WO2008018838A1

WO2008018838A1 - Method for preparing 10b enriched polyhedron boron clusters

Info

Publication number: WO2008018838A1
Application number: PCT/SG2007/000242
Authority: WO
Inventors: Yinghuai Zhu; Effendi Widjaja
Original assignee: Agency For Science, Technology And Research
Priority date: 2006-08-08
Filing date: 2007-08-08
Publication date: 2008-02-14

Abstract

A method for preparing 10B enriched polyhedron boron clusters is described starting from polyhedron boron clusters making use of 10B enriched precursors in the presence of transition metal nanoparticles and an ionic liquid. Further pharmaceutical compositions including the one or more of the10B enriched polyhedron boron clusters of the invention are also described. The 10B enriched polyhedron boron clusters may be used in boron neutron capture therapy.

Description

METHOD FOR PREPARING ¹⁰B ENRICHED POLYHEDRON BORON

CLUSTERS

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of priority of US provisional application No. 60/836,594 filed August 8, 2006, the contents of which is hereby incorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION [0002] The present invention relates to a method for preparing ¹⁰B enriched polyhedron boron clusters. In particular the present method makes use of transition metal nanoparticles for facilitating the ¹⁰B transfer from a ¹⁰B precursor to the ¹¹B polyhedron boron clusters.

BACKGROUND OF THE INVENTION

[0003] To-date, malignant brain tumours such as Glioblastoma Multiforme remain virtually untreatable and inevitably lethal. Brain cancer strikes more than 10,000 Americans each year and kills half of them within 12 months. The cancer infiltrates the brain so aggressively that surgeons are rarely able to remove all the cancerous tissue. In addition, the cancer is resistant to standard radiation treatment and chemotherapy.

[0004] In the last few years, possibilities of compounds for boron neutron capture therapy (BNCT) delivery such as carbohydrates, folic acids, polyphyrins, dendrimers, nucleosides, liposomes, etc. with varying success (J. F. Valliant, et al, Coordination Chem. Rev., 2002, 232, 173-230) have been widely explored. However, there are still efforts necessary to reach clinical usage.

[0005] In neutron capture therapy, a neutron capture agent is injected into the patient and is selectively taken into the malignant tissue. The administration of a pharmaceutical containing the neutron capture agent is preferably direct administration into the bloodstream of the patient. At an appropriate time after administration of the neutron capture agent, the treatment volume (i.e., the anatomical structure to be treated) is exposed to a field of thermal neutrons produced by application of an external neutron beam. Because boron-10 is commonly used as the capture agent, the technology has come to be known as boron neutron capture therapy, or BNCT.

[0006] The thermal neutrons interact with the boron-10 in the compound, which has a very high capture cross-section in the thermal energy range. Ideally, the boron-10 is present only in the malignant cells so that boron-neutron interactions will occur only in malignant cells. When non-radioactive ¹⁰B (in nature 20% of elemental boron) absorbs thermal neutrons (0.025 eV) in the beam, as shown, for example, in Figures 1 and 2, a nuclear fission of ¹⁰B atom occurs. Helium-4 (alpha particles) and Lithium-7 nuclei emanate from this reaction with a range of 4 - 9 micrometers, sharing between them 2.3-2.8 MeV. The main effect is due to the alpha particle, due to its longer range compared to the ⁷Li nucleus. The gamma radiation produced, contributes very little to the local or normal tissue effect. Since these nuclei only travel a very short range (about one cellular diameter) and deposit all their energy in the tumour, thereby damage is done to the tumour cell only, while largely sparing healthy tissue (A. H. Soloway, et al., Chem. Rev. 1998, 98 (No 4), 1515-1562; V. I. Bregadze, Chem. Rev. 1992, 92, 209-223; J. F. Valliant, et al, Coordination Chem. Rev., 2002, 232, 173-230). Thus, boron-neutron interaction provides a high probability of cell inactivation by direct DNA damage. [0007] The BNCT technique requires targeting malignant tumour with a carrier of ¹⁰B, then exposing the area to a beam of neutrons. To meet the criteria of selectivity and effectiveness, there must be a significant differential boron-10 concentration in tumour vs. normal tissues (¹⁰B concentration ratio should be at least 5/1 ) and approximately 20-30 micrograms per gram of ¹⁰B in cancer cells. There are two typical BNCT drugs, Sodium borocaptate (BSH) and Boronophenylalanine (BPA), which were developed about 30 years ago and are undergoing clinical tests now. But their metabolism and the precise biochemical basis of tumour accretion are not completely understood. The drawback of them is the relatively low selectivity of the tumour/blood and tumour/normal brain cells is only about 3/1.

[0008] Although many classes of boron-containing compounds have been developed and their development is still ongoing, it is unlikely that any single agent will target all or even most of the tumour cells. This is because for a boron delivery agent to be successful, these criteria must be met: (1 ) low systemic toxicity and normal tissue uptake with high tumour uptake and concomitantly high tumour/ brain and tumour/ blood concentration ratios (>3-4:1 ); (2) tumour concentrations of ~20ug ¹⁰B/g tumour; (3) rapid clearance from blood and normal tissues and persistence in tumour during BNCT. To date, there is no single boron delivery agent that fulfils all these criteria.

[0009] Over the past 20 years, other classes of boron delivery agents have been designed and synthesized that include boron-containing amino acids, biochemical precursors of nucleic acids, DNA-binding molecules, porphyrin derivatives, high molecular weight delivery agents, such as monoclonal antibodies and their fragments, and liposomes.

[0010] An important characteristic of agent synthesis is, regardless of the agent type and detailed synthesis employed. Agent synthesis rest upon the availability of ¹⁰B-enriched precursors which must eventually be available in large quantities of GMP purity. These precursors include borohydride ion([BH₄]^'), closo- dodecahydrododecaborate dianion ([c/oso-Bi2Hi2]^2"), closo- decahydrododecaborate dianion ([c/oso-B-ioH-io]^2') and decaborane (Bi₀Hi₄).

