A drug that was nearly killed by its own toxicity just posted Phase 3 results for the first-ever once-weekly oral HIV pill. Merck and Gilead's comeback story could reshape how millions of people manage the virus.
Take One Pill. Wait a Week. Repeat.
Imagine if your daily multivitamin only needed to be taken on Sundays. That's essentially what Merck and Gilead just pulled off for HIV treatment.
The two pharma giants reported positive Phase 3 results for islatravir/lenacapavir, a once-weekly oral pill for people already living with HIV. Both pivotal trials (ISLEND-1 and ISLEND-2) hit their primary endpoints at 48 weeks, showing the weekly tablet kept the virus suppressed just as well as taking a pill every single day. Statistically non-inferior. No major new safety red flags.
That alone would be noteworthy. But the real story is how this drug got here, because islatravir was basically left for dead four years ago.
A Comeback Nobody Saw Coming
Rewind to December 2021. Merck's islatravir was one of the most promising molecules in HIV, a novel drug that worked through a mechanism nobody else had. Then the FDA slammed the brakes.
Clinical data showed that islatravir was causing drops in CD4 T cells, the very immune cells that HIV destroys. A drug meant to fight HIV was mimicking one of its worst effects. The FDA placed clinical holds across treatment and prevention programs. Enrollment stopped. The monthly PrEP (prevention) program was eventually killed entirely.
It looked like islatravir might join the graveyard of promising molecules that couldn't survive their own side effects.
But Merck's scientists dug into the data and found a lifeline: the CD4 decline was dose-dependent. Higher doses caused bigger drops. Lower doses showed smaller changes, and some patients recovered after stopping. The fix wasn't to abandon islatravir; it was to turn down the volume.
Merck reformulated the drug at a lower dose and paired it with Gilead's lenacapavir, a first-in-class capsid inhibitor that attacks HIV at multiple stages of its life cycle. Think of lenacapavir as a Swiss Army knife against the virus: it blocks HIV from entering the cell nucleus, from assembling new copies of itself, and from forming the protective shell it needs to infect other cells.
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Together, they created something that had never existed before: a complete, two-drug, once-weekly oral HIV regimen in a single tablet.
What the Trials Actually Showed
The Phase 3 program consisted of two studies, each testing the weekly pill from a different angle.
ISLEND-1 was a double-blind trial. Patients who were already virus-suppressed on Biktarvy (Gilead's blockbuster daily pill) were randomized to either switch to the weekly tablet or keep taking their daily Biktarvy. At 48 weeks, the weekly regimen was non-inferior to daily Biktarvy at keeping the virus undetectable.
ISLEND-2 was open-label and broader. Patients suppressed on any standard daily regimen could switch to the weekly pill. Same result: non-inferior at 48 weeks.
The companies haven't released exact response rates, confidence intervals, or p-values yet (those are being saved for a major medical conference). But earlier Phase 2 data offer a preview of the ballpark. In that smaller study, 94.2% of patients on the weekly pill maintained viral suppression at 48 weeks, compared to 92.3% on daily Biktarvy. Zero patients in either group experienced virologic failure by the FDA's standard measurement.
The safety profile was described as "generally comparable" to daily regimens, with no repeat of the CD4 scare that derailed islatravir's earlier life. Full adverse event data are still pending.
Why "Once a Week" Is a Bigger Deal Than It Sounds
If you've never had to take a pill every single day for the rest of your life, it's easy to underestimate what a weekly option means.
About 40 million people worldwide live with HIV. Modern antiretrovirals are remarkably effective: take them daily, and the virus becomes undetectable. Undetectable means untransmittable. But "take them daily" is doing a lot of heavy lifting in that sentence.
Real-world adherence to daily antiretrovirals remains a persistent challenge. The reasons are deeply human. Depression. Substance use. Pill fatigue (the sheer psychological weight of a daily reminder that you have HIV). Stigma, too: a pill bottle in your medicine cabinet or a tablet taken at dinner can be an involuntary disclosure to a roommate, a partner, or a family member.
Every missed dose is a crack in the wall. Enough cracks, and the virus finds its way back.
