HIV Resistance Mutations & DTG/3TC Management

Educational study digest synthesizing key HIV resistance and two-drug regimen literature for fellows. The enrichment box is reviewer synthesis of additional evidence, each cited. Not medical advice.

In one line

DTG/3TC is a guideline-recommended two-drug regimen with a high barrier to resistance in patients without prior treatment failure — but using it in patients with historic M184V/I lamivudine resistance remains investigational, with emerging data showing maintained suppression but unresolved questions about proviral resistance dynamics and integrase resistance risk.

◆ What is M184V/I and why does it matter so much?

The M184V/I mutation is the most common NRTI resistance mutation in clinical practice. It is the first mutation to appear whenever a regimen containing lamivudine (3TC) or emtricitabine (FTC) begins to fail — before thymidine analogue-associated mutations (TAMs) or any other NRTI mutations emerge.¹ M184V confers high-level resistance to both 3TC and FTC, rendering them essentially inactive as standalone agents against the mutant virus.

But M184V is not simply bad news. The mutation comes with trade-offs that are clinically exploitable. First, it increases susceptibility to tenofovir (both TDF and TAF), making tenofovir-containing salvage regimens more effective in its presence. Second, it reduces viral replicative fitness, meaning the mutant virus replicates less efficiently than wild-type — a property that slows disease progression even when the virus is not fully suppressed. Third, M184V delays the accumulation of TAMs, those progressively debilitating mutations (M41L, D67N, K70R, L210W, T215Y/F, K219Q/E/N/R) that eventually confer broad NRTI cross-resistance.¹

These properties are why guidelines have long endorsed continuing 3TC or FTC in salvage regimens even when M184V is present — you lose the drug’s direct antiviral activity but retain its fitness cost on the virus.

◆ DTG/3TC as a two-drug regimen: what is the evidence base?

The approval of coformulated dolutegravir/lamivudine (Dovato) as a complete regimen rests on a solid trial program, all conducted in patients without known NRTI resistance.

The GEMINI-1 and GEMINI-2 trials randomized 1,433 treatment-naive patients with HIV RNA <500,000 copies/mL to DTG plus 3TC versus DTG plus TDF/FTC. At 96 weeks, DTG/3TC was noninferior (86% vs 89.5% with HIV RNA <50 copies/mL), and critically, no treatment-emergent NRTI or INSTI resistance occurred in either group. By week 144, noninferiority was maintained at 82% versus 84%.²

For the switch setting, the phase 3 TANGO trial demonstrated that switching from a TAF-based 3- or 4-drug regimen to DTG/3TC was noninferior through 144 weeks, with zero participants meeting confirmed virologic withdrawal criteria and no observed resistance.³

The SALSA trial broadened this to a more diverse population: 493 virologically suppressed adults (39% women, 39% aged ≥50, 19% African American/African heritage) were randomized to switch to DTG/3TC or continue their current antiretroviral regimen (including NNRTI-, INSTI-, and PI-based regimens, not just TAF-based). At week 48, only 1 participant (0.4%) in the DTG/3TC arm and 3 (1.2%) in the CAR arm had HIV RNA ≥50 copies/mL (adjusted difference −0.8%; 95% CI −2.4% to 0.8%), demonstrating noninferiority. Zero participants met confirmed virologic withdrawal criteria, so no resistance testing was needed.⁴

Drug-related adverse events in SALSA were higher with DTG/3TC early on (20% vs 6% through week 48), but after the adjustment period they equalized (5% vs 2% post–week 24). Weight gain averaged 2.1 kg with DTG/3TC versus 0.6 kg with continued therapy — a difference partly attributable to participants switching off TDF- and efavirenz-containing regimens known to suppress weight. Proximal tubular renal function and bone turnover biomarkers improved with the switch.⁴

◆ Can DTG/3TC work in patients with historic M184V/I?

This is the frontier question. The pivotal licensing studies for DTG/3TC excluded individuals with prior NRTI resistance, and current guidelines explicitly reflect this: DTG/3TC requires a genotype demonstrating 3TC sensitivity before initiation.^2,5 Yet M184V/I is so common that a substantial population of virologically suppressed patients carry it in their treatment history, and the simplicity of DTG/3TC makes it an attractive option if it can be used safely.

