Merck has FDA approval for a new combination drug for HIV, giving patients a new treatment option and an alternative to a blockbuster Gilead Sciences pill that comes with tolerability and side effect problems, some of them serious.
The Merck drug, brand name Idvynso, is specifically approved for adults who have reduced the virus to very low levels in the blood by taking antiretroviral drugs. For these patients, Merck’s new once-daily pill can be a replacement for those antiretrovirals. In the Tuesday approval announcement, Merck said Idvynso will be available in pharmacies after May 11.
Idvynso builds on doravirine, the already approved Merck drug that is branded as Pifeltro for use in combination with other antiretroviral agents and as Delstrigo as a standalone therapy. Doravirine is a non-nucleoside reverse transcriptase inhibitor (NNRTI), a small molecule drug that works by binding to and blocking an enzyme key to HIV replication. In Idvynso, doravirine is paired with a novel compound called islatravir, a nucleoside analog reverse transcriptase inhibitor (NRTI) that brings multiple mechanisms to block viral replication.
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Gilead’s top HIV treatment is Biktarvy, a product that combines three drugs in single once-daily tablet. For 2025, Gilead reported $14.3 billion in Biktarvy sales, a 7% increase compared to the prior year. But Biktarvy comes with side effect risks. The product’s label notes that use of Biktarvy or the components of this drug have led to some reports of high levels of lactic acid in the blood and severe cases of an enlarged liver with accumulation of fat. Some of these cases became fatal. When these complications arise, treatment with Biktarvy must be suspended. No such problems were reported in clinical tests of the new Merck drug.
Merck’s submission for Idvynso was based on two Phase 3 clinical trials. In both, results showed the study drug was non-inferior to an active comparator consisting of a standard antiretroviral therapy. In one of those studies, that active comparator was Biktarvy. In those who switched to Idvynso from Biktarvy in the clinical trial, results showed the study drug was non-inferior to the active comparator.
In 2021, the FDA placed a partial clinical hold on tests of the Merck HIV drug due to observations of lower levels of immune cells in treated patients. The hold was later lifted, with Merck cleared to proceed testing a lower dose. In the studies that supported the FDA submission, Merck said the average levels of immune cells were similar across treatment arms and there were no discontinuations attributed to declines in levels of these cells. Merck also said that weight changes following the start of treatment with Idvynso were modest overall.
In a research note posted last month, analytics and consulting company GlobalData said the market is shifting away from single-tablet HIV regimens comprised of three or four drugs, moving toward two-drug combinations that simplify treatment and reduce toxicity. That means Idvynso could become an alternative to GSK’s Dovato, a combo pill that pairs dolutegravir and lamividune.
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Clinicians told GlobalData that for the Merck pill to justify its use, it must show reduced side effects compared to standard therapies such as Biktarvy, particularly on effects such as weight gain and central nervous system tolerability. The firm projects Merck’s new HIV drug could reach sales of about $1.8 billion by 2033 across the U.S., France, Germany, Italy, Spain, the U.K., and Japan.
“As treatment-experienced populations age, demand for metabolically neutral, switch-friendly regimens will intensify, creating targeted but meaningful market expansion opportunities,” Anaelle Tannen, GlobalData infectious disease analyst, said in the note.
If Idvysno achieves GlobalData’s sales projection, it could become the top seller in what is currently a small Merck virology portfolio. There’s opportunity to expand Idvynso’s label to treatment-naive HIV patients. Merck is also evaluating the combination HIV drug in a pivotal trial that enrolled patients who have not previously received antiretroviral therapies. In Phase 3 results, Merck reported the study drug met the main viral suppression goal of showing non-inferiority compared to Biktarvy. These results were presented in February at the Conference on Retroviruses and Opportunistic Infections and published in The Lancet HIV.