Long-Acting PrEP With Lenacapavir: How a Capsid Inhibitor Could Reshape HIV Prevention Worldwide

Why Is the HIV Capsid Such an Important Drug Target?

For decades, antiretroviral drug development concentrated on viral enzymes — reverse transcriptase, protease, and integrase — each of which catalyzes a single discrete step in HIV replication. The capsid protein, by contrast, is a structural component that participates in a series of distinct biological processes: shielding viral RNA during cytoplasmic transit, facilitating transport through the nuclear pore complex, positioning the pre-integration complex near active host genes, and assembling into the protective shell of newly formed virions. Because one protein performs all these roles, a drug that binds it tightly can simultaneously impair multiple stages of the life cycle.

Lenacapavir binds to the interface between capsid protein subunits within the hexameric lattice that forms the mature viral core. Structural studies using cryo-electron microscopy have shown that the drug stabilizes the capsid lattice in a way that prevents the precisely timed disassembly (uncoating) required after the virus enters a new host cell. At the same time, lenacapavir destabilizes capsid assembly during the late phase of replication, leading to the production of malformed, non-infectious particles. This dual early-and-late-stage activity is distinct from any existing antiretroviral class and contributes to the unusually high genetic barrier to resistance observed in clinical use.

Resistance to lenacapavir requires specific mutations in the capsid gene that typically come at a substantial fitness cost to the virus. In vitro resistance selection experiments showed that the mutations conferring reduced susceptibility to lenacapavir also reduced viral replication capacity, making resistant variants less likely to persist and transmit. This property is particularly important for a PrEP agent, since any drug used for prevention in large populations must maintain its potency even under conditions of imperfect adherence and sporadic viral exposure.

How Does Twice-Yearly Dosing Work Pharmacologically?

The pharmacokinetic behavior of lenacapavir is governed by its physicochemical properties. The molecule has very low aqueous solubility, and when injected subcutaneously as a concentrated solution, it precipitates at the injection site into a microcrystalline depot. Drug is then absorbed gradually through dissolution-limited kinetics, producing a flat plasma concentration–time profile that remains above the threshold associated with antiviral activity for roughly six months. Phase 1 studies demonstrated that a single subcutaneous dose could sustain measurable plasma levels for more than 52 weeks, although the dosing interval selected for clinical development was 26 weeks to maintain concentrations well above the protein-adjusted 95% effective concentration (PA-EC95).

This depot-based delivery confers several practical advantages. Unlike oral medications where a single missed dose can cause a rapid decline in drug levels, the subcutaneous depot provides a pharmacokinetic buffer that tolerates minor variations in the timing of re-injection appointments. Population pharmacokinetic modeling suggests that protective levels are maintained even if a follow-up injection is delayed by several weeks, giving both patients and healthcare systems scheduling flexibility. The long half-life also means that drug concentrations do not fluctuate with food intake, gastrointestinal function, or interactions with common over-the-counter medications — variables that can affect oral PrEP efficacy.

The loading dose strategy — injections at day one, week four, and then every 26 weeks — was designed to rapidly achieve steady-state concentrations. Without a loading phase, the slow absorption from the depot would result in a lag period of inadequate drug levels during which a person would remain vulnerable to HIV acquisition. Clinical pharmacology data confirmed that the two-injection loading sequence brings plasma concentrations above the protective threshold within days, bridging the gap until depot absorption alone can sustain therapeutic levels.

What Sets the PURPOSE Results Apart From Earlier PrEP Trials?

Previous landmark PrEP trials, including iPrEx, Partners PrEP, and TDF2, established that daily oral tenofovir-based regimens reduce HIV risk by 44% to 86% depending on the population studied and adherence levels achieved. However, two major trials in African women — FEM-PrEP and VOICE — failed to demonstrate significant protection, primarily because objective drug-level monitoring revealed that a large proportion of participants were not taking the medication consistently. These findings highlighted a fundamental limitation: a highly efficacious drug is only useful if people can and do take it as prescribed.

