Antiretroviral Therapy — HIV Drug Classes

Antiretroviral therapy (ART) comprises multiple classes of drugs that inhibit different stages of the HIV replication cycle, fundamentally altering the natural history of HIV infection from a rapidly progressive fatal disease to a chronic manageable condition. The HIV pandemic affects approximately 39 million people globally, with nearly 86% of those diagnosed now receiving ART, achieving dramatic reductions in AIDS-related mortality and opportunistic infections. Modern combination ART (cART) typically utilizes three or more drugs from at least two different classes, termed highly active antiretroviral therapy (HAART), which suppresses viral replication to undetectable levels (<50 copies/mL) in the vast majority of adherent patients. Understanding the mechanisms of action, resistance patterns, pharmacokinetics, and side effect profiles of each drug class is essential for appropriate regimen selection, managing treatment failures, and predicting outcomes. For medical students and residents, mastery of ART fundamentals is critical for USMLE Step 2 CK and essential for clinical practice, as HIV management now represents routine internal medicine and primary care.

HIV replication requires a complex sequence of molecular events, each susceptible to pharmacologic intervention through distinct drug classes. Understanding where each class works in this lifecycle is essential for rational drug selection and resistance management.

HIV initiates infection by binding to the CD4 receptor and a co-receptor (CCR5 or CXCR4) on target cells, primarily CD4+ T lymphocytes, macrophages, and dendritic cells. This dual-receptor engagement undergoes conformational changes in the viral envelope glycoproteins (gp120 and gp41), triggering fusion of viral and cellular membranes. Entry inhibitors (CCR5 antagonists like maraviroc) block the co-receptor, while fusion inhibitors (enfuvirtide) bind gp41 and prevent membrane fusion. The efficiency of entry depends on tropism (R5-tropic vs X4-tropic strains), which has profound implications for drug selection and resistance patterns. Once fusion occurs, the viral core enters the cytoplasm, and this step represents the first potential therapeutic intervention point.

Following cell entry, the viral RNA genome must be converted to DNA to integrate into the host genome. Reverse transcriptase (RT) catalyzes this step and is the target of two major drug classes: nucleoside/nucleotide RT inhibitors (NRTIs/NtRTIs) and non-nucleoside RT inhibitors (NNRTIs). NRTIs (zidovudine, lamivudine, emtricitabine, tenofovir) are nucleoside analogs that, once phosphorylated intracellularly, compete with natural nucleotides and cause chain termination during DNA synthesis. NNRTIs (efavirenz, rilpivirine, doravirine) bind directly to the RT enzyme at an allosteric site, non-competitively inhibiting catalytic activity. The reverse transcription step is error-prone (error rate ~1 in 10,000 nucleotides), generating genetic diversity that drives resistance evolution. Once reverse transcription is complete, the pre-integration complex translocates to the nucleus where integrase catalyzes integration into chromosomal DNA, a process blocked by integrase strand transfer inhibitors (INSTIs) like dolutegravir, bictegravir, and raltegravir, which prevent the strand transfer reaction essential for integration.

Once integrated, the HIV genome is transcribed into mRNA, which is translated by host ribosomes into viral proteins (Gag, Pol, Env polyproteins). These proteins are assembled into immature virions that bud from the cell surface as infectious but non-replicating particles. Maturation requires HIV protease, an aspartic protease that cleaves the Gag and Pol polyproteins into functional enzymes and structural proteins. Protease inhibitors (PIs) (ritonavir, darunavir, lopinavir, atazanavir) competitively inhibit protease activity, resulting in production of non-infectious immature particles that cannot establish productive infection in newly infected cells. This class has extraordinary potency and a high genetic barrier to resistance, making PIs valuable in treatment-experienced patients with resistant virus.

HIV preferentially infects and destroys CD4+ T lymphocytes, which orchestrate adaptive immunity. The destruction occurs through direct cytopathic effects (viral replication causes cell lysis), chronic immune activation (driving exhaustion and apoptosis), and bystander killing of uninfected CD4 cells. With declining CD4 counts below 200 cells/μL, patients lose protection against opportunistic infections (Pneumocystis jirovecii, Cytomegalovirus, toxoplasmosis). Effective ART halts viral replication, allowing CD4 count reconstitution through thymic production and peripheral proliferation of existing cells, typically recovering 50-100 cells/μL per year once viremia is suppressed. The CD4 nadir (lowest count ever measured) and current CD4 count remain the best predictors of opportunistic infection risk and inform prophylaxis decisions.

HIV infection results from transmission of the virus through specific bodily fluids and routes; antiretroviral therapy is indicated universally for all HIV-positive patients regardless of risk factors.

