CD4 count improvement following tenofovir to abacavir switch in a patient with persistent lymphopenia despite an undetectable viral load.

Many different antiretroviral regimens can be used as initial therapy for infection with HIV. While all recommended regimens have been shown to be highly effective in suppressing HIV replication to undetectable levels, some differences may exist with regard to the level of immune reconstitution (eg, CD4+ cell population) that occurs. We report a case of a patient with profound and prolonged lymphopenia, despite undetectable HIV RNA levels, that reversed following a switch from a fixed-dose combination of tenofovir/emtricitabine to abacavir/lamivudine in the patient's regimen.

Plasma amprenavir pharmacokinetics and tolerability following administration of 1,400 milligrams of fosamprenavir once daily in combination with either 100 or 200 milligrams of ritonavir in healthy volunteers.

Once-daily (QD) fosamprenavir (FPV) at 1,400 mg boosted with low-dose ritonavir (RTV) at 200 mg is effective when it is used in combination regimens for the initial treatment of human immunodeficiency virus infection. Whether a lower RTV boosting dose (i.e., 100 mg QD) could ensure sufficient amprenavir (APV) concentrations with improved safety/tolerability is unknown. This randomized, two 14-day-period, crossover pharmacokinetic study compared the steady-state plasma APV concentrations, safety, and tolerability of FPV at 1,400 mg QD boosted with either 100 mg or 200 mg of RTV QD in 36 healthy volunteers. Geometric least-square (GLS) mean ratios and the associated 90% confidence intervals (CIs) were estimated for plasma APV maximum plasma concentrations (Cmax), the area under the plasma concentration-time curve over the dosing period (AUC0-tau), and trough concentrations (Ctau) during each dosing period. Equivalence between regimens (90% CIs of GLS mean ratios, 0.80 to 1.25) was observed for the plasma APV AUC0-tau (GLS mean ratio, 0.90 [90% CI, 0.84 to 0.96]) and Cmax (0.97 [90% CI, 0.91 to 1.04]). The APV Ctau was 38% lower with RTV at 100 mg QD than with RTV at 200 mg QD (GLS mean ratio, 0.62 [90% CI, 0.55 to 0.69]) but remained sixfold higher than the protein-corrected 50% inhibitory concentration for wild-type virus, with the lowest APV Ctau observed during the 100-mg QD period being nearly threefold higher. The GLS mean APV Ctau was 2.5 times higher than the historical Ctau for unboosted FPV at 1,400 mg twice daily. Fewer clinical adverse drug events and smaller increases in triglyceride levels were observed with the RTV 100-mg QD regimen. Clinical trials evaluating the efficacy and safety of FPV at 1,400 mg QD boosted by RTV at 100 mg QD are now under way with antiretroviral therapy-naïve patients.

Genetic barriers to resistance and impact on clinical response.

The development of drug resistance and cross-resistance continues to pose a challenge to successful long-term antiretroviral therapy despite the availability of new antiretroviral agents. The genetic barrier to resistance of a regimen does not directly correlate with its effectiveness. For some regimens with a low genetic barrier to resistance, however, the emergence of only 1 or 2 key resistance mutations may confer drug resistance not only to that regimen but also to other agents, thereby limiting subsequent treatment options. In addition to the genetic barrier to resistance, factors such as efficacy, safety, tolerability, convenience, and adherence must be considered when choosing a regimen.

Use of acid-reducing agents in protease inhibitor-based HAART and the potential for negative treatment outcomes.

A number of protease inhibitors (PIs) are dependent on an acidic gastric pH for optimal drug dissolution and absorption. As a result, the potential for negative drug interactions with acid-reducing agents exists and could lead to subtnerapeutic drug concentrations, viral breakthrough, and development of drug resistance. Pharmacokinetic evaluations of a number of PIs given with acid-reducing agents have been performed and show varying degrees of effect. Given the possibility of decreases in PI exposures, clinicians should be aware of the potential for a negative interaction when selecting PI-based HAART for patients taking acid-reducing agents.

Treatment strategies for highly treatment-experienced HIV-infected patients.

The management of highly treatment-experienced HIV-infected patients is often complicated by baseline antiretroviral drug resistance, patient intolerabilities, drug-drug interactions and quality-of-life issues; which are all factors that can limit the ability to construct a potent regimen. The mainstay of treatment has been to use new agents with activity against resistant virus. New agents, such as enfuvirtide and tipranavir/ritonavir, have shown promising results in highly active antiretroviral treatment regimens among patients with extensive treatment histories and resistance profiles, especially when used in combination with other active agents. Other strategies include mega-highly active antiretroviral treatment, double-boosted protease inhibitors, structured treatment interruptions and maintaining a replicative compromised virus. The future development of newer agents with activity against resistant virus is desperately needed, and many new compounds and classes of antiretrovirals are currently being investigated.

Genetic barriers to resistance and impact on clinical response.

The development of drug resistance and cross-resistance continues to pose a challenge to successful long-term antiretroviral therapy despite the availability of new antiretroviral agents. The genetic barrier to resistance of a regimen does not directly correlate with its effectiveness. For some regimens with a low genetic barrier to resistance, however, the emergence of only 1 or 2 key resistance mutations may confer drug resistance not only to that regimen but also to other agents, thereby limiting subsequent treatment options. In addition to the genetic barrier to resistance, factors such as efficacy, safety, tolerability, convenience, and adherence must be considered when choosing a regimen.

K65R-associated virologic failure in HIV-infected patients receiving tenofovir-containing triple nucleoside/nucleotide reverse transcriptase inhibitor regimens.

High rates of early virologic failure associated with the emergence of the K65R mutation in HIV-1 reverse transcriptase (RT) have been reported among HIV-infected patients who received novel, tenofovir-containing, triple-nucleoside/nucleotide reverse transcriptase inhibitor (NRTI/NtRTI) regimens as their initial therapy. This review surveys the findings of prospective and retrospective studies in this regard, examines the significance of the K65R mutation and other factors associated with reports of early virologic failure among patients receiving tenofovir-containing NRTI/NtRTI regimens, and discusses clinical approaches to preventing and managing HIV drug resistance and treatment failure associated with the K65R mutation.

Enfuvirtide.

Enfuvirtide (Fuzeon, T-20), jointly developed by Trimeris Inc. and Roche Pharmaceuticals, is the first of a new class of antiretroviral agents called fusion inhibitors that block HIV-1 entry into the host cell by binding to the gp41 subunit of the HIV-1 envelope glycoprotein. In vitro and in vivo studies have demonstrated potent antiretroviral activity among HIV-positive patients, including those with multi-drug resistant virus. The pharmacokinetic profile of subcutaneously administered enfuvirtide allows for twice-daily administration, although the possibility of once-daily dosing has not been excluded. Phase II and III clinical studies conducted to date have confirmed that enfuvirtide is an effective and safe drug for treating both adult and pediatric HIV-1-positive patients, with only mild or moderate adverse effects being reported.