Human immunodeficiency virus type 1 (HIV-1) is a lentivirus whose replication cycle provides multiple pharmacologically distinct targets. Understanding the viral lifecycle is prerequisite to understanding why any single antiretroviral agent fails when used as monotherapy and why combination antiretroviral therapy (ART) is obligatory in all treated patients.
HIV-1 is an enveloped ribonucleic acid (RNA) retrovirus that targets cells expressing the CD4 (cluster of differentiation 4) receptor, primarily CD4-positive T lymphocytes, macrophages, and dendritic cells. The viral envelope glycoprotein gp120 binds CD4 and a co-receptor, either CCR5 (C-C chemokine receptor type 5) or CXCR4 (C-X-C chemokine receptor type 4), triggering membrane fusion mediated by gp41. After entry, the viral RNA genome is reverse transcribed into double-stranded deoxyribonucleic acid (DNA) by reverse transcriptase (RT), an error-prone RNA-dependent DNA polymerase that lacks proofreading activity. This high error rate, producing approximately 10-5 mutations per nucleotide per replication cycle, generates enormous viral genetic diversity and is the molecular basis for resistance emergence.1
The viral DNA is then transported into the nucleus and integrated into host chromosomal DNA by integrase (IN), forming the provirus. Proviral DNA serves as a template for transcription of viral RNA, which is exported, translated into polyproteins, and cleaved by HIV protease (PR) into mature structural and enzymatic proteins. New virions bud from the host cell surface and require protease-mediated maturation to become infectious. Each of these steps, including entry, reverse transcription, integration, and protease-mediated maturation, constitutes a drug target.12
Without treatment, HIV-1 produces approximately 10 billion viral particles per day. Given the RT error rate, virtually every possible single-point mutation exists within the replicating viral population at any moment. A drug targeting a single site at a concentration that does not fully suppress replication exerts selective pressure that enriches pre-existing resistant mutants within days to weeks. This is why monotherapy invariably leads to virologic failure and resistance. The goal of combination ART is to reduce viral replication below the limit of detection (defined as fewer than 50 copies/mL by standard assays), preventing the generation of new resistance mutations and preserving immune function over decades.23
Current guidelines recommend initiating ART in all HIV-infected individuals regardless of CD4 count. The target is plasma HIV RNA below 50 copies/mL, maintained indefinitely. Patients who achieve and sustain undetectable viral loads do not transmit HIV sexually (U=U: Undetectable = Untransmittable) and have near-normal life expectancy with modern regimens. Failure to suppress, or intermittent viremia above 200 copies/mL, permits resistance evolution and immune deterioration.
The drug classes available for ART reflect the major enzymatic steps of the HIV lifecycle. Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) both target reverse transcriptase but through mechanistically distinct mechanisms. Protease inhibitors (PIs) block viral maturation. Integrase strand transfer inhibitors (INSTIs) prevent chromosomal integration. Entry inhibitors block viral fusion or co-receptor binding. Each class is covered in this and subsequent modules of Chapter 36. The current preferred first-line regimens for treatment-naive adults, per United States Department of Health and Human Services (DHHS) guidelines, consist of two NRTIs plus an integrase strand transfer inhibitor (INSTI), reflecting the superior resistance barrier and tolerability of INSTI-based regimens.3
A critical pharmacological principle underlying ART is that all agents in a regimen must be active against the patient's virus. Archived resistance mutations from prior treatment or transmitted resistance can render one or more components inactive, effectively reducing the regimen to functional monotherapy or dual therapy with a predictable virologic outcome. Baseline HIV genotype resistance testing is therefore mandatory before initiating or changing ART in all patients in whom it is feasible, and the results must be interpreted in the context of the patient's full treatment history.3
NRTIs (nucleoside reverse transcriptase inhibitors) and the single approved nucleotide analogue, tenofovir, constitute the backbone of nearly every recommended antiretroviral therapy (ART) regimen. Their shared mechanism, obligate intracellular phosphorylation, distinguishes them mechanistically and pharmacokinetically from all other antiretroviral classes.
