Biodistribution and toxicity of 2,4-divinyl-nido-o-carboranyldeuteroporphyrin IX in mice

In Vivo Application of Carboranes for Boron Neutron Capture Therapy (BNCT): Structure, Formulation and Analytical Methods for Detection

Carboranes have emerged as one of the most promising boron agents in boron neutron capture therapy (BNCT). In this context, in vivo studies are particularly relevant, since they provide qualitative and quantitative information about the biodistribution of these molecules, which is of the utmost importance to determine the efficacy of BNCT, defining their localization and (bio)accumulation, as well as their pharmacokinetics and pharmacodynamics. First, we gathered a detailed list of the carboranes used for in vivo studies, considering the synthesis of carborane derivatives or the use of delivery system such as liposomes, micelles and nanoparticles. Then, the formulation employed and the cancer model used in each of these studies were identified. Finally, we examined the analytical aspects concerning carborane detection, identifying the main methodologies applied in the literature for ex vivo and in vivo analysis. The present work aims to identify the current strengths and weakness of the use of carboranes in BNCT, establishing the bottlenecks and the best strategies for future applications.

Boron Neutron Capture Therapy: Current Status and Challenges

Boron neutron capture therapy (BNCT) is a re-emerging therapy with the ability to selectively kill tumor cells. After the boron delivery agents enter the tumor tissue and enrich the tumor cells, the thermal neutrons trigger the fission of the boron atoms, leading to the release of boron atoms and then leading to the release of the α particles (⁴He) and recoil lithium particles (⁷Li), along with the production of large amounts of energy in the narrow region. With the advantages of targeted therapy and low toxicity, BNCT has become a unique method in the field of radiotherapy. Since the beginning of the last century, BNCT has been emerging worldwide and gradually developed into a technology for the treatment of glioblastoma multiforme, head and neck cancer, malignant melanoma, and other cancers. At present, how to develop and innovate more efficient boron delivery agents and establish a more accurate boron-dose measurement system have become the problem faced by the development of BNCT. We discuss the use of boron delivery agents over the past several decades and the corresponding clinical trials and preclinical outcomes. Furthermore, the discussion brings recommendations on the future of boron delivery agents and this therapy.

Towards carborane-functionalised structures for the treatment of brain cancer

Boron neutron capture therapy (BNCT) is a promising targeted chemoradiotherapeutic technique for the management of invasive brain tumors, such as glioblastoma multiforme (GBM). A prerequisite for effective BNCT is the selective targeting of tumour cells with ¹⁰B-rich therapeutic moieties. To this end, polyhedral boranes, especially carboranes, have received considerable attention because they combine a high boron content with relative low toxicity and metabolic inertness. Here, we review progress in the molecular design of recently investigated carborane derivatives in light of the widely accepted performance requirements for effective BNCT.

Hemorrhage in mouse tumors induced by dodecaborate cluster lipids intended for boron neutron capture therapy

The potential of boron-containing lipids with three different structures, which were intended for use in boron neutron capture therapy, was investigated. All three types of boron lipids contained the anionic dodecaborate cluster as the headgroup. Their effects on two different tumor models in mice following intravenous injection were tested; for this, liposomes with boron lipid, distearoyl phosphatidylcholine, and cholesterol as helper lipids, and containing a polyethylene glycol lipid for steric protection, were administered intravenously into tumor-bearing mice (C3H mice for SCCVII squamous cell carcinoma and BALB/c mice for CT26/WT colon carcinoma). With the exception of one lipid (B-THF-14), the lipids were well tolerated, and no other animal was lost due to systemic toxicity. The lipid which led to death was not found to be much more toxic in cell culture than the other boron lipids. All of the lipids that were well tolerated showed hemorrhage in both tumor models within a few hours after administration. The hemorrhage could be seen by in vivo magnetic resonance and histology, and was found to occur within a few hours. The degree of hemorrhage depended on the amount of boron administered and on the tumor model. The observed unwanted effect of the lipids precludes their use in boron neutron capture therapy.

Polyhedral boron derivatives of porphyrins and phthalocyanines

The synthesis of compounds containing polyhedral boron cages and porphyrin or phthalocyanine units connected covalently in one molecule is reviewed. The importance of these compounds arises, on the one hand, from the use of polyhedral boron derivatives in neutron capture therapy for cancer; on the other hand, porphyrins and phthalocyanines are known as photosensitizers in photodynamic tumor therapy. Current interest in the binding of polyhedral boron compounds to porphyrins and phthalocyanines is due to the observation that porphyrins and phthalocyanines show improved uptake and good persistence in tissues. Medical applications of compounds containing polyhedral boron cages and porphyrin or phthalocyanine units in one molecule are briefly discussed.