Three prospective studies have now addressed this question directly, all using the same strategy: switch virologically suppressed patients with historic M184V/I (not detectable on current baseline Sanger sequencing) to DTG/3TC.

VOLVER-GESIDA

An open-label, single-arm phase 2a trial at 17 centers enrolled 121 participants with documented historic M184V/I (confirmed absent on baseline proviral Sanger sequencing). At week 96, 87% maintained HIV RNA <50 copies/mL, 3.3% had low-level viremia but remained on study, and 9.9% had missing data. Two individuals (1.7%) experienced confirmed virologic failure, but neither developed integrase resistance. The study used next-generation sequencing (NGS) to evaluate M184V/I dynamics and found a statistically significant increase in the frequency of M184V/I detection in proviral DNA between baseline and week 96.⁶

ART-PRO and SOLAR-3D

Both studies prospectively followed suppressed patients with historic M184V/I switched to DTG/3TC through 144 weeks and found no virologic failures and no treatment-emergent resistance.⁶

The proviral reservoir signal

A finding that demands attention, even if its clinical meaning is uncertain.

The VOLVER-GESIDA study’s observation of increasing M184V/I frequency in proviral DNA over time is the most consequential result from these trials. Proviral reservoir dynamics are known to be influenced by clonal proliferation and sampling variability, and changes in M184V/I frequency in proviral DNA do not necessarily reflect ongoing viral replication.⁶ However, the possibility that this increase reflects incomplete suppression by a regimen that relies on a drug (3TC) against which the archived virus is highly resistant cannot be excluded. A larger study or longer follow-up might reveal more virologic failures — and some of those could involve integrase resistance, an outcome with significant downstream consequences.⁶

◆ Why does integrase resistance matter so disproportionately?

The concern about DTG/3TC with historic M184V/I is not just about the risk of virologic failure — it is about what kind of resistance could emerge if it does. Integrase strand transfer inhibitors (INSTIs) are the backbone of modern HIV treatment. DTG and bictegravir (BIC) anchor nearly every recommended initial regimen, and long-acting cabotegravir (CAB-LA) is the only injectable treatment option. If a patient develops integrase resistance on DTG/3TC, they lose access to the entire dominant drug class.

The experience with dolutegravir monotherapy is instructive. Even this highly potent agent selected for integrase resistance in a small but clinically meaningful proportion of patients when used alone — a risk that has been deemed unacceptable.⁶ DTG/3TC in a patient with M184V/I is not quite monotherapy, since 3TC still exerts some selective pressure and fitness cost, but it is closer to functional monotherapy than a standard three-drug regimen.

By contrast, in settings where a slightly higher resistance risk is tolerated — such as long-acting CAB/RPV — the trade-off is justified by substantial quality-of-life advantages: reduced self-stigma, improved privacy, and the convenience of injectable dosing.⁶ DTG/3TC has not demonstrated convincing advantages over tenofovir-based three-drug oral regimens, which are well tolerated, durable, active against hepatitis B, and associated with minimal toxicity. In that context, even a small increase in integrase resistance incidence carries outsized importance.⁶

◆ What do current guidelines say?

The HHS Guidelines for Antiretroviral Agents in Adults and Adolescents (2026) recommend DTG/3TC (AI rating) as a preferred initial regimen for most people with HIV, with four explicit exceptions:⁵

HIV RNA >500,000 copies/mL
HBV coinfection (unless another HBV-active drug is added)
Prior CAB-LA PrEP exposure without documented INSTI sensitivity
Starting ART before genotypic resistance testing results for reverse transcriptase are available

For the switch setting, the guidelines endorse DTG/3TC as a maintenance strategy in people with ongoing virologic suppression and no history of prior virologic failure or resistance to these agents (AI), citing TANGO, SALSA, and smaller studies.⁵

Importantly, the guidelines do not currently endorse DTG/3TC in the presence of documented M184V/I resistance, whether historical or current. The editorial assessment of the VOLVER-GESIDA data concludes that “these data are not sufficient to warrant a change in routine clinical practice or guideline recommendations” and that broader adoption “should await additional, preferably comparative, data with longer follow-up and clearer reassurance regarding virologic failure and integrase resistance risk.”⁶

◆ What about cost and tenofovir toxicity — the arguments for DTG/3TC?

