George Walter Koszalka

Potent and Selective Inhibition of Human Cytomegalovirus Replication by 1263W94, a Benzimidazole l-Riboside with a Unique Mode of Action

ABSTRACT Benzimidazole nucleosides have been shown to be potent inhibitors of human cytomegalovirus (HCMV) replication in vitro. As part of the exploration of structure-activity relationships within this series, we synthesized the 2-isopropylamino derivative (3322W93) of 1H-β-d-ribofuranoside-2-bromo-5,6-dichlorobenzimidazole (BDCRB) and the biologically unnatural l-sugars corresponding to both compounds. One of the l derivatives, 1H-β-l-ribofuranoside-2-isopropylamino-5,6-dichlorobenzimidazole (1263W94), showed significant antiviral potency in vitro against both laboratory HCMV strains and clinical HCMV isolates, including those resistant to ganciclovir (GCV), foscarnet, and BDCRB. 1263W94 inhibited viral replication in a dose-dependent manner, with a mean 50% inhibitory concentration (IC50) of 0.12 ± 0.01 μM compared to a mean IC50 for GCV of 0.53 ± 0.04 μM, as measured by a multicycle DNA hybridization assay. In a single replication cycle, 1263W94 treatment reduced viral DNA synthesis, as well as overall virus yield. HCMV mutants resistant to 1263W94 were isolated, establishing that the target of 1263W94 was a viral gene product. The resistance mutation was mapped to the UL97 open reading frame. The pUL97 protein kinase was strongly inhibited by 1263W94, with 50% inhibition occurring at 3 nM. Although HCMV DNA synthesis was inhibited by 1263W94, the inhibition was not mediated by the inhibition of viral DNA polymerase. The parent benzimidazole d-riboside BDCRB inhibits viral DNA maturation and processing, whereas 1263W94 does not. The mechanism of the antiviral effect of l-riboside 1263W94 is thus distinct from those of GCV and of BDCRB. In summary, 1263W94 inhibits viral replication by a novel mechanism that is not yet completely understood.

Anti-human immunodeficiency virus synergism by zidovudine (3'-azidothymidine) and didanosine (dideoxyinosine) contrasts with their additive inhibition of normal human marrow progenitor cells.

The anti-human immunodeficiency virus (HIV) activity and hemopoietic toxicity of zidovudine (AZT) and didanosine (dideoxyinosine;ddI), alone and in combination, were assessed in a variety of cell types. AZT was more potent than ddI as an inhibitor of HIV in vitro. Synergistic inhibition of HIV by the combination of these agents was observed in MT4 cells, peripheral blood lymphocytes, and macrophages. Toxicity assessment in vitro by using progenitor (erythroid and granulocyte-macrophage) colony-forming assays with normal human bone marrow showed ddI to be less toxic than AZT. Addition of inhibitory concentrations of ddI to AZT resulted in additive inhibition of progenitor CFUs. These in vitro findings suggest that combinations of ddI and AZT at appropriately modified doses may provide an enhanced degree of selectivity in anti-HIV chemotherapy.

Preclinical and Toxicology Studies of 1263W94, a Potent and Selective Inhibitor of Human Cytomegalovirus Replication

ABSTRACT 1263W94 is a novel benzimidazole compound being developed for treatment of human cytomegalovirus infection. No adverse pharmacological effects were demonstrated in safety pharmacology studies with 1263W94. The minimal-effect dose in a 1-month rat study was 100 mg/kg/day, and the no-effect dose in a 1-month monkey study was 180 mg/kg/day. Toxic effects were limited to increases in liver weights, neutrophils, and monocytes at higher doses in female rats. 1263W94 was not genotoxic in the Ames or micronucleus assays. In the mouse lymphoma assay, 1263W94 was mutagenic in the absence of the rat liver S-9 metabolic activation system, with equivocal results in the presence of the S-9 mix. Mean oral bioavailability of 1263W94 was >90% in rats and ∼50% in monkeys. Clearance in rats and monkeys was primarily by biliary secretion, with evidence of enterohepatic recirculation. In 1-month studies in rats and monkeys, mean peak concentrations and exposures to 1263W94 increased in near proportion to dose. Metabolism of 1263W94 to its primary metabolite, an N-dealkylated analog, appeared to be mediated via the isozyme CYP3A4 in humans. 1263W94 was primarily distributed in the gastrointestinal tract of rats but did not cross the blood-brain barrier. In monkeys, 1263W94 levels in the brain, cerebrospinal fluid, and vitreous humor ranged from 4 to 20%, 1 to 2%, and <1%, of corresponding concentrations in plasma, respectively. The high level of binding by 1263W94 to human plasma proteins (primarily albumin) was readily reversible, with less protein binding seen in the monkey, rat, and mouse. Results of these studies demonstrate a favorable safety profile for 1263W94.

