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The compound class of 3-carboranyl thymidine analogues (3CTAs) are boron delivery agents for boron neutron capture therapy (BNCT), a binary treatment modality for cancer. Presumably, these compounds accumulate selectively in tumor cells via intracellular trapping, which is mediated by hTK1. Favorable in vivo biodistribution profiles of 3CTAs led to promising results in preclinical BNCT of rats with intracerebral brain tumors. This review presents an overview on the design, synthesis, and biological evaluation of first- and second-generation 3CTAs. Boronated nucleosides developed prior to 3CTAs for BNCT and non-boronated N3-substituted thymidine conjugates for other areas of cancer therapy and imaging are also described. In addition, basic features of carborane clusters, which are used as boron moieties in the design and synthesis of 3CTAs, and the biological and structural features of TK1-like enzymes, which are the molecular targets of 3CTAs, are discussed.

Invasive fungal disease constitutes a growing health burden and development of novel antifungal drugs with high potency and selectivity against new fungal molecular targets are urgently needed. Previously, an aminothiazole derivative, designated as 41F5, was identified in our laboratories as highly active against Histoplasma yeast (MIC50 0.4-0.8 µM) through phenotypic high-throughput screening of a commercial library of 3600 purine mimicking compounds (Edwards, JA et al. Antimicrob. Agents Chemother. 2013, 57:4349-5359). Consequently, 68 analogues of 41F5 were designed and synthesized or obtained from commercial sources and their MIC50s of growth inhibition were evaluated in Histoplasma capsulatum to establish a basic Structure-Activity-Relationship (SAR) for this potentially new class of antifungals. The growth inhibiting potentials of smaller subsets of this library were also evaluated in Cryptococcus neoformans and human hepatocyte HepG2 cells, the latter to obtain Selectivity Indices (SIs). The results indicate that a thiazole core structure with a naphth-1-ylmethyl group at the 5-position and cyclohexylamide-, cyclohexylmethylamide-, or cyclohexylethylamide substituents at the 2-position caused the highest growth inhibition of Histoplasma yeast with MIC50s of 0.4 µM. For these analogues, SIs of 92 - >100 indicated generally low host toxicity. Substitution at the 3- and 4-position decreased antifungal activity. Similarities and differences were observed between Histoplasma and Cryptococcus SARs. For Cryptococcus, the naphth-1-ylmethyl substituent at the 5-position and smaller cyclopentylamide or cyclohexylamide groups at the 2-position were important for activity. In contrast, slightly larger cyclohexylmethyl- and cyclohexylethyl substituents markedly decreased activity.

Pan-genotypic nucleoside HCV inhibitors display a high genetic barrier to drug resistance and are the preferred direct-acting agents to achieve complete sustained virologic response in humans. Herein, we report, the discovery of a β-d-2'-Cl,2'-F-uridine phosphoramidate nucleotide 16, as a nontoxic pan-genotypic anti-HCV agent. Phosphoramidate 16 in its 5'-triphosphate form specifically inhibited HCV NS5B polymerase with no marked inhibition of human polymerases and cellular mitochondrial RNA polymerase. Studies on the intracellular half-life of phosphoramidate 16-TP in live cells demonstrated favorable half-life of 11.6 h, suggesting once-a-day dosing. Stability in human blood and favorable metabolism in human intestinal microsomes and liver microsomes make phosphoramidate 16 a prospective candidate for further studies to establish its potential value as a new anti-HCV agent.

The synthesis and initial biological evaluation of 3-carboranyl thymidine analogues (3CTAs) that are (radio)halogenated at the closo-carborane cluster is described. Radiohalogenated 3CTAs have the potential to be used in the radiotherapy and imaging of cancer, as they may be selectively entrapped in tumor cells through monophosphorylation by human thymidine kinase 1 (hTK1). Two strategies for the synthesis of a 127I-labeled form of a specific 3CTA, previously designated as N5, are described: 1) direct iodination of N5 with iodine monochloride (ICl) and aluminum chloride (AlCl3) to obtain N5-127I and 2) initial monoiodination of o-carborane to 9-iodo-o-carborane followed by its functionalization to N5-127I. The former strategy produced N5-127I in low yields along with di-, tri-, and tetra-iodinated N5 as well as decomposition products, whereas the latter method produced only N5-127I in high yields. N5-127I was subjected to nucleophilic halogen- and isotope exchange reactions using Na79/81Br and Na125I, respectively, in the presence of Herrmann's catalyst to obtain N5-79/81Br and N5-125I, respectively. Two intermediate products formed using the second strategy, 1-(tert-butyldimethylsilyl)-9-iodo-o-carborane and 1-(tert-butyldimethylsilyl)-12-iodo-o-carborane, were subjected to x-ray diffraction studies to confirm that substitution at a single carbon atom of 9-iodo-o-carborane resulted in the formation of two structural isomers. To the best of our knowledge, this is the first report of halogen and isotope exchange reactions of B-halocarboranes that have been conjugated to a complex biomolecule. Human TK1 phosphorylation rates of N5, N5-127I, and N5-79/81Br ranged from 38.0 % to 29.6% relative to that of thymidine, the endogenous hTK1 substrate. The in vitro uptake of N5, N5-127I, and N5-79/81Br in L929 TK1 (+) cells was 2.0 ×, 1.8 ×, and 1.4 × greater than that in L929 TK1 (−) cells.

