-R, R-K-R-R-R-R, R-R-K-R-R-R, R-R-R-K-R-R, R-R-R-R-K-R, R-R-R-R-R-K, K-K-R-R-R-R, K-R-K-R-R-R, K-R-R-K-R-R, K-R-R- R-K-R, K-R-R-R-R-K, R-K-K-R-R-R, R-K-R-K-R-R, R-K-R-R-K-R, R-K-R-R-R-K, R-R-K-K-R-R, R-R-K-R-K-R, R-R-K-R-R-K, R-R-R-K-K-R, R-R-R-K-R-K, and R-R- R-R-K-K.

4. The cyclic compound according to claim 3, wherein Z¹ is selected from the group consisting of R-R-R, R-R-R-R, K-R-R-R, R-K-R-R, R-R-K-R, R-R-R-K, R-R-R-R-R,

K-R-R-R-R, R-K-R-R-R, R-R-K-R-R, R-R-R-K-R, R-R-R-R-K, R-R-R-R-R-R, K-R- R-R-R-R, R-K-R-R-R-R, R-R-K-R-R-R, R-R-R-K-R-R, R-R-R-R-K-R, and R-R-R-R- R-K.

5. The cyclic compound according to any one of the preceding claims, wherein Z² is selected from the group consisting of phenylalanine, tyrosine, tryptophan, phenylglycine, 1-naphtylalanine, 2-naphtylalanine, thienylalanine, β-2-thienylalanine, cyclohexylalanine, pyrrolidinylalanine, piperidin-3-ylalanine, piperidin-4-ylalanine l,2,3,4-tetrahydroisoquinoline-3-carboxylic acid, p-substituted phenylalanine, preferably Z is tryptophan.

6. The cyclic compound according to any one of the preceding claims, wherein the compound further comprises Z³, wherein Z³ connects Z² to X² if X² is present or to Z¹ if X² is not present, and wherein Z³ represent an amino acid or equivalent thereof having a thiol group in the side chain.

7. The cyclic compound according to claim 6, wherein Z³ is selected from the group consisting of cysteine, homocysteine and penicillamine, preferably Z³ is cysteine.

8. The cyclic compound according to any one of the preceding claims, wherein X² comprises 1 or 2 amino acids, preferably independently selected from the group consisting of glycine, alanine, serine, threonine, proline, aminohexanoic acid, aminopentanoic acid, amino butanoic acid and aminopropanoic acid, more preferably X² is GIy- Ala.

9. The cyclic compound according to any one of the preceding claims, wherein X¹ comprises 2 or 3 amino acids, preferably independently selected from the group consisting of glycine, alinine, valine, serine, threonine, asparagine and glutamine, more preferably X¹ is independently selected from valine, serine and glutamine, more preferably X¹ is Ser-Val-Gln.

10. The cyclic compound according to any one of the preceding claims comprising 7- 12 amino acids or equivalents thereof, preferably comprising 8-11 amino acids or equivalents thereof.

11. A composition comprising a cyclic compound as defined in any one of claims 1 - 10.

12. A composition according to claim 11, further comprising a pharmaceutically acceptable carrier.

13. A composition according to claims 11 or 12, further comprising antimicrobial agent, whereby preferably the antimicrobial agent is an antibiotic, an antifungal, an antiparasitic or an antiviral agent.

14. A cyclic compound as defined in any one of claims 1 - 10 or a composition as defined in claims 11 - 13 for use as a medicament.

15. Use of a cyclic compound as defined in any one of claims 1 - 10 or a composition as defined in claims 11 - 13 for the manufacture of a medicament for preventing or treating an infectious disease.

16. A method for preventing or treating infectious disease in a patient, comprising administering to said patient a prophylactic or therapeutic dose of a cyclic compound as defined in any one of claims 1 - 10 or a composition as defined in claims 11 - 13.