🧬 Antiretroviral Regimen Selection Simulator

Baseline Assessment — Building the Clinical Picture Before Prescribing

Before any antiretroviral is chosen, clinicians assemble a baseline profile: plasma HIV RNA (viral load), CD4+ T-cell count, an HIV genotypic resistance test, a review of comorbid conditions and concomitant medications, and — for people who can become pregnant — a pregnancy-potential assessment. This baseline determines both urgency and constraints on regimen selection.

• 5–7: Baseline tests recommended (VL, CD4, genotype, renal/hepatic panel, HLA-B*5701, HBV/HCV serology)
• Same day–7 days: Time to ART initiation (rapid-start strategies now widely favored)
• ~10–15%: Transmitted resistance prevalence (to any drug class, region-dependent)
• <200 / 200–500 / >500: CD4 count categories (cells/µL — informs OI prophylaxis need)

What baseline testing establishes

Plasma HIV-1 RNA (viral load) quantifies how much virus is replicating and serves as the primary marker tracked after treatment starts — the explicit goal is durable suppression below the assay's lower limit of quantification.

CD4+ T-cell count stages immunologic damage and flags the need for opportunistic-infection prophylaxis (e.g., Pneumocystis jirovecii prophylaxis below 200 cells/µL) and heightens vigilance for immune reconstitution inflammatory syndrome (IRIS) after starting therapy.

Genotypic resistance testing (protease, reverse transcriptase, and increasingly integrase) sequences the circulating virus to detect transmitted or archived resistance mutations before any drug is chosen — this single test can redirect the entire third-agent decision in Stage 3.

Comorbidities screened at baseline include renal function (eGFR), hepatic function and viral hepatitis coinfection (HBV/HCV), cardiovascular risk factors, psychiatric history, bone density risk, and drug-drug interaction potential with existing medications.

Pregnancy potential is assessed because certain agents carry different safety and dosing considerations across the reproductive lifecycle, and regimen choice may be adjusted accordingly under current perinatal guidelines.

Modern guidelines increasingly favor "rapid ART start" — initiating treatment within days of diagnosis, sometimes before resistance results return — using regimens with a high barrier to resistance so that an unexpected mutation profile is unlikely to compromise the initial choice.

The NRTI Backbone — Two Drugs That Anchor Almost Every Regimen

Nearly all contemporary antiretroviral regimens are built on a two-drug nucleos(t)ide reverse transcriptase inhibitor (NRTI) "backbone." The dominant modern pairing is tenofovir (as tenofovir alafenamide, TAF, or tenofovir disoproxil fumarate, TDF) combined with emtricitabine — with lamivudine-based pairings (including abacavir/lamivudine) as common alternatives.

• TAF/FTC: Most-used backbone (tenofovir alafenamide + emtricitabine)
• Improved: TAF vs TDF bone/renal signal (TAF associated with fewer renal & bone effects)
• HLA-B*5701: Abacavir prerequisite (screen before use — hypersensitivity risk)
• ~10 h: Backbone half-life (FTC) (supports once-daily dosing)

Why a two-NRTI backbone remains the default architecture

NRTIs are chain-terminating nucleoside/nucleotide analogs incorporated by HIV reverse transcriptase into the growing proviral DNA strand, halting further elongation. Pairing two NRTIs with non-overlapping resistance pathways provides redundancy: a single resistance mutation rarely disables both drugs simultaneously.

Tenofovir alafenamide (TAF) has largely supplanted tenofovir disoproxil fumarate (TDF) in guideline-preferred regimens because it achieves higher intracellular drug concentrations at a lower plasma dose, translating into a more favorable renal and bone-mineral-density safety profile, while TDF remains widely used globally for its lower cost and extensive experience.

Emtricitabine (FTC) and lamivudine (3TC) are cytidine analogs with near-identical resistance profiles (both selected for by the M184V mutation) and are considered largely interchangeable backbone partners.

Abacavir/lamivudine is an alternative backbone reserved for patients who screen negative for HLA-B*5701 (to avoid abacavir hypersensitivity reaction) and who have lower baseline viral loads, per some guideline pathways, plus adequate cardiovascular risk consideration.

Two-drug regimens without a separate NRTI backbone (e.g., an INSTI plus lamivudine) exist as guideline-endorsed alternatives for selected patients, but the three-drug, NRTI-backbone-anchored architecture remains the default starting framework this simulator models.

Choosing the Third Agent — Why INSTIs Lead Current Guidelines

The third drug completes the regimen and is chosen from three main classes: integrase strand transfer inhibitors (INSTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs), or boosted protease inhibitors (PIs). Current DHHS and EACS guidelines recommend an INSTI-based regimen for most treatment-naive patients, owing to potent and rapid viral suppression, a favorable tolerability profile, few drug-drug interactions, and — for second-generation agents — a high genetic barrier to resistance.

• INSTI: Guideline-preferred class (bictegravir / dolutegravir-based regimens)
• Fastest: Time to VL <50 c/mL (INSTIs vs NNRTI/PI comparators in trials)
• High: 2nd-gen INSTI resistance barrier (bictegravir, dolutegravir)
• Alternative: Boosted-PI role today (reserved for select resistance/adherence cases)

Integrase strand transfer inhibitors (INSTIs) — the preferred class

INSTIs block the strand-transfer step of viral integrase, preventing proviral DNA from inserting into the host genome — a mechanistically late and highly vulnerable step in the viral life cycle.

Second-generation INSTIs (bictegravir, dolutegravir) combine potent antiviral activity with a high genetic barrier to resistance: multiple mutations are typically required before clinically meaningful resistance emerges, which is why they anchor most guideline-preferred single-tablet regimens today.

