AIDSWEEKLY Plus; Monday, August 23, 2004
Staff Medical Writers

According to recent research published in the Journal of Clinical Pharmacology, "Enfuvirtide (Fuzeon) is an HIV fusion inhibitor, the first drug in a new class of antiretrovirals.

"The HIV protease inhibitors ritonavir and saquinavir both inhibit cytochrome P450 (CYP450) isoenzymes, and low-dose ritonavir is often used to boost pharmacokinetic exposure to full-dose protease inhibitors. These two studies were designed to assess whether ritonavir and ritonavir-boosted saquinavir influence the steady-state pharmacokinetics of enfuvirtide."

"Both studies were single-center, open-label, one-sequence crossover clinical pharmacology studies in 12 HIV-1-infected patients each. Patients received enfuvirtide (90 mg twice daily [bid], subcutaneous injection) for 7 days and either ritonavir (200 mg bid, ritonavir study orally) or saquinavir/ritonavir (1000/100 mg bid, saquinavir/ritonavir study, orally) for 4 days on days 4 to 7.

"Serial blood samples were collected tip to 24 hours after the morning dose of enfuvirtide on days 3 and 7. Plasma concentrations for enfuvirtide, enfuvirtide metabolite, saquinavir, and ritonavir were measured using validated liquid chromatogrophy tandem mass spectrometry methods," wrote K. Ruxrungtham and coworkers.

"Efficacy and safety were also monitored. Bioequivalence criteria require the 90% confidence interval (CI) for the least squares means (LSAM) of C-max and AUC(12hours) to be between 80% and 125%," said investigators.

"In the present studies," the authors reported, "analysis of variance showed that when coadministered with ritonavir, the ratio of LSM for enfuvirtide was 124% for C-max (90% confidence interval [CI]: 109-141%), 122% for AUC(12 hours) (90% CI: 108-137%), and 114% for C-trough (90% CI: 102-128%).

"Although the bio-equivalence criteria were not met," the authors continued, "the increase in enfuvirtide exposure was small (< 25%) and not clinically relevant.

"When administered with ritonavir-boosted saquinavir, the ratio of LSM for enfuvirtide was 107% for C-max (90% CI: 94.3-121%) and 114% for AUC(12 hours) (90% CI: 105-124%), which therefore met bioequivalence criteria, and 126% for C-trough (90% CI: 117-135%)."

Ruxrungtham concluded, "The pharmacokinetics of enfuvirtide are affected to a small extent when coadministered with ritonavir at a dose of 200 mg bid but not when coadministered with a saquinavir-ritonavir combination (1000/100 mg bid).

"However, previous clinical studies have shown that such increases in enfuvirtide exposure are not clinically relevant. Thus, no dosage adjustments are warranted when enfuvirtide is coadministered with low-dose ritonavir or saquinavir boosted with a low dose of ritonavir."

Ruxrungtham and colleagues published their study in Journal of Clinical Pharmacology (Lack of interact-ion between enfuvirtide and ritonavir or ritonavir-boosted saquinavir in HIV-1-infected patients. J Clin Pharmacol. 2004 Jul;44(7):793-803.

For additional information, contact K. Ruxrungtham, Chulalongkom Hospital, Department of Medicine, Rama 4 Rd., Bangkok 10330, Thailand.

The publisher's contact information for the Journal of Clinical Pharmacology is: Sage Publications Inc., 2455 Teller Rd., Thousand Oaks, CA 91320 USA.

The information in this article comes under the major subject areas of HIV/AIDS, Pharmacokinetics, and Antiretroviral Therapy.

This article was prepared by AIDS Weekly editors from staff and other reports.

Reference

Ruxrungtham K, Boyd M, Bellibas SE, et al., "Lack of interaction between enfuvirtide and ritonavir or ritonavir-boosted saquinavir in HIV-1-infected patients", J Clin Pharmacol. 2004 Jul;44(7):793-803.

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