[0011] Decaborane, due in part to its relative stability and solid state, is one of the key compounds and is the most useful of the boron hydrides. Decaborane is prepared from sodium borohydride (NaBhU) and is employed in the syntheses of borane species such as [c/oso-1 ,2-C₂BiOHi₂], ([CZOSO-BI₂HI₂]^2'), ([C/OSO-BI_OHIO]^2"). Currently, each of these precursors is derived from commercially available ¹⁰B- enriched boric acid, B(OH)₃.

[0012] Prior to the late 1970's, the principal processes for the preparation of decaborane involved pyrolytic or high pressure reactions using lower boron hydrides, such as diborane (B₂H₆) or tetraborane (B₄Hi₀). Further along, a non- pyrolytic method involving the reaction of an alkali metal pentaborane with diborane at temperatures below -2O⁰C was developed. These methods all required elaborate equipment and potentially hazardous reagents. [0013] Currently, ¹⁰B-enriched decaborane is prepared from ¹⁰B-enriched boric acid using the Dunks et al. method. The current synthesis method comprises of 4 steps with a low overall yield of 50% (Figure 3). Since the ¹⁰B starting materials such as NaBH₄ and boric esters (B(OR)₃) are very expensive, it is not economically beneficial to synthesize ¹⁰B-enriched decaborane and ¹⁰B-enriched carborane directly from those materials owing to the low and unreliable yields.

[0014] In the last decades, high boron content carboranes (C₂B₁₀H₁₂) carbon-boron clusters arranged in an icosahedral geometry in which the carbon and boron atoms are hexacoordinated) were studied for use in BNCT for cancer {J. F. Valliant, et al, Coordination Chem. Rev., 2002, 232, 173-230). Carboranes, themselves, are remarkably stable to oxidising agents, alcohols and strong acids, and up to 400 ⁰C. However, they are biologically inactive since they are too hydrophobic to make ionic or hydrogen bonding (V. I. Bregadze, Chem. Rev. 1992, 92, 209-223). Their use as hydrophobic pharmacophores in biologically active molecules has been reported and reviewed (/. M. Wyzlic, et al., Int. J. Radial Oncol. Biol. Phys. 1994, 28, 1203-1213).

[0015] In addition, decaborane is difficult to obtain commercially, explosive in combination with certain other compounds, and difficult to produce by known methods i.e. scale-up problem.

[0016] With the limitations as mentioned above, and since each of the boron delivery agents mentioned shares the production and precursor procurement requirements i.e. they are derived from one or more of the ¹⁰B-enriched precursor species, there is a still need to find new routes of synthesis to prepare the ¹⁰B- enriched boron cages conveniently and with higher yields.

SUMMARY OF THE INVENTION

[0017] In a first aspect, the present invention refers to a method for preparing ¹⁰B enriched polyhedron boron clusters comprising reacting a ¹⁰B enriched precursor with a polyhedron boron cluster in the presence of transition metal nanoparticles and an ionic liquid.

[0018] This specific reaction process enables the easy preparation of ¹⁰B enriches polyhedron boron clusters.

[0019] In a further aspect, the present invention refers to a pharmaceutical composition comprising at least one ¹⁰B enriched polyhedron boron cluster prepared according to the process of the present invention. [0020] Another aspect of the present invention is directed to the use of the ¹⁰B enriched polyhedron boron clusters prepared according to the process of the present invention as boron neutron capture therapy agent.

[0021] A still further aspect of the present invention is directed to a process for preparing transition metal nanoparticles comprising reacting a transition metal compound with a reducing agent in the presence of an ionic liquid.

BRIEF DESCRIPTION OF THE FIGURES

[0022] Figure 1 shows the ¹⁰B[n, a] ⁷Li reaction, which is the basis of Boron Neutron Capture Therapy.

[0023] Figure 2 is a schematic sketch of the interaction reaction of the thermal neutrons with the boron-IO in the compound showing the relative ranges the reaction products.

[0024] Figure 3 shows the synthesis of ¹⁰B enriched B₁₀Hi₄ and 1-R¹-2-R²- c/oso-C₂B₁₀H₁₀ according to the prior art.

[0025] Figure 4 shows the preparation of ¹⁰B enriched carborane and ¹⁰B enriched decaborane according to one embodiment of the present invention.

[0026] Fig. 5a shows Ru nanoparticles in different ionic liquids.

[0027] Fig. 5b is a Transmission Electron Microscopy (TEM) picture of Ru nanoparticles.

[0028] Fig. 5c shows X-ray Diffraction (XRD) spectra of Ru, an ionic liquid and Ru in an ionic liquid.

[0029] Fig. 5d is a X-ray Photoelectron Spectroscopy (XPS) spectrum of Ru nanoparticles. [0030] Fig. 6a illustrates the molecular structure of 1 ,2-c/oso-C₂BioHi2 and

B10H-14.

[0031] Fig. 6b illustrates the molecular structure of trihexyltetradecylphosphonium dodecylbenzenesulfonate and 1-n-butyl-3- methylimidazolium hexafluorophosphate. [0032] Fig. 7a shows a Raman spectrum in different runs of B₁₀H₁₄.

[0033] Fig. 7b shows a ¹⁰B-NMR spectrum of Bi₀Hi₄.

[0034] Fig. 8a shows a Raman spectrum in different runs of 1 ,2-C₂B₁₀Hi₂.

