HIV Pharmacology: NRTIs and NNRTIs

HIV-1 is an RNA retrovirus that targets CD4+ T lymphocytes and uses reverse transcriptase to convert its RNA genome into DNA that integrates permanently into the host chromosome. This lifecycle provides multiple drug targets, and understanding it explains why combination antiretroviral therapy (ART) is obligatory.

HIV-1 enters cells by binding its envelope glycoprotein gp120 to the CD4 receptor, then to a co-receptor — either CCR5 (C-C chemokine receptor type 5) or CXCR4 (C-X-C chemokine receptor type 4). The viral RNA is then reverse transcribed into double-stranded DNA by reverse transcriptase (RT), an error-prone enzyme that lacks proofreading. This high error rate generates enormous viral diversity, including drug-resistant variants, before any drug is ever taken. The viral DNA integrates into host chromosomal DNA via integrase, forming the provirus, which serves as template for new viral RNA and proteins. Newly budded virions require protease cleavage of polyproteins to become infectious.

The error-prone RT explains why monotherapy always fails. Without treatment, HIV produces roughly 10 billion virions per day, and every possible single-point mutation exists in the viral population before treatment begins. A drug targeting only one viral protein rapidly selects for pre-existing resistant variants. Combination ART forces the virus to simultaneously carry resistance mutations at two or more independent targets, which is statistically improbable at achievable viral loads. The goal of ART is plasma HIV RNA below 50 copies/mL, which prevents new resistance mutations and preserves immune function.

NRTI toxicity is mechanistically informative: the class-wide concern is off-target inhibition of mitochondrial DNA polymerase gamma, with risk varying substantially by agent. Hepatitis B virus (HBV) co-infection creates a critical safety obligation when discontinuing certain NRTIs. Resistance follows two mechanistically opposite pathways with directly opposing effects on drug susceptibility.

Mitochondrial toxicity arises because NRTIs can inhibit mitochondrial DNA polymerase gamma (pol-gamma), the enzyme that replicates mitochondrial DNA. Impaired mitochondrial function produces a spectrum of effects: lactic acidosis (most severe, potentially fatal), hepatic steatosis, peripheral neuropathy, and lipoatrophy. Risk is highest with ZDV and was greatest historically with stavudine (d4T) and didanosine (ddI), no longer used in resource-rich settings. TDF, FTC, and 3TC have negligible mitochondrial toxicity at therapeutic concentrations. TDF's renal proximal tubular toxicity is a distinct phenomenon: TDF accumulates in proximal tubular cells and impairs mitochondria locally, causing Fanconi syndrome (glucosuria without hyperglycemia, phosphaturia, proteinuria). TAF avoids this because plasma tenofovir concentrations are far lower.

Lamivudine and emtricitabine are active against HBV in addition to HIV. This creates two critical obligations. First, patients co-infected with HIV and HBV must receive an ART regimen containing TDF or TAF plus FTC or 3TC to treat both infections simultaneously. Second, abrupt discontinuation of 3TC or FTC in HBV co-infected patients can precipitate severe HBV flares with potentially fatal hepatic decompensation. Screen all patients for hepatitis B surface antigen (HBsAg) before starting ART and plan any future regimen changes accordingly.

NRTI resistance follows two mechanistic pathways. Discrimination mutations alter the RT active site to exclude NRTI triphosphates. The most important is M184V, selected by 3TC or FTC, which causes high-level resistance to both agents but paradoxically increases susceptibility to ZDV and TDF. K65R, selected by tenofovir, confers resistance to TDF, TAF, and ABC while preserving ZDV susceptibility. Excision mutations — the thymidine analogue mutations (TAMs) — are selected by ZDV and enhance RT's ability to remove the incorporated NRTI, restoring chain elongation. TAMs accumulate over time and confer broad NRTI cross-resistance.

The most clinically significant NNRTI toxicities and interactions stem from CYP3A4 induction by efavirenz and nevirapine, which substantially lowers concentrations of many co-medications. Second-generation NNRTIs are CYP3A4 substrates without inducing activity, giving them a far cleaner interaction profile — a key practical advantage in polypharmacy patients.

Efavirenz CNS toxicity is the most common reason for regimen switching. Symptoms peak in the first 2–4 weeks and include vivid or disturbing dreams, insomnia, dizziness, and impaired concentration. Bedtime dosing reduces subjective severity. About 10–15% of patients have persistent symptoms at 6 months. Depression, suicidal ideation, and psychosis are rare but documented; efavirenz is relatively contraindicated in patients with pre-existing psychiatric illness. Efavirenz's historical Food and Drug Administration (FDA) Pregnancy Category D classification (based on primate neural tube defect data) has been substantially revised — current guidelines accept it throughout pregnancy when no preferred alternative is available, though INSTI-based regimens are preferred in pregnancy.

The most clinically dangerous NNRTI interaction is efavirenz plus methadone. Efavirenz reduces methadone plasma concentrations by 50–60% through CYP3A4 and CYP2B6 induction, precipitating opioid withdrawal within 1–2 weeks of efavirenz initiation in patients on stable methadone maintenance. Coordinate with the methadone prescriber before starting efavirenz — dose escalation is routinely required. INSTI-based regimens have no clinically significant interaction with methadone and are strongly preferred in this population.

Rilpivirine interactions are driven by its CYP3A4 substrate status. Strong CYP3A4 inducers — rifampin, carbamazepine, phenytoin, phenobarbital, and St. John's Wort — reduce rilpivirine to subtherapeutic levels and are contraindicated. For doravirine, rifampin and rifabutin are both contraindicated due to magnitude of AUC reduction. Both rilpivirine and doravirine are far better choices than efavirenz in patients on methadone or other drugs sensitive to CYP3A4 induction.

Transmitted resistance shapes NNRTI selection at diagnosis. K103N is present in approximately 2–8% of newly diagnosed treatment-naive patients in the U.S. This single mutation confers high-level resistance to efavirenz and nevirapine but not to rilpivirine or doravirine. Baseline genotypic resistance testing is therefore required before selecting an NNRTI. Patients with multiple NNRTI resistance mutations have compromised susceptibility across the entire class and should be treated with an INSTI-based regimen instead.