Synthesis and Comparative Toxicology of a Series of Polyhedral Borane Anion-Substituted Tetraphenyl Porphyrins

Three structurally similar tetraphenylporphyrins bearing polyhedral borane anions have been synthesized and their toxicological profiles obtained in rats. These conjugates were found to have quite different acute toxicities as manifested at the maximum tolerated dose (MTD). When given at the MTD and observed over 28 days, the most acutely toxic porphyrin was found to be devoid of toxicity, as measured by blood chemistry panels. The remaining two less acutely toxic compounds both elicited significant changes, characterized by moderate to severe thrombocytopenia, failure to gain weight normally and changes in liver enzymes indicative of mild hepatotoxicity. All toxic effects were transient, with platelets rebounding to above normal levels at day 28. We conclude that thrombocytopenia is the dose limiting toxicity for boronated porphyrins in mammals and suggest that these effects may be due to the porphyrin, not the borane or carborane.

Total syntheses of three copper (II) tetracarboranylphenylporphyrins containing 40 or 80 boron atoms and their biological properties in EMT-6 tumor-bearing mice

Three carboranyltetraphenylporphyrins containing 40 or 80 boron atoms were synthesized and evaluated for their biodistribution and toxicity in EMT-6 tumor-bearing mice. Copper (II) meso-5,10,15,20-tetrakis[3-methoxy-4-(o-carboranylmethoxy)phenyl]porphyrin, 6, and copper (II) meso-5,10,15,20-tetrakis[3-hydroxy-4-(o-carboranylmethoxy)phenyl]porphyrin, 8, are B40 congeners with different lipophilicities, each less than their B80 congener, copper (II) meso-5,10,15,20-tetrakis[m-(3,5-di-o-carboranylmethoxybenzyloxy)phenyl]porphyrin, 18. Two days after the last of a series of i.p. injections in BALB/c mice bearing EMT-6 mammary tumors, a dose of 185 mg/kg 6 (54 mg/kg B) delivered over 3.5 times the concentration of boron to tumor (169 microg/g B) than did 118 mg/kg 8 (36 mg/kg B), which delivered 35 microg/g B, or 87 mg/kg 18 (30 mg/kg B), which delivered 46 microg/g B. The tumor-to-blood and tumor-to-brain boron concentration ratios at that time for all three porphyrins exceeded 80:1. Two days after the last injection, there resulted moderate thrombocytopenia that essentially disappeared two days later from 6 and 18, and mild leukocytosis from 6, 8, and 18, all of which were clinically inconsequential. Thus, 6 may rank among the most clinically promising carboranyl porphyrins ever made to deliver 10B to tumors for boron neutron-capture therapy (BNCT) that has also been tested for its toxicity in vivo.

Synthesis, cellular uptake and animal toxicity of a tetra(carboranylphenyl)-tetrabenzoporphyrin

A water-soluble nido-carboranyl-tetrabenzoporphyrin has been synthesized in 43% overall yield, by condensation of butanopyrrole with a carboranylbenzaldehyde, followed by metal insertion, oxidation, demetallation and deboronation reactions. This compound accumulated within human glioblastoma T98G cells to a significant higher extent than a structurally related nido-carboranylporphyrin, and localized preferentially in the cell lysosomes. Animal toxicity studies using male and female BALB/c mice revealed that both compounds are non-toxic even at a dose of 160 mg/kg, administered intraperitoneally as a single injection at a concentration of 4 mg/mL. It is concluded that the tetra(carboranylphenyl)-tetrabenzoporphyrin is a promising new sensitizer for the treatment of malignant tumors.

Synthesis of copper octabromotetracarboranylphenylporphyrin for boron neutron capture therapy and its toxicity and biodistribution in tumour-bearing mice

Copper tetracarboranyltetraphenylporphyrin (CuTCPH) is a minimally toxic carborane-containing porphyrin that has safely delivered high concentrations of boron for experimental boron neutron capture therapy (BNCT). Copper octabromotetracarboranylphenylporphyrin (CuTCPBr), synthesized by bromination of CuTCPH, is one of several new minimally toxic analogues of CuTCPH being studied in our laboratory, which could possess comparable or better tumour-targeting properties with enhanced tumour cytotoxicity. Its biodistribution, biokinetics and toxicity in mice with subcutaneous EMT-6 (mammary) or SCCVII (squamous cell) carcinomas were compared with those of CuTCPH. The administration of approximately 200 mg kg(-1) of either porphyrin in six intraperitoneal injections over 2 days had no apparent effect, but administration of approximately 400 mg kg(-1) slightly lowered body weights, elevated alanine and aspartate transaminase activities in blood plasma, and depressed blood platelet counts for several days. Enzymes and platelets returned to normal within 5 days after those injections and body weights returned to normal within 2 weeks. High average concentrations of boron from either porphyrin were achieved in the two tumour models from a total dose of approximately 200 mg kg(-1). The high tumour boron concentration decreased slowly while concentrations in blood decreased rapidly. Boron concentrations in brain and skin were consistently lower than in tumour by a factor of 10 or more. Although either CuTCPH or CuTCPBr can be labelled with (64)Cu for imaging by positron emission tomography (PET), CuTCPBr can also be labelled by (76)Br, another PET-imageable nuclide.