Two factors could shift the calculus. TDF-related renal and bone toxicity is cumulative and becomes more important as the HIV population ages, particularly in patients with other risk factors. The SALSA trial showed improved proximal tubular renal function and bone turnover biomarkers in participants who switched off TDF to DTG/3TC.⁴ However, TAF does not carry the same degree of toxicity risk as TDF, making this a less pressing argument where TAF is available.

Cost is the other lever. With generic dolutegravir expected within 3–5 years in the United States, DTG/3TC will offer substantial savings over branded options. This is especially relevant in resource-limited settings where TLD (tenofovir/lamivudine/dolutegravir) is already the default regimen for most people with HIV — a fully powered study comparing DTG plus TDF/3TC versus DTG/3TC in virologically suppressed patients with prior treatment failure would greatly help inform care in that context.⁶

Notably, the INSTINCT trial (recently published) randomized patients to switch from DTG/3TC to BIC/TAF/FTC or continue DTG/3TC and did not find significant increases in weight, lipids, or other key metabolic outcomes with the change to a TAF-containing regimen — partially allaying earlier concerns about TAF-associated metabolic effects.⁶

Enrichment — The NRTI resistance landscape for the fellow

Reviewer addition (beyond the primary articles). Grounded in current guidance and foundational literature.

Understanding M184V/I in isolation misses the broader architecture of NRTI resistance. The six thymidine analogue-associated mutations (TAMs) — M41L, D67N, K70R, L210W, T215Y/F, and K219Q/E/N/R — confer progressively broader NRTI cross-resistance as they accumulate, with the degree of resistance to each NRTI increasing with the number of TAMs present. There are two major TAM pathways: the M41L/L210W/T215Y pathway, which causes higher-level zidovudine resistance and greater cross-resistance, and the D67N/K70R/K219Q pathway, which causes lower-level resistance with a greater decrease when M184V is also present.¹

The K65R mutation is the signature tenofovir resistance mutation. It can also emerge with abacavir, though L74V is more common. K65R reduces susceptibility to tenofovir, abacavir, didanosine, and to some degree 3TC/FTC, but increases susceptibility to zidovudine — a reciprocal relationship with the TAM pathway that is clinically exploitable in salvage therapy.¹

For NNRTIs, K103N is the most common resistance mutation and confers high-level cross-resistance across the class (efavirenz, nevirapine). Sequential NNRTI use after failure with resistance is generally not effective with first-generation agents. NNRTI resistance mutations do not reduce viral fitness, unlike some NRTI mutations — so there is no benefit to maintaining an NNRTI in a failing regimen, and doing so only accumulates additional mutations that may compromise second-generation NNRTIs like etravirine and doravirine.¹

PI resistance has become less common with the widespread use of ritonavir- and cobicistat-boosted regimens. Failure with boosted darunavir, lopinavir, and atazanavir is infrequently associated with PI resistance emergence, unlike unboosted-PI-era patterns where D30N (nelfinavir) and L90M accumulated readily. The PI mutations at codons 82, 84, and 90 are associated with significant cross-resistance across the class.¹

Resistance testing at diagnosis is now standard of care. Guidelines recommend genotypic testing at the time of HIV diagnosis regardless of whether treatment will begin immediately. US surveillance data have shown transmitted resistance to at least one drug class in approximately 14.5% of newly diagnosed treatment-naive individuals, including NRTI resistance in 7.1%, NNRTI resistance in 8.4%, and PI resistance in 2.8%. Transmitted resistant virus persists for prolonged periods because reversion to wild-type requires back mutation rather than selection of pre-existing wild-type strains. Genotypic analysis is preferred over phenotypic testing for baseline screening because it is more sensitive for detecting mixtures of susceptible and resistant virus.^1,5

A specific emerging concern is M184V/I in the context of PrEP breakthrough infections. In a cohort of people with newly diagnosed HIV in New York City, 2% had M184V/I, and those with prior oral PrEP use were four to seven times more likely to harbor this resistance. This reinforces why DTG/3TC should not be started without genotypic resistance test results.⁵

Enrichment — Genotypic vs phenotypic resistance testing

Reviewer addition. Foundational testing concepts relevant to the clinical decisions above.

