Alternative strategies for anti-HIV treatment

Determination of Amprenavir Total and Unbound Concentrations in Plasma by High-Performance Liquid Chromatography and Ultrafiltration

Amprenavir is an HIV-1 protease inhibitor with high protein binding (90%) in human plasma. This study was designed to develop an assay to measure the concentration of unbound amprenavir, to study variation with time in patients, and to investigate whether ritonavir and lopinavir, other protease inhibitors that could be combined, interact with amprenavir protein binding in vitro. A reverse-phase high-performance liquid chromatography assay to UV detection was developed and validated to measure total amprenavir in plasma, and this method was adapted to quantitate low concentrations of unbound amprenavir in ultrafiltrate aqueous fluid. Equilibrium dialysis and ultrafiltration were used and compared with separate unbound fraction. The latter method was easier to use and was, therefore, subsequently adopted. In 10 patients who received amprenavir 600 mg bid combined with ritonavir, mean amprenavir free-fraction in plasma was 8.6% (range, 4.4-20%). When added to pooled human plasmas at concentrations close to those found in treated patients, the unbound amprenavir fraction was increased in the presence of lopinavir, but remained unaffected by ritonavir. It remains to be seen whether measurement of unbound concentrations, rather than total concentrations, could improve therapeutic drug monitoring.

Once-Daily Administration of Antiretrovirals

Adherence to therapy is of critical importance for the long-term success of the treatment of HIV infection. Once-daily administration of antiretroviral agents is appealing, as it may increase patient's adherence. The pharmaceutical industry is making huge efforts to develop drugs or combinations of drugs with pharmacokinetic properties allowing once-daily administration. The major pharmacokinetic requirement for once-daily administration is that the intracellular concentration of the antiretroviral or its active metabolite remains above the minimal concentration that can inhibit viral replication during the entire 24-hour period. Soon, all three major classes of antiretroviral agents will be available as once-daily formulations. However, only a few clinical trials have yet assessed the efficacy and safety of truly once-daily antiretroviral combinations. Preliminary results from these small pilot studies suggest that once-daily administration of antiretrovirals is a feasible approach. Large comparative trials are needed before the real benefits of such a strategy can be fully assessed.