Contributions to the chemistry of anionic water-soluble tetraarylporphyrins and their metal complexes

Recent contributions to the coordination chemistry of tetraanionic water-soluble tetraarylporphyrins, namely 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, [1]4−, 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, [2]4−, and 5,10,15,20-tetrakis(4-hydroxidophosphorylmethylphenyl)porphyrin, [ H 43]4−, are reported. Problems with the isolation of sodium and potassium salts of [2]4− are discussed. The tetraphosphonoporphyrin H 8[3] is precipitated as a dihydrochloride H 8[3]·2 HCl , its metal complexes H 8[3-M] as such. The synthesis of salts containing Ce and Os complexes of [1]4−, Al , Ga , In , Ge , Sn , As , Sb , Mn , Mo , W , Re , Os , and Pt complexes of [2]4−, and Ni , Cu , Zn , Pt and Os complexes of [ H 43]4− as documented in publications and doctoral theses from the Darmstadt laboratory are reviewed. Some properties of the new complex salts are reported. The results of crystal structure determinations of the potassium salt of tetrakis(4-sulfonatophenyl)porphyrinatoplatinate(II), K 8[2-Pt]2·10 DMSO ·6 H 2O· C 7 H 8, and the mixed sodium-potassium salt of tetrakis(4-sulfonatophenyl)porphyrinatoaluminate, K 2 Na [ Al ( DMSO )2( tpps 4)]·8( DMSO )·2 H 2 O , are shown.

Biodistribution of a carborane-containing porphyrin as a targeting agent for Boron Neutron Capture Therapy of oral cancer in the hamster cheek pouch

Boron Neutron Capture Therapy (BNCT) is a bimodal cancer treatment based on the selective accumulation of 10B in tumors and concurrent irradiation with thermalized neutrons. The short-range, high-LET radiation produced by the capture of neutrons by 10B could potentially control tumor while sparing normal tissue if the boron compound targets tumor selectively within the treatment volume. In previous studies, we proposed and validated the hamster cheek pouch model of oral cancer for BNCT studies, proved that absolute and relative uptake of the clinically employed boron compound boronophenylalanine (BPA) would be potentially therapeutic in this model and provided evidence of the efficacy of in vivo BPA-mediated BNCT to control hamster oral mucosa tumors with virtually no damage to normal tissue. We herein present the biodistribution and pharmacokinetics of a lipophilic, carborane-containing tetraphenylporphyrin (CuTCPH) in the hamster oral cancer model. CuTCPH is a novel, non-toxic compound that may be advantageous in terms of selective and absolute delivery of boron to tumor tissues. For potentially effective BNCT, tumor boron concentrations from a new agent should be greater than 30 ppm and tumor/blood and tumor/normal tissue boron concentration ratios should be greater than 5/1 without causing significant toxicity. We administered CuTCPH intraperitoneally (i.p.) as a single dose of 32 microg/g body weight (b.w.) (10 microg B/g b.w.) or as four doses of 32 microg/g b.w. over 2 days. Blood (Bl) and tissues were sampled at 3, 6, 12, 24, 48, and 72 h in the single-dose protocol and at 1-4 days after the last injection in the multidose protocol. The tissues sampled were tumor (T), precancerous tissue surrounding tumor, normal pouch (N), skin, tongue, cheek and palate mucosa, liver, spleen, parotid gland and brain. The maximum mean B ratios for the single-dose protocol were T/N: 9.2/1 (12h) and T/Bl: 18.1/1 (72 h). The B value peaked to 20.7+/-18.5 ppm in tumor at 24h. The multidose protocol maximum mean ratios were T/N: 11.9/1 (3 days) and T/Bl: 235/1 (4 days). Absolute boron concentration in tumor reached a maximum value of 116 ppm and a mean value of 71.5+/-48.3 ppm at 3 days. The fact that absolute and relative B values markedly exceeded the BNCT therapeutic threshold with no apparent toxicity may confer on this compound a therapeutic advantage. CuTCPH-mediated BNCT would be potentially useful for the treatment of oral cancer in an experimental model.