Unprotected receptive anal intercourse carries the highest per-act transmission risk (~1.4%), followed by receptive vaginal intercourse (~0.08%), insertive anal intercourse (~0.06%), and insertive vaginal intercourse (~0.04%). Mucosal surfaces in the genital and rectal tracts contain high densities of CD4+ T cells and macrophages, providing cellular targets for viral infection. Inflammatory conditions (other sexually transmitted infections, ulcerative genital lesions) increase transmission risk by recruiting activated lymphocytes and disrupting epithelial barriers. Sexual transmission accounts for approximately 80% of new HIV infections globally and remains the predominant route in developed nations. The concept of "undetectable = untransmittable" (U=U) has revolutionized HIV management; patients with sustained viral loads <50 copies/mL achieve zero risk of sexual transmission (Prevention of Sexual Transmission of HIV in Serodifferent Couples with Antiretroviral Therapy, PARTNER study).

Sharing of needles, syringes, or injection equipment among people who inject drugs carries ~0.63% per-act transmission risk and accounts for approximately 5-10% of global HIV infections. Healthcare-associated transmission through needlestick injury carries ~0.3% per-act risk and is now extraordinarily rare due to universal precautions and post-exposure prophylaxis (PEP). Vertical transmission from mother-to-child occurs in 15-45% of untreated pregnancies but is reduced to <1% when the pregnant patient receives ART with undetectable viral load. These transmission routes emphasize that effective ART in the source patient or PrEP/PEP in the exposed individual prevents transmission across all routes.

Genetic factors including CCR5-Δ32 deletion (conferring protection against R5-tropic virus) and HLA alleles influence disease progression rates. Viral factors such as tropism (R5 vs X4), genetic barrier to resistance, and replication capacity affect clinical course and treatment response. However, universal ART eligibility means that all individuals warrant treatment regardless of these prognostic variables, as suppressed viral replication eliminates the opportunity for these factors to influence disease trajectory.

ART is administered to suppress viral replication and prevent disease progression; the clinical presentation varies by treatment adherence, degree of viremia suppression, and CD4 count recovery.

Acute retroviral syndrome (ARS) occurs in 40-90% of newly infected individuals 2-4 weeks after exposure, presenting with constitutional symptoms mimicking acute viral illness: fever, fatigue, myalgias, arthralgias, and pharyngitis. The syndrome reflects robust viral replication (often >100,000 copies/mL) and vigorous immune activation; patients may have transient CD4 decline but usually recover spontaneously as adaptive immunity develops. Persistent viremia over months to years leads to progressive CD4 depletion, with clinical manifestations appearing when CD4 falls below 200 cells/μL: opportunistic infections (Pneumocystis jirovecii pneumonia, oropharyngeal candidiasis, cryptococcal meningitis), malignancies (Kaposi sarcoma, non-Hodgkin lymphoma), and wasting syndrome. The pre-ART era's "AIDS-defining illnesses" (CDC category C diseases) represent the spectrum of severe immunosuppression.

Patients achieving and maintaining viral suppression (<50 copies/mL) experience normalization of immune function, cessation of opportunistic infection risk (when CD4 >200 cells/μL), and normal life expectancy. Immune reconstitution typically manifests as rising CD4 counts (50-100 cells/μL per year for first 2 years, then plateauing at 500-900 cells/μL). Immune reconstitution inflammatory syndrome (IRIS) occurs in a subset of patients, typically those with very low CD4 counts at ART initiation; paradoxically, immune recovery can trigger exaggerated inflammatory responses to opportunistic pathogens or their antigens, manifesting as worsening clinical symptoms despite improving CD4 counts and declining viral load.

Oral candidiasis (white plaques on tongue, erythema, dysphagia) indicates CD4 <100 cells/μL in untreated patients; successfully treated patients rarely develop this finding. Lipodystrophy (facial fat loss, dorsal hump, central obesity) historically occurred with certain ART regimens (particularly protease inhibitors and thymidine NRTIs) but is less common with modern regimens. Hepatosplenomegaly may indicate chronic immune activation or opportunistic infections. Kaposi sarcoma presents as painless violaceous macules or nodules on skin or mucous membranes, typically in patients with CD4 <50 cells/μL with HHV-8 co-infection.

Beyond therapeutic benefit, some ART drugs produce direct toxicities: nucleoside reverse transcriptase inhibitors can cause mitochondrial toxicity (lactic acidosis, lipoatrophy, neuropathy); protease inhibitors may cause gastrointestinal upset and metabolic abnormalities; non-nucleoside reverse transcriptase inhibitors can cause neuropsychiatric symptoms and rash. These toxicities must be distinguished from viral disease progression, as they may necessitate regimen modification.

