A Study to Compare Three Different Formulations of Tenofovir 1% Gel When Administered Rectally

This is a double-blinded, randomized, safety, acceptability, pharmacokinetic, and ex vivo efficacy study of three rectally-applied tenofovir-based microbicide formulations. Approximately 18 total evaluable HIV-negative men and women (\~9 per site) will be enrolled across two study sites: University of California at Los Angeles (UCLA) and Magee-Womens Research Institute (MWRI) at University of Pittsburgh. Each participant will experience seven rectal exposures to the rectal-specific formulation (RF) and seven rectal exposures to the reduced glycerin vaginal formulation (RGVF) of tenofovir 1% gel, but only one exposure to the vaginal formulation (VF), which will be coupled with six preceding exposures to the Universal HEC Placebo Gel to balance out the VF study stage. Participant accrual will take approximately 6 months and each participant will be on study for approximately 3 months. The total duration of the study will be approximately 1 year. The primary objectives of the study are safety, acceptability, and pharmacokinetics, specifically: * To evaluate the safety of each tenofovir-based microbicide gel formulation when applied rectally * To evaluate the acceptability of each tenofovir-based microbicide gel formulation when applied rectally * To compare systemic and compartment pharmacokinetics among the three tenofovir-based microbicide gel formulations when applied rectally Secondary objective of the study is to evaluate the mucosal immunotoxicity of each tenofovir-based microbicide gel formulation when applied rectally. And the exploratory objective of the study is to assess the preliminary (ex vivo) efficacy of each tenofovir-based microbicide gel formulation using biopsy explants after each product is applied rectally.