NRTIs are prodrugs that must undergo intracellular phosphorylation to their active triphosphate forms by host cell kinases. The resulting nucleoside triphosphate analogues are recognized by human immunodeficiency virus (HIV) reverse transcriptase (RT) as substrates but, lacking a 3-prime hydroxyl group on the ribose ring, act as obligate chain terminators when incorporated into the growing viral deoxyribonucleic acid (DNA) strand. RT cannot extend the chain after incorporation of a nucleoside/nucleotide reverse transcriptase inhibitor (NRTI) triphosphate, halting reverse transcription. Tenofovir disoproxil fumarate (TDF) and tenofovir alafenamide (TAF) are prodrugs of tenofovir, a nucleotide (already monophosphorylated) that requires only two additional phosphorylation steps to reach the active diphosphate form.4
The phosphorylation requirement has important pharmacological consequences. Plasma drug concentrations measured by standard pharmacokinetic methods do not reflect intracellular concentrations of the active triphosphate species, which may differ by orders of magnitude. Intracellular half-lives of the triphosphate forms are substantially longer than plasma half-lives, enabling once-daily dosing even for drugs with short plasma half-lives. Lamivudine (3TC), for example, has a plasma half-life of approximately 5 to 7 hours but an intracellular triphosphate half-life of 10 to 19 hours, permitting once-daily dosing. Emtricitabine (FTC) has an even longer intracellular half-life of approximately 39 hours.5
Tenofovir exists in two prodrug formulations with markedly different pharmacokinetic and safety profiles. TDF achieves high plasma tenofovir concentrations that distribute broadly, including penetration into gut-associated lymphoid tissue (GALT), but is associated with renal proximal tubular toxicity and bone mineral density loss through systemic tenofovir exposure. TAF uses a novel phosphonamidate prodrug chemistry that delivers tenofovir preferentially into lymphocytes, resulting in intracellular tenofovir diphosphate concentrations 4-fold higher than TDF at one-tenth the plasma tenofovir concentration. This pharmacokinetic advantage translates into substantially lower rates of renal and bone adverse effects at the approved 25 mg dose of tenofovir alafenamide (TAF) versus 300 mg TDF, with equivalent antiviral efficacy.4511
TAF is preferred over TDF in patients with estimated glomerular filtration rate (eGFR) below 60 mL/min/1.73m2, osteoporosis or significant bone density loss, concurrent nephrotoxic drug use, or older age. TDF remains preferred in patients with hepatitis B virus (HBV) co-infection when TAF-containing co-formulations are unavailable, and has more data supporting pre-exposure prophylaxis (PrEP) in certain contexts. Both are contraindicated when eGFR falls below 30 mL/min unless on dialysis with careful monitoring.