Syntheses and preliminary biological studies of four meso-Tetra[(nido-carboranylmethyl)phenyl]porphyrins

Two meso-tetra[(nido-carboranylmethyl)phenyl]porphyrins (para- and meta-regioisomers) and their corresponding Zn(II) complexes have been synthesized with the aim of studying the effect of carborane distribution and metalation on the biological properties of this series of compounds. In vitro cell toxicity, uptake/efflux, and subcellular localization using rat 9L, mouse B16 and/or human U-373MG cells were evaluated. All four amphiphilic porphyrins display very low cytotoxicities and time- and concentration-dependent uptake by cells, which is influenced by serum proteins. Preliminary subcellular localization studies suggest that one of these compounds localizes in close proximity to the cell nucleus. All four nido-carboranylporphyrins show promise as boron-carriers for the boron neutron capture therapy of cancers, particularly the metal-free nido-carboranylporphyrins 5 and 12, which are able to deliver higher amount of boron to cells in vitro than the corresponding zinc complexes.

Boron neutron capture therapy of a murine mammary carcinoma using a lipophilic carboranyltetraphenylporphyrin

The first control of a malignant tumor in vivo by porphyrin- mediated boron neutron capture therapy (BNCT) is described. In mice bearing implanted EMT-6 mammary carcinomas, boron uptake using a single injection of either p-boronophenylalanine (BPA) or mercaptoundecahydrododecaborane (BSH) was compared with either a single injection or multiple injections of the carboranylporphyrin CuTCPH. The BSH and BPA doses used were comparable to the highest doses of these compounds previously administered in a single injection to rodents. For BNCT, boron concentrations averaged 85 microg (10)B/g in the tumor and 4 microg (10)B/g in blood 2 days after the last of six injections (over 32 h) that delivered a total of 190 microg CuTCPH/g body weight. During a single 15, 20, 25 or 30 MW-min exposure to the thermalized neutron beam of the Brookhaven Medical Research Reactor, a tumor received average absorbed doses of approximately 39, 52, 66 or 79 Gy, respectively. A long-term (>200 days) tumor control rate of 71% was achieved at a dose of 66 Gy with minimal damage to the leg. Equivalent long-term tumor control by a single exposure to 42 Gy X rays was achieved, but with greater damage to the irradiated leg.

Evaluation of carborane-containing porphyrins as tumour targeting agents for boron neutron capture therapy

A number of carborane-containing porphyrins were administered to mice bearing subcutaneously transplanted mammary carcinomas. Administration was via serial intraperitoneal (i.p.) injections to assess their relative toxicities and tumour affinities. Three analogues of the natural porphyrin heme and four tetraphenylporphyrins (TPPs) were given at total doses of 78-245 micrograms g-1 body weight. The water-insoluble TPPs were less toxic to mice, and delivered greater amounts of boron to tumour than did the water-soluble TPPS and the heme analogues. One such compound, NiTCP-H, delivered more than 100 micrograms B g-1 to tumour tissue with a tumour:blood boron concentration ratio greater than 500:1 and a tumour: brain boron concentration ratio greater than 50:1, 4 days after the last of six i.p. injections given over 2 days. Another TPP analogue, NiTCP, delivered approximately 50 micrograms B g-1 to tumour with similar boron concentrations in normal tissues. Neither compound was toxic to mice at total doses of approximately 200 micrograms g-1 body weight. In contrast, the heme analogues were toxic and, with the exception of VCDP, delivered less boron to tumour than NiTCP and NiTCP-H. The two porphyrins with the greatest potential for application to boron neutron capture therapy (BNCT), NiTCP and NiTCP-H, yielded higher tumour:blood and tumour:brain boron concentration ratios in mice than could be achieved with p-boronophenylalanine (BPA) and sodium mercaptoundecahydrododecaborate (BSH), the compounds which are currently being used in clinical trials of BNCT in the treatment of glioblastoma. The boron delivered by each of the porphyrins tested remained in tumour tissue longer than did boron delivered by either BPA or BSH. The copper and nickel chelates of these porphyrins behave identically in vivo. The former offer the potential for imaging by 67Cu-mediated single photon emission computed tomography (SPECT) to aid BNCT treatment planning.

