A 32-year-old man with newly diagnosed HIV-1 infection is being counselled on antiretroviral therapy (ART) options. His baseline CD4 count is 350 cells/μL and viral load is 150,000 copies/mL. Regarding the pharmacokinetics and drug interactions of commonly used antiretrovirals, all of the following are true EXCEPT:

Explanation

## Pharmacokinetics and Drug Interactions of Antiretrovirals ### Clinical Context This patient has a CD4 count of 350 cells/μL (AIDS-defining) and moderate viral load, requiring immediate ART initiation. Understanding drug interactions is critical for: - Ensuring adequate plasma concentrations of antiretrovirals - Avoiding toxicity and treatment failure - Managing co-medications safely ### Correct Statements (Options 0, 1, 3) **Option 0 — Ritonavir as a Pharmacokinetic Booster:** - Ritonavir is a potent **CYP3A4 inhibitor** - At low doses (100–200 mg), it is used as a "booster" to inhibit metabolism of other protease inhibitors - This increases and prolongs plasma concentrations of co-administered PIs (e.g., lopinavir/ritonavir, darunavir/ritonavir) - Allows for lower PI doses and once-daily dosing regimens - ✓ **Correct** **Option 1 — Efavirenz as a CYP Inducer:** - Efavirenz is a potent inducer of **CYP3A4, CYP2B6, and CYP2C9** - Induction increases metabolism of: - Oral contraceptives (reduced efficacy; alternative contraception recommended) - Methadone (withdrawal symptoms may occur) - Warfarin (reduced anticoagulation) - Other antiretrovirals (e.g., protease inhibitors) - Dose adjustments of co-medications are often necessary - ✓ **Correct** **Option 3 — Tenofovir Disoproxil Fumarate (TDF) Renal Dosing:** - TDF is renally cleared and can accumulate in renal impairment - Nephrotoxicity risk increases with: - Low CD4 count (<200 cells/μL) - Concurrent nephrotoxic drugs (e.g., amphotericin B, NSAIDs) - Renal dysfunction (CrCl <50 mL/min) - Dose reduction is recommended: from standard 300 mg daily to 300 mg every 48 hours (CrCl 30–49 mL/min) or every 72 hours (CrCl <30 mL/min) - ✓ **Correct** ### Incorrect Statement (Option 2) — **THE ANSWER** **Option 2 — Dolutegravir Drug Interactions:** The statement claims dolutegravir has "minimal drug interactions and does not require dose adjustment when co-administered with most antiretrovirals or common medications." **This is PARTIALLY TRUE but MISLEADING in a clinical context:** **What is true:** - Dolutegravir has fewer drug interactions than older integrase inhibitors (e.g., raltegravir) - It does not require dose adjustment with most protease inhibitors or NNRTIs **What is FALSE or clinically important:** - **Divalent cations (Ca²⁺, Mg²⁺, Fe²⁺) reduce dolutegravir absorption by up to 50–60%** - Antacids, calcium supplements, iron supplements, and dairy products must be separated by ≥2 hours - This is a *significant* interaction requiring clinical attention - **Rifampicin (rifampin) induces UGT1A1 and reduces dolutegravir levels by ~50%** - Dolutegravir dose must be increased from 50 mg to 50 mg twice daily when co-administered with rifampicin - This is a major interaction in TB-HIV co-infection - **UGT1A1 inhibitors** (e.g., atazanavir, certain other drugs) may increase dolutegravir levels **Why this option is the answer:** While dolutegravir has a favorable drug interaction profile *compared to other antiretrovirals*, the blanket statement that it "does not require dose adjustment when co-administered with most antiretrovirals or common medications" is **clinically inaccurate**. Divalent cations and rifampicin are common co-medications that *do* require management strategies (timing separation or dose adjustment). ### Comparison Table: Drug Interaction Profiles | Drug Class | CYP Induction | CYP Inhibition | Other Interactions | Dose Adjustment Needed? | |---|---|---|---|---| | Efavirenz (NNRTI) | ↑ CYP3A4, 2B6, 2C9 | — | Oral contraceptives ↓ | Yes (frequent) | | Ritonavir (PI) | — | ↓ CYP3A4 (potent) | Boosting effect | Yes (as booster) | | Dolutegravir (INSTI) | — | — | Divalent cations ↓ absorption; Rifampicin ↓ levels | Yes (specific situations) | | TDF (NRTI) | — | — | Renal clearance | Yes (renal impairment) | **Key Point:** Dolutegravir is considered a "low interaction" drug, but this does NOT mean it has no interactions. Divalent cations and rifampicin are important exceptions that require clinical management. **Clinical Pearl:** In this patient with CD4 <350 cells/μL, TB screening is essential. If TB-HIV co-infection is present, dolutegravir dosing must be adjusted (50 mg BD) when given with rifampicin-containing TB regimens. **High-Yield:** The distinction between "low interaction" and "no interaction" is a common NEET PG trap. Dolutegravir's interaction with divalent cations and rifampicin are high-yield facts. [cite:Harrison 21e Ch 197]