Effect of stereoisomerism on the cellular pharmacology of beta-enantiomers of cytidine analogs in Hep-G2 cells.

The beta-L enantiomers of 2',3'-dideoxycytidine (beta-L-ddC) and its 5-fuoro derivative, 2',3'-dideoxy-5-fluorocytidine (beta-L-FddC), were demonstrated to be active against human immunodeficiency virus (HIV) and hepatitis B virus (HBV) replication in vitro. In the present study, we investigated the cellular pharmacology of beta-L-ddC and beta-L-FddC and compared it with that of beta-D-2',3'-dideoxy-5-fluorocytidine (beta-D-FddC). Beta-L-FddC (10 microM) was found to be phosphorylated rapidly in Hep-G2 cells to its 5'-mono-, di-, and triphosphate derivatives with intracellular triphosphate levels achieving 26.6 +/- 10.9 pmol/10(6) cells after 72 hr. In contrast, the active 5'-phosphorylated derivative of beta-D-FddC achieved lower levels with triphosphate levels of only 2.3 +/- 0.5 pmol/ (10(6) cells under the same conditions. Beta-L-ddC was also phosphorylated rapidly. A 5'-diphosphocholine (18 +/- 5.8 pmol/10(6) cells) and a 5'-diphosphoethanolamine (13.6 +/- 0.9 pmol/10(6) cells) derivative were detected in beta-D-FddC-treated cells after 72 hr, whereas in beta-L-FddC- and beta-L-ddC-treated cells, only the 5'-diphosphocholine derivative (10.9 +/- 2.8 and 60.4 +/- 5.7 pmol/10(6) cells, respectively) was detected. Beta-L-FddC-5'-triphosphate (beta-L-FddCTP), beta-D-FddC-5'-triphosphate (beta-D-FddCTP), and beta-L-ddC-5'-triphosphate (beta-L-ddCTP) followed a single phase elimination process with an intracellular half-life (T1/2) of 10.5, 5.7, and 12.3 hr, respectively. Furth ermore, beta-L-FddCTP, beta-D-FddCTP, and beta-L-ddCTP levels of 6.7 +/- 2.3, 0.3 +/- 0.1, and 12.0 pmol/10(6) cells, respectively, were still detectable 24 hr following drug removal. The higher intracellular 5'-triphosphate levels of beta-L-FddC and the extended T1/2 of its 5'-triphosphate are consistent with the more potent in vitro antiviral activity of beta-L-FddC in Hep-G2 cells when compared with its beta-D enantiomer, beta-D-FddC.