Interaction between fosamprenavir, with and without ritonavir, and nevirapine in human immunodeficiency virus-infected subjects

Clinical management of drug interaction with antiretroviral agents

PURPOSE OF REVIEW

Combination antiretroviral therapy has improved the morbidity and mortality of HIV-infected patients worldwide. As patients live longer, management of HIV infection extends to treatment of a wide spectrum of co-morbid conditions. Pharmacokinetic interactions are common among antiretroviral drugs when they are used in combination and along with treatments for other conditions. This review discusses the clinical significance of drug interactions among antiretroviral drugs and other medications, resources to use in assessing drug interaction potential, and some key principles to follow when managing patients prescribed potentially interacting drugs.

RECENT FINDINGS

Targeted pharmacokinetic drug interaction studies and extrapolations on the basis of potential mechanism of interactions provide an initial basis for recommendations regarding use of certain drug combinations. Some unexpected interactions have emerged in the literature through case reports in which untoward effects were observed.

SUMMARY

Management of patients on multiple drug therapy can be a challenge. The key to safe and effective therapy relies on the clinician's vigilance in their ongoing assessment of interaction potential among drugs prescribed to each patient, the significance for such interactions, the need for modification to therapy, and close follow up to assess safety and toxicity.

Factors Influencing Efavirenz and Nevirapine Plasma Concentration: Effect of Ethnicity, Weight and Co-Medication

Background

The aim of this study was to examine factors influencing plasma concentration of efavirenz and nevirapine.

Methods

Data from the Liverpool Therapeutic Drug Monitoring (TDM) registry were linked with the UK Collaborative HIV Cohort (CHIC) Study. For each patient, the first measurement of efavirenz (600 or 800 mg/day) or nevirapine (400 mg/day) plasma concentration was included. Linear regression was used to evaluate the association of dose, gender, age, weight, ethnicity and concomitant antiretroviral drugs or rifampicin with log-transformed drug concentration, adjusted for time since last intake.

Results

Data from 339 patients on efavirenz (34% black, 17% rifampicin) and 179 on nevirapine (27% black, 6% rifampicin) were included. Multivariable models revealed the following predictors for efavirenz concentration: black ethnicity (59% higher; P<0.001), weight (10% lower per additional 10 kg; P=0.002), 800 mg/day (52% higher; P=0.027), rifampicin (35% lower; P=0.039), and zidovudine (25% lower; P=0.010). Notably, without adjustment for other factors, patients on rifampicin had 48% higher efavirenz concentration, as these patients were mostly black and on 800 mg/day. For nevirapine the predictors were black ethnicity (39% higher; P=0.002), rifampicin (40% lower; P=0.002), protease inhibitor (28% higher; P=0.008) and tenofovir (22% higher; P=0.024).

Conclusions

We observed clear associations between ethnicity and concentrations of nevirapine and efavirenz. Our analyses confirm that concomitant rifampicin substantially decreases concentration of both efavirenz and nevirapine; however, for efavirenz this effect was more than counterbalanced by the effect of ethnicity and increased efavirenz dose. There was also an additional impact of weight, which should be considered when determining optimal dosage. Other associations from our analysis (between tenofovir or protease inhibitor and nevirapine, and zidovudine and efavirenz), require confirmation in formal pharmacokinetic studies.

Fosamprenavir calcium plus ritonavir for HIV infection

Fosamprenavir is a protease inhibitor (PI) approved for the treatment of HIV-1 infection. Fosamprenavir is a prodrug of amprenavir developed to reduce the pill burden yet maintain the unique resistance pattern and efficacy associated with amprenavir. In a head-to-head, noninferiority trial in antiretroviral treatment-naive HIV-infected patients, the antiviral efficacy and tolerability of ritonavir-boosted fosamprenavir was not inferior to ritonavir-boosted lopinavir, when the PIs were combined with two other nucleoside reverse transcriptase inhibitors. There are fewer studies published about fosamprenavir use in antiretroviral treatment-experienced HIV-infected patients. The high genetic barrier to the development of resistance to fosamprenavir and the low level of cross-resistance between ritonavir-boosted fosamprenavir and other PI regimens are notable. As with amprenavir, gastrointestinal disturbance and rash are the most frequent short-term treatment-limiting events with fosamprenavir. Treatment with ritonavir-boosted fosamprenavir can produce a durable response. To date, fosamprenavir is one of the recommended preferred PI components for the treatment of antiretroviral-naive HIV-infected patients.