Protease inhibitors (PIs) target the HIV type 1 (HIV-1) aspartyl protease (PR), an enzyme essential for the post-translational processing of the Gag and Gag-Pol polyproteins into mature structural and enzymatic proteins. Without functional protease, newly budded virions are morphologically immature and non-infectious. PIs are among the most potent antiretroviral agents available but require pharmacokinetic boosting to achieve therapeutic plasma concentrations.
HIV-1 protease is a homodimeric aspartyl protease that cleaves the Gag polyprotein (producing matrix, capsid, nucleocapsid, and p6 proteins) and the Gag-Pol polyprotein (producing reverse transcriptase, integrase, and protease itself). Cleavage occurs at specific amino acid sequences that differ from host cellular substrates, providing the basis for selective inhibition. PIs are peptidomimetic compounds that mimic the transition state of the protease cleavage reaction, binding the active site with high affinity and competitively blocking substrate access. Because PR activity is required for virion maturation after budding rather than during intracellular replication, PIs produce non-infectious particle release rather than blocking replication outright, a mechanistic distinction from reverse transcriptase and integrase inhibitors.1
All currently used PIs are metabolized extensively by cytochrome P450 3A4 (CYP3A4) and are also substrates of P-glycoprotein (P-gp), an efflux transporter expressed in the gut wall, liver, and blood-brain barrier. Without pharmacokinetic enhancement, oral bioavailability of most PIs is low and variable, and plasma concentrations are insufficient for sustained viral suppression. The pharmacokinetic boosting strategy exploits the potent CYP3A4 and P-gp inhibitory activity of low-dose ritonavir or cobicistat to dramatically increase protease inhibitor (PI) plasma concentrations and extend half-lives, enabling twice-daily or once-daily dosing at substantially lower PI doses than would otherwise be required.2
Ritonavir, originally developed as an antiretroviral PI in its own right, is now used exclusively as a pharmacokinetic enhancer (booster) at sub-therapeutic doses of 100 to 200 mg. At these doses it provides no meaningful antiviral activity itself but produces near-complete inhibition of CYP3A4, raising co-administered PI area under the concentration-time curve (AUC) by 5 to 20-fold depending on the PI. Cobicistat is a mechanistic inhibitor of CYP3A4 developed specifically as a booster without intrinsic antiviral activity; it shares ritonavir's CYP3A4 inhibitory potency but lacks ritonavir's broad cytochrome P450 (CYP) inhibition of CYP2D6 (cytochrome P450 2D6), CYP2C9 (cytochrome P450 2C9), and other isoforms, resulting in a somewhat narrower drug interaction profile. However, cobicistat inhibits renal tubular secretion of creatinine via inhibition of multidrug and toxin extrusion protein 1 (MATE1), causing an apparent rise in serum creatinine of approximately 0.1 to 0.2 mg/dL without affecting true glomerular filtration rate (GFR); this must not be misinterpreted as nephrotoxicity.4
Both ritonavir and cobicistat are CYP3A4 inhibitors used as pharmacokinetic boosters. Ritonavir additionally inhibits CYP2D6, CYP2C9, and other isoforms — a broader interaction profile. Cobicistat inhibits MATE1, causing a predictable serum creatinine rise of 0.1 to 0.2 mg/dL that does not reflect true renal impairment; this artifact can complicate interpretation of renal function in clinical monitoring. Neither booster should be combined with the other, and boosted regimens are incompatible with strong CYP3A4 inducers including rifampin.
