Despite substantial progress in the development of antiretroviral regimens that durably suppress Human Immunodeficiency Virus (HIV) infection, new agents that maintain high efficacy while further optimizing the safety of lifelong, chronic therapy are needed. Tenofovir alafenamide (TAF; formerly known as GS-7340) is a novel prodrug of the antiviral acyclic nucleoside phosphonate tenofovir (TFV) with improved properties relative to tenofovir disoproxil fumarate (TDF). Although potent and generally well tolerated, TDF therapy has been associated with changes in markers of renal function, decreases in bone mineral density and a rare occurrence of serious renal adverse events, including Fanconi's Syndrome. The renal and bone toxicity observed with TDF is associated with high circulating plasma levels of TFV. TAF was discovered to be a more efficient prodrug able to further refine HIV therapy and better address life-long therapy in an older and increasingly comorbid HIV infected population. By enhancing stability in biological matrices while being rapidly activated in cells, TAF produces higher levels of intracellular TFV diphosphate, the pharmacologically active metabolite, in HIV-target cells at substantially reduced oral doses of TFV equivalents. All TFV released in the body is eventually eliminated renally; therefore, lowering the TFV equivalents administered reduces off-target kidney exposure. Effective therapy is thus achieved at approximately 90% lower systemic exposure to TFV, translating to statistically and clinically significant improvement in safety parameters associated with bone mineral density and markers of renal function.

Drug-associated dysfunction of mitochondria is believed to play a role in the etiology of the various adverse symptoms that occur in human immunodeficiency virus (HIV)-infected patients treated with the nucleoside reverse transcriptase inhibitors (NRTIs). Tenofovir, a nucleotide analog recently approved for use in the treatment of HIV infection, was evaluated in vitro for its potential to cause mitochondrial toxicity and was compared to currently used NRTIs. Treatment with tenofovir (3 to 300 M) for up to 3 weeks produced no significant changes in mitochondrial DNA (mtDNA) levels in human hepatoblastoma (HepG2) cells, skeletal muscle cells (SkMCs), or renal proximal tubule epithelial cells. The potencies of inhibition of mtDNA synthesis by the NRTIs tested were zalcitabine (ddC) > didanosine (ddI) > stavudine > zidovudine (ZDV) > lamivudine ‫؍‬ abacavir ‫؍‬ tenofovir, with comparable relative effects in the three cell types. Unlike ddC and ddI, tenofovir did not affect cellular expression of COX II and COX IV, two components of the mitochondrial cytochrome c oxidase complex. Lactate production was elevated by less than 20% in HepG2 cells or SkMCs following treatment with 300 M tenofovir. In contrast, lactate synthesis increased by >200% in the presence of 300 M ZDV. Thus, treatment of various human cell types with tenofovir at concentrations that greatly exceed those required for it both to have in vitro anti-HIV type 1 activity in peripheral blood mononuclear cells (50% effective concentration, 0.2 M) and to achieve therapeutically relevant levels in plasma (maximum concentrations in plasma, 0.8 to 1.3 M) is not associated with mitochondrial toxicity.

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-didehydrothymidine (D4T) is a thymidine nucleoside analog which has potent antihuman immunodeficiency virus activity in vitro. We have studied its metabolism in cells to assist in determining its mechanism of action. D4T is metabolized in cells to the mono-, di-, and triphosphate nucleotides. Our data suggest that the initial conversion to the monophosphate is catalyzed by thymidine kinase. This enzyme has an affinity for D4T 600-fold lower than for thymidine and catalyzes the rate-limiting step in production of the triphosphate. Nevertheless, intracellular concentrations of the triphosphate approximately equal to the reported K, for human immunodeficiency virus reverse transcriptase are attained with extracellular concentrations of free drug as low as 0.05 ,uM. The pattern of phosphorylation is different from that of 3'-azido-3'-deoxythymidine (AZT), which has an affinity for thymidine kinase equivalent to that of thymidine and is easily phosphorylated. The rate-limiting step in formation of AZT triphosphate is conversion of monoto diphosphate, and thus the monophosphate accumulates. On removal of D4T or AZT from the media, both triphosphates have an intracellular half-life of about 200 min, and this rate ultimately controls the rate of elimination of the drugs from cells. The differences in metabolism of D4T and AZT observed in vitro may be responsible for the differences in toxicity seen in vitro and in vivo and support the exploration of the clinical utility of D4T as an anti-human immunodeficiency virus agent.

The pharmacokinetics of cidofovir (HPMPC; (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine) were examined at five dose levels in three phase I/II studies in a total of 42 human immunodeficiency virus-infected patients (with or without asymptomatic cytomegalovirus infection). Levels of cidofovir in serum following intravenous infusion were dose proportional over the dose range of 1.0 to 10.0 mg/kg of body weight and declined biexponentially with an overall mean ؎ standard deviation terminal half-life of 2.6 ؎ 1.2 h (n ‫؍‬ 25). Approximately 90% of the intravenous dose was recovered unchanged in the urine in 24 h. The overall mean ؎ standard deviation total clearance of the drug from serum (148 ؎ 25 ml/h/kg; n ‫؍‬ 25) approximated renal clearance (129 ؎ 42 ml/h/kg; n ‫؍‬ 25), which was significantly higher (P < 0.001) than the baseline creatinine clearance in the same patients (83 ؎ 21 ml/h/kg; n ‫؍‬ 12). These data indicate that active tubular secretion played a significant role in the clearance of cidofovir. The steady-state volume of distribution of cidofovir was approximately 500 ml/kg, suggesting that the drug was distributed in total body water. Repeated dosing with cidofovir at 3.0 and 10.0 mg/kg/week did not alter the pharmacokinetics of the drug. Concomitant administration of intravenous cidofovir and oral probenecid to hydrated patients had no significant effect on the pharmacokinetics of cidofovir at a 3.0-mg/kg dose. At higher cidofovir doses, probenecid appeared to block tubular secretion of cidofovir and reduce its renal clearance to a level approaching glomerular filtration.Cidofovir (HPMPC; (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine) is an acyclic nucleotide analog with potent activity against a broad spectrum of herpesviruses, including cytomegalovirus (CMV). The in vivo and in vitro antiviral activities of cidofovir have been reviewed (1). Unlike ganciclovir and other nucleoside analogs currently used for clinical therapy of human herpesvirus infections, cidofovir does not depend on phosphorylation by viral nucleoside kinases to exert its antiviral effect (2). Instead, the drug is phosphorylated to its active form by cellular enzymes. In vitro studies have suggested that the resulting active metabolites are cleared slowly from the intracellular space (2).Preclinical pharmacokinetic studies with radiolabelled cidofovir in rats and mice (10) and in rabbits and monkeys (3) have demonstrated that the majority of the drug is distributed to the kidneys and is excreted in the urine within 24 h of intravenous administration. In monkeys, a fraction of the radioactive dose (approximately 10%) was excreted in a slow elimination phase, with a terminal elimination half-life of 24 to 35 h. This slower excretion phase may reflect the long intracellular half-life of the phosphorylated metabolites of cidofovir (2). In both monkeys and rabbits, approximately 98% of the excreted radioactive dose was present in the urine as unchanged drug. The oral bioavailability of the drug was estimated to be 3% i...