Merck’s once-daily DOR/ISL HIV therapy delivers sustained viral control and weight stability in Phase 3 results

Merck has announced new findings from two pivotal Phase 3 studies evaluating its investigational, once-daily oral two-drug regimen of doravirine/islatravir (DOR/ISL) in adults with virologically suppressed HIV-1 infection. The latest data, unveiled ahead of the 20th European AIDS Conference, revealed that the therapy not only maintained durable viral suppression but also demonstrated minimal changes in weight, lipid profiles, and insulin resistance compared to standard triple-drug regimens. The results highlight the company’s ambition to deliver simpler HIV treatments that support long-term metabolic health without compromising efficacy.

How did Merck’s DOR/ISL regimen maintain viral suppression while avoiding weight and lipid changes in Phase 3 trials?

The two Phase 3 trials—MK-8591A-051 and MK-8591A-052—evaluated adults already suppressed on standard antiretroviral therapy who were switched to DOR/ISL. The goal was to determine whether a simplified, non-integrase strand transfer inhibitor (non-InSTI) regimen could maintain virologic suppression while reducing the metabolic complications that have become common with long-term use of current HIV therapies.

Results from both studies confirmed that DOR/ISL met its primary efficacy endpoints for noninferiority at 48 weeks. Participants who switched to DOR/ISL remained virologically suppressed at rates comparable to those continuing their baseline regimens. Crucially, no treatment-emergent resistance to either doravirine or islatravir was observed.

The secondary analyses focused on body composition, fasting lipids, and insulin resistance. The mean weight change from baseline at Week 48 was essentially neutral—just –0.03 kilograms in the DOR/ISL group versus +0.28 kilograms in the comparator arm. Approximately 14.6 percent of patients who switched to DOR/ISL experienced a weight gain of 5 percent or more, compared with 16 percent among those who remained on bictegravir/emtricitabine/tenofovir alafenamide (BIC/FTC/TAF). Similarly, only 3.5 percent of the DOR/ISL group recorded a 10 percent weight increase versus 2.5 percent for the control arm.

These findings indicate that switching to DOR/ISL did not lead to the steady weight increase often reported with InSTI-based or tenofovir-containing therapies. Measures of lean body mass, peripheral fat, trunk fat, and waist-to-hip ratio all showed negligible differences between treatment arms. Mean changes in fasting glucose, insulin, triglycerides, and cholesterol fractions were likewise modest and statistically comparable between groups.

Why metabolic neutrality could redefine long-term HIV maintenance therapy and patient adherence

For many adults living with HIV, maintaining viral suppression has become a manageable, lifelong goal—but long-term metabolic complications remain a persistent challenge. Modern HIV therapy has significantly extended life expectancy, yet it has also introduced new comorbidities, including obesity, dyslipidemia, and insulin resistance. These side effects can increase the risk of cardiovascular disease and diabetes, undermining adherence and quality of life.

The concept of metabolic neutrality—the ability of a regimen to sustain viral control without causing meaningful metabolic disruption—has emerged as a key differentiator in treatment design. Merck’s data suggest that DOR/ISL could address this unmet need. Because the therapy avoids an InSTI backbone and uses two agents with distinct mechanisms—doravirine, a non-nucleoside reverse transcriptase inhibitor (NNRTI), and islatravir, a nucleoside reverse transcriptase translocation inhibitor (NRTTI)—it offers a unique pharmacologic profile less likely to interfere with weight regulation or lipid metabolism.

These outcomes could resonate strongly with clinicians who seek balance between long-term efficacy and metabolic safety. In a field where small differences in weight or lipid outcomes can drive prescribing decisions, DOR/ISL’s near-neutral metabolic footprint may position it as an alternative for patients struggling with side effects of their current regimens.

However, while the 48-week results are encouraging, experts caution that the data are still exploratory. The trial populations were composed of stable, virologically suppressed adults—typically the lowest-risk cohort for metabolic complications. To fully validate DOR/ISL’s metabolic neutrality, longer-term follow-up and inclusion of more diverse patient groups will be essential.

How does Merck’s regulatory strategy strengthen confidence in DOR/ISL as an HIV maintenance option?