Darunavir (DRV) is the preferred PI in current guidelines when a PI-based regimen is indicated. Boosted with ritonavir (darunavir/ritonavir 800/100 mg once daily in treatment-naive patients) or cobicistat (darunavir/cobicistat 800/150 mg once daily), darunavir has an exceptionally high genetic resistance barrier: no single mutation confers high-level resistance, and at least three major PI resistance mutations must accumulate simultaneously for clinical failure. Darunavir binds the protease active site with very high affinity (dissociation constant approximately 4.5 × 10-12 mol/L) and forms a hydrogen bond network with the protease backbone that is disrupted only by combinations of active-site mutations. For treatment-naive patients, 800 mg once-daily darunavir/ritonavir or darunavir/cobicistat is preferred; for treatment-experienced patients with suspected PI resistance, 600 mg twice-daily darunavir/ritonavir provides higher trough concentrations.3
Atazanavir (ATV) was the first once-daily PI and was widely used before darunavir-based regimens became dominant. Its most distinctive pharmacological feature is inhibition of uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), the enzyme responsible for hepatic bilirubin conjugation. This results in unconjugated hyperbilirubinemia in most patients (mean total bilirubin approximately 2 to 3 mg/dL), causing scleral icterus and visible jaundice in up to 10% of patients. The hyperbilirubinemia is benign and reversible on discontinuation, but it affects quality of life and is a common reason for regimen switching. Atazanavir also causes nephrolithiasis (kidney stones composed of atazanavir crystals) in approximately 1 to 3% of patients with long-term use. Unboosted atazanavir requires an acidic gastric environment for absorption and cannot be co-administered with proton pump inhibitors (PPIs); boosted atazanavir tolerates PPIs at reduced doses.4
| PI Agent | Standard Boosted Dose | Resistance Barrier | Distinctive Toxicity | Key Interaction |
|---|---|---|---|---|
| Darunavir/r or /c | 800/100 mg QD (naive); 600/100 mg BID (experienced) | Very high | Rash (sulfa moiety); GI | Avoid rifampin; reduce statins |
| Atazanavir/r or /c | 300/100 mg QD or 300/150 mg QD | Moderate | Hyperbilirubinemia; nephrolithiasis | PPIs: unboosted ATV contraindicated; boosted reduce PPI dose |
| Lopinavir/ritonavir | 400/100 mg BID | Moderate | GI intolerance; dyslipidemia; QTc | Rifampin: contraindicated; no solution for adults |
| Tipranavir/r | 500/200 mg BID | High (salvage) | Hepatotoxicity; intracranial hemorrhage (rare) | Many; induces CYP3A4 despite being substrate |
The toxicity profile of protease inhibitors extends well beyond individual agent-specific effects. As a class, PIs are associated with metabolic perturbations including dyslipidemia, insulin resistance, and lipodystrophy that reflect both direct drug effects and the consequences of cytochrome P450 3A4 (CYP3A4) inhibition by boosting agents. Their drug interaction burden, mediated primarily through booster-driven CYP3A4 inhibition, is the most extensive of any antiretroviral class.
Protease inhibitor (PI)-associated dyslipidemia is most pronounced with lopinavir/ritonavir (LPV/r) and older, less-favored PIs. Triglyceride elevations of 2 to 5-fold above baseline and low-density lipoprotein (LDL) cholesterol increases of 20 to 40% are common with LPV/r; darunavir-based regimens produce substantially less dyslipidemia. The mechanism involves inhibition of hepatic lipid metabolism and upregulation of sterol regulatory element-binding protein (SREBP)-mediated lipogenic pathways. Statin management in patients on boosted PI regimens requires careful attention: simvastatin and lovastatin are contraindicated because CYP3A4 inhibition by the booster raises statin plasma concentrations by 20 to 50-fold, dramatically increasing myopathy and rhabdomyolysis risk. Atorvastatin may be used at the lowest effective dose with monitoring. Rosuvastatin and pravastatin are the preferred statins in patients on boosted PIs because they are minimally metabolized by CYP3A4; rosuvastatin concentrations increase modestly with some PIs via organic anion transporting polypeptide 1B1 (OATP1B1) inhibition but remain manageable.2
Lipodystrophy, a syndrome of abnormal fat distribution, was a prominent feature of older PI regimens and thymidine analogue-containing backbones. It manifests as lipoatrophy (loss of subcutaneous fat in the face, limbs, and buttocks) and lipohypertrophy (central fat accumulation, dorsocervical fat pad, visceral adiposity). Lipoatrophy is now rare with modern regimens that avoid thymidine analogues (stavudine, zidovudine). Central fat accumulation persists as a more common phenomenon, particularly with older PIs and with integrase inhibitor-based regimens that promote weight gain, discussed further in Section 3.