[0035] Fig. 8b shows a ¹⁰B-NMR spectrum off ,2-C₂Bi₀Hi₂. DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0036] In the following description non-limiting embodiments of the process of the invention will be explained. [0037] According to the present invention, it has been surprisingly found that it is possible to conveniently and effectively prepare ¹⁰B enriched polyhedron boron clusters in high yields by using the method as recited in independent claim 1 and the claims dependent thereon, as compared to the methods described in the prior art. Thus, ¹⁰B enriched compounds can be prepared which are useful in BNCT therapy.

[0038] In the context of the present invention, the term "comprising" or

"comprises" means including, but not limited to, whatever follows the word

"comprising". Thus, the use of the term "comprising" indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0039] Thus, in one embodiment the present invention provides a method for preparing ¹⁰B enriched polyhedron boron clusters comprising: reacting a ¹⁰B enriched precursor with a polyhedron boron cluster in the presence of transition metal nanoparticles and an ionic liquid. [0040] According to the present invention, a ¹⁰B enriched polyhedron boron cluster is a polyhedron cluster which has a sufficient high amount of ¹⁰B in the molecule. ¹⁰B and ¹¹B are the two stable isotopes of the element boron. Isotopes are any of the several different forms of an element each having different atomic mass. Isotopes of an element have nuclei with the same number of protons but different numbers of neutrons. Therefore, isotopes have different mass numbers, which give the total number of nucleons - the number of protons plus neutrons.

¹⁰Boron has a natural abundance of 18.8%, whereas ¹¹B has a natural abundance of 81.2%. Thus, it is necessary to increase the amount of the ¹⁰B isotope to provide useful compounds for BNCT therapy. [0041] According to the process of the present invention, the ¹⁰B enriched precursor which provides the source of ¹⁰B to be transferred to the respective polyhedron boron cluster may be a ¹⁰B enriched borane. A borane is a chemical compound of boron and hydrogen. Several borane compounds may be useful in the process of the present invention, for example, the ¹⁰B enriched borane may be selected from the group consisting of BH₃, B_nH_n+6 or B_nH_n+4, wherein n is an integer from 2 to 6. Examples of suitable boranes include, but are not limited to, BH₃, B₂H6, B₄H-10, B₅H₉, B₅H-I-I, B₆Hi₀, and B₆H₁₂. In one embodiment, the borane may be BH₃, B₂H₆ or B₅H₉. The ¹⁰B enriched boranes may be prepared according to processes described in the prior art (cf. Examples).

[0042] In one embodiment of the present invention, the polyhedron boron cluster is a cluster consisting of boron atoms or is a cluster composed of boron and carbon. Polyhedron in this respect means the structural configuration having flat faces and straight edges. The polyhedron boron cluster may be selected from B_zH₂₊₆, BzH₂₊₄, 1 ,7-C₂B₁₀H₁₂, 1 ,2-C₂B₁₀H₁₂, 1 , 12-C₂B₁₀H₁₂₁I J-[C₂BzH₂(R¹XR²)], or 1 ,2-[C₂B_ZH_Z(R¹)(R²)], wherein z is an integer from 7 to 12 and R¹ and R² may be the same or different and may be selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, cyano, nitro, OH, amino, halide, (-C_v(R³R⁴)Si_w(R⁵R⁶)-)_yNH₂ and [(-C_v(R³R⁴)Si_w- (R⁵R⁶HyNH₃)J⁺[X^"]; wherein R³, R⁴, R⁵ and R⁶ may be the same or different and may be selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, cyano, nitro, OH, amino and halide; v is an integer from 0 to 5 and w is an integer from 0 to 5, wherein at least one of v and w is 1 ; y is an integer from 1 to 5; X is selected from the group consisting of Cl^", Br^", NO₃ ^" and CH₃COO^"; said R¹ and R² being each attached to a different carbon atom. In one embodiment R¹ and R² may be an alkyl group and/or an alkenyl group. In this regard it is noted that in case the polyhedron boron compound is able to form a salt, it is possible to use such a salt of the boron compound in the method of the present invention.

[0043] Further examples of suitable polyhedron boron compounds that can be used in the method of the present invention are bis(amino)-or//7o-dicarbaborane cluster compounds (also referred to as 1 ,2-carborane compounds with respect to the position of the carbon atoms in the borane cluster) that are described in International Patent application WO 2007/058630, Woodhouse and Rendina, Dalton Trans., 2004, 3669-3677, Zakharkin et al. Pharmaceutical Chemistry Journal 2000; 34 (6):301-304, or in Malmquist et al. Inorganic Chemistry 1992; 31 :2534- 2537, for example. Bis(amino)-c/oso-dicarbaborane (also referred to as 1 ,7 carborane compounds with respect to the position of the carbon atoms in the borane cluster) as described in Woodhouse and Rendina, Dalton Trans., 2004, 3669-3677, for example can also be used in the method of the invention. Illustrative, non limiting examples of such compounds include bis(aminomethyl)- ortho-carborane (either in free form or salt form such as the hydrochloride) 1 ,2- bis(aminopropyl)1 ,2-carborane (either in free form as a salt thereof such as the hydrochloric salt), bis(aminomonosilyl)-ortho-carborane (either in free form or salt form), bis(aminodisilyl)-ortho-carborane (either in free or salt form) 1 ,7- bis(aminopropyl)1 ,7-carborane (either in free form or salt form such as the hydrochloride) or multinuclear platinum (ll)-amine complexes derived from 1 ,7- bis(aminopropyl)1 ,7-carborane (the latter were found to be promising anti-cancer agents by Woodhouse and Rendina, Dalton Trans., 2004, 3669-3677). [0044] For example, the polyhedron boron cluster according to formulas of

B_zH_z+₆ and B_zH_z+4 may be, but is not limited to, B₈Hi₂, BgHi₅, B₁0H₁₄ and BioHi₆. In one embodiment of the present invention, the borane may be decarborane (B₁₀H₁₄). In other embodiments, the polyhedron boron compound may be 1 ,7- C2B10H12, 1 ,2-C-2BioHi2, or 1 ,12-C-2BioHi2, wherein these three compounds may also be optionally substituted at least at one of the carbon atoms.

