Ritonavir is included at low dose in many protease inhibitor regimens even when its own antiviral contribution is negligible. The reason is:

• A It potently inhibits CYP3A4, increasing exposure of the companion protease inhibitor ✓
• B It competitively blocks protease inhibitor efflux at the blood brain barrier
• C It induces glucuronidation, smoothing trough levels of the companion drug
• D It directly inhibits HIV reverse transcriptase at low concentrations

Explanation

Ritonavir is one of the most potent inhibitors of CYP3B4 among marketed drugs. At low booster doses it dramatically reduces first pass and systemic clearance of co-administered protease inhibitors such as lopinavir, darunavir and atazanavir, allowing fewer doses and higher troughs that overcome resistance. This same inhibition makes ritonavir a notorious source of drug interactions with statins and midazolam. It has no reverse transcriptase activity.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

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Written and medically reviewed by the StethoPrep medical team.