PI resistance mutations cluster in the protease active site and flanking regions. Major PI resistance mutations directly impair inhibitor binding, while minor mutations restore viral fitness in the context of major mutations. Because human immunodeficiency virus (HIV) protease must maintain substrate binding for its essential catalytic function, there is an inherent fitness cost to PI resistance mutations, limiting how many can accumulate while preserving replicative capacity. Darunavir resistance requires accumulation of at least three of eleven defined major resistance-associated mutations (RAMs) including I50V (Ile50Val), I54M/L (Ile54Met/Leu), L76V (Leu76Val), and I84V (Ile84Val); the high barrier reflects darunavir's extensive hydrogen bond network with the protease backbone that is disrupted only by multiple simultaneous active-site alterations. Cross-resistance among PIs is extensive; once high-level PI resistance is present (typically 5 or more major mutations), susceptibility to all available PIs is substantially reduced and the class should be abandoned in favor of other drug classes.3
Simvastatin and lovastatin are absolutely contraindicated with boosted PI regimens. CYP3A4 inhibition raises simvastatin plasma concentrations by up to 50-fold, producing catastrophic myopathy and rhabdomyolysis risk. Atorvastatin may be used cautiously at the lowest effective dose. Rosuvastatin and pravastatin are the preferred statins: rosuvastatin is not CYP3A4-metabolized (though OATP1B1 inhibition modestly raises its levels); pravastatin is neither CYP3A4-metabolized nor a significant OATP1B1 substrate and is the safest choice with lopinavir/ritonavir.
The drug interaction landscape of boosted PI regimens is the most complex in antiretroviral pharmacology and extends to virtually every major drug class. Boosted PIs raise plasma concentrations of drugs that are CYP3A4 substrates, including most azole antifungals (voriconazole is paradoxically reduced by ritonavir-boosted PIs due to CYP2C19 induction), rifabutin (dose must be reduced from 300 mg to 150 mg every other day or three times weekly with boosted PI co-administration), macrolide antibiotics (clarithromycin area under the concentration-time curve (AUC) increases substantially), immunosuppressants (cyclosporine, tacrolimus, sirolimus concentrations increase dramatically — critical in transplant patients), and direct oral anticoagulants (DOACs such as rivaroxaban and apixaban are contraindicated with boosted PIs due to CYP3A4 and P-gp inhibition). Rifampin is contraindicated with all boosted PIs because it reduces PI AUC by 75 to 90% through potent CYP3A4 induction, undermining the entire boosting strategy.24
Boosted PIs also interact with contraceptives, though the direction of effect varies by PI and contraceptive. Lopinavir/ritonavir reduces ethinyl estradiol concentrations by approximately 40%, potentially impairing oral contraceptive efficacy; alternative or additional contraceptive methods are recommended. Darunavir/ritonavir has a more modest effect. Hormonal emergency contraception (levonorgestrel) may have reduced efficacy with enzyme-inducing PIs; copper intrauterine devices (IUDs) are not affected by PI interactions and are the preferred emergency contraception in patients on boosted PI regimens.4
Integrase strand transfer inhibitors (INSTIs) are the cornerstone of current preferred first-line antiretroviral therapy (ART) regimens. They inhibit the strand transfer step of human immunodeficiency virus (HIV) integration, exhibit rapid antiviral activity, have minimal off-target toxicity, and — for the second-generation agents dolutegravir and bictegravir — present a resistance barrier so high that virologic failure with resistance selection is rare even with suboptimal adherence.