The PURPOSE 1 trial addressed this directly by comparing a regimen that requires no daily adherence against the established oral standards. Conducted at sites in South Africa and Uganda among cisgender women aged 16 to 25 — a population where previous oral PrEP trials had struggled — the trial reported that no participant randomized to lenacapavir acquired HIV. The lenacapavir arm met the pre-specified primary endpoint of superiority over background HIV incidence and also demonstrated statistically significant superiority over both Descovy and Truvada, the two FDA-approved oral PrEP options. This was the first time any PrEP regimen had achieved 100% observed efficacy in a randomized Phase 3 trial.

PURPOSE 2 broadened the evidence base by enrolling cisgender men who have sex with men, transgender women, transgender men, and gender non-binary individuals across sites in the Americas, Europe, Africa, and Asia-Pacific. Lenacapavir again showed superior efficacy compared to daily oral Truvada. Importantly, the multi-regional design demonstrated consistent protection across diverse epidemiological settings, sexual practices, and healthcare delivery contexts, supporting the drug's utility as a universal PrEP option rather than one limited to specific populations or regions.

How Does Lenacapavir Compare With Cabotegravir, the Other Long-Acting PrEP Option?

Cabotegravir long-acting (brand name Apretude), approved by the FDA in December 2021, was the first injectable PrEP option and represented a major advance over daily oral regimens. Administered as an intramuscular gluteal injection every two months after a loading phase, cabotegravir demonstrated superior efficacy to daily oral Truvada in the HPTN 083 and HPTN 084 trials among men who have sex with men and cisgender women respectively. However, the every-two-month dosing schedule still requires six healthcare visits per year for injections, and the intramuscular injection can cause significant injection site pain that some patients find burdensome.

Lenacapavir's every-six-month dosing reduces the injection frequency to just two visits per year after the loading phase, a threefold reduction compared to cabotegravir. The subcutaneous route of administration may also be more acceptable to some individuals than intramuscular injection. From a resistance perspective, lenacapavir's novel capsid-targeting mechanism means it retains full activity against HIV strains carrying integrase resistance mutations that could theoretically compromise cabotegravir, and vice versa. This mechanistic independence means both drugs could potentially be used sequentially or in complementary prevention strategies.

The choice between cabotegravir and lenacapavir in clinical practice will depend on multiple factors including availability, cost, patient preference, and regional regulatory status. In resource-limited settings, the reduced number of healthcare visits required for lenacapavir could translate into substantial savings in healthcare worker time and clinic capacity, which are often the binding constraints on PrEP program scale-up. Both agents represent transformative additions to the HIV prevention toolkit, and their availability creates opportunities for tailored prevention strategies that match individual needs and circumstances.

What Are the Barriers to Equitable Global Rollout?

The concentrations of new HIV infections are starkly geographic: sub-Saharan Africa accounted for approximately 65% of all new infections in 2023 according to UNAIDS data, yet these are precisely the settings with the least capacity to pay high drug prices or implement complex new delivery programs. Gilead Sciences entered voluntary licensing agreements through the Medicines Patent Pool in 2024 to allow generic manufacturers in India and other countries to produce lenacapavir for distribution in 120 low- and middle-income countries. Independent analyses have estimated that the active pharmaceutical ingredient could be manufactured generically for as little as $40 per person per year, a fraction of the US list price.

However, manufacturing a long-acting injectable formulation is more complex than producing conventional oral tablets. The crystalline depot formulation requires precise control of particle size, concentration, and excipient composition to ensure consistent six-month drug release. Generic manufacturers will need to demonstrate bioequivalence through pharmacokinetic studies, a process that typically takes longer for depot injectables than for oral generics. The WHO's prequalification process, which enables procurement by international funding agencies, adds additional timelines. Realistic projections suggest that affordable generic lenacapavir may not reach the highest-burden countries until 2027 or 2028.

Health system readiness is another critical factor. Twice-yearly injections must be administered by trained healthcare workers in clinical settings, which represents a shift from the pharmacy-dispensed or community-distributed oral PrEP models that many programs currently use. Countries will need to integrate lenacapavir delivery into existing service platforms — such as reproductive health clinics, HIV testing centers, and primary care facilities — and train providers in subcutaneous injection technique and supply chain management for a cold-chain product. Despite these challenges, modeling by the Clinton Health Access Initiative and others suggests that the reduced visit frequency could ultimately make lenacapavir PrEP more cost-effective than oral PrEP in high-incidence settings, provided that upfront implementation investments are made.