Abacavir (ABC) is a carbocyclic NRTI that does not require a ribose ring and is phosphorylated by adenosine kinase to the active carbovir triphosphate. Its most consequential pharmacological feature is its association with a potentially fatal hypersensitivity reaction (HSR) occurring in approximately 5 to 8% of patients who carry the human leukocyte antigen B*57:01 (HLA-B*57:01) allele. The HSR typically presents within the first 6 weeks of therapy with fever, rash, constitutional symptoms, and gastrointestinal manifestations. Rechallenge after confirmed HSR is contraindicated and has caused fatal hypotension. Prospective HLA-B*57:01 screening before abacavir initiation reduces the incidence of clinically suspected HSR to near zero and is mandatory prior to abacavir prescription.6
Zidovudine (ZDV, formerly AZT) was the first approved antiretroviral agent (1987) and is a thymidine analogue. Its primary clinical use is now limited to prevention of mother-to-child transmission (PMTCT) during labor and delivery and neonatal post-exposure prophylaxis. ZDV is associated with mitochondrial toxicity, causing bone marrow suppression (macrocytic anemia, neutropenia), myopathy, and peripheral neuropathy through inhibition of mitochondrial DNA polymerase gamma (pol-gamma). Stavudine (d4T) and didanosine (ddI) carry the highest risk of mitochondrial toxicity and have been largely withdrawn from use in resource-rich settings due to lactic acidosis risk.5
The pharmacokinetics of approved NRTIs vary considerably by agent and must be considered in patients with renal impairment, since most NRTI parent drugs and their metabolites are renally cleared. Dose adjustment is required for tenofovir (TDF formulation), lamivudine, emtricitabine, and abacavir at varying eGFR thresholds. Abacavir is the exception, being primarily hepatically metabolized by alcohol dehydrogenase and glucuronyl transferase, with less than 2% renal excretion of unchanged drug, making it the preferred NRTI backbone component in advanced chronic kidney disease (CKD). Lamivudine and emtricitabine are renally cleared and require dose reduction when eGFR falls below 50 mL/min and 30 mL/min respectively.5
| Agent | Key Activation Step | Intracellular t½ | Renal Dose Adj? | Primary Toxicity Signal |
|---|---|---|---|---|
| TDF (tenofovir DF) | 2 phosphorylations to TFV-DP | ~150 h | Yes (<60 mL/min) | Nephrotoxicity, bone loss |
| TAF (tenofovir AF) | 2 phosphorylations to TFV-DP | ~150 h | Yes (<15 mL/min) | Weight gain (less renal/bone) |
| Emtricitabine (FTC) | 3 phosphorylations to FTC-TP | ~39 h | Yes (<30 mL/min) | Hyperpigmentation (rare); HBV flare |
| Lamivudine (3TC) | 3 phosphorylations to 3TC-TP | ~10-19 h | Yes (<50 mL/min) | Generally well tolerated; HBV flare |
| Abacavir (ABC) | Phosphorylated to carbovir-TP | ~21 h | No | Hypersensitivity (HLA-B*57:01) |
| Zidovudine (ZDV) | 3 phosphorylations to ZDV-TP | ~7 h | Yes (severe renal) | Anemia, neutropenia, myopathy |
The toxicity profiles of NRTIs are mechanistically informative: most major adverse effects derive from off-target inhibition of host cell polymerases, particularly mitochondrial deoxyribonucleic acid (DNA) polymerase gamma, or from renal and bone effects of systemic tenofovir exposure. Resistance patterns are mutation-specific and have direct consequences for cross-resistance within and between classes.
Mitochondrial toxicity is the class-wide adverse effect of greatest historical concern for NRTIs, though its clinical relevance has diminished substantially with the elimination of high-risk agents such as stavudine and didanosine from standard regimens. All NRTIs have some affinity for mitochondrial DNA polymerase gamma (pol-gamma), with the thymidine analogues zidovudine (ZDV) and stavudine (d4T) having the highest affinity. Inhibition of pol-gamma impairs mitochondrial DNA (mtDNA) replication, reducing mitochondrial function and causing a spectrum of toxicities: lactic acidosis (most severe, can be fatal), hepatic steatosis, peripheral neuropathy, lipodystrophy (lipoatrophy of subcutaneous fat), and cardiomyopathy. The risk hierarchy from highest to lowest mitochondrial toxicity among currently used agents is: ZDV greater than abacavir (ABC), with tenofovir disoproxil fumarate (TDF), emtricitabine (FTC), and lamivudine (3TC) having negligible mitochondrial toxicity at therapeutic concentrations.5
Tenofovir nephrotoxicity deserves particular clinical attention. TDF is a substrate of renal organic anion transporter 1 (OAT1) and multidrug resistance-associated protein 2 (MRP2), which facilitate its uptake into proximal tubular cells where it can accumulate to toxic concentrations. The resulting Fanconi syndrome, characterized by glucosuria without hyperglycemia, hypophosphatemia from phosphate wasting, hyperuricosuria, and proteinuria with or without elevation of serum creatinine, occurs in approximately 1 to 3% of TDF recipients with long-term use. Risk factors include pre-existing renal disease, advanced age, low body weight, concurrent use of ritonavir or cobicistat (which inhibit MRP2-mediated tubular secretion of tenofovir, increasing intracellular accumulation), and concurrent nephrotoxic agents including non-steroidal anti-inflammatory drugs (NSAIDs). Tenofovir alafenamide (TAF) largely avoids this toxicity due to its dramatically lower plasma tenofovir exposure.4