HIV integrase (IN) performs two sequential catalytic reactions to insert viral deoxyribonucleic acid (DNA) into the host chromosome. The first, 3-prime processing, removes two nucleotides from each 3-prime end of the viral DNA in the cytoplasm, generating reactive hydroxyl groups. The second, strand transfer, occurs in the nucleus: the processed viral DNA ends attack the host chromosomal DNA backbone in a concerted cleavage-ligation reaction, inserting the viral genome. INSTIs chelate two magnesium ions in the integrase active site through a pharmacophore containing two oxygen atoms (the diketo acid motif or its bioisosteric equivalents), blocking the metal coordination required for strand transfer. This mechanism is highly specific because the active site metal chelation geometry is unique to integrase; host cell DNA repair enzymes that perform analogous chemistry have sufficiently different active site architecture to maintain selectivity.5
Raltegravir (RAL) was the first approved integrase strand transfer inhibitor (INSTI) and established proof-of-concept for the class. It is dosed twice daily due to its short plasma half-life (approximately 9 hours) and has a low genetic resistance barrier: a single mutation at any of three signature pathways (Tyr143Cys/Arg [Y143C/R], Gln148His/Arg/Lys [Q148H/R/K], or Asn155His [N155H]) confers significant resistance. It does not inhibit or induce cytochrome P450 (CYP) enzymes and is eliminated by uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1)-mediated glucuronidation, making atazanavir (a UGT1A1 inhibitor) a significant interaction partner. Despite its limitations in resistance barrier, raltegravir retains clinical utility in specific populations including pregnancy (most safety data of any INSTI) and patients requiring twice-daily dosing flexibility.6
Elvitegravir (EVG) is available only in fixed-dose combinations co-formulated with cobicistat as a pharmacokinetic booster (Stribild: EVG/cobicistat/tenofovir disoproxil fumarate (TDF)/emtricitabine (FTC); Genvoya: EVG/cobicistat/tenofovir alafenamide (TAF)/FTC). Elvitegravir requires cobicistat boosting because its half-life without boosting is insufficient for once-daily dosing. The cobicistat component introduces the full interaction burden of a cytochrome P450 3A4 (CYP3A4) inhibitor, including the creatinine artifact, interactions with statins, and the incompatibility with rifampin. Elvitegravir shares cross-resistance with raltegravir at the glutamine-148 (Q148) and asparagine-155 (N155) pathways; exposure to one agent may compromise the other.5
Dolutegravir (DTG) is a second-generation INSTI with a substantially higher genetic resistance barrier than raltegravir or elvitegravir, attributable to its flexible molecular structure that adapts to minor integrase conformational changes induced by first-generation resistance mutations. Single mutations that confer high-level raltegravir resistance (Y143, N155H, Q148H) reduce dolutegravir susceptibility minimally (less than 2-fold). Clinically meaningful resistance to dolutegravir in treatment-naive patients is extraordinarily rare; in the landmark SAILING (Study of Active Antiretroviral Naive Patients) and FLAMINGO (Dolutegravir Versus Darunavir/Ritonavir in Antiretroviral-Naive HIV-Infected Individuals) trials, no treatment-emergent integrase resistance mutations were identified in dolutegravir recipients at virologic failure. Dolutegravir is metabolized primarily by UGT1A1 with minor CYP3A4 contribution; it is neither a significant CYP inhibitor nor inducer at therapeutic doses, though it inhibits renal tubular secretion of creatinine via organic cation transporter 2 (OCT2) — the same artifact as cobicistat but through a different transporter mechanism.5
An early safety signal from the Tsepamo study in Botswana (2018) suggested an increased risk of neural tube defects (NTDs) in infants born to women who conceived while taking dolutegravir, with a prevalence of approximately 0.9% versus 0.1% in non-dolutegravir-exposed pregnancies. Subsequent larger analyses from the same cohort and from multinational surveillance have shown NTD rates of approximately 0.19% with dolutegravir, no longer statistically significantly different from background rates in most analyses. Current guidelines accept dolutegravir throughout pregnancy, including at conception, but this evolving evidence base should be discussed with patients of childbearing potential when selecting a regimen.
Bictegravir (BIC) is a second-generation INSTI available only as a fixed-dose combination with TAF and FTC (Biktarvy). It has a resistance profile equivalent to dolutegravir, with no clinically relevant treatment-emergent resistance reported in phase 3 trials in treatment-naive patients. Bictegravir is metabolized by CYP3A4 and UGT1A1 and is a weak inhibitor of OCT2 and multidrug and toxin extrusion protein 1 (MATE1), producing the creatinine secretion artifact similar to dolutegravir and cobicistat. It is not a clinically significant CYP inhibitor or inducer. Cabotegravir (CAB) is a long-acting injectable INSTI with a resistance profile similar to dolutegravir, discussed in Section 6.5
Weight gain associated with INSTI-based regimens has emerged as a clinically significant concern since these agents became the preferred first-line therapy. Pooled analyses from switch trials demonstrate that patients switching from protease inhibitor (PI) or non-nucleoside reverse transcriptase inhibitor (NNRTI)-based regimens to INSTI-based regimens gain an average of 2 to 4 kg over 48 to 96 weeks, with greater weight gain observed with integrase inhibitors combined with TAF rather than TDF. The mechanism is not fully established but likely involves a combination of immune reconstitution effects (the return-to-health phenomenon), direct metabolic effects of integrase inhibition on adipogenesis, and the elimination of TDF-mediated appetite suppression and weight neutrality. The clinical significance of this weight gain in terms of cardiovascular and metabolic outcomes remains an active area of investigation. Women and persons of African ancestry appear to experience greater INSTI-associated weight gain than men and persons of European ancestry in current data.7
Despite the high resistance barriers of second-generation INSTIs, clinically meaningful drug interactions — particularly with polyvalent cations and enzyme-inducing rifamycins — require careful management in routine practice. The integrase strand transfer inhibitor (INSTI) class also has specific pharmacological considerations in pregnancy, tuberculosis co-treatment, and patients with advanced renal or hepatic impairment.