[0045] As explained above, in the formula 1 ,2-[C₂B_ZH_Z(R¹)(R²)], R¹ and R² may be the same or different and may be selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, cyano, nitro, OH, amino, halide, (-C_v(R³R⁴)Si_w(R⁵R⁶)-)_yNH₂ and [(-C_v(R³R⁴)Siw- (R⁵R⁶H_yNH₃)I⁺[X^"]; said R¹ and R² being each attached to a different carbon atom. Such compounds are disclosed in International Patent application WO 2007/058630 (see above). The polyhedron compound may also have the formula 1 ,7-[C₂B₂Hz(R¹XR²)] wherein R¹ and R² may be the same or different and may be selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, cyano, nitro, OH, amino, halide, (-C_v(R³R⁴)Siw(R⁵R⁶)-)yNH₂ and [(-C_v(R³R⁴)Si_w-(R⁵R⁶)-)_yNH₃)]⁺[χ-]; said R¹ and R² being each attached to a different carbon atom. As mentioned above, such as compounds are exemplarily described in Woodhouse and Rendina, Dalton Trans., 2004, 3669-3677 and can also be synthesised adapting the synthesis route for 1 ,2-[C₂B₂Hz(R¹ )(R²)] compounds disclosed in WO 2007/058630. [0046] In one embodiment of the present invention, in which compounds of the formula 1 ,2-[C₂B_ZH_Z(R¹)(R²)], 1 , 7-[C₂B₂H₂(R¹ )(R²)] or 1 ,12-[C₂B₂H₂(R¹ )(R²)] are used, v can be 0, 1 or 2, w can be 0, 1 , 2 or 3 and y can also be 1 or 2. In a further embodiment v can be 1 , 2, 4 or 5 and w and y are 0. In a still further embodiment w can be 1 , 2, 3, 4 or 5 and v and y are 0. In one embodiment R¹ and R² independently from each other include, but are not limited to, -CH₂-NH₂, -C₂H₄- NH₂, -CH₂-NH₃ ⁺X^' and -C₂H₄-NH₃ ⁺X^". In a further embodiment, R¹ and R² can independently from each other also include, but are not limited to, -SiH₂-NH₂, -Si₂H₄-NH₂, -Si(CHs)₂-NH₂, -Si₂(CHs)₄-NH₂, -SiH₂-NH₃ ⁺X-, -Si₂H₄-NH₃ ⁺X^", -Si(CHs)₂-NH₃ ⁺X and -Si₂(CHs)₄-NH₃ ⁺. In a still further embodiment, R¹ and R² may independently from each other be -CH₂-Si(CH₃)₂-NH₂, -Si(CH₃)₂-CH₂-NH₂ -CH₂-Si(CH₃)₂-NH₃ ⁺ or -Si(CH₃)₂-CH₂-NH₃ ⁺. Thus, in accordance with the above, in a compound of the invention R¹ can for example be -C₂H₄-NH₂ and R2 can for example by -CH₂-NH₂ or -SiH₂-NH₂.

[0047] Examples of suitable alkyl groups include, but are not limited to, are methyl, ethyl, propyl, isopropyl, n-butyl, tert.-butyl and isobutyl. Exemplary alkenyl groups include, but are not limited to, for example -CH=CH₂ and -CH₂-CH=CH₂, whereas exemplary alkynyl groups include, but are not limited to, -C≡CH and -CH₂-C≡CH. Examples of aromatic groups include, but are not limited to, benzyl, phenyl, toluenyl and naphthyl. The heteroaryl may be selected from pyridyl, thienyl or the like. The halide may be selected from fluoride, chloride or bromide. Optionally substituted means that the respective substituent may be further substituted with an alkyl or aromatic group defined as above. The substituents R³, R⁴, R⁵ and R⁶ can be as explained above. In one further embodiment of the present invention R³, R⁴, R⁵ and R⁶ are all -H. [0048] X can be any pharmaceutically acceptable anion in the above formula (I). Illustrative examples of pharmaceutical anions include, but are not limited to, Cl^", Bf, CN^", SCN^", OH^", NO₂ ^", NO₃-, MeO^", EtO^", citrate, oxalate, tatrate or CH₃COO^' and the like. In one illustrative embodiment of the present invention X is a halogen such as Cl^" or Br^".

[0049] The above mentioned polyhedron boron cluster compounds, may be optionally substituted on one or more of the boron atoms of the cluster cage. For example, such substituents may be selected from the group consisting of substituted or unsubstituted C₁-C₁₂- aliphatic or aromatic group, a halide, an O- alkyl or an N-alkyl group. Examples of suitable aliphatic groups for this purpose include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl. The aliphatic group can further be substituted by an aromatic group such as phenyl, for example. Exemplary aromatic groups that can be used include, but are not limited to, phenyl, toluoyl and naphthyl. The halide can be selected from fluoride, chloride or bromide. Examples of suitable O-alkyl groups are methoxy, ethoxy, propoxy or butoxy, whereas the N-alkyl group is selected from -NHMe, -N(Me)₂, -N(Ethyl)₂ or -N(Propyl)₂. The preparation of carborane compounds having substituents at the boron atoms is described in US Patent 7,053,158 (see Figure 8 thereof, for example) or on pages 197 and 198 (in particular paragraph 9.2) of Carboranes (Russell N. Grimes, Academic Press, New York, 1970, ISBN 75-127684).

