Background:  Combination treatment with pegylated interferon (peg-IFN) and ribavirin (RBV) delivers sustained virological response to HCV infection but is associated with significant toxicity. Anemia and neutropenia limit therapy or force reductions in RBV dose potentially compromising efficacy. Intracellular phosphorylation of RBV and the nucleoside analogues defines their efficacy and toxicity. ZDV, D4T, and RBV are competing substrates for a thymidine phosphorylation pathway raising concerns regarding adverse interactions and altered efficacy.

Methods:  A pharmacokinetic investigation of the effect of combination peg-IFN/ RBV therapy (1000/1200 mg) on intracellular ZDV (n = 6), d4T (n = 5), and 3TC (n = 9) triphosphate levels in HIV suppressed, HCV-co-infected patients. Blood was reserved at 5 time points 0 to 8 hours, at baseline and on days 3 and 14. Intracellular triphosphates and endogenous nucleotide triphosphate levels in PBMC were determined using an enzymatic template primer extension assay.

Results:  In nongenotype 1 patients there was an 86% response (12 week). Hemoglobin decreased (9.6±1.4 vs13.9±0.7 g/L; p <0.001) and lactate dehydrogenase increased (860±250 vs 420±100U; p <0.001) in patients receiving ZDV compared with other nucleoside analogues. This was managed without RBV dose reduction. There was a 44% reduction in the ZDVTP/dTTP ratio (3.5±0.8 vs 2.18±0.2; p <0.02) from day 3 onwards. Interestingly this represented a 1.8-fold increase in absolute ZDVTP but a 2.95-fold increase in dTTP (for ZDV) levels. Both the d4TTP/dTTP ratio and dTTP (for d4T) concentrations were unchanged over the study period. The 3TCTP/dCTP ratio was increased 1.4-fold on day 3 reflecting an increase in levels of 3TCTP compared with baseline, this normalised by day 14.

Conclusions:  Consistent with in vitro findings RBV decreased ZDVTP/dTTP phosphorylation. This was not associated with loss of ZDV efficacy. Reports suggest that RBV increases the endogenous dTTP pool providing negative feed back on thymidine kinase, reducing ZDV phosphorylation. However in this study absolute ZDVTP levels increased which may contribute to the increased toxicity observed in this group. In our study dTTP (for d4T) was unaltered failing to support a universal rise in dTTP levels as a result of RBV therapy. However, up-regulation of thymidine kinase by the combined presence of both RBV and ZDV but not d4T may explain both the increase in ZDVTP, increased dTTP (for ZDV) levels and lack of effect on dTTP for d4T.

Keywords: Intracellular; NRTIs; Ribavirin