INSTI resistance follows distinct mutational pathways that reflect the agent-specific binding geometries within the integrase active site. For raltegravir and elvitegravir, three independent resistance pathways exist: Y143C/R (selected primarily by raltegravir), Q148H/R/K (selected by both, with high-level cross-resistance), and N155H (selected by both, with partial cross-resistance). Accessory mutations at positions E92 (Glu92), G140 (Gly140), E138 (Glu138), and others restore viral fitness in the context of these primary mutations and further reduce susceptibility. The glutamine-148 (Q148) pathway with secondary mutations (particularly G140S + Q148H) produces the highest-level raltegravir and elvitegravir resistance and substantially reduces susceptibility to dolutegravir (8 to 25-fold), making it the resistance pathway of greatest clinical concern when second-generation INSTI use is anticipated after first-generation failure. For dolutegravir and bictegravir, no consistent single-mutation resistance pathway has been identified; Arginine-to-lysine at codon 263 (R263K) is the most commonly observed mutation under dolutegravir selective pressure in cell culture but produces only modest resistance and impairs viral fitness substantially.6
The most clinically important drug interaction for all INSTIs is chelation with polyvalent metal cations. All INSTIs contain a diketo acid or equivalent pharmacophore that chelates Mg2+ in the integrase active site; this same pharmacophore also chelates dietary and supplemental divalent and trivalent metal ions (Ca2+, Mg2+, Al3+, Fe2+/3+, Zn2+) in the gastrointestinal (GI) tract, forming insoluble complexes that dramatically reduce INSTI absorption. Antacids containing magnesium or aluminum hydroxide, calcium supplements, and iron supplements must be separated from INSTI administration. For dolutegravir and raltegravir, co-administration with cation-containing products must be separated by at least 2 hours before or 6 hours after the INSTI. Bictegravir can be taken with calcium- or iron-containing supplements if taken with food; food stabilizes bictegravir absorption sufficiently to overcome modest cation chelation. Antacids containing calcium carbonate must be separated from bictegravir by 2 hours. This interaction is frequently encountered in clinical practice because antiretroviral patients, particularly those with bone density loss from tenofovir disoproxil fumarate (TDF) exposure, are commonly prescribed calcium and vitamin D supplementation.6
Calcium, magnesium, iron, and aluminum supplements and antacids dramatically reduce INSTI absorption through GI chelation. Timing rules: dolutegravir and raltegravir must be separated from these products by 2 hours before or 6 hours after. Bictegravir may be taken with calcium or iron supplements if taken with food. Elvitegravir (in Genvoya/Stribild) should be taken with food — food reduces the cation interaction. Failure to counsel patients on supplement timing is a common cause of subtherapeutic INSTI trough concentrations with otherwise-adherent patients.