[0050] In the present invention, transition metal nanoparticles are used as a kind of catalyst to facilitate the ¹⁰B transfer from the ¹⁰B enriched precursor to the polyhedron boron cluster. The transition metal nanoparticles may be prepared by reducing a transition metal compound. For example, the transition metal of the transition metal compounds may be, but is not limited to, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Cu, Ti or Cr and mixtures thereof. However, any other transition metal suitable for the intended purpose may be used. [0051] In one embodiment the transition metal compound is a metallocene or a metallocene-like compound. A metallocene is a compound with the general formula (CsRs)₂M consisting of two cyclo pentad ienyl anions (Cp) bound to a metal center in the oxidation state II. According to the definition by IUPAC, a metallocene contains a transition metal and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e. the two cyclopentadienyl anions are co-planar with equal bond lengths and strengths. A metallocene-like compound is a compound which has a metallocene as basic structure but which may have further transition metals and further ligands included in the system. [0052] For example, in one embodiment of the present invention, the metallocene may include, but is not limited to, Cp₂Ti, Cp₂Fe, Cp₂Ru, Cp₂Ni, and Cp₂Co. Cp may be a substituted or unsubstituted cyclopentadienyl ligand, for example C₅H₅ or C-sMes. However, any further Cp ligands with one or more different ligands may also be used for the purpose of the present invention. For example, one or more of the hydrogen atoms of C₅H₅ may be substituted with ethyl, Cp or phenyl.

[0053] The metallocene-like compound of the present invention may be, but is not limited to, [CpRuCpRuCp][PF₆], [CpFeCpFeCp][PF₆], [CpFeCpRuCp][PF₆], [CpFeRUCpOsCp][PF₆], and [CpRuCpOsCp][PF₆], wherein Cp is defined as given above.

[0054] In a further embodiment, the transition metal compound may be a metal carbonyl compound. Metal carbonyl compounds are coordination complexes of transition metals with carbon monoxide. These complexes may be homoleptic, i.e. contain only CO ligands, but may also contain a mix of different ligand besides carbonyl ligands, for example Cp, cyclobutadiene, cyclooctadiene, cyclooctatetraene, phosphate ligands, ethylene and the like. Such compounds may include, but are not limited to, Fe(CO)₅, Ni(CO)₄, V(CO)₆, Cr(CO)₆, Mo(CO)₆, W(CO)₆, Co₂(CO)₈, CpMn(CO)₃ or the like. [0055] The transition metal nanoparticles of the present invention may be prepared by reducing a transition metal compound as explained above in the presence of an ionic liquid. The reducing agent may be any reducing agent capable of reducing the transition metal compound, for example to the oxidation state (0). Examples of such reducing agents include, but not limited to, NaBH₄, LiAIH₄, NaSO₃, diisobutylaluminium hydride (DIBAH) and K₃[Fe(CN)₆]. The reducing agent may be used in an amount sufficient to reduce the transition metal of the transition metal compound to the required oxidation state. The reducing agent is typically used in an about equimolar ratio to the transition metal compound or in excess. For example, the molar ratio of transition metal:reducing agent may be in the range of about 1 :1 to about 1 :5, for example about 1 :2, about 1 :3 or about 1 :4.

[0056] The preparation of the transition metal nanoparticles may be carried out in the presence of a solvent. Any solvent which is inert under the reaction conditions may be used. For example, the solvent may be, but is not limited to, pentane, hexane, benzene, toluene, dichloromethane, chloroform, methanol, ethanol, ethylene glycol, diglyme, tetrahydrofuran, and so on.

[0057] After the reduction of the transition metal compound, the obtained intermediate compound is heated slowly in the presence of the ionic liquid. For example, the reaction may be heated to about 100⁰C or to about 120⁰C or to about 18O⁰C₁ depending on the used solvents and the used ionic liquid. The skilled man in the art will recognize which temperature will be sufficient for completing the reaction. The reaction time may be in the range of about 1h to about 5h, for example about 2h to about 4h. Again, the skilled man will be capable of adjusting the reaction time.

[0058] According to the present invention, an ionic liquid is used in the preparation of the ¹⁰B enriched polyhedron boron cluster and the transition metal nanoparticles. An ionic liquid in the sense of the present invention is a liquid that contains essentially only ions. Some ionic liquids are in a dynamic equilibrium where at any time more than 99.99% of the liquid is made up of ionic rather than molecular species. All ionic liquid which can stabilize the transition metal nanoparticles and favor the isotope exchange reaction may be used in the process of the present invention. For example, the ionic liquid may be selected from the group consisting of trihexyltetradecylphosphonium dodecylbenzenesulfonate, 1-n- butyl-3-methylimidazolium hexafluorophosphate, imidazole based compounds such as 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-ethylpyridinium hexafluorophosphate, N-methyl-N-methylpyrrolidinium hexafluorophosphate, N- methyl-N-ethylpyrrolidinium hexafluorophosphate and ammonium hexafluorophosphate. In the above mentioned compounds, tetrafluoroborate may be also be used as anion instead of hexafluorophosphate. In one embodiment of the present invention trihexyltetradecylphosphonium dodecylbenzenesulfonate is used as the ionic liquid.

[0059] The process of the present invention may be carried out in the presence of a solvent. Any solvent which is inert under the reaction conditions may be used. For example, the solvent may be, but is not limited to, pentane, hexane, benzene, toluene, dichloromethane, chloroform, methanol, ethanol, ethylene glycol, diglyme, tetrahydrofuran, and so on. [0060] As the ¹⁰B content is important for the use as BNCT agent, it is possible to exchange at least about 50% of the ¹¹B of the polyhedron boron cluster with ¹⁰B by the process of the present invention. In one embodiment at least about 75% of the ¹¹B may be exchanged. It is also possible to exchange at least about 80%, for example at least about 85% of the ¹¹B. In one embodiment, at least about 90% of the ¹¹B may be exchanged. Thus, the compounds prepared by the inventive process will be highly effective in the BNCT therapy. The ¹¹B exchange rate can be monitored, for example, by Raman spectroscopy.

