Merck Starts Phase III Studies on Once-Monthly Pill for HIV Prevention

Merck MRK recently launched two late-stage clinical studies to evaluate MK-8527, its investigational once-monthly oral pill for HIV pre-exposure prophylaxis (PrEP).

The EXPrESSIVE-11 study, which starts enrolment next month, will assess the drug in individuals at high risk of HIV-1 exposure across 16 countries. The second study, EXPrESSIVE-10, will focus on sexually active women aged 16 to 30 in Kenya, South Africa and Uganda, with enrollment expected to begin in the coming months. Combined, the two studies will enrol nearly 9,000 participants and pit MK-8527 against Gilead Sciences’ GILD daily oral PrEP pill Truvada.

MK-8527 is designed to block an enzyme called reverse transcriptase, which HIV uses to replicate. The initiation of the two late-stage studies is supported by results from a phase II study that showed similar rates of adverse events between the treatment and placebo arms. Detailed data from this study will be presented tomorrow at the 13th International AIDS Society Conference on HIV Science.

Merck is sponsoring both late-stage studies, with grant funding from the Gates Foundation.

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MRK's Efforts to Boost HIV Program

Merck is building a competitive HIV franchise, targeting both prevention and treatment. In addition to MK-8527, the company is also advancing once-daily and once-weekly oral treatments for the disease.

Last week, Merck announced that the FDA had accepted a regulatory filing seeking approval for its investigational, once-daily, two-drug, single-tablet regimen of doravirine/islatravir (DOR/ISL) for treating virologically suppressed adults with HIV-1 infection. A final decision is expected by April 28, 2026.

Doravirine, in combination with other antiretrovirals and as a single agent, is approved under the trade name Pifeltro for treating adults with HIV-1 in the United States. It is also approved as a component of a single-tablet regimen under the trade name Delstrigo for treating HIV-1.

In collaboration with Gilead, Merck is assessing the investigational once-weekly, two-drug, single tablet regimen of islatravir/lenacapavir (ISL/LEN) across two late-stage studies in virologically suppressed people with HIV-1.

MRK is also evaluating the combination of islatravir with another investigational drug called ulonivirine in a phase II study as an oral once-weekly treatment for HIV-1.

Collectively, these efforts also reflect Merck’s intent to diversify beyond oncology and become a major player in the HIV therapeutic space.

Stiff Competition in the HIV Space

Merck’s growing HIV portfolio positions it to compete with Gilead, which has a market-leading portfolio of HIV drugs. At present, there are two FDA-approved daily oral medications for PrEP — Truvada and Descovy — both marketed solely by GILD. The successful development and potential approval of MK-8527 is likely to challenge the dominance of Gilead in the PrEP space.

Gilead also markets the flagship drug Biktarvy – the number-one prescribed regimen for both treatment-naïve and switch patients. If approved, Merck’s DOR/ISL would compete with Biktarvy for market share.

Gilead recently achieved a breakthrough in the fight against the HIV epidemic after the FDA approved its twice-yearly injectable HIV-1 capsid inhibitor, lenacapavir, for the prevention of HIV. This marks the first and only twice-yearly PrEP option available in the United States. The drug will be marketed under the brand name Yeztugo.

Another major player in the HIV space is GSK plc GSK, which also markets multiple drugs in this space. GSK’s HIV portfolio sales are being driven by strong patient demand for Cabenuva, Apretude and Dovato. The company is also focused on developing the next generation of HIV innovation with integrase inhibitors for treatment as well as prevention.

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