Advantages driving guideline preference: fast viral load decline, low rates of treatment-emergent resistance in treatment-naive patients, a comparatively clean drug-interaction profile (fewer CYP450-mediated interactions than boosted PIs), and generally favorable lipid and gastrointestinal tolerability versus older agents.

Considerations: some INSTIs carry interaction caveats with polyvalent cation-containing products (antacids, some supplements) which can reduce absorption if not appropriately spaced, and a subset of patients report neuropsychiatric side effects warranting monitoring.

Alternatives — NNRTIs and boosted PIs

NNRTIs (e.g., doravirine, efavirenz, rilpivirine) bind an allosteric pocket on reverse transcriptase. Newer agents like doravirine offer improved tolerability and a cleaner resistance profile than earlier-generation NNRTIs, but the class overall has a lower genetic barrier to resistance than second-generation INSTIs — a single mutation can compromise efficacy for some NNRTIs.

Boosted protease inhibitors (e.g., darunavir/ritonavir or darunavir/cobicistat) inhibit the viral protease enzyme responsible for cleaving polyprotein precursors into mature, infectious virions. Boosted PIs retain a high barrier to resistance and remain valuable in patients with extensive prior resistance or adherence concerns, but carry a heavier drug-interaction burden (via CYP3A4 inhibition) and a less favorable metabolic/GI tolerability profile than INSTIs.

When baseline genotype flags resistance-associated mutations — as toggled in this simulator's resistance slider — the algorithm favors an alternative, resistance-informed regimen (often built around a boosted PI or a resistance-tailored INSTI/NNRTI selection) rather than the standard first-line INSTI-based single-tablet regimen.

Single-Tablet Regimens — Consolidating the Backbone and Third Agent

Where the backbone and third agent are chemically and pharmacokinetically compatible, manufacturers co-formulate them into a single fixed-dose combination pill taken once daily. Reducing pill burden from three or more tablets to one is one of the most consistently demonstrated levers for improving adherence — and adherence is the single strongest predictor of durable viral suppression.

• Higher: STR adherence advantage (vs multi-tablet regimens in adherence studies)
• 1 tablet/day: Typical STR pill count (backbone + third agent co-formulated)
• 2–4 tablets/day: Multi-tablet fallback (when components can't be co-formulated)
• Multiple: Approved STR combinations (INSTI-, NNRTI-, and PI-based options exist)

Why fewer pills translates into better outcomes

Adherence to antiretroviral therapy needs to be consistently high to maintain viral suppression and prevent the emergence of resistance mutations; even modest lapses in dosing can allow viral replication to resume under selective drug pressure.

Single-tablet regimens (STRs) reduce the daily decision points and logistical burden of treatment: one pill, once daily, is simpler to remember, easier to travel with, and less stigmatizing to take in public or shared living situations than a multi-pill regimen.

STRs also simplify prescribing and reduce the chance of partial-regimen errors (e.g., a patient inadvertently continuing only part of a multi-drug regimen after a prescription lapse), and they are associated in observational and trial data with modestly higher rates of sustained viral suppression compared with equivalent multi-tablet regimens.

Not every effective combination can be co-formulated — differences in dosing frequency, food requirements, or chemical compatibility sometimes require a multi-tablet regimen even when each individual drug remains guideline-preferred. In those cases, once-daily dosing of all components is still prioritized where possible to preserve simplicity.

When a resistance-adjusted regimen is required (see the resistance slider), a true single-tablet option may not exist for that specific combination — reflected in this simulator as a higher pill-burden metric even after reaching the single-tablet-consideration stage.

Individualized Finalization — Tailoring the Regimen to the Person

The last step folds every earlier input back together with patient-specific factors: renal and hepatic function (which can require dose adjustment or exclude certain agents), potential drug-drug interactions with existing medications, the genotype-derived resistance profile, and the patient's own preferences and priorities. The result is a regimen that is not just guideline-concordant, but individually appropriate.

• eGFR-based: Renal-adjustment triggers (TDF and some NRTIs need dose/agent review)
• Multiple: Interaction screening sources (CYP450, transporter, and QT-interval checks)
• Ongoing: Guideline update cadence (DHHS/EACS/WHO revise recommendations regularly)
• Central: Shared decision-making (patient priorities weighed alongside clinical data)

The factors that shape the final prescription

Renal function: reduced eGFR can necessitate dose adjustment or avoidance of certain agents (historically a key consideration for TDF); TAF-based regimens are often preferred when renal impairment is a concern, within labeled thresholds.

Hepatic function and viral hepatitis coinfection: some agents require caution or dose modification in hepatic impairment, and choices may be coordinated with hepatitis B or C treatment plans when coinfection is present, since certain NRTIs (tenofovir, lamivudine, emtricitabine) are also active against HBV.

Drug-drug interactions: concomitant medications (anticonvulsants, acid-reducing agents, other chronic therapies) are screened against the proposed regimen; boosted PIs and, to a lesser extent, some INSTIs carry more interaction potential than others, which can tip the final agent choice.

Resistance mutations: any mutations flagged at baseline (or emerging on therapy) are cross-referenced against the proposed regimen's genetic barrier, steering selection toward agents unaffected by the specific mutation pattern detected.

Patient preference: dosing frequency, pill size, food requirements, prior drug experience, and personal priorities (e.g., minimizing specific side-effect risks) are incorporated through shared decision-making, since a regimen a patient can sustain long-term will outperform a theoretically optimal one that is poorly tolerated or inconvenient.

Regimen selection is not a one-time decision — ongoing monitoring of viral load, tolerability, and any new interacting medications means the "final" regimen may be revisited and re-individualized over the course of long-term care.