Rifamycin interactions with INSTIs are mechanistically distinct from those with PIs and NNRTIs. Rifampin induces uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1) and cytochrome P450 3A4 (CYP3A4), the primary elimination pathways for dolutegravir and bictegravir, reducing their area under the concentration-time curve (AUC) by approximately 54% and 75% respectively. For dolutegravir, the approved strategy is dose doubling from 50 mg once daily to 50 mg twice daily, which restores adequate plasma exposures confirmed in pharmacokinetic studies. Bictegravir is contraindicated with rifampin because dose doubling has not been studied in the fixed-dose combination context and the magnitude of interaction is greater. Rifabutin has substantially less UGT1A1 and CYP3A4 inducing activity than rifampin and can generally be used with standard INSTI doses, though modestly increased monitoring is appropriate. Raltegravir AUC is reduced by approximately 40% with rifampin; twice-daily raltegravir at the standard 400 mg dose has been shown in pharmacokinetic studies to maintain sufficient trough concentrations and is used in some resource-limited settings for human immunodeficiency virus (HIV)-tuberculosis co-treatment.4
In pregnancy, dolutegravir has the most extensive safety data among second-generation INSTIs. Following resolution of the initial neural tube defect concern (discussed in Section 3), dolutegravir is now recommended throughout pregnancy by United States Department of Health and Human Services (DHHS) guidelines. Dolutegravir crosses the placenta (umbilical cord-to-maternal plasma ratio approximately 1.2, indicating active transport), achieving concentrations in the fetus that are at least equivalent to maternal levels. Raltegravir remains an important alternative in pregnancy, with extensive safety data from clinical trials and observational cohorts; raltegravir pharmacokinetics are modestly altered in the third trimester with lower trough concentrations, but clinical outcomes remain favorable. Cabotegravir-rilpivirine long-acting injectable (LA) antiretroviral therapy (ART) is not recommended in pregnancy due to absent safety data and the prolonged pharmacokinetic tail (discussed in Section 6) that would persist if adverse effects require discontinuation.9
Hepatic impairment affects INSTI pharmacokinetics modestly. Dolutegravir AUC increases approximately 1.5-fold in moderate hepatic impairment (Child-Pugh B); no dose adjustment is required but monitoring is appropriate. Dolutegravir is not recommended in severe hepatic impairment (Child-Pugh C) due to insufficient data. Raltegravir undergoes UGT1A1-mediated glucuronidation in the liver but its pharmacokinetics are not substantially altered by hepatic impairment, making it the preferred INSTI in severe hepatic disease when an INSTI is required. For renal impairment, all INSTIs can be used without dose adjustment at any eGFR level, including dialysis, because renal elimination of unchanged parent drug is minimal. The cobicistat-containing fixed-dose combination Genvoya (EVG/c/TAF/FTC) requires an eGFR above 30 mL/min due to the TAF component restriction rather than the INSTI itself.7
Entry inhibitors encompass mechanistically diverse agents that block human immunodeficiency virus (HIV) infection before integration occurs. They target the sequential steps of HIV attachment and entry: cluster of differentiation 4 (CD4) binding, co-receptor engagement, and membrane fusion. While this class includes some of the most mechanistically elegant antiretrovirals, its clinical use remains limited to specific patient populations due to the requirement for tropism testing, the injectable route of administration of fusion inhibitors, and the specialized indications for newer attachment inhibitors.
HIV type 1 (HIV-1) uses one of two co-receptors for membrane fusion: CCR5 (C-C chemokine receptor type 5) is used by R5-tropic virus, which is the predominant strain in early and established HIV infection; CXCR4 (C-X-C chemokine receptor type 4) is used by CXCR4-tropic (X4-tropic) virus, which emerges in approximately 50% of patients with advanced disease and long-term infection as a consequence of immune system evolution and CXCR4-expressing target cell expansion. Dual-tropic or mixed virus populations express co-receptor flexibility. Maraviroc (MVC) is a CCR5 antagonist that blocks the conformational change in CCR5 required for gp41-mediated fusion. Because it targets a host cell receptor rather than a viral protein, it has a mechanistically different resistance profile: resistance requires viral tropism shift from R5 to X4 or dual-tropic rather than mutation of the drug-binding site. Maraviroc has no activity against X4-tropic or dual-tropic virus; prior to prescribing, a validated co-receptor tropism assay (standard: Trofile assay or genotypic inference from the V3 loop sequence) must confirm exclusively R5 tropism. The presence of any CXCR4-using virus, even at low frequency, predicts virologic failure with maraviroc-based regimens.8
Maraviroc is metabolized primarily by cytochrome P450 3A4 (CYP3A4) and is a P-glycoprotein (P-gp) substrate. Its dose must be adjusted based on the co-administered antiretroviral agents: 300 mg twice daily is the standard dose; it is reduced to 150 mg twice daily when used with potent CYP3A4 inhibitors such as ritonavir-boosted or cobicistat-boosted PIs (because the booster dramatically raises maraviroc exposure); and it is increased to 600 mg twice daily when used with potent CYP3A4 inducers such as efavirenz, rifampin, or rifabutin (because induction reduces maraviroc exposure to subtherapeutic levels). The principal adverse effects of maraviroc include cough, upper respiratory infections, and orthostatic hypotension; hepatotoxicity with systemic allergic reaction has been reported rarely but can be severe.8
Maraviroc must never be prescribed without prior confirmation of exclusively CCR5 tropism by a validated assay. Any detectable CXCR4-using virus in the pre-treatment sample predicts failure. Tropism can shift over time with disease progression; a tropism result obtained years before prescribing may not reflect current viral population. Repeat tropism testing before initiating maraviroc-based regimens after any period of viremia or treatment interruption is strongly recommended.