[0061] Thus, a further embodiment of the present invention is directed to a pharmaceutical composition comprising at least one ¹⁰B enriched polyhedron boron cluster prepared according to the process of the present invention. A "pharmaceutical composition" refers to a mixture of one or more of the compounds described herein, or physiologically/pharmaceutically acceptable salts thereof, with other chemical components, such as physiologically/pharmaceutically acceptable carriers, excipients and diluents. The purpose of a pharmaceutical composition is to facilitate administration of a compound to an organism. The pharmaceutical composition may be used in the treatment of diseases, such as cancer, as will be explained in more detail below. The pharmaceutical composition may comprise further pharmaceutically active compounds which will be useful in the treatment of the respective disease.

[0062] A compound prepared by the process of the present invention or a pharmaceutically acceptable salt thereof can be administered as such to a human patient or can be administered in pharmaceutical compositions in which the foregoing materials are mixed with suitable carriers, excipient(s) or diluents. Techniques for formulation and administration of drugs may be found in "Remington^'s Pharmaceutical Sciences, "Mack Publishing Co., Easton, PA., 18^th edition, June 1995.

[0063] As used herein, "administer" or "administration" refers to the delivery of a compound prepared by the process of the present invention or a pharmaceutically acceptable salt thereof or of a pharmaceutical composition containing such a compound or a pharmaceutically acceptable salt thereof of this invention to an organism. [0064] Suitable routes of administration may include, without limitation, oral, rectal, transmucosal or intestinal administration or intramuscular, subcutaneous, intramedullary, intrathecal, direct intraventricular, intravenous, intravitreal, intraperitoneal, intranasal, or intraocular injections. A compound of the invention can either be administered in a systemic or also in a rather manner, for example, via injection of the compound directly into a solid tumour. This can, for example, be done using a depot or sustained release formulation or using magnetic nanoparticles as explained below. When included into magnetic nanoparticles, the compounds of the invention can be targeted to the desired site/tumour by applying an external magnetic field as described in Alexiou et al., Locoreginal Cancer Treatment with Magnetic Drug Targeting, Cancer Research, 60, 6641-6648, December 2000).

[0065] The ¹⁰B enriched polyhedron boron clusters prepared according to the process of the present invention can be used for the treatment in conjunction with boron neutron capture therapy. The boron neutron capture therapy may be carried out in a manner known to the skilled man in the art (for review see Soloway et al., "The chemistry of neutron capture therapy", Chem. Rev. 1998, 98 (No 4), 1515-1562. For example, as it is known that BNCT is an experimental form of radiotherapy that utilizes a neutron beam that interacts with boron injected to a patient and that BNCT depends on the interaction of slow neutrons with boron-10 to produce alpha particles, another type of radiation, patients can first be given, for example, an intravenous injection of a boron-10 tagged compound according to the present invention that preferentially binds tumor cells. The neutrons are created either in a nuclear reactor or by colliding high-energy protons into a lithium target. The neutrons pass through a moderator, which shapes the neutron energy spectrum suitable for BNCT treatment. Before entering the patient the neutron beam is shaped by a beam collimator. While passing through the tissue of the patient, the neutrons are slowed by collisions and become low energy thermal neutrons. The thermal neutrons undergo reaction with the boron-10 nuclei, forming a compound nucleus (excited boron-11) which then promptly disintegrates to lithium-7 and an alpha particle. Both the alpha particle and the lithium ion produce closely spaced ionizations in the immediate vicinity of the reaction, with a range of approximately 10 micrometers, or one cell diameter. This technique is advantageous since the radiation damage occurs over a short range and thus normal tissues can be spared. Also, there are two mechanisms for tumor selectivity, since both the boron compound is made to bind to tumor cells and the neutron beam is aimed at the location of the tumor. Further methods known in the art with respect to BNCT are also applicable to the compounds of the present invention.

[0066] The compounds prepared according to the process of the present invention and their natural biochemicals as well as related magnetic nanoparticles may have high potential as BNCT agents for, for example, malignant brain tumor, head and neck cancer treatment.

[0067] The compounds prepared according to the process of the present invention may be used as boron neutron capture therapy agents alone or with other BNCT agents known in the art.

[0068] For use as neutron capture therapy agents the compounds prepared according to the process of the present invention can also be used as building blocks to constitute natural biochemicals or magnetic nanoparticles. These compounds can be chemically or physically coupled to such biochemicals or contained or coupled to such nanoparticles.

[0069] The biochemicals which may be covalently or non-covalently coupled may include, but are not limited to, carbohydrates, folic acids or nucleosides. These compounds may be catabolized, i.e. that for example a nucleoside to which a dicarbaborane compound of the invention is coupled, will be integrated into DNA of a cancer cell and can selectively exercise its effect directly at the DNA of the respective cell. [0070] Magnetic nanoparticles in the meaning of the present invention may be used as drug carriers. The magnetic nanoparticles may be magnetic particles bearing, for example, a phosphate group or a sulfonate group being negatively charged. The compounds of the present invention may then be immobilized on the magnetic nanoparticle by, for example, electrostatic interactions. So obtained magnetic particles may, for example, be directly injected into the tumor tissue or into the blood stream of the patient with a pharmaceutical composition as explained above. With the aid of a magnetic source outside the body of the patient, the particles may be successfully directed to the advanced sarcomas without the associated organ toxicity. Thus, the amount of systemic distribution of the cytotoxic drug may be reduced and thus, the associated side-effects are also reduced. Further, the dosage required for a more efficient, localized targeting of the drug can also be reduced. The use of magnetic nanoparticles is generally well tolerated in most of the patients treated with such a method. Examples of magnetic nanoparticles include, but are not limited to, carbon nanotubes, fullerenes, layered double hydroxides and dendrimers. The compounds of the invention may also be encapsulated in polymeric nanoparticles that are suitable as dug delivery systems, as described, for example, in US patent application US 2005/0277739, the content thereof is incorporated herein by reference.

