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2'-Deoxy-3'-thiacytidine is a dideoxycytidine analog with a sulfur in place of the 3' carbon of the ribose. There are two enantiomeric forms of the compound, both of which inhibit human immunodeficiency virus type 1 and 2 replication in vitro. However, the (-) enantiomer (lamivudine) appears to be significantly less cytotoxic to uninfected lymphocytes than is the (+) enantiomer. Lamivudine has entered initial clinical trials, and the present study was designed to describe the pharmacokinetic behavior of this compound in both plasma and cerebrospinal fluid (CSF) of primates. Lamivudine was administered as an intravenous bolus dose of 20 mg/kg for 24 h for administration. Urine samples were also obtained from two animals. The same dose of the racemate (BCH-189) was administered to one animal. The drug was quantitated in CSF and plasma with a reverse-phase high-pressure liquid chromatography technique. Elimination of lamivudine from plasma was biexponential, with a mean alpha phase half-life of 5.4 min, a mean beta phase half-life of 84 min, and a total clearance of 6.1 liters/h. The total clearance of the same dose of BCH-189 in a single animal was 11.0 liters/h. In two animals from which urine was obtained for 12 h postadministration, 32 and 59% of the drug was recovered unchanged. The deamination product of lamivudine was not detected. The CSF/plasma ratio of lamivudine was significantly higher when the drug was measured in the lumbar CSF (mean, 0.41) than when it was measured in the ventricular CSF (mean, 0.079). The measured CSF/plasma ratio for ventricular CSF is equivalent to that of other dideoxycytidine analogs, confirming the importance of the nucleobase in determining the degree of CSF penetration. The difference in lamivudine exposure in ventricular and lumbar CSF suggests that there is a transport mechanism for efflux of cytidine analogs from ventricular CSF.

Pharmacokinetics of lamivudine and BCH-189 in plasma and cerebrospinal fluid of nonhuman primates