Merck has already filed a New Drug Application for DOR/ISL with the U.S. Food and Drug Administration, which accepted the submission in July 2025. The PDUFA target date is April 28, 2026. This submission builds on Merck’s growing HIV portfolio, which includes development programs for weekly and monthly oral regimens as well as long-acting prevention therapies.

The company’s strategy is to provide treatment flexibility across dosing intervals—daily, weekly, and monthly—while maintaining robust safety and efficacy. In this context, DOR/ISL serves as the cornerstone of its “simplify and sustain” approach. If approved, it would become the first oral two-drug regimen without an integrase inhibitor to achieve noninferiority versus standard triple-therapy regimens.

Merck’s emphasis on metabolic health aligns with broader clinical and regulatory priorities. The FDA has been receptive to regimens that minimize long-term toxicity, especially as the population of people living with HIV ages. Given that comorbid metabolic disease now represents one of the most significant healthcare costs in HIV management, the potential for a metabolically neutral maintenance therapy carries strong economic and clinical implications.

What historical safety context shaped islatravir’s development and dosing refinements?

While islatravir is now advancing toward market approval in combination with doravirine, its clinical development path has not been without challenges. Earlier studies in 2021 and 2022 observed declines in CD4 and total lymphocyte counts in some participants receiving higher islatravir doses. This prompted Merck to pause multiple trials and re-evaluate dosing strategies.

Subsequent pharmacokinetic modeling and dose optimization allowed the company to identify lower, therapeutically effective concentrations that avoided the immunologic issues seen in prior studies. The ongoing Phase 3 data provide reassurance that these adjustments have successfully mitigated those risks. No new safety signals related to lymphocyte counts were reported in the current 48-week analyses.

By acknowledging and addressing these earlier safety concerns, Merck strengthened the credibility of its current DOR/ISL data package. The company’s transparent approach has been seen by clinical observers as a positive example of adaptive drug development, balancing scientific caution with a commitment to innovation.

How investor sentiment reflects confidence in Merck’s HIV franchise and its pursuit of differentiated therapies

From a market standpoint, the DOR/ISL results arrive at a strategic moment for Merck. The company’s HIV franchise, while smaller than those of established leaders like Gilead Sciences and ViiV Healthcare, remains an important growth segment in its diversified portfolio. The DOR/ISL data bolster investor confidence that Merck can compete effectively in the evolving post-InSTI landscape.

Analysts view the findings as supportive of Merck’s efforts to carve out a niche in patients who require metabolic stability or simplified treatment regimens. Though DOR/ISL is unlikely to unseat integrase-based therapies immediately, its potential as a differentiated alternative could yield steady adoption among select populations. Market watchers note that the company’s strong R&D pipeline and upcoming prevention candidates could further expand its relevance in the HIV space.

Still, investor enthusiasm remains tempered by known risks. The metabolic data, though positive, are exploratory and limited in duration. Regulatory review will likely hinge on confirming long-term safety and resistance durability. Pricing and reimbursement dynamics may also shape commercial outcomes, as payers increasingly demand comparative evidence of long-term benefit.

Overall, sentiment is cautiously optimistic. Merck’s Phase 3 results have reaffirmed its scientific credibility and underscored a disciplined strategy to deliver clinically meaningful differentiation rather than incremental iteration.

What the latest results reveal about the future of HIV care and the shift toward simplified, patient-centric therapy

The DOR/ISL program represents a significant step in the evolution of HIV treatment from triple-drug complexity toward streamlined regimens that prioritize tolerability and patient quality of life. The notion of a metabolically neutral, once-daily oral option directly addresses the next frontier of HIV management—sustainability over decades of continuous therapy.

If approved, DOR/ISL could play a pivotal role in helping patients and physicians tailor treatment choices to individual metabolic profiles. Its apparent neutrality on weight and lipid markers, combined with robust viral suppression, could make it especially appealing to patients with existing cardiovascular or metabolic risks.

For Merck, the DOR/ISL data also serve as a foundation for its broader vision: advancing innovative regimens that integrate viral durability with systemic health. The company’s work in longer-acting formulations and prevention could extend these benefits even further, potentially reshaping adherence patterns and long-term outcomes.

While further research will determine how broadly these benefits translate across real-world populations, the message from Merck’s latest trials is clear—metabolic stability matters as much as viral control in defining the next generation of HIV therapies.