Genotypic testing identifies specific mutations in the viral genome and uses rule-based algorithms to predict drug susceptibility. It is faster, less expensive, and more sensitive for detecting mixtures of resistant and wild-type virus (critical when reversion has begun). It is the preferred test for baseline screening and for most cases of treatment failure.¹

Phenotypic testing directly measures the fold-change in IC₅₀ required to inhibit the patient’s virus compared with a wild-type reference. It provides quantitative information and can assess the net effect of interactions among multiple mutations — something that becomes increasingly important as resistance patterns grow complex. For tenofovir, the best virologic response occurs with fold-change <1.4, is partially reduced at 1.4–4.0, and essentially absent above 4.0.¹ Phenotypic testing is most valuable after multiple regimen failures when mutation patterns are complex, when there may be limited options making even partial drug activity worth pursuing, and for evaluating susceptibility to newer agents or non-subtype-B virus.

All currently available resistance tests share important limitations: they cannot detect minority populations of virus accounting for less than approximately 20% of the sample, and they cannot detect resistant virus archived in viral reservoirs. Because wild-type virus replaces mutant virus when drug pressure is withdrawn, resistance tests are most reliable at predicting activity of drugs the patient is currently taking. A patient with a distant history of NNRTI failure may no longer show K103N on genotyping, but its presence should be inferred — it would re-emerge rapidly if NNRTI treatment were reinitiated. The cumulative resistance profile, assembled from all prior and current resistance tests plus the treatment history, is what should guide regimen decisions.^1,5

Trial	Design	Population	Key result	Resistance observed
GEMINI-1 & -2	Phase 3 RCT, noninferiority	1,433 treatment-naive, VL <500k	DTG/3TC noninferior at wk 96 (86% vs 89.5%) and wk 144 (82% vs 84%)	None in either arm
TANGO	Phase 3 RCT, switch, noninferiority	Suppressed, TAF-based 3/4DR	DTG/3TC noninferior through 144 wk; 0 met CVW criteria	None
SALSA	Phase 3 RCT, switch, noninferiority	493 suppressed, diverse CARs (50% NNRTI, 40% INSTI, 10% PI)	DTG/3TC noninferior at wk 48 (adj diff −0.8%; 95% CI −2.4, 0.8)	None; 0 met CVW criteria
VOLVER-GESIDA	Phase 2a, single-arm, open-label	121 suppressed with historic M184V/I	87% suppressed at wk 96; 2 (1.7%) virologic failures	No INSTI resistance; increase in proviral M184V/I frequency
ART-PRO	Prospective, historic M184V/I	Suppressed, historic M184V/I	No virologic failures through 144 wk	None
SOLAR-3D	Prospective, historic M184V/I	Suppressed, heavily treatment-experienced	No virologic failures through 144 wk	None

Sources

1. Gallant JE. Antiretroviral drug resistance and resistance testing. Top HIV Med. 2005;13(5):138–142. PMID 16377851.
2. Cahn P, et al. Durable efficacy of dolutegravir plus lamivudine in antiretroviral treatment-naive adults with HIV-1 infection: 96-week results from GEMINI-1 and GEMINI-2. J Acquir Immune Defic Syndr. 2020;83(3):310–318. PMID 31834000.
3. van Wyk J, et al. Efficacy and safety of switching to dolutegravir/lamivudine vs continuing a tenofovir alafenamide-based 3- or 4-drug regimen (TANGO). Clin Infect Dis. 2020;71(8):1920–1929. doi:10.1093/cid/ciz1243.
4. Llibre JM, et al. Efficacy and safety of switching to DTG/3TC vs continuing a 3- or 4-drug regimen (SALSA): week 48 results. Clin Infect Dis. 2023;76(4):720–729. doi:10.1093/cid/ciac130. PMID 35235656.
5. Panel on Antiretroviral Guidelines for Adults and Adolescents. Guidelines for the use of antiretroviral agents in adults and adolescents with HIV. HHS. Updated September 2025.
6. Green AS, Li JZ, Sax PE. Is 2-drug therapy with dolutegravir and lamivudine an appropriate option for people with HIV and historic lamivudine resistance? Clin Infect Dis. 2026;83(1):158–159. doi:10.1093/cid/ciag061. PMID 41665225.