GILD, MRK Once-Weekly HIV Pill: $162B Cannibalization Risk

Gilead Sciences (GILD) closed July 24 at $129.31, valuing the company at $162.5 billion. That price is built on $20.8 billion in 2025 HIV revenue, a Biktarvy franchise generating $14.3 billion annually, and Yeztugo (lenacapavir for PrEP) on track for $1 billion in 2026 sales. Now Gilead and Merck (MRK) have presented Phase 3 data for a once-weekly oral HIV treatment that could redefine the $30 billion HIV market. The irony: Gilead’s biggest competitive threat in long-acting HIV treatment is a drug built on its own lenacapavir molecule.

The ISLEND Data: Non-Inferiority in Two Registrational Trials

On July 21 and 29, 2026, Gilead and Merck presented detailed Week 48 results from the Phase 3 ISLEND-1 and ISLEND-2 trials at the AIDS 2026 conference in Rio de Janeiro. Both trials evaluated islatravir 2 mg plus lenacapavir 300 mg (ISL/LEN) as a once-weekly single tablet for virologically suppressed adults switching from daily therapy.

ISLEND-1 (NCT06630286) was a double-blind, active-controlled trial in 607 participants suppressed on Biktarvy for at least 6 months. Participants were randomized 1:1 to switch to once-weekly ISL/LEN or remain on Biktarvy. The primary endpoint was the percentage of participants with HIV-1 RNA at least 50 copies/mL at Week 48, defined by the FDA snapshot algorithm.

The result: zero participants (0%) on ISL/LEN had detectable virus at Week 48, compared to one participant (0.3%) on Biktarvy. ISL/LEN was statistically non-inferior. CD4 counts changed by an average of minus 10 cells/uL on ISL/LEN versus minus 18 cells/uL on Biktarvy. Treatment-related adverse events were 13.5% for ISL/LEN and 13.2% for Biktarvy. No participant discontinued due to CD4 or lymphocyte decline.

ISLEND-2 (NCT06630299) was an open-label trial in 626 participants suppressed on various daily standard-of-care regimens. At Week 48, 0.3% of ISL/LEN participants had detectable virus versus 1.3% on standard of care. Non-inferior. CD4 counts changed by minus 45 cells/uL on ISL/LEN versus minus 8 cells/uL on standard care, though the ISL/LEN group had higher baseline CD4 counts and the groups converged at Week 48.

The safety signal is the key data point. Islatravir previously saw clinical holds in 2021 due to CD4 lymphocyte declines at higher doses. The ISLEND trials used a 2 mg dose, significantly lower than the doses that triggered those concerns. In both trials, investigators reported no between-group differences in lymphocyte counts at Week 48. The safety profile was comparable to daily oral therapy.

This is the first oral once-weekly HIV regimen to show non-inferiority in two registrational Phase 3 trials. If approved, ISL/LEN would be the first complete weekly pill for HIV treatment.

Why This Is a Gilead Story, Not Just a Merck Story

The ISL/LEN regimen combines Merck’s islatravir (a nucleoside analog reverse transcriptase translocation inhibitor) with Gilead’s lenacapavir (a capsid inhibitor). Gilead sponsors the trials. Gilead will lead regulatory filings. The economics of the partnership have not been disclosed in detail, but Gilead is clearly positioning lenacapavir as the backbone of its long-acting strategy across both treatment and prevention.

Lenacapavir is already FDA-approved as Yeztugo for twice-yearly injectable PrEP, where it showed at least 99.9% of participants remained HIV negative in the PURPOSE 1 and PURPOSE 2 trials. Gilead has also filed for an oral once-weekly PrEP formulation of lenacapavir, with FDA acceptance in June 2026. The molecule is becoming a platform.

The strategic logic is clear. Gilead already dominates HIV treatment with Biktarvy, which holds over 52% U.S. market share and generated $14.3 billion in 2025. A once-weekly pill built on lenacapavir would extend Gilead’s dominance into the long-acting treatment space while Merck gains its first meaningful HIV franchise foothold through islatravir.

But here is the tension. If ISL/LEN cannibalizes Biktarvy, Gilead is trading a $14.3 billion daily pill for a weekly pill at an unknown price point. The switch economics depend on pricing. A weekly regimen will likely command a premium over daily Biktarvy, but if payers push back or if the premium is modest, Gilead could see net revenue dilution from patients switching from a high-margin daily pill to a weekly one.

The Competitive Threat: ViiV and the Long-Acting Injectables

ViiV Healthcare, the GSK-Pfizer-Shionogi joint venture, is the direct competitor in long-acting HIV treatment. Cabenuva (cabotegravir plus rilpivirine) is an every-4-week or every-8-week injectable regimen already on the market. ViiV is presenting new data at AIDS 2026, including phase IIIb data comparing Dovato to Biktarvy in treatment-naive adults and 96-week results from the LATA trial of every-8-weeks cabotegravir plus rilpivirine in adolescents in sub-Saharan Africa.