Enfuvirtide (T-20) is a 36-amino acid synthetic peptide fusion inhibitor that mimics the HR2 (heptad repeat 2) region of gp41, binding the heptad repeat 1 (HR1) region and preventing the six-helix bundle formation required for membrane fusion. It is the only antiretroviral that must be administered by subcutaneous injection twice daily, which substantially limits its use to salvage regimens where oral options are exhausted. Injection site reactions (ISRs) occur in virtually all patients receiving enfuvirtide — nodules, cysts, ecchymoses, and pain at injection sites are universal; bacterial pneumonia occurred at higher rates in enfuvirtide recipients than comparators in clinical trials, possibly due to immune modulation at injection sites. Resistance emerges through mutations in the gp41 HR1 region, primarily at positions G36 (Gly36), I37 (Ile37), V38 (Val38), Q39 (Gln39), Q40 (Gln40), and N43 (Asn43), with single mutations conferring 10 to 50-fold reduced susceptibility. Given its injectable route and injection site reaction (ISR) burden, enfuvirtide has been largely superseded by newer salvage agents.8
Ibalizumab (IBA) is a humanized monoclonal antibody that binds domain 2 of the CD4 receptor, blocking post-attachment conformational changes required for co-receptor engagement without preventing CD4-mediated immune function (which requires domain 1 binding). It is administered intravenously every 2 weeks and is approved for treatment-experienced adults with multi-drug resistant (MDR) HIV-1 infection. Because it targets a host cell protein rather than viral proteins, its resistance profile is atypical: resistance emerges through mutations in the variable loop 5 (V5 loop) of gp120 that restore the ability of gp120-CD4 complexes to engage co-receptors despite ibalizumab binding at domain 2. Fostemsavir is an oral attachment inhibitor (prodrug of temsavir) that binds gp120 directly, blocking CD4 receptor engagement; it is approved for heavily treatment-experienced adults with MDR HIV-1 failing their current regimen, where at least one other active antiretroviral can be combined.8
The evolution of preferred first-line antiretroviral therapy (ART) reflects three decades of pharmacological learning: regimens have moved from complex multi-pill schedules with significant toxicity toward single-tablet regimens with high tolerability, durable efficacy, and resistance barriers so high that treatment failure with resistance selection is rare. Long-acting injectable regimens now offer monthly or bimonthly administration as a complete departure from the daily oral pill paradigm.