Both lamivudine and emtricitabine have potent activity against hepatitis B virus (HBV) in addition to human immunodeficiency virus (HIV). This dual activity has two important clinical consequences. First, patients co-infected with HIV and HBV must receive an antiretroviral therapy (ART) regimen containing either TDF or TAF plus FTC or 3TC to treat both infections simultaneously; failing to treat HBV with active agents while treating HIV risks immune reconstitution hepatitis. Second, abrupt discontinuation of lamivudine or emtricitabine in HBV co-infected patients can precipitate severe HBV flares with potentially fatal hepatic decompensation as viral replication rapidly resurges. Clinicians must check HBV surface antigen (HBsAg) status before initiating ART and plan accordingly.5
Any patient co-infected with HIV and HBV must receive an ART backbone containing tenofovir (TDF or TAF) plus emtricitabine or lamivudine. Never discontinue these agents without a plan to maintain HBV suppression. Interruption of effective anti-HBV therapy in a co-infected patient can cause fatal hepatic decompensation. If a regimen switch is planned, HBV-active agents must be continued or substituted with another active HBV regimen simultaneously.
Nucleoside/nucleotide reverse transcriptase inhibitor (NRTI) resistance is driven by two principal mechanisms: discrimination mutations and excision mutations. Discrimination mutations alter the reverse transcriptase (RT) active site to preferentially exclude NRTI triphosphates relative to natural deoxynucleoside triphosphates. The most clinically significant discrimination mutations are M184V (methionine to valine at codon 184) and K65R (lysine to arginine at codon 65). M184V emerges rapidly with lamivudine or emtricitabine monotherapy and confers high-level resistance to both agents but simultaneously increases sensitivity to ZDV, TDF, and ABC. K65R is selected by tenofovir and confers resistance to TDF, TAF, and ABC while retaining susceptibility to ZDV. These interactions form the pharmacological rationale for the recommended combination of TDF/TAF (tenofovir formulation) plus FTC/3TC (emtricitabine or lamivudine) in backbone design.7
Excision mutations, primarily thymidine analogue mutations (TAMs), are selected by ZDV and d4T and work by enhancing RT's ability to remove the incorporated NRTI through a pyrophosphorolysis reaction, allowing chain elongation to resume. The six canonical thymidine analogue mutations (TAMs) are M41L (Met41Leu), D67N (Asp67Asn), K70R (Lys70Arg), L210W (Leu210Trp), T215F/Y (Thr215Phe/Tyr), and K219Q/E (Lys219Gln/Glu). TAMs accumulate over time and confer cross-resistance to multiple NRTIs; five or more TAMs substantially reduce susceptibility to the entire NRTI class. The K65R and TAMs pathways are largely mutually exclusive because K65R reduces the efficiency of ZDV pyrophosphorolysis, explaining the antagonism between tenofovir and ZDV resistance pathways.7
Drug interactions involving NRTIs are generally less extensive than those of other antiretroviral classes because NRTIs are not substrates, inhibitors, or inducers of cytochrome P450 (CYP) enzymes. Their interactions are primarily at the level of renal transporters. Concurrent use of probenecid reduces renal tubular secretion of multiple NRTIs, increasing plasma concentrations. NSAIDs compete with TDF for OAT1-mediated renal uptake into tubular cells but also reduce renal perfusion through prostaglandin inhibition, potentiating tenofovir nephrotoxicity. Boosting agents ritonavir and cobicistat increase plasma TDF concentrations by approximately 30% through inhibition of MRP2-mediated tubular secretion; this interaction does not require dose adjustment but does mandate closer renal monitoring. Abacavir bioavailability is reduced by approximately 20% when co-administered with ethanol through competition for alcohol dehydrogenase, though this is not considered clinically significant at moderate alcohol intake.4
NNRTIs (non-nucleoside reverse transcriptase inhibitors) bind a hydrophobic allosteric pocket on the p66 subunit of human immunodeficiency virus (HIV) reverse transcriptase (RT), approximately 10 angstroms from the polymerase active site. Unlike NRTIs, they do not require intracellular phosphorylation, are not incorporated into viral deoxyribonucleic acid (DNA), and act by inducing conformational changes that reduce the catalytic rate of reverse transcription. They are active only against HIV type 1 (HIV-1) and have no activity against HIV type 2 (HIV-2).
