Anti-human immunodeficiency virus type 1 activity, intracellular metabolism, and pharmacokinetic evaluation of 2 '-deoxy-3 '-oxa-4 '-thiocytidine

The racemic nucleoside analogue 2'-deoxy-3'-oxa-4'-thiocytidine (dOTC) is i n clinical development for the treatment of human immunodeficiency virus (H IV) type 1 (HIV-1) infection. dOTC is structurally related to lamivudine (3 TC), but the oxygen and sulfur in the furanosyl ring are transposed. Intrac ellular metabolism studies showed that dOTC is phosphorylated within cells via the deoxycytidine kinase pathway and that approximately 2 to 5% of dOTC is converted into the racemic triphosphate derivatives, which had measurab le half-lives (2 to 3 hours) within cells. Both 5'-triphosphate (TP) deriva tives of dOTC were more potent than 3TC-TP at inhibiting HIV-1 reverse tran scriptase (RT) in vitro. The K-i values for dOTC-TP obtained against human DNA polymerases alpha, beta, and gamma were 5,000-, 78-, and 571-fold great er, respectively, than those for HIV RT (28 nM), indicating a good selectiv ity for the viral enzyme. In culture experiments, dOTC is a potent inhibito r of primary isolates of HIV-1, which were obtained from antiretroviral dru g-naive patients as well as from nucleoside therapy-experienced (3TC- and/o r zidovudine [AZT]-treated) patients. The mean 50% inhibitory concentration of dOTC for drug-naive isolates was 1.76 mu M, rising to only 2.53 and 2.5 mu M for viruses resistant to 3TC and viruses resistant to 3TC and AZT, re spectively. This minimal change in activity is in contrast to the more dram atic changes observed when 3TC or AZT was evaluated against these same vira l isolates. In tissue culture studies, the 50% toxicity levels for dOTC, wh ich were determined by using [H-3]thymidine uptake as a measure of logarith mic-phase cell proliferation, was greater than 100 mu M for all cell lines tested. In addition, after 14 days of continuous culture, at concentrations up to 10 mu M, no measurable toxic effect on HepG2 cells or mitochondrial DNA replication within these cells was observed. When administered orally t o rats, dOTC was well absorbed, with a bioavailability of approximately 77% , with a high proportion (approximately 16.5% of the levels in serum) found in the cerebrospinal fluid.