Synthesis of a nickel tetracarboranylphenylporphyrin for boron neutron-capture therapy: biodistribution and toxicity in tumor-bearing mice

Nickel-2,3,7,8,12,13,17,18-octaacetic acid-5,10,15,20-tetra-[3-carboranyl-methoxyphenyl]-porphyrin octamethylester (NiTCP) was given in a Cremophor EL, a polyethoxylated castor oil, and propylene glycol emulsion to BALB/c mice bearing transplanted s.c. KHJJ mammary carcinomas. A total dose of 244 microg NiTCP/gram body weight (gbw) (54 microg B/gbw) was given in 6 i.p. injections over a 32 hr period. Observations of behavior and changes in body weight and chemical and hematological blood tests indicated little or no toxicity from NiTCP over a period of 6-90 hr after injections. Boron concentrations near tumor margins were 160-180 microg B/g at 41-90 hr after the last injection. Tumor:normal brain boron concentration ratios reached approx. 10:1 and tumor:blood ratios reached approx. 250:1 after 4 days. There was no evidence of thrombocytopenia or other potentially important toxicities. Our findings place NiTCP among the leading candidates for pre-clinical experiments aimed toward improvement upon the compounds being tested for boron neutron-capture therapy of glioblastoma multiforme.

Progress in the field of compounds for boron neutron capture therapy

For successful application of boron neutron capture therapy to the treatment of cancer, selective accumulation of boron is required. This can be achieved by the preparation of suitable substances, which must contain boron and at the same time accumulate or be retained in tumor tissue. The radiobiological rationale for selection of suitable compounds is discussed. Examples of useful new compounds are given for which boronated analogues exist.

A comparative study on the pharmacokinetics and biodistribution of boronated porphyrin (BOPP) and sulfhydryl boron hydride (BSH) in the RG2 rat glioma model

Boron neutron capture therapy is a treatment modality for cancer that depends on the specific uptake of boron by the tumor cells. The infiltrative growth of malignant gliomas requires that boron reach and accumulate in migrating cells outside the margin of the tumor; thus, it is important that the biodistribution of new boron compounds is also studied in the surrounding healthy brain tissue. This study is undertaken in the present work, in which the biodistribution and pharmacokinetics of sulfhydryl boron hydride (BSH) and boronated porphyrin (BOPP) in the RG2 rat glioma model are investigated. This model mimics the characteristics of human glioma with cells migrating into the surrounding brain. The animals were infused intravenously with either BSH (25 micrograms or 175 micrograms of boron per gram of body weight) or BOPP (12 micrograms of boron per gram body weight). For the low dose of BSH, the maximum tumor-boron content was 8 ppm at approximately 9 hours after the infusion with a tumor-to-blood ratio of 0.6. At the higher dose, the corresponding figures were 15 ppm after 12 hours with a tumor-to-blood ratio of 0.5. For BOPP, a tumor-boron concentration of 81 ppm was achieved 24 hours after the infusion and sustained in that range for at least 72 hours. The tumor-to-blood ratio at 24 hours was slightly above 6, but continued to increase as the blood was cleared. These results indicate that both compounds are spread into the normal brain tissue following the same pathways as the migrating tumor cells and in this way can be taken up even in distant tumor cell foci.

Selectivity of boron carriers for boron neutron capture therapy: pharmacological studies with borocaptate sodium, L-boronophenylalanine and boric acid in murine tumors

Borocaptate sodium (BSH) and L-boronophenylalanine (L-BPA) are two boron carriers used for boron neutron capture therapy (BNCT) in the treatment of glioblastoma and melanoma, respectively. The suitability of these two compounds was evaluated on the basis of pharmacokinetic studies aiming at characterizing their biodistribution, tumor uptake and tumor selectivity. Boric acid was also used as a reference compound since it is nonselective and relatively freely diffusible. The compounds were investigated in two tumor models, a B16 pigmented melanoma and the RIF1 sarcoma. Mice were sacrificed after different boron doses at various post-injection times and tissue and plasma levels measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The proposed minimum effective tumor boron concentration of 15 ppm was achieved in both tumor models for the three compounds tested, although only for L-BPA in the melanoma was this achieved when tumor-plasma ratios were above 1. In the RIF1 model, maximum tumor concentrations of 44 and 31 ppm B were reached after administration of 50 micrograms B/g body weight for boric acid and BSH, respectively. After administration of 12.5 micrograms B/g of L-BPA, maximum concentrations of 15 and 21 ppm were found in the RIF1 and B16 models, respectively. Tumor-plasma ratios (TPR) for BSH remained close to or below unity at all times studied in both tumors. Brain levels of BSH were very low, however, leading to tumor-brain ratios markedly greater than 1 at all times. L-BPA and boric acid showed TPR values above unity in both tumor models, reaching 3.2 in B16.(ABSTRACT TRUNCATED AT 250 WORDS)