The allosteric binding pocket exploited by NNRTIs does not exist in HIV-2 RT or in human DNA polymerases, conferring class selectivity. However, the structural flexibility of this pocket means that single amino acid substitutions at the binding interface can dramatically reduce non-nucleoside reverse transcriptase inhibitor (NNRTI) binding affinity. The K103N (lysine to asparagine at codon 103) mutation, for example, is the most common transmitted resistance mutation globally and confers high-level resistance to efavirenz and nevirapine while leaving later-generation NNRTIs, rilpivirine and doravirine, largely susceptible. This difference in genetic resistance barrier between first- and second-generation NNRTIs is a key pharmacological distinction.8
Efavirenz (EFV) was the dominant NNRTI in first-line regimens for nearly two decades. It is a potent inducer of cytochrome P450 3A4 (CYP3A4) and cytochrome P450 2B6 (CYP2B6) and is itself metabolized primarily by CYP2B6 with contributions from CYP3A4. CYP2B6 is highly polymorphic; the 516G greater than T single nucleotide polymorphism, occurring in approximately 20 to 30% of individuals of African ancestry, produces slow metabolizer phenotypes with efavirenz plasma concentrations 2 to 3 times higher than in extensive metabolizers. High efavirenz concentrations are associated with central nervous system (CNS) toxicity including vivid dreams, insomnia, dizziness, and depression. As a CYP3A4 inducer, efavirenz reduces plasma concentrations of numerous co-medications including most protease inhibitor (PI)-based regimens (requiring dose adjustment), azole antifungals, and combined oral contraceptives. Efavirenz reduces rifabutin concentrations, requiring rifabutin dose increase; its interaction with rifampin is complex because both are CYP3A4 inducers.89
Rilpivirine (RPV) is a second-generation NNRTI with a narrower spectrum of activity against common resistance mutations but a substantially improved CNS tolerability profile compared to efavirenz. Rilpivirine is metabolized by CYP3A4 and is neither an inducer nor a significant inhibitor of cytochrome P450 (CYP) enzymes, producing a cleaner drug interaction profile. Two pharmacokinetic requirements are non-negotiable: rilpivirine requires co-administration with a meal of at least 390 kcal to achieve adequate absorption (food increases rilpivirine area under the curve by approximately 40%), and proton pump inhibitors (PPIs) are contraindicated because gastric acid suppression reduces rilpivirine dissolution and absorption. Histamine-2 receptor antagonists (H2RAs) may be used with appropriate timing (at least 12 hours before or 4 hours after rilpivirine). Rilpivirine is contraindicated in patients with pre-treatment viral load above 100,000 copies/mL or cluster of differentiation 4 (CD4) count below 200 cells/microL due to higher virologic failure rates in these populations.9
PPIs are contraindicated with rilpivirine regardless of timing because they raise gastric pH throughout the day. H2RAs may be used if taken at least 12 hours before or 4 hours after rilpivirine. Antacids must be separated from rilpivirine by at least 2 hours before or 4 hours after. This interaction is particularly relevant in aging HIV-positive patients with high rates of gastroesophageal reflux disease who are commonly prescribed PPIs.