Current Department of Health and Human Services (DHHS) guidelines identify two preferred initial regimens for treatment-naive adults without significant resistance, co-infection, or contraindications. Bictegravir/TAF/FTC (Biktarvy) is a single-tablet once-daily regimen combining the highest-barrier integrase strand transfer inhibitor (INSTI) available with the preferred tenofovir prodrug formulation; it requires no fasting or food requirements, has no significant cytochrome P450 (CYP) interactions, and is appropriate for most patients initiating ART. Dolutegravir plus tenofovir alafenamide (TAF)/emtricitabine (FTC) (or tenofovir disoproxil fumarate (TDF)/FTC when TAF is unavailable or not preferred) is the alternative preferred regimen, requiring two pills daily but offering component-level flexibility for patients with specific contraindications to individual agents. Both regimens achieve virologic suppression (below 50 copies/mL) in approximately 90% of treatment-naive adults by week 48 in phase 3 trials, with no resistance observed at virologic failure in either arm in registrational studies.7
The dominance of INSTIs over PIs and NNRTIs in first-line therapy is pharmacologically justified on multiple grounds. Second-generation INSTIs (dolutegravir, bictegravir) have a resistance barrier so high that treatment-naive patients essentially cannot develop drug resistance during virologic failure, whereas NNRTIs have low barriers and PIs, though highly resistant themselves, require pharmacokinetic boosting with its attendant interaction burden. INSTI-based regimens produce virologic suppression faster (median time to suppression approximately 4 weeks shorter with dolutegravir (DTG) versus efavirenz-based therapy in the SINGLE (Single Pill Once Daily HIV Treatment) trial) and have tolerability profiles superior to both protease inhibitor (PI) and non-nucleoside reverse transcriptase inhibitor (NNRTI) comparators in head-to-head trials. The primary remaining concerns are weight gain (discussed in Section 3), drug interactions via uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1) induction, and the evolving neural tube defect data with dolutegravir in early pregnancy.67
Cabotegravir plus rilpivirine (CAB+RPV) long-acting injectable (LA) ART represents the most significant pharmacological innovation in human immunodeficiency virus (HIV) treatment delivery since the first fixed-dose combination. Cabotegravir is a second-generation INSTI closely related to dolutegravir, formulated as a nanosuspension for intramuscular injection. Rilpivirine is the same NNRTI used in oral formulations but similarly reformulated for injection. The approved regimen consists of oral lead-in with cabotegravir 30 mg plus rilpivirine 25 mg once daily for 4 weeks (to assess tolerability before the long-acting commitment), followed by cabotegravir 600 mg plus rilpivirine 900 mg intramuscularly once monthly, or the more recently approved every-2-month (Q2M) regimen of cabotegravir 600 mg plus rilpivirine 1,200 mg. Phase 3 trials (ATLAS, FLAIR, ATLAS-2M) demonstrated non-inferiority to daily oral ART in virologically suppressed adults, and patient-reported outcomes consistently showed strong preference for the injectable regimen.9
The prolonged pharmacokinetic tail of both cabotegravir and rilpivirine after injection discontinuation (cabotegravir detectable for up to 12 months; rilpivirine for up to 4 years in some individuals) creates a functional monotherapy window if injections are stopped abruptly. Patients who wish to discontinue injectable therapy must transition to oral ART promptly — DHHS recommends starting oral ART the day after the last injection. Contraindications include: NNRTI or INSTI resistance mutations in the treatment history, potent UGT1A1/CYP3A4 inducers (rifamycins), pre-treatment VL above 100,000 copies/mL or CD4 below 200 (same as oral rilpivirine), and pregnancy (insufficient safety data).
The pharmacokinetic profile of long-acting CAB+RPV merits detailed understanding. After intramuscular gluteal injection, both drugs are absorbed slowly from the injection site depot (T-max approximately 7 days for cabotegravir, 3 to 4 days for rilpivirine LA), providing sustained plasma concentrations throughout the dosing interval. Cabotegravir LA at 600 mg monthly maintains trough concentrations well above the protein-adjusted 90% inhibitory concentration (PA-IC90) throughout the 4-week interval in most patients. Rilpivirine LA trough concentrations are lower relative to its PA-IC90, and the rilpivirine component imposes the same contraindications as oral rilpivirine: exclusively wild-type virus is required, and patients with pre-treatment baseline RPV resistance mutations are ineligible. The injection site pain associated with intramuscular administration (particularly rilpivirine, which requires a large injection volume) decreases substantially with successive injections and is the most common reason for discontinuation in clinical trials, affecting approximately 20% of recipients but leading to discontinuation in fewer than 1%.910
Lenacapavir (LEN) represents an additional long-acting antiretroviral with a novel mechanism: capsid inhibitor. Lenacapavir disrupts the HIV type 1 (HIV-1) capsid protein through multiple mechanisms including inhibition of capsid assembly, nuclear import of the pre-integration complex, and impairment of capsid-nucleoporin interactions required for nuclear pore traversal. It is formulated for subcutaneous injection every 6 months and approved in combination with an optimized background regimen for heavily treatment-experienced adults with multi-drug resistant (MDR) HIV-1. Lenacapavir is both a cytochrome P450 3A4 (CYP3A4) substrate and a moderate CYP3A4 inhibitor, requiring careful attention to drug interactions. Its 6-month dosing interval represents the longest between antiretroviral injections approved to date, and ongoing trials are evaluating lenacapavir as a component of long-acting first-line regimens.911
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