Role of Purine Nucleoside Phosphorylase in Interactions between 2′,3′-Dideoxyinosine and Allopurinol, Ganciclovir, or Tenofovir

Abstract

The level of systemic exposure to 2,3-dideoxyinosine (ddI) is increased 40 to 300% when it is coadministered with allopurinol (Allo), ganciclovir (GCV), or tenofovir. However, the mechanism for these drug interactions remains undefined. A metabolic route for ddI clearance is its breakdown by purine nucleoside phosphorylase (PNP). Consistent with previous reports, enzymatic inhibition assays showed that acyclic nucleotide analogs can inhibit the phosphorolysis of inosine. It was further established that the mono-and diphosphate forms of tenofovir were inhibitors of PNP-dependent degradation of ddI (K i s, 38 nM and 1.3 M, respectively). Allo and its metabolites were found to be relatively weak inhibitors of PNP (K i s, >100 M). Coadministration of tenofovir, GCV, or Allo decreased the amounts of intracellular ddI breakdown products in CEM cells, while they increased the ddI concentrations (twofold increase with each drug at approximately 20 M). While inhibition of the physiological function of PNP is unlikely due to the ubiquitous presence of high levels of enzymatic activity, phosphorylated metabolites of GCV and tenofovir may cause the increased level of exposure to ddI by direct inhibition of its phosphorolysis by PNP. The discrepancy between the cellular activity of Allo and the weak enzyme inhibition by Allo and its metabolites may be explained by an indirect mechanism of PNP inhibition. This mechanism may be facilitated by the unfavorable equilibrium of PNP and the buildup of one of its products (hypoxanthine) through the inhibition of xanthine oxidase by Allo. These findings support the inhibition of PNP-dependent ddI degradation as the molecular mechanism of these drug interactions.Current treatment regimens for human immunodeficiency virus (HIV) infection call for the use of three or more antiretrovirals of different classes. Other agents are also required for the treatment of opportunistic infections that occur as a result of immunosuppression. The use of multiple treatments increases the potential for drug-drug interactions and, as a result, treatment complications (9). One such interaction is an increase in the level of systemic exposure to the anti-HIV drug 2Ј,3Ј-dideoxyinosine (ddI; Videx; Bristol-Myers Squibb) when it is coadministered with allopurinol (Allo)

Intracellular Metabolism of the Nucleotide Prodrug GS-9131, a Potent Anti-Human Immunodeficiency Virus Agent

“…Cells were preincubated for 24 h with 0.3 M GS-9131, washed three times in fresh culture medium, and incubated in the absence of compound for 2, 4, 24, and 48 h. In all experiments, an aliquot of cells (2 ϫ 10 6 to 3 ϫ 10 6 cells) was collected at each time point, counted, pelleted by centrifugation, resuspended in 0.5 ml of the original treatment medium, and layered onto 0.5 ml of Nyosil M25 oil. Oil separation of cells from extracellular medium was done essentially as described previously (24). The samples were spun in a microcentrifuge for 20 s at the maximum speed (approximately 800 ϫ g).…”

“…A colorimetric 96-well assay was performed to assess the inhibition of human purine nucleoside phosphorylase (PNP)-mediated phosphorolysis of ddI, essentially as described previously (9,24). Briefly, inhibition constants (K i ) were generated by obtaining binding curves for ddI (measuring the rate of phosphorolysis between 100 M and 4 mM ddI) in the presence of different concentrations of GS-9148 or its phosphorylated metabolites or the positive control TFV-MP.…”

Role of the Pharmacist in Caring for Patients with HIV/AIDS: Clinical Practice Guidelines

“…An example of an undesirable HIV-related pharmacodynamic drug-drug interaction occurs with the combination of didanosine and tenofovir. Despite virologic suppression, paradoxical decreases in CD4 cell counts 32,33 and increased toxic effects of didanosine have been observed with this combination, 34 and coadministration of these 2 drugs should therefore be avoided.…”

Measurement of Intracellular Didanosine and Tenofovir Phosphorylated Metabolites and Possible Interaction of the Two Drugs in Human Immunodeficiency Virus-Infected Patients

“…LbPeB9026, 2002). The mechanism underlying this interaction is not known, but recently it was hypothesized that this could result from inhibition of the cellular enzyme purine nucleotide phosphorylase by TDF mono-and diphosphate (18). Since purine nucleotide phosphorylase plays a critical role in the catabolism of purine nucleoside, its inhibition may result in an increase in plasma ddI concentration.…”