A weekly pill doesn't solve all of those problems. But it reduces 365 dosing decisions per year to 52. That's a meaningful difference for someone battling pill fatigue, someone who travels constantly, or someone who simply doesn't want a daily physical reminder of their diagnosis.
The Goldilocks Option
The HIV treatment landscape is increasingly splitting into three lanes, and the weekly pill parks itself right in the middle.
Lane one: daily pills. Biktarvy and similar single-tablet regimens remain the gold standard. They work brilliantly for most people. But they require daily commitment, and Gilead's dominance here (the company controls over 70% of the HIV treatment market) means the ceiling for growth is limited.
Lane three: long-acting injectables. ViiV Healthcare's Cabenuva requires intramuscular injections every one or two months. Gilead's own lenacapavir (branded as Sunlenca for treatment, Yeztugo for prevention) only needs a subcutaneous shot twice a year. For patients who hate pills or can't reliably take them, injectables are transformative. But they require clinic visits, trained staff, cold-chain storage, and a strict schedule; miss an injection window and you risk breeding drug-resistant virus.
Lane two: once-weekly oral. This is where islatravir/lenacapavir lives. It offers the convenience boost of less frequent dosing without the logistical baggage of injections. You take it at home, on your own schedule. No needles, no clinic dependency. If something goes wrong, stopping is straightforward (unlike injectables, which linger in your body for months after the last dose).
For the patient who's tired of daily pills but doesn't want to commit to injections, this is the Goldilocks option. Not too frequent, not too complicated. Just right.
Wall Street's Verdict: Nice, But Show Me the Money
Analysts greeted the Phase 3 results with polite applause rather than standing ovations.
The consensus: this is a de-risking event that validates the program and supports regulatory filing. Merck and Gilead have filed for U.S. regulatory approval and plan to submit in additional markets. But the commercial upside is viewed as "meaningful, not transformational."
For context, Merck's earlier daily islatravir combo (paired with doravirine instead of lenacapavir) carried analyst estimates of roughly $1.7 billion in peak annual sales. The weekly formulation is considered more compelling, so the ceiling could be somewhat higher. Rough directional estimates from available commentary suggest a $1 to $3 billion peak sales range under favorable conditions.
For Gilead, the weekly pill reinforces a multi-modal long-acting HIV platform that also includes twice-yearly injectable lenacapavir for prevention, which posted a stunning 100% efficacy rate (zero infections) in women in the PURPOSE 1 trial. The oral weekly regimen is complementary, not cannibalistic.
For Merck, the stakes are arguably higher. The company faces a looming patent cliff on Keytruda, its cancer blockbuster, later this decade. Every non-oncology growth asset matters for the post-Keytruda narrative. Analysts framed the ISL/LEN data as "supportive" of Merck's diversification story, though it won't single-handedly plug a Keytruda-sized hole.
The announcement also came alongside less cheerful news: Merck and Gilead are terminating a Phase 3 trial of Trodelvy plus Keytruda in first-line lung cancer, which tempered the day's overall enthusiasm.
What to Watch Next
Several questions will shape how big this story gets:
Safety details. Full adverse event data, including any signals around CD4 counts or bone marrow effects, will be presented at an upcoming medical conference. Given islatravir's history, this data will get intense scrutiny.
Guideline positioning. Will U.S. and European HIV treatment guidelines list the weekly pill as a "preferred" regimen or merely an "alternative"? The difference matters enormously for prescribing behavior.
Pricing and payer response. Can Merck and Gilead charge a premium for convenience, or will payers demand price parity with daily generics? This single question could be the difference between a $1 billion product and a $3 billion one.
Real-world adherence. Non-inferiority in a clinical trial is one thing. But will people actually remember to take a pill once a week? Daily habits are easy to automate; weekly ones can be surprisingly slippery. (Quick: did you take your weekly vitamin last Sunday?)
The Bottom Line
A drug that was nearly killed by its own toxicity just posted positive Phase 3 results in a formulation that could change how millions of people manage HIV. It's not a cure. It's not even clearly better than what exists. But it's different in a way that matters: fewer pills, fewer reminders, fewer moments of vulnerability.
In a disease where the biggest enemy is often the daily grind of treatment itself, taking one pill instead of seven each week isn't just convenient. It might be the difference between staying suppressed and falling through the cracks.
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