Doravirine (DOR) is the newest approved NNRTI, distinguished by its activity against the most common transmitted NNRTI resistance mutations including K103N, E138K (Glu138Lys), and K101E (Lys101Glu). Doravirine is metabolized primarily by CYP3A4 but is neither an inducer nor a clinically significant inhibitor of CYP3A4. It has no food requirements and a favorable CNS tolerability profile comparable to rilpivirine. The major limitation of doravirine is its susceptibility to rifampin-mediated CYP3A4 induction, reducing doravirine area under the concentration-time curve (AUC) by approximately 88%, making the combination contraindicated; rifabutin reduces doravirine AUC by approximately 50%, also contraindicated. Doravirine is available as a fixed-dose combination with tenofovir disoproxil fumarate (TDF) and lamivudine (3TC) (Delstrigo) and with tenofovir alafenamide (TAF) and 3TC in newer formulations.910
Nevirapine (NVP) is an older NNRTI now rarely used in resource-rich settings due to its toxicity profile. Its most serious adverse effect is severe hepatotoxicity, occurring at highest risk in treatment-naive women with CD4 counts greater than 250 cells/microL and in men with CD4 counts greater than 400 cells/microL at the time of initiation. This risk is not observed with standard once-daily lead-in dosing followed by maintenance therapy but reflects a pharmacogenomic hypersensitivity reaction. Nevirapine is also associated with severe cutaneous reactions including Stevens-Johnson syndrome (SJS). Like efavirenz, nevirapine is a CYP3A4 inducer and reduces concentrations of many co-administered drugs. It is retained in some resource-limited settings due to cost, and in prevention of mother-to-child transmission (PMTCT) programs for single-dose administration to the neonate.8
| Agent | Resistance Barrier | Key Resistance Mutations | CYP Effect | Primary Clinical Concern |
|---|---|---|---|---|
| Efavirenz | Low | K103N (high-level), Y188L, G190A | 3A4/2B6 inducer | CNS toxicity; contraceptive interactions |
| Rilpivirine | Low-moderate | E138K, K101E, V179L (K103N retained) | 3A4 substrate only | Food/PPI requirement; VL >100k contraindicated |
| Doravirine | Moderate | V106A, F227L (K103N retained) | 3A4 substrate only | Rifamycin interaction (contraindicated) |
| Nevirapine | Very low | K103N (high-level), Y181C | 3A4 inducer | Hepatotoxicity (CD4-dependent risk); SJS |
The toxicity and interaction profiles of NNRTIs are dominated by their effects on cytochrome P450 (CYP) enzymes, primarily CYP3A4 (cytochrome P450 3A4) and CYP2B6 (cytochrome P450 2B6). First-generation non-nucleoside reverse transcriptase inhibitors (NNRTIs) are CYP inducers that reduce concentrations of numerous co-medications; second-generation agents are substrates without significant inducing or inhibiting activity. Understanding these distinctions is essential for managing complex co-medication scenarios common in human immunodeficiency virus (HIV)-positive patients.
