What is Lenacapavir and How Does It Work as PrEP?

Pre-exposure prophylaxis, or PrEP, refers to the use of antiretroviral medications by people who are HIV-negative but at substantial risk of acquiring human immunodeficiency virus (HIV) infection through sexual contact or injection drug use. Traditional PrEP options, such as daily oral pills containing tenofovir disoproxil fumarate and emtricitabine (often branded as Truvada), have proven effective but face challenges related to adherence. Many individuals struggle with taking a pill every day, leading to gaps in protection.

Enter lenacapavir, a groundbreaking long-acting injectable medication developed by Gilead Sciences. Unlike conventional antiretrovirals that target enzymes like reverse transcriptase or integrase, lenacapavir is the first-in-class capsid inhibitor. The HIV capsid is a protein shell that protects the virus's genetic material and is crucial for its replication cycle. By binding to the capsid protein, lenacapavir disrupts multiple stages of the viral life cycle, including entry into host cells, uncoating, nuclear import, and assembly of new virions. This multi-stage interference makes it highly potent with a high barrier to resistance.

Administered as a subcutaneous injection in the abdomen every six months, lenacapavir offers a dosing schedule that addresses adherence barriers head-on. Initiation requires an oral loading dose—two 300 mg tablets on day 1 alongside two 1.5 mL injections (total 927 mg subcutaneously), followed by two more tablets on day 2. Protection begins rapidly thereafter, typically within hours of completing the loading regimen. Maintenance injections occur every 26 weeks, with flexibility of up to two weeks early or late. If delayed beyond that, oral bridging doses bridge the gap until the next injection.

This regimen's simplicity could transform HIV prevention, particularly for populations facing stigma, instability, or daily routine disruptions. Clinical trials have demonstrated its potential, though recent analyses highlight rare breakthrough infections that warrant careful consideration.

📊 PURPOSE 1 Trial: Pioneering Results in Cisgender Women

The PURPOSE 1 trial, a phase 3, randomized, active-controlled superiority study, focused on cisgender adolescent girls and young women aged 16 to 25 in South Africa and Uganda—regions with high HIV incidence rates exceeding 3% annually. Over 5,300 participants were enrolled, reflecting a population disproportionately affected by HIV, where young women account for a significant portion of new infections.

Participants received either twice-yearly lenacapavir or daily oral emtricitabine/tenofovir alafenamide (F/TAF) or tenofovir disoproxil fumarate/emtricitabine (F/TDF). Early interim results, announced in June 2024, stunned the medical community: zero HIV infections in the lenacapavir arm among over 2,000 participants. This translated to 100% efficacy compared to background incidence and superiority over daily oral PrEP.

Final 52-week data presented at the Conference on Retroviruses and Opportunistic Infections (CROI) 2026 refined this picture. Lenacapavir showed an HIV incidence of 0.07 per 100 person-years (ppy), with two seroconversions out of more than 2,000 participants. In contrast, the oral PrEP arms had incidences of 1.98 ppy (F/TAF, 77 infections) and 1.94 ppy (F/TDF). While not flawless, these rates underscore lenacapavir's exceptional performance, reducing infections by over 96% relative to daily options.

Notably, PURPOSE 1 was the first major PrEP trial to include pregnant and lactating individuals from the outset, addressing a critical evidence gap. No safety signals emerged in these groups, paving the way for broader recommendations.

PURPOSE 2 Trial: Efficacy Across Diverse Gender Identities

Building on PURPOSE 1, the PURPOSE 2 trial targeted cisgender men who have sex with men (MSM), transgender men, transgender women, and gender non-binary individuals aged 16 and older. Conducted across Argentina, Brazil, Mexico, Peru, Puerto Rico, South Africa, Thailand, and the United States, it enrolled 3,265 participants at elevated HIV risk.

