A new treatment for hepatitis C virus can reduce the effectiveness of certain HIV medications if taken together and vice versa, the FDA warned in a drug safety announcement.

An interaction between the HCV drug boceprevir (Victrelis) and HIV protease inhibitors boosted with ritonavir results in reduced blood levels of the medications, data from a pharmacokinetic study involving 39 patients indicated.

“Healthcare professionals who have started patients infected with both chronic HCV and HIV on [boceprevir] and antiretroviral therapy containing a ritonavir-boosted protease inhibitor should closely monitor patients for HCV treatment response and for potential HCV and HIV virologic rebound,” the agency wrote.

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Boceprevir, a novel direct-acting agent targeting the NS3/4A hepatitis C protease, was approved last year as an adjuvant to ribavirin and pegylated interferon-alfa for patients with genotype 1 disease.

Another novel HCV drug, telaprevir (Incivek), was also approved last year, and already contains labeling information warning of potential interactions with HIV medications. Neither boceprevir nor telaprevir was approved to treat patients with both HCV and HIV infections.

In the pharmacokinetic study, boceprevir reduced average trough concentrations of ritonavir-boosted atazanavir (Reyataz), lopinavir (Kaletra), and darunavir (Prezista) by 49%, 43% and 59%, respectively. When these three HIV drugs were administered with boceprevir, peak concentrations fell an average of 25% to 36% and exposure declined 34% to 44%.

Coadministering boceprevir with ritonavir-boosted atazanavir did not reduce boceprevir exposure; however, giving it with lopinavir/ritonavir or darunavir/ritonavir resulted in boceprevir exposure decreases of 45% and 32%, respectively.

Clinicians should report any adverse events related to the use of these products to the FDA’s MedWatch Safety Information and Adverse Event Reporting Program.