Merck & Co. Inc. (MRK) announced Wednesday that its investigational HIV treatment held its own against the current heavyweight champion in a key Phase 3 trial. The once-daily oral pill combining doravirine and islatravir (DOR/ISL) matched the performance of Gilead Science Inc.'s (GILD) Biktarvy in treatment-naïve adults with HIV-1 infection—people who haven't previously received antiretroviral treatment.

Here's what makes this interesting: Merck is achieving similar results with a two-drug regimen while Biktarvy uses three drugs (bictegravir/emtricitabine/tenofovir alafenamide, or BIC/FTC/TAF for those keeping score at home). Sometimes less really is more, or at least equally effective.

The trial hit its primary efficacy target, measuring the percentage of participants with HIV-1 RNA levels below 50 copies/mL at Week 48. Merck's DOR/ISL demonstrated non-inferiority to Gilead's combination, meaning it performed just as well by the statistical standards that matter to regulators.

The safety profile also looked good. The trial met its primary safety objective, with DOR/ISL showing comparable safety to BIC/FTC/TAF. When you're treating a chronic condition requiring daily medication, safety matters as much as efficacy.

The FDA has already accepted Merck's New Drug Application for DOR/ISL, though with a specific use case: treating HIV-1 infection in adults to replace their current antiretroviral regimen if they're already virologically-suppressed on a stable treatment. The agency set a target decision date of April 28, 2026, under the Prescription Drug User Fee Act.

This isn't Merck's first rodeo with doravirine. In the U.S., doravirine is already approved for treating adults with HIV-1 when combined with other antiretrovirals. It's available as a single agent called PIFELTRO and as part of a single-tablet regimen called DELSTRIGO, which combines doravirine, lamivudine, and tenofovir disoproxil fumarate.

Back in March, Merck presented results from two other Phase 3 trials testing DOR/ISL in adults with HIV-1 who were already virologically suppressed. One trial looked at patients on BIC/FTC/TAF, the other at patients on broader antiretroviral therapy. Both trials showed DOR/ISL met non-inferiority criteria compared to their respective comparator therapies at week 48, with safety objectives also achieved.

The pattern across these trials suggests Merck has built a solid case for its two-drug approach, both for switching patients who are already controlled and potentially for treating newly diagnosed patients.

Merck stock slipped 1.48% to $95 following the announcement Wednesday.