ViiV’s cabotegravir long-acting injectable is also approved for PrEP, competing directly with Gilead’s Yeztugo. The difference: Yeztugo is twice-yearly, cabotegravir PrEP is every two months. Gilead’s PrEP access strategy includes partnerships with PEPFAR and the Global Fund to deliver lenacapavir to up to 3 million people in high-incidence countries through 2028. That global health access play gives Gilead a scale advantage in LMIC markets where ViiV has committed to tripling annual supply of cabotegravir PrEP.

In the treatment market, the competition is between a once-weekly oral pill (ISL/LEN) and a monthly or bimonthly injectable (Cabenuva). Patients who prefer oral dosing over injections will favor ISL/LEN. Patients who want the longest possible dosing interval will favor Cabenuva. The market will likely split, with ISL/LEN capturing the oral-preferring switch population and Cabenuva holding the injection-accepting population.

Merck’s independent HIV strategy adds another layer. The FDA approved Merck’s Idvynso (doravirine plus islatravir) on April 21, 2026, as a once-daily two-drug regimen for virologically suppressed adults. Idvynso is the first non-INSTI, tenofovir-free regimen to show non-inferiority to Biktarvy. Merck is also developing islatravir plus ulonivirine (MK-8507) as an independent once-weekly regimen in Phase 2b, and MK-8527 as a once-monthly oral PrEP candidate in Phase 3. Merck is building an HIV franchise that does not depend on Gilead.

Valuation: What Is ISL/LEN Worth to Gilead?

GILD at $129.31 trades at roughly 6.2x 2025 HIV revenue of $20.8 billion, or 5.6x total 2025 product sales of $28.9 billion. The company guided 8% HIV sales growth in 2026, driven by Biktarvy demand and Yeztugo launch momentum. Yeztugo alone is forecast at $1 billion in 2026, up from an initial $800 million projection, with $166 million in Q1 2026 sales already on the board.

ISL/LEN has no PDUFA date yet. Regulatory filings are planned but not submitted. Assuming a 2027 approval, the once-weekly treatment market opportunity depends on switch rates from daily therapy. There are approximately 1.2 million people living with HIV in the United States and roughly 39 million globally. Even a 10% switch rate in the U.S. market at a $15,000 annual price point would generate $1.8 billion in peak sales. Globally, with Gilead’s access strategy, the number could be multiples higher but at lower margins.

The risk to Gilead’s valuation is not that ISL/LEN fails. The data are strong. The risk is that ISL/LEN succeeds but accelerates the erosion of Biktarvy revenue faster than Gilead can offset with premium pricing on the weekly regimen. Biktarvy at $14.3 billion is the goose. ISL/LEN is the golden egg. You cannot sell the egg without cooking some goose.

Compare Gilead at $162.5 billion to Merck at approximately $330 billion market cap (at $131.07 per share). Merck’s HIV franchise is nascent: Idvynso just launched in May 2026, and the ISL/LEN partnership with Gilead gives Merck a piece of the long-acting market without bearing the full development cost. For Merck, ISL/LEN is pure upside on top of Keytruda and the HPV franchise. For Gilead, ISL/LEN is both upside and cannibalization risk.

The Verdict

Gilead at $129 is a hold for existing shareholders and a buy on any pullback below $120. The HIV franchise generates $20.8 billion annually with 6 to 8% growth. Yeztugo is the fastest launch in Gilead’s history. ISL/LEN adds a long-acting oral treatment option that extends the lenacapavir platform. The bear case is Biktarvy cannibalization, but that risk is already years away and Gilead controls the timeline.

The real risk is not clinical. It is competitive. Merck is building an independent HIV pipeline with islatravir at the core. If Merck’s ulonivirine combination succeeds in Phase 2b, Merck will have a once-weekly regimen that does not need Gilead’s lenacapavir. That would break Gilead’s monopoly on long-acting oral HIV treatment. Watch the ulonivirine Phase 2b data. That is the threat Gilead cannot control.

For more on Gilead’s HIV franchise, see our lenacapavir PrEP approval coverage and our analysis of the Trodelvy plus Keytruda CHMP recommendation. For the broader biotech catalyst calendar driving these stocks, our guide to investing in biotech covers the fundamentals.

ISL/LEN is real. The data are clean. The once-weekly pill is coming. The question for Gilead shareholders is whether the company that built a $14 billion franchise on a daily pill can survive the transition to weekly dosing without losing the plot. The ISLEND trial details are registered on ClinicalTrials.gov (ISLEND-1: NCT06630286, ISLEND-2: NCT06630299).