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A library of sixteen 2nd generation amino- and amido-substituted carboranyl pyrimidine nucleoside analogues, designed as substrates and inhibitors of thymidine kinase 1 (TK1) for potential use in boron neutron capture therapy (BNCT) of cancer, was synthesized and evaluated in enzyme kinetic-, enzyme inhibition-, metabolomic-, and biodistribution studies. One of these 2nd generation carboranyl pyrimidine nucleoside analogues (YB18A [3]), having an amino group directly attached to a meta-carborane cage tethered via ethylene spacer to the 3-position of thymidine, was approximately 3–4 times superior as a substrate and inhibitor of hTK1 than N5-2OH (2), a 1st generation carboranyl pyrimidine nucleoside analogue. Both 2 and 3 appeared to be 5′-monophosphorylated in TK1(+) RG2 cells, both in vitro and in vivo. Biodistribution studies in rats bearing intracerebral RG2 glioma resulted in selective tumor uptake of 3 with an intratumoral concentration that was approximately 4 times higher than that of 2. The obtained results significantly advance the understanding of the binding interactions between TK1 and carboranyl pyrimidine nucleoside analogues and will profoundly impact future design strategies for these agents.

bRibonucleoside analog inhibitors (rNAI) target the hepatitis C virus (HCV) RNA-dependent RNA polymerase nonstructural protein 5B (NS5B) and cause RNA chain termination. Here, we expand our studies on ␤-D-2=-C-methyl-2,6-diaminopurine-ribonucleotide (DAPN) phosphoramidate prodrug 1 (PD1) as a novel investigational inhibitor of HCV. DAPN-PD1 is metabolized intracellularly into two distinct bioactive nucleoside triphosphate (TP) analogs. The first metabolite, 2=-C-methyl-GTP, is a wellcharacterized inhibitor of NS5B polymerase, whereas the second metabolite, 2=-C-methyl-DAPN-TP, behaves as an adenosine base analog. In vitro assays suggest that both metabolites are inhibitors of NS5B-mediated RNA polymerization. Additional factors, such as rNAI-TP incorporation efficiencies, intracellular rNAI-TP levels, and competition with natural ribonucleotides, were examined in order to further characterize the potential role of each nucleotide metabolite in vivo. Finally, we found that although both 2=-C-methyl-GTP and 2=-C-methyl-DAPN-TP were weak substrates for human mitochondrial RNA (mtRNA) polymerase (POLRMT) in vitro, DAPN-PD1 did not cause off-target inhibition of mtRNA transcription in Huh-7 cells. In contrast, administration of BMS-986094, which also generates 2=-C-methyl-GTP and previously has been associated with toxicity in humans, caused detectable inhibition of mtRNA transcription. Metabolism of BMS-986094 in Huh-7 cells leads to 87-fold higher levels of intracellular 2=-C-methyl-GTP than DAPN-PD1. Collectively, our data characterize DAPN-PD1 as a novel and potent antiviral agent that combines the delivery of two active metabolites. Chronic infection with hepatitis C virus (HCV) represents a significant disease burden on global health. It is estimated that 3% of the human population carries a chronic HCV infection, which translates to roughly 170 million infections worldwide and 3.2 million infections in the United States (1). Although up to 25% of acutely infected individuals clear the virus spontaneously, chronic HCV infection develops in the remaining 75% of those infected and can lead to the development of liver cirrhosis and hepatocellular carcinoma (2-4).HCV contains a 9.6-kb positive-strand RNA genome that codes for three structural and seven nonstructural proteins. The nonstructural protein 5B (NS5B) is the viral RNA-dependent RNA polymerase and is responsible for genome replication. Because of the high mutation rate of this enzyme, as well as the high virus replication rate, HCV propagation results in the generation of thousands of viral quasispecies (5, 6). Therefore, drug combination therapy is required in order to overcome development of resistance and to achieve successful viral clearance. HCV treatment comprised of ribavirin and pegylated interferon alpha, with or without protease inhibitors, can achieve sustained virologic responses in 40 to 80% of treated patients (7-9). However, curative outcomes historically have been suboptimal due to host polymorphisms, viral genotypic variability, and onset ...