Interactions of enantiomers of 2',3'-didehydro-2',3'-dideoxy-fluorocytidine with wild type and M184V mutant HIV-1 reverse transcriptase.

Both the beta-D-(+) and beta-L-(-)-enantiomers of 2',3'-didehydro-2',3'-dideoxy-5-fluorocytidine (D4FC) are clinically relevant compounds because of their potent anti-HIV and anti-HBV activities. Cross-resistance to L-D4FC with HBV containing a mutation in the conserved polymerase YMDD region has been observed. In order to better understand the effects of stereochemistry on planar 5-fluorinated cytidine analogs and to gain insight into resistance caused by YMDD mutations in HIV-1 reverse transcriptase (RT), a combination of transient kinetic studies and computer modeling were employed. In contrast to studies with the (+) and (-) isomers of 3TC-TP and FTC-TP, it was found that wild type RT had a high enantiomeric selectivity between the D-(+) and L-(-) isomers of D4FC-TP. While no resistance was conferred by the methionine 184 to valine mutation to D-D4FC-TP, L-D4FC-TP was incorporated 50- to 70-fold less efficiently. The kinetic parameters of incorporation in the presence of L-D4FC-TP by RT(WT) and the mechanism of resistance by RT(M184V) were found to be distinct from those seen with the corresponding L-isomers containing an oxathiolane ring: (-)-3TC-TP and (-)-FTC-TP. Molecular modeling suggests that L- and D-D4FC-TP are positioned in the active site favorably for incorporation by RT(WT) and that L-D4FC-TP, but not D-D4FC-TP, is sterically hindered by the addition of a beta branched amino acid at position 184 of RT(M184V).