EXAMPLES

[0071] In the following, a typical preparation process for ¹⁰B enriched polyhedron boron clusters from ¹¹B enriched precursors, is explained and shown as an example in Figure 4.

Example 1

[0072] (a) Synthesis of ¹⁰B₂H₆: ¹⁰B enriched diborane (¹⁰B₂H₆) was produced following the protocol described by Narayana & Periasamy, Journal of Organometallic Chemistry, 323, (1987), pages 145-147 using ¹⁰B enriched sodium borohydride (Na¹⁰BH₄). In brief, under argon atmosphere, a solution of iodine (20 mmol) in dry diglyme 30 ml was dropwise added to a solution of Na¹⁰BH₄ (40 mmol) in dry diglyme 30 ml at room temperature with continuous stirring. The produced ¹⁰BaHe and H₂ were carried off through a side tube and connected to a series of four cooling traps for purifying and trapping the product. The first trap was immersed in a -78⁰C bath to remove traces of diglyme and iodine which were entrained in the diborane stream. The other three traps were cooled with liquid nitrogen (-196°C) to collect diborane. The outlet from the latest trap was vented through a mercury bubbler and a trap containing adequate amount of acetone to destroy excess diborane. When the reaction had finished, the bubbler was removed under argon and replaced by a reactor for next step.

[0073] (b) Synthesis of ruthenium nanoparticle in ionic liquid: Nano- scale ruthenium was prepared by reduction of metallocene complex, [(η⁵- C₅H₅)Ru(μ,η⁵-C5Me5)Ru(η⁵-C5Me₅)][PF₆] (10.0 mg) (which complex is described in Kudinov et al, Journal of Organometallic Chemistry, 336, (1987) pages 187-197) in ethylene glycol (-10.0 ml) in the presence of NaBhU (~1.5 mg) under hydrogen atmosphere to produce a red brown solution. The solution was heated slowly to 180 ⁰C and ionic liquid trihexyltetradecylphosphonium dodecylbenzenesulfonate (-4.0 ml) was added during that time. After keeping at that temperature for 3h, the mixture was cooled to room temperature and kept in static overnight. After removing of the excess ethylene glycol and washing with a mixture of hexane and diethyl ether, a black sticky residue was obtained. The produced ruthenium nanoparticles were subjected to analysis with XRD, XPS, and TEM (Figures 5a to 5d). The XRD spectra show bulk Ru(O) model with broad peaks owing to small size. XPS spectra show typical Ru(O) absorption at 280.08 and 284.80 eV for 3d_5/2 and 3d_3/2 respectively. TEM show the prepared ruthenium nanoparticles are smaller than 5 nm in the range of 2-4 nm with uniform distribution. The above synthesized system is stable for more than one month under argon atmosphere.

Example 2: Evaluation of the catalytic activity

[0074] (a) Catalytic ¹⁰B/¹¹B isotope exchange between ¹⁰B₂H₆ and B-₁₀H₁₄: Catalytic isotope process was undergoing in a solution of decaborane (14) (0.1 g, 0.82 mmol) in above prepared catalytic system of 4.0 mg of Ru nanoparticles in 5.0 ml ionic liquid trihexyltetradecylphosphonium dodecylbenzenesulfonate and 10.0 ml co-solvent dichloromethane. The ¹⁰B enriched diborane (41.00 mmol) as produced in Example 1 was introduced to the 11 reaction flask. The resulting mixture was then heated to 50 °C to undergo reaction (P≤1.08 atm) for 6 hrs. After the reaction process, the diborane was released and destroyed by bubbling through excess acetone. The residue was dried under reduced pressure and the obtained residue was extracted with hexane (25 ml x 3) and combined to dryness with a recovery of decaborane (14) >97.3%. The same operation was repeated 6 times to same samples. The obtained ¹⁰B enriched B-₁₀H₁₄ sample was then analyzed with Raman spectroscopy and ¹⁰B- NMR (Figures 7a and 7b). After 6 runs, the total conversion of natural abundance cluster Bi₀H-_I4 to ¹⁰B enriched ¹⁰B-_I0H-_M is more than 80% (based on Raman spectra). The Raman scattering spectra were measured at room temperature using a JY Horiba LabRAM Raman microscope equipped with liquid nitrogen cooled charge-coupled device (CCD) multichannel detector (256 pixels x 1024 pixels) and a high grade Olympus microscope (objective 10Ox). The spectra were measured using the visible 514.5 nm argon ion laser as the scattering excitation source. The laser power on the sample was about 6 mW. The spectral acquisition time for each Raman spectrum was about 120 seconds with spectral resolution around 1.5 - 2 cm^"1.

[0075] From Raman spectra, the ¹⁰B enriched > 80% compared with natural abundance (¹⁰B/¹¹B = 19.9/80.1). The ¹⁰B-NMR show same peaks with the ¹¹B- NMR which confirms the identical boron cluster.

[0076] (b) Catalytic ¹⁰B/¹¹ B isotope exchange between ¹⁰B₂H₆ and 1,2- C₂BioHi₂: A similar process as (a) was used to produce ¹⁰B-enriched 1 ,2-C-2BioHi2 with a reaction time of 12 hrs per operation (with a recovery of 1 ,2-C₂Bi₀Hi₂ >98.4% per operation) for 5 times.. After 5 runs, the final conversion of natural abundance carborane cluster 1 ,2-C₂Bi₀Hi₂ to ¹⁰B enriched 1 ,2-C₂Bi₀H₁₂ is more than 80% (based on Raman spectra, Figures 8a and 8b).

