Innovations recently introduced into the field of systemic PrEP are long-acting (LA) formulations of antiretrovirals that stably release drugs over many weeks either as nanocrystal-based-formulations or intravaginal rings. These approaches offer major benefits including: (a) the ability to mitigate poor patient compliance with daily systemic PrEP dosing; (b) the potential for a reduced reliance of HIV prevention on the front-line HIV therapeutic drugs; and (c) their ability to be utilized somewhat discreetly without requiring a partner’s knowledge or consent. In addition, similar advantages have been considered for the use of LA formulations for HIV therapy. Our long-term goal of this collaborative effort between Drs. Garcia, Benhabbour, Wahl and Kovarova is to develop a delivery systems for LA therapy and PrEP that can offer durable and sustained viral suppression and/or protection from HIV transmission while providing flexibility in the choice of active ingredient, high efficacy of HIV inhibition and increased user compliance. Animal models are essential for the in vivo evaluation of HIV prevention approaches. Significant progress has been made in the development and implementation of both non-human primate (NHP) and humanized mouse models of SIV/HIV infection for the evaluation of topical microbicides, systemic pre-exposure prophylaxis and HIV treatment. Although both systems have been shown to provide insight into the process of HIV acquisition and the effectiveness of different interventions, one notable advantage of using humanized mice is the ability to use highly relevant transmitted/founder human viruses and human infected cells for challenge experiments in the presence of human semen. In this regard, bone marrow/liver/thymus (or BLT) mice have become widely utilized to evaluate the efficacy of novel prevention approaches and treatment to HIV infection. Concordant results between humans, NHP and BLT mice when using the same drug and the same route of challenge (i.e. rectal vs. vaginal) confirm the suitability of this model for the pre-clinical efficacy evaluation of HIV prevention and therapy interventions as proposed in the following Specific Aims: Specific Aim 1) To develop and characterize novel, safe and effective polymer- based ultra-long acting in-situ forming implants (ISFI) for HIV treatment and prevention. Specific Aim 2: In vivo assessment of the efficacy of ultra-long acting EFdA ISFI formulation to prevent HIV transmission. Specific Aim 3: In vivo assessment of the efficacy of a combination ultra-long- acting antiretroviral formulation to control HIV replication in vivo.