PROJECT SUMMARY/ ABSTRACT Long‐acting injectables for antiretroviral therapy (ART) or pre‐exposure prophylaxis (PrEP) are the most exciting recent advance in HIV medicine. The roll‐out of cabotegravir long‐acting (CAB LA) PrEP will be expanded massively globally with the WHO advising this PrEP option in late July 2022. For any new PrEP tool, determining the best way to avert failures is key, especially with CAB LA PrEP which can fail with integrase inhibitor mutations, with implications for future management. The trials demonstrating that CAB LA was superior to oral PrEP across populations employed single plasma levels of CAB as exposure metrics to adjudicate failures. However, half of the breakthroughs in the trials were not explained by plasma CAB levels, especially with on‐time injections, likely because of limitations of plasma levels. Drugs delivered via injection elute from depot compartments at variable rates over time, making long‐term metrics of exposure important. Our UCSF group has been working on long‐acting exposure metrics of antiretrovirals (ARVs) in hair samples (which are easy to collect, store and ship) during the first iteration and first renewal of this R01 grant. We have been highly productive over this R01 to date (publishing 72 papers), demonstrating important uses of hair levels in the fields of HIV treatment, prevention, over the perinatal period, and in low‐income countries. As PrEP is revolutionized with the availability (e.g. CAB) and promise (e.g. lenacapavir) of LA agents, drug levels will play a critical role in determining the onset of protection, adjudicating failures, and assessing risks with discontinuation; this renewal seeks to contribute to this important field. As an exploratory objective, we will also develop an antibody to CAB to create a point‐of‐care urine assay, as we have with tenofovir. This proposal will leverage three important trials to explore key knowledge gaps that will arise in the next phase of LA PrEP. The Sustainable East Africa Research in Community Health (SEARCH) Sapphire trial is evaluating a number of interventions to prevent and treat HIV, including the provision of CAB LA to 600 at‐ risk individuals across 10 communities in East Africa (Aim 1). The Botswana Harvard AIDS Institute Partnership will launch a new study to provide women in the immediate postpartum period CAB LA PrEP and follow mothers and their infants for 96 weeks, evaluating incident HIV and resistance, as well as the pharmacokinetics (PK) of CAB and its transfer during breastfeeding (Aim 2). The PURPOSE 1 and 2 trials (Gilead) will study lenacapavir LA (a new capsid inhibitor given every 6 months as a subcutaneous injection) for PrEP in diverse populations and will incorporate hair collection in their open‐label extension phases. All 3 trials will collect hair and plasma for drug levels, and track robust outcomes, to allow us to 1) investigate hair levels as predictors of failure; 2) determine maternal to infant trans...