Background:  PD754 is a deoxycytidine analogue with activity against HIV-1, including isolates resistant to other available NRTI. SPD754 and 3TC have additive antiviral activity against wild-type HIV-1 in vitro, but share a common intracellular anabolic pathway. In vitro studies were conducted to investigate the effects of SPD754 and 3TC on each other’s intracellular phosphorylation, and to establish whether any such effect on SPD754 influences its antiviral activity. The plasma pharmacokinetic profiles of SPD754 and 3TC were determined in human volunteers after their administration alone or in combination. The pharmacokinetic profiles of 3TC-TP and SPD754-TP in PBMC were also determined in the same study.

Methods:  PBMC were incubated in vitro in the presence of fixed concentrations of [³H]SPD754 and unlabelled 3TC, or [³H]3TC and unlabelled SPD754. Intracellular concentrations of labeled phosphorylated metabolites were determined by HPLC. The antiviral activity of SPD754 against a recombinant HIV-1 containing the M184V mutation was assessed in activated PBMC at a range of 3TC concentrations. In the human volunteer study, 21 male HIV-ve subjects received either SPD754 600 mg twice daily, 3TC 300 mg once daily or both in random order for 4 days (with ≥7 day washout interval). Extensive PBMC and plasma sampling was undertaken after the final dose in each period. Parent plasma and intracellular TP concentrations were assayed by HPLC.

Results:  The entry of SPD754 and 3TC into PBMC was unaffected by each other’s presence. 3TC reduced intracellular SPD754-TP concentrations in a dose dependent manner, with 3 mM 3TC reducing SPD754-TP by 4- to 6-fold relative to its concentration determined in the absence of 3TC. SPD754 had no effect on intracellular 3TC-TP concentrations. The IC₅₀ value of SPD754 against M184V HIV-1 was increased 2- to 4-fold by the addition of 3TC. The human plasma PK of SPD754 and 3TC, and the intracellular pharmacokinetics of 3TC-TP, were unaffected by co-administration. In contrast, intracellular SPD754-TP concentrations were reduced approximately 6-fold in the presence of 3TC.

Conclusions:  The plasma pharmacokinetics of SPD754 and 3TC are unaffected by co-administration. However 3TC markedly reduces intracellular SPD754-TP concentrations in vitro and in vivo, resulting in antagonism of their antiviral activity. Since plasma pharmacokinetic studies of NRTI are unable to assess effects on intracellular TP levels, evaluation of TP concentrations is essential when assessing the drug-drug interactions of NRTI.

Keywords: SPD754; 3TC; Combination