Which pharmacokinetic feature best distinguishes protease inhibitors (PIs) from non-nucleoside reverse transcriptase inhibitors (NNRTIs) in antiretroviral therapy?

Explanation

## Distinguishing Feature: Hepatic Metabolism and Drug Interactions ### Protease Inhibitors (PIs) **Key Point:** PIs are substrates and potent inhibitors of CYP3A4, leading to clinically significant drug–drug interactions. - Hepatic metabolism via CYP3A4 (ritonavir, lopinavir, atazanavir, darunavir) - Extensive protein binding (>95%) - Require dose adjustment with many co-medications - Ritonavir used as a pharmacokinetic booster at sub-therapeutic doses ### Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs) **Key Point:** NNRTIs are also hepatically metabolized but are primarily CYP3A4 inducers or substrates with fewer bidirectional interactions. - Efavirenz: CYP3A4 inducer (increases metabolism of other drugs) - Rilpivirine: CYP3A4 substrate (fewer interactions than PIs) - Doravirine: minimal CYP interactions - Moderate protein binding (95–99%) ### Comparison Table | Feature | Protease Inhibitors | NNRTIs | | --- | --- | --- | | **Primary metabolism** | CYP3A4 substrate + inhibitor | CYP3A4 substrate or inducer | | **Drug–drug interactions** | Extensive (bidirectional inhibition) | Moderate to minimal | | **Protein binding** | >95% | 95–99% | | **Renal excretion** | <5% unchanged | <5% unchanged | | **Boosting requirement** | Ritonavir boost common | Not required | | **CNS penetration** | Poor | Variable (efavirenz: good) | **High-Yield:** The hallmark of PIs is **potent CYP3A4 inhibition**, making them prone to clinically significant interactions with statins, antiarrhythmics, and immunosuppressants. This is the single best discriminator. **Clinical Pearl:** Ritonavir (a PI) is often used not for its antiretroviral activity but as a pharmacokinetic booster to elevate levels of other PIs—a unique feature among antiretrovirals. [cite:Harrison 21e Ch 197]