Efavirenz central nervous system (CNS) toxicity is the most clinically consequential adverse effect of the NNRTI (non-nucleoside reverse transcriptase inhibitor) class and deserves systematic attention. Symptoms include vivid or disturbing dreams, insomnia, dizziness, impaired concentration, and depersonalization. These effects typically peak in the first 2 to 4 weeks and improve in most patients, but persist in approximately 10 to 15% at 6 months. Bedtime dosing reduces subjective severity by ensuring peak plasma concentrations occur during sleep. Depression, suicidal ideation, and psychosis are rare but documented. Patients with pre-existing psychiatric conditions require careful monitoring; efavirenz is considered a relative contraindication in this population and alternatives should be prioritized. The CNS toxicity is plasma concentration-dependent and more severe in CYP2B6 slow metabolizers. Pre-emptive CYP2B6 genotyping is not standard practice but may be informative in patients with unexplained severe CNS toxicity.8
The teratogenicity classification of efavirenz has evolved. Earlier case reports suggesting a risk of neural tube defects prompted an FDA (Food and Drug Administration) warning and its classification as FDA Pregnancy Category D. Subsequent larger epidemiological studies, including a meta-analysis of over 2,000 first-trimester exposures, found no statistically significant increase in neural tube defect risk above the background rate of 0.1 to 0.2%. Current Department of Health and Human Services (DHHS) guidelines classify efavirenz as acceptable for use throughout pregnancy, including during the first trimester, based on this reassuring data. Nonetheless, rilpivirine or integrase strand transfer inhibitor (INSTI)-based regimens are generally preferred in pregnant patients initiating antiretroviral therapy (ART) for their cleaner data profiles.3
The most clinically impactful NNRTI drug interactions arise from CYP3A4 induction by efavirenz and nevirapine. Efavirenz reduces plasma concentrations of most protease inhibitors to subtherapeutic levels, requiring dosage adjustment (e.g., lopinavir/ritonavir dose must be increased). Efavirenz reduces plasma concentrations of methadone by approximately 50 to 60%, precipitating opioid withdrawal in patients on stable methadone maintenance therapy; methadone dose escalation is routinely required. Rifampin induces CYP3A4 and CYP2B6 and reduces efavirenz area under the concentration-time curve (AUC) by approximately 25%; current guidelines recommend increasing the efavirenz dose from 600 mg to 800 mg daily in patients weighing more than 60 kg on rifampin-based tuberculosis (TB) therapy, though this guidance is not uniformly followed given modest pharmacokinetic benefit and increased CNS toxicity risk.9
Efavirenz reduces methadone plasma concentrations by 50 to 60% through CYP3A4 induction. This interaction is clinically significant and typically symptomatic within 1 to 2 weeks of efavirenz initiation in patients on stable methadone. Methadone dose requirements increase substantially; close coordination with the prescribing methadone clinic is mandatory. Patients should be warned to expect withdrawal symptoms and instructed to contact their providers immediately. INSTI-based regimens without CYP-inducing activity are strongly preferred in patients on methadone maintenance therapy.
Rilpivirine's interaction profile is primarily driven by its CYP3A4 substrate status without inducing or inhibiting effects. Strong CYP3A4 inducers including rifampin, rifabutin, carbamazepine, phenytoin, phenobarbital, and St. John's Wort reduce rilpivirine plasma concentrations to subtherapeutic levels and are contraindicated. Rilpivirine prolongs the QTc interval at supratherapeutic concentrations (3 and 12 times the recommended dose) but not at therapeutic doses; nonetheless, caution is warranted when combining rilpivirine with other QTc-prolonging agents, particularly in patients with pre-existing cardiac conduction abnormalities or electrolyte disturbances.9
NNRTI resistance mutations merit additional discussion in the context of transmitted resistance. Lysine-to-asparagine mutation at codon 103 (K103N) is the single most commonly transmitted HIV resistance mutation globally, present in approximately 2 to 8% of newly diagnosed, treatment-naive patients in the United States and Europe depending on the population studied. A single K103N mutation confers high-level resistance to efavirenz and nevirapine but retains susceptibility to rilpivirine and doravirine, making baseline resistance testing critical for NNRTI selection. The glutamate-to-lysine mutation at codon 138 (E138K) confers resistance to rilpivirine and is also found in treatment-naive patients, though at lower frequencies than K103N. Doravirine's resistance profile is largely non-overlapping with K103N and E138K, supporting its utility in patients with common transmitted NNRTI resistance. Multiple NNRTI resistance mutations reduce susceptibility to all approved agents, at which point the class should be abandoned in favor of regimens not reliant on NNRTI activity.7
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