Lenacapavir at the Spatiotemporal Limit: Tissue Pharmacokinetics, Pharmacokinetic Tail Dynamics, and the Emergence of Capsid Resistance in Long-Acting HIV Prevention

Background: Lenacapavir (LEN), a first-in-class HIV-1 capsid inhibitor administered subcutaneously twice yearly, has demonstrated near-perfect efficacy in Phase 3 trials, generating justified enthusiasm for its potential to reconfigure HIV prevention. However, its revolutionary success obscures critical spatiotemporal vulnerabilities. We critically examined the protective envelope of LEN, specifically focusing on the gap between systemic pharmacokinetics (PK) and mucosal distribution, the dynamics of the prolonged PK tail, and the clinical emergence of capsid resistance. Methods: A systematic narrative synthesis was conducted through April 2026. We integrated screening and data parameters from the PURPOSE and CAPELLA trials alongside advanced PK-PD modeling and structural biology datasets to comprehensively map the spatiotemporal parameters of lenacapavir-based PrEP. Findings: We characterize the Limitations of Systemic Surrogacy, highlighting that no published empirical data exist characterizing LEN concentrations in human mucosal sanctuary sites; the sole registered tissue distribution trial (Pro00043856) remains unpublished four years post-registration. While modeling baseline assumptions establish a wild-type 95% preventive plasma concentration (EC 95 ) of 5.8 ng/mL, the terminal absorption-limited elimination phase creates a definitive 106– 235 day Mutant Selection Window (MSW) following drug discontinuation. Based on the 58 studies included in the full synthesis, falling sub-therapeutic drug concentrations during this tail phase exert selective pressure that favors high-fitness capsid substitutions (eg, Q67H), lowering the genetic barrier for complex, high-level resistance mutations (eg, N74D). Clinical breakthrough infections observed across the CAPELLA trial (19%) and rare PURPOSE infections (0.07 per 100 person-years) validate this corridor of vulnerability, highlighted by a recorded seroconversion occurring 16 months post-injection. Conclusion: Optimizing the public health impact of lenacapavir requires transitioning from simple serum-based monitoring toward tissue-informed clinical stewardship. Unlike conventional oral PrEP, twice-yearly lenacapavir involves an extended pharmacological commitment of nearly 12 months, requiring pre-planned clinical protocols for bridging therapy during drug discontinuation. Implementation requires the evaluation of high-sensitivity viral surveillance tools ( LOD ≤ 10 copies/mL) and global access reconfigurations to resolve the economic and licensing barriers separating low-cost generic synthesis from international list prices, protecting the long-term viability of the capsid inhibitor class across all implementation sectors. A flowchart detailing three phases: active prophylaxis, discontinuation and clinical stewardship. Keywords: lenacapavir, HIV pre-exposure prophylaxis, pharmacokinetics, drug resistance, viral, global health equity, capsid inhibitor, area under curve