Fosamprenavir and Rifabutin interaction
Fosamprenavir and Rifabutin: both prescribing labels flag this combination with the strongest language, such as a contraindication, a boxed warning, or an instruction to avoid it.
Do not combine without a prescriber's guidance. The label language below is the strongest FDA uses.
What the FDA labels say
From the Rifabutin label · effective 2026-09-01
Therefore, co-administration with antiretroviral and anti-HCV drugs metabolized by CYP3A is not recommended or there may be a need to increase the dose of antiretroviral or anti-HCV drugs (see PRECAUTIONS-Drug Interactions ).
Fosamprenavir/ritonavir 700 mg twice a day plus ritonavir 100 mg twice a day for 2 weeks 150 mg every other day for 2 weeks Healthy subjects (15) ↔ AUC compared to rifabutin 300 mg once a day alone ↓ C max by 15% ↑ AUC by 35% compared to historical control (fosamprenavir/ritonavir 700/100 mg twice a day) , ↑ C max by 36%, ↑ C min by 36% Reduce rifabutin dose…
The concomitant use of protease inhibitors may require at least a 50% reduction in rifabutin dose, and depending on the protease inhibitor, an adjustment of the antiretroviral drug dose.
Co-administration with antiretroviral drugs metabolized by CYP3A, including but not limited to products containing bictegravir, elvitegravir, oral rilpivirine, or doravirine and anti-HCV drugs including but not limited to sofosbuvir (alone or in combination) may decrease plasma concentrations of those drugs, which may lead to loss of virologic response and possible…
Effect of Other Drugs on Rifabutin Pharmacokinetics Some drugs that inhibit CYP3A may significantly increase the plasma concentration of rifabutin.
From the Fosamprenavir (Fosamprenavir Calcium) label · effective 2026-06-30
Antimycobacterial: Rifabutin a ↑ Rifabutin and rifabutin metabolite A complete blood count should be performed weekly and as clinically indicated to monitor for neutropenia.
Fosamprenavir calcium: A dosage reduction of rifabutin by at least half the recommended dose is required.
Fosamprenavir calcium/ritonavir: Dosage reduction of rifabutin by at least 75% of the usual dose of 300 mg/day is recommended (a maximum dose of 150 mg every other day or 3 times per week).
Initiation of medications that inhibit or induce CYP3A may increase or decrease concentrations of fosamprenavir calcium/ritonavir, respectively.
Coadministration of fosamprenavir calcium with drugs that induce CYP3A4 may decrease amprenavir (active metabolite) concentrations leading to potential loss of virologic activity.
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Open in the checkerNot medical advice. Interaction Checker quotes FDA label text and NIH fact sheets. Whether an interaction matters for you depends on doses, timing, kidney and liver function, and other conditions. Confirm with a pharmacist or prescriber.