A New Era in HIV Treatment: The First Potent Once-Weekly Pill Is on the Horizon

In both trials, success was measured at Week 48 by the proportion of participants who maintained an undetectable viral load (HIV-1 RNA < 50 copies/mL), the globally accepted marker of effective HIV treatment.

The Verdict on Efficacy: Non-Inferiority at 48 Weeks

The headline result was unambiguous. At Week 48, the once-weekly ISL/LEN regimen met its primary endpoint in both ISLEND-1 and ISLEND-2, demonstrating statistical non-inferiority to daily oral therapy. This means the weekly pill was proven to be no less effective than the established daily standard.

This is particularly notable because ISL/LEN is the first two-drug regimen to demonstrate non-inferior efficacy to Biktarvy in a Phase 3 pivotal study. Biktarvy is a potent three-drug single-tablet regimen, so matching its performance with only two drugs is a critical scientific achievement.

The Phase 3 success was strongly foreshadowed by earlier Phase 2 data (NCT05052996). In that study, at 48 weeks, 94.2% of participants who switched to weekly islatravir plus lenacapavir maintained viral suppression, compared to 92.3% who stayed on Biktarvy. Equally important, no participants across these studies developed treatment-emergent resistance to the HIV drugs.

Safety: A Cleaner Profile at the Right Dose

A critical chapter in islatravir's development involved a safety concern. Earlier trials testing higher doses of islatravir were paused after some participants experienced decreases in total lymphocyte and CD4+ T-cell counts. The Phase 3 and late-stage Phase 2 trials used a refined weekly dose of 2 mg of islatravir combined with 300 mg of lenacapavir.

At this dose, the results were reassuring. The Phase 2 week 48 data confirmed no clinically relevant decreases in CD4+ T-cells or lymphocytes were observed. Gilead’s summary of the Phase 3 safety profile noted it was “generally comparable” to the daily comparator regimens, with no new safety signals identified.

Additionally, patient experience data points to a meaningful quality-of-life benefit. A patient-reported outcomes (PRO) analysis from the Phase 2 study showed that after 48 weeks, numerically more participants on the weekly regimen reported that the treatment fit better into their lifestyle, was less of a daily reminder of their HIV status, and caused less worry than taking a daily pill.

A Two-Pronged Mechanism: How Islatravir and Lenacapavir Work

The regimen’s power comes from two drugs with distinct targets in the HIV lifecycle, combining to form a high-barrier-to-resistance partnership.

• Islatravir (ISL): Developed by Merck, this is a first-in-class nucleoside reverse transcriptase translocation inhibitor (NRTTI). It works through multiple mechanisms—it blocks the reverse transcriptase enzyme and also terminates the viral DNA chain, a process that effectively halts the virus’s ability to replicate and integrate into human cells.
• Lenacapavir (LEN): Gilead's capsid inhibitor takes a different approach. Instead of targeting enzymes, it binds directly to the protective shell (capsid) of the HIV virus, disrupting several critical stages of the viral lifecycle, including capsid assembly, viral DNA transport into the cell nucleus, and the release of new virus particles.

This dual mechanism of action is a core reason the regimen is so robust with just two agents.

The Path to Approval and a Changing Landscape

Gilead and Merck had already filed the combination for regulatory review in the United States, with a Prescription Drug User Fee Act (PDUFA) target action date set for April 28, 2026. With the Phase 3 studies now providing the required pivotal efficacy data, a decision is imminent.

The significance of this milestone extends beyond a single drug. If approved, the ISL/LEN pill would be the first fully oral once-weekly treatment for HIV. While injectable long-acting therapies like cabotegravir/rilpivirine are available, they require clinic visits every month or two. A once-weekly pill could combine the convenience of long-acting dosing with the familiarity of an oral tablet, removing logistical barriers for those who cannot easily access injection services or simply prefer a pill.

This innovation is part of a wider industry push toward treatment simplification. Gilead itself is in a quiet race, developing another once-weekly oral candidate—a combination of bictegravir and lenacapavir (BIC/LEN)—with results from its ARTISTRY-1 and ARTISTRY-2 Phase 3 trials on the horizon. For people living with HIV, the future of treatment is rapidly moving away from the paradigm of a rigid daily pill burden toward a flexible menu of options, from weekly pills to long-acting injections, tailored to fit their lives.