Dideoxynucleosides, which are potent inhibitors of HIV reverse transcriptase and other viral DNA polymerases, are a common component of highly active anti-retroviral therapy (HAART) (ref. 1). Six reverse transcriptase inhibitors have been approved for human use: azidothymidine; 2'3'-dideoxycytidine; 2'3'-dideoxyinosine; 2', 3'-didehydro-3'deoxythymidine; 2',3'-dideoxy-3'-thiacytidine; and 4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene-1-++ +metha nol. Although drug-resistant HIV strains resulting from genetic mutation have emerged in patients treated with HAART (ref. 1), some patients show signs of drug resistance in the absence of drug-resistant viruses. In our study of alternative or additional mechanisms of resistance operating during antiviral therapy, overexpression and amplification of the MRP4 gene correlated with ATP-dependent efflux of PMEA (9-(2-phosphonylmethoxyethyl)adenine) and azidothymidine monophosphate from cells and, thus, with resistance to these drugs. Overexpression of MRP4 mRNA and MRP4 protein severely impaired the antiviral efficacy of PMEA, azidothymidine and other nucleoside analogs. Increased resistance to PMEA and amplification of the MRP4 gene correlated with enhanced drug efflux; transfer of chromosome 13 containing the amplified MRP4 gene conferred resistance to PMEA. MRP4 is the first transporter, to our knowledge, directly linked to the efflux of nucleoside monophosphate analogs from mammalian cells.

Bis(isopropyloxymethylcarbonyl) 9-R-(2-phosphonomethoxypropyl)adenine [bis(POC)PMPA] has been identified as a novel prodrug of PMPA. The anti-human immunodeficiency virus activity of bis(POC)PMPA was >100-fold greater than that of PMPA in both an established T-cell line and primary peripheral blood lymphocytes. This improved efficacy was shown to be due to a rapid intracellular uptake of the prodrug resulting in an increased intracellular accumulation of PMPA diphosphate (PMPApp), the pharmacologically active metabolite. PMPApp levels in bis(POC)PMPA-treated cells exceeded by >1,000-fold the levels seen in cells treated with unmodified PMPA in both resting and activated peripheral blood lymphocytes. Significant differences in the intracellular catabolism of PMPA metabolites were noted between the resting and activated lymphocytes. The half-life for the disappearance of PMPApp, derived from either bis(POC)PMPA or PMPA, was 12 to 15 h in the activated lymphocytes and 33 to 50 h in the resting lymphocytes. This long persistence of PMPApp, particularly in resting lymphocytes, may be unique to the nucleoside phosphonate analogs and indicates that effective levels of the active metabolite can be achieved and maintained with relatively infrequent administration of the parent drug.

Purpose: GS-9219, a novel prodrug of the nucleotide analogue 9-(2-phosphonylmethoxyethyl)guanine (PMEG), was designed as a cytotoxic agent that preferentially targets lymphoid cells. Our objective was to characterize the antiproliferative activity, pharmacokinetics, pharmacodynamics, and safety of GS-9219. Experimental Design: GS-9219 was selected through screening in proliferation assays and through pharmacokinetic screening. The activation pathway of GS-9219 was characterized in lymphocytes, and its cytotoxic activity was evaluated against a panel of hematopoietic and nonhematopoietic cell types. To test whether the prodrug moieties present in GS-9219 confer an advantage over PMEG in vivo, the pharmacokinetics, pharmacodynamics (lymph node germinal center depletion), and toxicity of equimolar doses of GS-9219 and PMEG were evaluated after i.v. administration to normal beagle dogs. Finally, proof of concept of the antitumor efficacy of GS-9219 was evaluated in five pet dogs with spontaneous, advanced-stage non^Hodgkin's lymphoma (NHL) following a single i.v. administration of GS-9219 as monotherapy. Results: In lymphocytes, GS-9219 is converted to its active metabolite, PMEG diphosphate, via enzymatic hydrolysis, deamination, and phosphorylation. GS-9219 has substantial antiproliferative activity against activated lymphocytes and hematopoietic tumor cell lines. In contrast, resting lymphocytes and solid tumor lines were less sensitive to GS-9219. GS-9219, but not PMEG, depleted the germinal centers in lymphoid tissues of normal beagle dogs at doses that were tolerated. In addition, GS-9219 displayed significant in vivo efficacy in five dogs with spontaneous NHL after a single administration, with either no or low-grade adverse events. Conclusion: GS-9219 may have utility for the treatment of NHL.

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)