An enzymic synthesis of purine d-Arabinonucleosides

Abstract A method is described for the synthesis of purine d -arabinonucleosides that uses purine bases and 2,2′-anhydro-(1-β- d -arabinofuranosylcytosine), AraC-an, as the starting materials. AraC-an was chosen as the precursor to the d -arabinosyl donor, because it is more readily available than any of the products that may be sequentially derived from it, namely, 1-β- d -arabinofuranosylcytosine (AraC), 1-β- d -arabinofuranosyluracil (AraU), and α- d -arabinofuranosyl-1-phosphate (Ara f 1-P), a d -arabinofuranosyl donor. Four reactions were involved in the overall process; ( a ) AraC-an was nonenzymically hydrolyzed at alkaline pH to AraC which was then ( b ) deaminated by cytidine deaminase to AraU, a nucleoside, ( c ) phosphorylyzed by uridine phosphorylase to Ara f 1-P, and ( d ) this ester caused to react with a purine base to afford a purine d -arabinonucleoside, the reaction being catalyzed by purine nucleoside phosphorylase. All four reactions occurred in situ , the first and second being performed sequentially, whereas the third and fourth were combined in a single step. The three enzyme catalysts were purified from Escherichia coli . The efficiency of the method is exemplified by the synthesis of the d -arabinonucleosides of 2,6-diaminopurine and adenine; the overall yields, based on AraC-an, were 60 and 80%, respectively.

Development of Novel Benzimidazole Riboside Compounds for Treatment of Cytomegalovirus Disease

Benzimidazole ribosides are a new class of compounds with novel mechanisms of action against CMV. One compound in this series, BDCRB, inhibits CMV DNA processing by the UL89 gene product (putative terminase), but rapid metabolism to an inactive compound makes it unsuitable for development as a medicine. Another benzimidazole analogue, 1263W94, has many characteristics that make it an attractive candidate for development, including high potency in vitro, selectivity, good oral bioavailability, and lower toxicity than therapies currently available for treatment of CMV disease. Initial clinical trials have provided encouraging results, including good tolerability and linear pharmacokinetics over a wide dose range. Ongoing and planned clinical trials that will study the safety and tolerability of repeated dosing and evaluate the in vivo antiviral activity and ocular penetration of 1263W94, will help to determine the potential of this drug as an improved therapy for CMV disease.

6-Methoxypurine arabinoside as a selective and potent inhibitor of varicella-zoster virus.

Seven 6-alkoxypurine arabinosides were synthesized and evaluated for in vitro activity against varicella-zoster virus (VZV). The simplest of the series, 6-methoxypurine arabinoside (ara-M), was the most potent, with 50% inhibitory concentrations ranging from 0.5 to 3 microM against eight strains of VZV. This activity was selective. The ability of ara-M to inhibit the growth of a variety of human cell lines was at least 30-fold less (50% effective concentration, greater than 100 microM) than its ability to inhibit the virus. Enzyme studies suggested the molecular basis for these results. Of the seven 6-alkoxypurine arabinosides, ara-M was the most efficient substrate for VZV-encoded thymidine kinase as well as the most potent antiviral agent. In contrast, it was not detectably phosphorylated by any of the three major mammalian nucleoside kinases. Upon direct comparison, ara-M was appreciably more potent against VZV than either acyclovir or adenine arabinoside (ara-A). However, in the presence of an adenosine deaminase inhibitor, the arabinosides of adenine and 6-methoxypurine were equipotent but not equally selective; the adenine congener had a much less favorable in vitro chemotherapeutic index. Again, this result correlated with data from enzyme studies in that ara-A, unlike ara-M, was a substrate for two mammalian nucleoside kinases. Unlike acyclovir and ara-A, ara-M had no appreciable activity against other viruses of the herpes group. The potency and selectivity of ara-M as an anti-VZV agent in vitro justify its further study.

Studies Designed to Increase the Stability and Antiviral Activity (HCMV) of the Active Benzimidazole Nucleoside, TCRB

The potent activity of 2,5,6-trichloro-1-(beta-D-ribofuranosyl)benzimidazole (TCRB) against Human Cytomegalovirus with the concomitant low cellular toxicity at concentrations that inhibit viral growth prompted considerable interest in this research area. This interest was moderated by the pharmacokinetic studies of TCRB in rats and monkeys that revealed the instability of TCRB in vivo. These studies suggested that the instability was due to a cleavage of the glycosidic bond in vivo which released the heterocycle (2,5,6-trichlorobenzimidazole) into the bloodstream. This prompted us to initiate synthetic studies designed to increase the stability of the glycosidic bond of TCRB and BDCRB. Several synthetic approaches to address this and other problems are presented.

Synthesis and Biological Evaluation of Pyrimidine and Purine α-L-2′,3′-Dideoxy Nucleosides

Abstract A series of α-L-2′,3′-dideoxy nucleosides was prepared as potential antiviral agents. The pyrimidine nucleosides were prepared by standard Vorbruggen coupling reactions. The purine analogues were prepared by enzymatic transfer of the dideoxy sugar from a pyrimidine to a purine base. These compounds were inactive against HIV-1, HBV, HSV-1 and -2, VZV, and HCMV.

Identification and structure-activity studies of novel ultrashort-acting benzodiazepine receptor agonists

The synthesis and evaluation of novel ultrashort-acting benzodiazepine (USA BZD) agonists is described. A BZD scaffold was modified by incorporation of amino acids and derivatives. The propionate side chain of glutamic acid tethers an enzymatically labile functionality where the metabolite carboxylic acid displays markedly reduced BZD receptor affinity. The USA BZDs were characterized by full agonism profiles. Copyright2000 Elsevier Science Ltd.

Purine Salvage Enzymes in Man and Leishmania donovani

A comparison of the enzymes of pathogenic protozoa to those of man is of fundamental importance to the search for much needed chemotherapeutic agents. The enzymes involved in purine salvage are of particular interest because most pathogenic protozoa lack the ability to synthesize purines de novo and consequently are obligate salvagers of preformed purines.