Again, lenacapavir was compared to daily F/TDF. At 52 weeks, lenacapavir achieved an incidence of 0.11 ppy with three seroconversions among 2,179 participants, versus 0.92 ppy and 12 infections in the oral arm. This represents a 96% reduction versus background incidence and 89% superiority over daily PrEP. Published in the New England Journal of Medicine in November 2024, these findings confirmed lenacapavir's robustness in a population where injectable cabotegravir (Apretude, every two months) had previously shown promise but required more frequent dosing.

The trial's diversity—encompassing various sexual practices, gender-affirming hormone use, and regional differences—enhances generalizability. Adherence modeling suggested lenacapavir's levels remained protective even with minor delays, a boon for real-world implementation.

🔬 Rare Breakthrough Infections: What We Know

Across PURPOSE 1 and 2, five seroconversions occurred in lenacapavir recipients, four with on-time injections and detectable drug levels. This rarity—amid thousands of participant-years—affirms high efficacy, yet prompts scrutiny.

• In PURPOSE 1: One case followed a missed injection at week 26, with transition to oral PrEP; seroconversion at week 95. The second occurred at week 52 despite on-time dosing.
• In PURPOSE 2: All three had on-time injections; one at week 52 with target blood levels.

Of these, four harbored lenacapavir-associated resistance mutations (e.g., N74D, Q67H/K70R in the capsid gene), likely emerging during PrEP exposure rather than via transmission, as such strains are not prevalent. One case lacked genotyping due to low viral load. No unifying cause—behavioral, biological, or viral—has emerged, though investigations continue.

Experts emphasize context: Breakthroughs occur with all PrEP modalities (e.g., cabotegravir saw infections despite adherence). Stephanie Cox noted at CROI 2026, "We don’t know why these occurred… but overall efficacy is extremely high." Hyman Scott added, "The breakthrough infections are important to evaluate but are extremely rare." This positions lenacapavir as a durable option, not infallible but superior in low burden.

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Safety Profile and Practical Considerations

Safety data from over 8,500 participants across trials reveal a favorable profile. Injection-site reactions (redness, swelling) affected 63-69%, mostly mild and transient, resolving within days. No discontinuations due to reactions in PURPOSE trials. Systemic adverse events mirrored placebo rates; no immune reconstitution issues or hepatotoxicity signals.

Drug interactions are manageable: Adjustments needed for rifampin, atorvastatin, or certain antifungals. Compatible with gender-affirming hormones, contraceptives, and tuberculosis meds (with monitoring). Renal/hepatic impairment contraindications apply, per labeling.

Monitoring mirrors other PrEP: HIV testing every six months (antigen/antibody, RNA if acute symptoms), STI screens, risk reassessment. For those switching from cabotegravir or tenofovir-based PrEP, streamlined testing suffices.

Regulatory Milestones and Global Rollout

The U.S. FDA approved lenacapavir (Yeztugo) for PrEP in June 2025, granting Breakthrough Therapy Designation earlier. The CDC issued clinical recommendations in September 2025 for adults/adolescents ≥35 kg at risk. WHO followed in July 2025, endorsing it alongside cabotegravir and daily pills for key populations like sex workers, MSM, transgender individuals, and people who inject drugs.

Implementation emphasizes community delivery, rapid HIV testing, and data collection on uptake. For full CDC guidelines, consult official sources. WHO's guidelines stress integration into combination prevention.

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Future Directions and Access Challenges

Ongoing trials (PURPOSE 3-5, PURPOSE 365) probe people who inject drugs, U.S. women, and once-yearly intramuscular dosing—early data promising. Modeling predicts millions of averted infections with equitable access.

Challenges persist: High initial pricing ($28,000/year in U.S.), though generics via Gates Foundation/Unitad partnerships target $40/year in 120 low-income countries by 2027. Advocacy focuses on voluntary licensing for scalability.

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Shaping the Future of HIV Prevention

Lenacapavir PrEP exemplifies innovation, offering near-perfect efficacy with minimal dosing. Rare breakthroughs, under study, do not eclipse its promise—especially versus adherence-limited alternatives. As rollout accelerates, integration with testing, counseling, and other tools will maximize impact.

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