Synergistic inhibition of human immunodeficiency virus isolates (including 3'-azido-3'-deoxythymidine-resistant isolates) by foscarnet in combination with 2',3'-dideoxyinosine or 2',3'-dideoxycytidine

Selective excision of chain-terminating nucleotides by HIV-1 reverse transcriptase with phosphonoformate as substrate

A major mechanism for human immunodeficiency virus 1 (HIV-1) reverse transcriptase (RT) resistance to nucleoside analogs involves the phosphorolytical removal of the chain-terminating nucleotide from the 3'-end of the primer. In this work, we analyzed the effect of phosphonoformate (PFA) and other pyrophosphate (PP(i)) analogs on PP(i)- and ATP-dependent phosphorolysis catalyzed by HIV-1 RT. Our experimental data demonstrated that PFA did not behave as a linear inhibitor but as an alternative substrate, allowing RT to remove AZT from a terminated primer through a PFA-dependent mechanism. Interestingly, in non-terminated primers, PFA was not a substrate for this reaction and competitively inhibited PP(i)- and ATP-dependent phosphorolysis. In fact, binding of PFA to the RT.template/primer complex was hindered by the presence of a chain terminator at the 3'-end of the primer. Other pyrophosphate analogs, such as phosphonoacetate, were substrates for the excision reaction with both terminated and nonterminated primers, whereas pamidronate, a bisphosphonate that prevents bone resorption, was not a substrate for these reactions and competitively inhibited the phosphorolytic activity of RT. As expected from their mechanisms of action, pamidronate (but not PFA) synergistically inhibits HIV-1 RT in combination with AZT-triphosphate in the presence of PP(i) or ATP. These results provide new clues about the mechanism of action of PFA and demonstrate that only certain pyrophosphate analogs can enhance the effect of nucleosidic inhibitors by blocking the excision of chain-terminating nucleotides catalyzed by HIV-1 RT. The relevance of these findings in combined chemotherapy is discussed.

In Vitro Evaluation of the Anti-HIV Activity and Metabolic Interactions of Tenofovir and Emtricitabine

Background

Tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) are nucleoside reverse transcriptase inhibitors (NRTIs) with once-daily dosing approved for use in HIV-1 infection. We evaluated this combination for anti-HIV activity and intracellular metabolic interactions in vitro.

Methods

Intracellular anabolism of combinations of FTC and tenofovir (TFV) were studied in the human T leukaemic cell line CEM and human peripheral blood mononuclear cells (PBMCs). The anti-HIV activity of the combination was studied in the human T leukaemic MT-2 cell line against wild-type or mutant virus (K65R and M184V) and in PBMCs against wild-type virus. Antiviral synergy was quantitated by isobologram and MacSynergy methods.

Results

Both TFV and FTC were efficiently converted to their active metabolites in PBMCs and CEM cells. In CEM cells, there was a statistically significant increase in the levels of TFV diphosphate ( P=0.047) and FTC triphosphate ( P=0.0069) following a 24 h incubation with the combination compared with the levels seen with the individual drugs. In PBMCs, similar levels of active metabolites were observed for each drug when they were incubated individually or in combination. The combination of TFV and FTC displayed additive to synergistic activity against HIV replication in PBMCs and resulted in strongly synergistic anti-HIV activity in MT-2 cells against both wild-type and mutant virus.

Conclusions

The combination of TFV and FTC showed additive to synergistic anti-HIV activity in vitro, which correlated with the levels of intracellular phosphorylation observed. These results support the use of these drugs as a dual NRTI backbone in combination therapy for the treatment of HIV-1.

Hydroxyurea enhances the activities of didanosine, 9-[2-(phosphonylmethoxy)ethyl]adenine, and 9-[2-(phosphonylmethoxy)propyl]adenine against drug-susceptible and drug-resistant human immunodeficiency virus isolates

We assessed the effects of hydroxyurea (HU) at a concentration of 50 microM on the in vitro activities of 2',3'-dideoxyinosine (ddI), 9-[2-(phosphonylmethoxy)ethyl]adenine (PMEA), and 9-[2-(phosphonylmethoxy)propyl]adenine (PMPA) against a wild-type human immunodeficiency virus (HIV) type 1 (HIV-1) laboratory isolate and a panel of five well-characterized drug-resistant HIV isolates. Fifty micromolar HU significantly increased the activities of ddI, PMEA, and PMPA against both the wild-type and the drug-resistant HIV-1 isolates. In fixed combinations, both ddI and PMEA were synergistic with HU against wild-type and drug-resistant viruses.

Highly drug-resistant HIV-1 clinical isolates are cross-resistant to many antiretroviral compounds in current clinical development

OBJECTIVES

To assess the in-vitro drug susceptibility of a panel of five well-characterized drug-resistant HIV variants to recently developed anti-HIV compounds including seven reverse transcriptase (RT) inhibitors and seven protease inhibitors.

METHODS

Drug-resistant viral strains were selected on the basis of the prevalence of these mutants in patient samples from local area HIV clinics. The isolates included one multinucleoside-resistant virus containing the Q151M mutation, and four clinical isolates containing multiple RT and protease resistance mutations. The activity of the experimental compounds against these isolates was determined using drug susceptibility assays and measuring the viral antigen p24 end-point.

RESULTS

These clinically relevant highly drug-resistant viruses were resistant to many of the new compounds in clinical development. In most cases the resistance mutations of the clinical isolate were different from those selected in vitro for the particular experimental compound.