Diagnosis of HIV infection precedes and is distinct from treatment decisions, but understanding viral load, CD4 count, and resistance patterns is essential for tailoring ART.

The current CDC-recommended HIV testing algorithm begins with a 4th-generation antigen/antibody combo test that detects HIV antibodies and p24 antigen simultaneously, reducing the window period (time between infection and detection) to approximately 18-45 days. Positive combo tests trigger a HIV-1/HIV-2 differentiation immunoassay to distinguish HIV-1 (responsible for >99% of global infections) from HIV-2 (West African endemic). If results are discordant or inconclusive, reflex to HIV-1 nucleic acid testing (NAT/RNA) provides definitive diagnosis and simultaneously measures viral load. This algorithm achieves >99.9% specificity, virtually eliminating false-positive diagnoses. Sensitivity approaches 100% after 3 weeks of infection.

HIV RNA (viral load) is measured by reverse-transcription polymerase chain reaction (RT-PCR), with standard assays detecting 40-10,000,000+ copies/mL (detection limit varies by assay). Viral load correlates directly with disease progression rate and infectivity; each log increase in viral load increases AIDS risk approximately 1.5-fold. ART goal is <50 copies/mL (below the level of quantification for most assays, defined as "virologically suppressed" or "undetectable"). Viral loads should be monitored 2-4 weeks after ART initiation, then monthly until suppression achieved, then every 3 months once suppressed and stable. Viral rebound (detected viral load in previously suppressed patient) suggests treatment failure from non-adherence, inadequate drug levels, or emergence of resistant virus.

CD4+ T cell count (normal range 500-1,500 cells/μL) predicts risk of opportunistic infections with specific thresholds: <500 cells/μL increases infection risk broadly, <200 cells/μL mandates Pneumocystis jirovecii prophylaxis (trimethoprim-sulfamethoxazole or alternatives), <100 cells/μL warrants cytomegalovirus and toxoplasmosis prophylaxis consideration, and <50 cells/μL indicates MAC (Mycobacterium avium complex) prophylaxis. CD4 recovery upon ART typically occurs over months to years; initial recovery is often rapid (50-100 cells/μL per month for first 3 months) due to redistribution of existing lymphocytes, followed by slower recovery from new thymic production. CD4 count should be measured at baseline, 2-4 weeks into ART, then every 3 months until >200 cells/μL on ART for >3 months, then every 6-12 months. CD4 count does NOT need to be rechecked after immune reconstitution on stable suppressive ART.

Genotypic resistance testing (sequencing of reverse transcriptase, integrase, and protease genes) identifies mutations conferring resistance to specific drugs. Genotypic testing is performed at baseline (prior to ART initiation) to guide initial regimen selection and upon suspected treatment failure (with detectable viral load >500-1,000 copies/mL) to identify which drugs have become ineffective. Interpretation requires understanding resistance pathways: for example, the M184V mutation in reverse transcriptase confers lamivudine/emtricitabine resistance; thymidine analog mutations (TAMs) like M41L and D67N confer zidovudine resistance. Phenotypic testing (measuring in vitro virus susceptibility to drugs) provides additional information but is less commonly used due to complexity and cost. Tropism testing (determining CCR5 vs CXCR4 tropism via genotype or phenotype) is required before initiating CCR5 antagonists like maraviroc, as this class is only effective against R5-tropic virus.

At baseline, evaluate: (1) CD4 count and percentage; (2) viral load (quantitative HIV RNA); (3) genotypic resistance testing; (4) tropism testing (if considering entry inhibitors); (5) hepatitis B and C serologies; (6) renal function (creatinine, eGFR) because some drugs require dose adjustment; (7) lipid panel and glucose to establish cardiovascular and metabolic risk; (8) bone mineral density screening in older adults. Hepatitis B co-infection requires special consideration, as certain ART drugs (lamivudine, emtricitabine, tenofovir) have activity against HBV, and abrupt discontinuation risks hepatitis B reactivation. These baseline tests inform drug selection and establish monitoring parameters.

Modern ART utilizes combination therapy with ≥3 drugs from ≥2 different classes, tailored based on baseline characteristics, comorbidities, and preferences. The paradigm has shifted from rigid standardized regimens to individualized selection, with an unprecedented number of highly effective options.

The major ART drug classes target distinct steps in the HIV replication cycle: (1) Nucleoside/Nucleotide Reverse Transcriptase Inhibitors (NRTIs/NtRTIs): zidovudine (AZT), lamivudine (3TC), emtricitabine (FTC), tenofovir disoproxil fumarate (TDF), tenofovir alafenamide (TAF), abacavir (ABC),