What does the M184V mutation in HIV-1 indicate and how does it change antiretroviral therapy management?

M184V Mutation in HIV: Clinical Significance and ART Management

The M184V mutation indicates high-level resistance to lamivudine (3TC) and emtricitabine (FTC), but paradoxically, continuing these agents even with M184V present is often beneficial and recommended, particularly when combined with high-barrier agents like dolutegravir or bictegravir.

What M184V Indicates

The M184V mutation is a single-base change in the YMDD domain of HIV reverse transcriptase that:

• Confers high-level resistance to 3TC and FTC 1
• Is rapidly selected when these drugs are used with inadequate viral suppression 1
• Represents one of the most common NRTI resistance mutations in treated patients 2
• Increases risk of virological failure (1.87 times higher) and viral blips (2.26 times higher) compared to wild-type virus 3

However, M184V has several favorable characteristics that distinguish it from other resistance mutations:

• Reduces viral fitness, slowing HIV replication 4, 2
• Delays or reverses zidovudine (AZT) resistance when both mutations are present 4
• Increases susceptibility to tenofovir, particularly tenofovir alafenamide (TAF) 5
• Retains partial activity of 3TC/FTC even in the presence of the mutation 6

How M184V Changes ART Management

For Treatment-Naïve Patients with Transmitted M184V

Avoid 3TC/FTC-based regimens initially, as these drugs will have reduced efficacy. However, this scenario is uncommon since M184V reduces viral fitness and is rarely transmitted.

For Patients with Virological Failure and Newly Detected M184V

The 2025 IAS-USA guidelines recommend continuing 3TC or FTC even when M184V is present, particularly when combined with integrase inhibitors 6. Here's the algorithmic approach:

Step 1: Assess Current Regimen Backbone

• If on dolutegravir or bictegravir-based regimen: Resume the same regimen if adherence can be improved, even before resistance results return 6
• These high-barrier agents maintain efficacy despite NRTI resistance, including M184V

Step 2: For Regimen Changes with M184V Present

• Preferred approach: Dolutegravir or bictegravir + tenofovir alafenamide/FTC (or 3TC) 6
  • This combination showed comparable or superior suppression rates to boosted PI + 2 NRTIs, even with NRTI resistance present 6
  • TAF has a 4-fold higher intracellular concentration than TDF, providing better activity against TAM + M184V combinations 5
  • Continue 3TC/FTC because they retain partial activity even with M184V 6

Step 3: Special Consideration for Switching to B/F/TAF

• Bictegravir/emtricitabine/tenofovir alafenamide maintains 98% viral suppression in patients with preexisting M184V 7
• This is a highly effective switch option even when M184V is documented 7

Critical Nuance: The "3TC Paradox"

Despite high-level phenotypic resistance, continuing 3TC/FTC with M184V present provides clinical benefit through multiple mechanisms 4, 2:

1. Preserves the fitness-reducing M184V mutation under drug pressure
2. Maintains partial antiviral activity
3. Prevents reversion to wild-type (more fit) virus
4. Enhances activity of companion drugs, particularly tenofovir

The 2025 IAS-USA guidelines explicitly recommend continued treatment with NRTIs such as 3TC/FTC since they retain partial activity even in the presence of extensive resistance mutations 6.

For Multiclass Resistance Including M184V

When M184V exists alongside extensive resistance (TAMs, NNRTI mutations, INSTI resistance):

• Use novel agents: ibalizumab, fostemsavir, or lenacapavir 6
• Still continue 3TC/FTC for their partial activity 6
• Aim for at least 2 fully active drugs in the regimen 6

Common Pitfalls to Avoid

1. Don't automatically discontinue 3TC/FTC when M184V is detected - This is the most important clinical pearl. The mutation's fitness cost and residual drug activity justify continuation 6, 4, 2

2. Don't use 3TC/FTC as one of your "fully active" drugs when counting active agents in salvage therapy, but do include them for their beneficial effects 6

3. Prioritize adherence assessment over immediate regimen change - Most virological failure with modern regimens stems from adherence issues, not resistance 6

4. Consider TAF over TDF when M184V is present, especially with thymidine analog mutations, due to higher intracellular drug levels 5

5. Don't wait for resistance results before resuming a dolutegravir or bictegravir-based regimen if adherence has been addressed 6