Low-dose ritonavir is commonly combined with other protease inhibitors in antiretroviral regimens. The purpose of adding ritonavir in this setting is:

• A To add a second antiviral mechanism acting at the protease active site
• B To block viral entry by downregulating CCR5 on lymphocyte membranes
• C To induce CYP2C9 and accelerate clearance of competing drugs
• D To inhibit CYP3A4 mediated metabolism of the companion protease inhibitor and raise its plasma concentrations ✓

Explanation

Ritonavir is one of the most potent CYP3A4 inhibitors available. At subtherapeutic antiviral doses it markedly reduces the first-pass and systemic clearance of co-administered protease inhibitors such as darunavir and atazanavir, allowing less frequent dosing and higher trough levels, the concept of pharmacokinetic boosting. Its own antiretroviral contribution at booster doses is minimal, and CCR5 blockade is the mechanism of maraviroc, a different class entirely.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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