Claims

CLAIMS:

1. A method for preparing ¹⁰B enriched polyhedron boron clusters comprising: reacting a ¹⁰B enriched precursor with a polyhedron boron cluster in the presence of transition metal nanoparticles and an ionic liquid.

2. The method according to claim 1 , wherein the ¹⁰B enriched precursor is a ¹⁰B enriched borane.

3. The method according to claim 2, wherein the ¹⁰B enriched borane is selected from BH₃, B_nH_n+₆ or B_nH_n+₄, wherein n is an integer from 1 to 6.

4. The method according to claim 3, wherein the ¹⁰B enriched borane is selected from the group consisting of BH₃, B₂H₆, B₄Hi₀, B₅H₉, B₅Hn, and

5. The method according to any of the preceding claims, wherein the polyhedron boron cluster is selected from the group consisting of B_zH_z+6,

B₂H₂₊₄, 1 ,7-[C₂B₂H₂(R¹XR²)], 1 ,12-[C₂B₂H_z(R¹XR²)], and 1 ,2-[C₂B₂H₂(R¹XR²)], wherein z is an integer from 7 to 12 and R¹ and R² may be the same or different and may be selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, cyano, nitro, OH, amino, halide, (-C_v(R³R⁴)Si_w(R⁵R⁶)-)_yNH₂ and [(- C_v(R³R⁴)Si_w(R⁵R⁶)-)_yNH₃)]⁺[X-]; wherein R³, R⁴, R⁵ and R⁶ may be the same or different and may be selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, cyano, nitro, OH, amino and halide; v is an integer from 0 to 5 and w is an integer from 0 to 5, wherein at least one of v and w is 1 ; y is an integer from 1 to 5; X is selected from the group consisting of Cl^", Br^", NO₃ ^" and CH₃COO^"; said R¹ and R² being each attached to a different carbon atom.

6. The method according to claim 5, wherein the polyhedron boron cluster is B₁₀H₁₄, (1 ,7-C₂Bi₀H₁₂), (1 ,12-C₂B₁₀H₁₂), or (1 ,2-C₂Bi₀Hi₂).

7. The method according to any of the preceding claims, wherein the transition metal nanoparticles are prepared by reducing a transition metal compound in the presence of an ionic liquid.

8. The method according to claim 7, wherein the transition metal of the transition metal compound is selected from the group consisting of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ti, Cr and mixtures thereof.

9. The method according to claim 7 or 8, wherein the transition metal compound is selected from the group consisting of metallocenes, metallocene-like compounds and metal carbonyl compounds.

10. The method according to claim 9, wherein the metallocene or metallocene- like compound is selected from the group consisting of Cp₂Ti, Cp₂Fe, Cp₂Ru,

Cp₂Ni, Cp₂Co, [CpRuCpRuCp][PF₆], [CpFeCpFeCp][PF₆],

[CpFeCpRuCp][PF₆], [CpFeRuCpOsCp][PF₆], and [CPRUCPOSCP][PF₆], wherein Cp is independently selected from C₅H₅ or C₅Me₅.

11. The method according to any of the preceding claims, wherein the ionic liquid is selected from the group consisting of trihexyltetradecylphosphonium dodecylbenzenesulfonate, 1 -n-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, 1- ethylpyridinium hexafluorophosphate, N-methyl-N-methylpyrrolidinium hexafluorophosphate, N-methyl-N-ethylpyrrolidinium hexafluorophosphate and ammonium hexafluorophosphate.

12. The method according to claim 11 , wherein tetrafluoroborate is used as anion.

13. The method according to any of the preceding claims, wherein the reaction is carried out in the presence of a solvent.

14. The method according to any of the preceding claims wherein at least 50% of the ¹¹B in the polyhedron boron clusters has been exchanged by ¹⁰B.

15. The method according to claim 14, wherein at least 75% of the ¹¹B has been exchanged.

16. A pharmaceutical composition comprising at least one ¹⁰B enriched polyhedron boron cluster as prepared according to any of claims 1 to 15.

17. The pharmaceutical composition according to claim 16, further comprising at least one pharmaceutically acceptable carrier and/or diluent.

18. Use of the ¹⁰B enriched polyhedron boron cluster as prepared according to any of claims 1 to 13 as boron neutron capture therapy agent.

19. The use of claim 18, wherein the ¹⁰B enriched polyhedron boron cluster is a building block for the constitution of natural biochemicals or nanoparticles.

20. The use according to claim 19, wherein the biochemicals are selected from the group consisting of carbohydrates, folic acids and nucleosides.

21. The use according to claim 19, wherein the nanoparticles are selected from the group consisting of carbon nanotubes, fullerenes, layered double hydroxides and dendrimers.

22. A process for preparing transition metal nanoparticles comprising: reacting a transition metal compound with a reducing agent in the presence of an ionic liquid.

23. The process according to claim 22, wherein the transition metal compound is selected from the group consisting of metallocenes, metallocene-like compounds and metal carbonyl compounds.

24. The process according to claim 22 or 23, wherein the reducing agent is selected from the group consisting of NaBH₄, LiAIH₄, NaSOβ, diisobutylaluminium hydride (DIBAH) and K₃[Fe(CN)₆].

25. The process according to any of claims 22 to 24, wherein the ionic liquid is selected from the group consisting of trihexyltetradecylphosphonium dodecylbenzenesulfonate, 1 -n-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, 1- ethylpyridinium hexafluorophosphate, N-methyl-N-methylpyrrolidinium hexafluorophosphate, N-methyl-N-ethylpyrrolidinium hexafluorophosphate and ammonium hexafluorophosphate.

26. Transition metal nanoparticles obtained by a process according to any of claims 22 to 25.