CONCLUSIONS

It is critical to expand the preclinical development of new drugs to include the assessment of their activity against currently circulating highly drug-resistant clinical strains, in order to develop appropriate salvage therapies for patients harboring resistant strains.

Pronounced anti-HIV-1 activity of foscarnet in patients without cytomegalovirus infection

Combined therapy using reverse transcriptase (RT) and protease inhibitors is the current established treatment for HIV-1 infection. Foscarnet is an RT inhibitor that is a product analogue, in contrast to the widely used nucleoside analogues. In this study, the anti-HIV-1 effect of foscarnet, 50 mg three times per day administered intravenously for 4 weeks, was evaluated in 10 patients with minor or no symptoms. Serious adverse events developed in 2 patients, although most patients experienced some side effects. The levels of HIV-1 RNA decreased from a median value of 4.7 to 2.6 10log copies/ml. The effect was sustained through 4 weeks. One week after cessation of treatment, HIV-1 RNA levels increased to baseline. In contrast, no increase in the number of CD4+ cells was observed. The anti-HIV-1 effect was considered to be a direct effect on HIV-1 replication because no patient had concomitant cytomegalovirus (CMV) infection.

Drug susceptibility of subtypes A,B,C,D, and E human immunodeficiency virus type 1 primary isolates

We determined the susceptibility to antiviral drugs of clinical isolates of human immunodeficiency virus type 1 (HIV-1) subtypes A, B, C, D, and E. Isolates from treated and untreated patients were tested for sensitivity to zidovudine (ZDV), lamivudine (3TC), didanosine (ddI), nevirapine (NVP), foscarnet (PFA), and ritonavir (RNV). The susceptibility to these different drugs was broadly similar between the different subtypes of HIV-1. Isolates of subtype D showed a tendency toward slightly lower susceptibility to all the antiviral drugs, which could be related to the rapid growth characteristics of these isolates.

Zalcitabine. An update of its pharmacodynamic and pharmacokinetic properties and clinical efficacy in the management of HIV infection

Zalcitabine is a dideoxynucleoside antiretroviral agent that is phosphorylated to the active metabolite 2',3'-dideoxycytidine 5'-triphosphate (ddCTP) within both uninfected and HIV-infected cells. At therapeutic concentrations, ddCTP inhibits HIV replication by inhibiting the enzyme reverse transcriptase and terminating elongation of the proviral DNA chain. The results of 3 large pivotal trials comparing zidovudine monotherapy with combination therapy have now clearly established that zalcitabine plus zidovudine combination with an improvement in viral load and CD4+ cell count compared with zidovudine monotherapy. More recently, clinical end-point and surrogate marker data have established the efficacy of zalcitabine in combination with the protease inhibitor saquinavir in zidovudine-experienced patients. Other studies have demonstrated the utility of zalcitabine in combination with ritonavir and the nucleoside analogue lamivudine. Importantly, early use of zalcitabine in the treatment sequence does not appear to limit the therapeutic efficacy of subsequent therapy with other nucleoside analogues such as lamivudine. Peripheral neuropathy is the most frequent dose-limiting adverse effect associated with zalcitabine therapy and is generally reversible on discontinuation of treatment. Stomatitis and mouth ulcers may occur frequently with zalcitabine therapy but tend to resolve with continuing treatment. Haematological toxicity, which is a common adverse effect associated with zidovudine, is reported infrequently with zalcitabine. Overall, combination therapy with zalcitabine plus zidovudine or saquinavir has been shown to have a tolerability profile comparable to that of either agent alone, although treatment with zidovudine plus zalcitabine was associated with a significant increase in the incidence of haematological toxicity compared with zidovudine monotherapy in one study. Therefore, current data suggest that zalcitabine is a useful antiretroviral agent for inclusion as a component of initial double combination therapy with zidovudine or as part of triple combination therapy including zidovudine plus a protease inhibitor in the management of patients with HIV infection.

Antiviral efficacy and toxicity of ribavirin and foscarnet each given alone or in combination in the murine AIDS model

The antiviral efficacy and toxicity of ribavirin, foscarnet (PFA), and combinations of both drugs at two different doses have been evaluated in the murine AIDS (MAIDS) model. Our results clearly demonstrated that infected mice treated with ribavirin at 100 mg/ kg/day were protected against splenomegaly, lymphadenopathy, and hypergammaglobulinemia whereas PFA alone at 180 or 360 mg/kg/day did not afford any protection. Treatment with drug combinations showed protective effects similar to those observed with ribavirin alone. Hyperplasia and deorganization of the lymphoid architecture were noted in spleen and lymph nodes of infected mice compared to those of the uninfected group. However, treatment with ribavirin restored the lymphoid tissue architecture and reduced the emergence of germinal centers. Electron microscopic examination of renal cortex of animals treated with PFA at 360 mg/kg/day revealed clear mitochondrial necrosis (bursting of mitochondria) of the distal tubules and vacuolization of the proximal tubules which was more striking with combination therapy. Regarding hematotoxicity, PFA did not cause significant hematotoxicity at both doses, whereas ribavirin was hematotoxic at both doses (50 and 100 mg/kg/day), this toxicity being more evident at the higher dose. In conclusion, treatment with ribavirin showed clear efficacy against MAIDS whereas PFA had no efficacy. Furthermore, ribavirin treatment caused hematoxicity and PFA treatment resulted in nephrotoxicity.