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

Inclusion Criteria: 1. ≥ Age of 18 at screening 2. Willing and able to communicate in English 3. Willing and able to provide written informed consent to take part in the study 4. Willing and able to provide adequate locator information 5. Understands and agrees to local sexually transmitted infection (STI) reporting requirements 6. HIV-1 uninfected at screening according to the standard DAIDS algorithm in Appendix II 7. Must have been vaccinated for or have natural immunity to Hepatitis B, which will be verified by a positive Hepatitis B surface antibody (HBsAb) test at screening (Note: One re-screen will be allowed for individuals who are non immune to Hepatitis B but undergo vaccination.) 8. Availability to return for all study visits, barring unforeseen circumstances 9. Willing to abstain from RAI and practices involving insertion of anything in rectum (drug, enema, penis, or sex toy) for 72 hours before and 72 hours after each flexible sigmoidoscopy and study product exposure. 10. Must agree to use study provided condoms for the duration of the study 11. Must be in general good health 12. Must agree not to participate in other concurrent interventional and/or drug trials 13. Per participant report at screening, a history of consensual RAI at least once in the last three months. In addition to the criteria listed above, female participants must meet the following criteria: 14. Willing to abstain from insertion of anything into vagina (drug, douche, penis, or sex toy) other than the swabs/sponges for study related specimen collection for 24 hours before and after each study product exposure 15. Post-menopausal or using (or willing to use) an acceptable form of contraception (e.g., intrauterine device (IUD), hormonal contraception, surgical sterilization, or vasectomization of male partner). If the female participant has female partners only, the method of contraception will be noted as a barrier method in the study documentation. Temporary abstinence due to absence of partner(s) for the duration of the study will be acceptable. Exclusion Criteria: 1. Abnormalities of the colorectal mucosa, or significant colorectal symptom(s), which in the opinion of the clinician represents a contraindication to biopsy (including but not limited to presence of any unresolved injury, infectious or inflammatory condition of the local mucosa, and presence of symptomatic external hemorrhoids). 2. At screening: participant-reported symptoms and/or clinical or laboratory diagnosis of active rectal or reproductive tract infection requiring treatment per current CDC guidelines or symptomatic urinary tract infection (UTI). Infections requiring treatment include symptomatic bacterial vaginosis, symptomatic vaginal candidiasis, other vaginitis, trichomoniasis, Chlamydia (CT), gonorrhea (GC), syphilis, active HSV lesions, chancroid, pelvic inflammatory disease, genital sores or ulcers, cervicitis, or symptomatic genital warts requiring treatment. Note that an HSV-1 or HSV-2 seropositive diagnosis with no active lesions is allowed, since treatment is not required. (Note: In cases of non-anorectal GC/CT identified at screening, one re-screening 2 months after screening visit will be allowed.) 3. Per participant report and/or clinical or laboratory diagnosis, anorectal STI within six months prior to the Screening Visit 4. At screening: 1. Hemoglobin \< 10.0 g/dL 2. Platelet count less than 100,000/mm3 3. White blood cell count \< 2,000 cells/mm3 or \> 15,000 cells/mm3 4. For females: calculated creatinine clearance less than 60 mL/min by the Cockcroft-Gault formula where creatinine clearance in mL/min (140- age in years) x (weight in kg) x (0.85 for female)/72 x (serum creatinine in mg/dL) 5. For males: calculated creatinine clearance less than 60 mL/min by the Cockcroft-Gault formula where creatinine clearance in mL/min = (140 - age in years) x (weight in kg) x (1 for male)/72 x (serum creatinine in mg/dL) 6. Serum creatinine \> 1.3× the site laboratory upper limit of normal (ULN) 7. Alanine transaminase (ALT) and/or aspartate aminotransferase (AST) \> 2.5× the site laboratory ULN 8. +1 glucose or +1 protein on urinalysis (UA) 9. History of bleeding problems (verified via prothrombin time (PT)/ International Normalized Ratio (INR) test) 10. Positive for Hepatitis B surface antigen (HBsAg) 5. History of significant gastrointestinal bleeding in the opinion of the investigator 6. Known allergic reaction to methylparaben, propylparaben, sorbic acid, glycerin, glycerol, or tenofovir 7. Current known HIV-infected partner(s) 8. By participant report at enrollment, history of excessive daily alcohol use (as defined by the CDC as heavy drinking consisting of an average consumption of more than 2 drinks per day for men, and more than 1 drink per day for women), frequent binge drinking or illicit drug use that includes any injection drugs, methamphetamines (crystal meth), heroin, or cocaine including crack cocaine, within the past 12 months 9. Per participant report at screening, anticipated use and/or unwillingness to abstain from the following medications during the period of study participation: 1. Heparin, including Lovenox® 2. Warfarin 3. Plavix® (clopidogrel bisulfate) 4. Rectally administered medications (including over-the-counter products) 5. Acyclovir, valacyclovir, famciclovir, and TDF 6. \>81 mg of aspirin per day AND unwillingness and/or inability to completely stop all use of aspirin or aspirin-containing medications for 3 days before and 3 days after the biopsy collection procedure 7. Non-steroidal anti-inflammatory drugs (NSAIDS) 8. Any other drugs that are associated with increased likelihood of bleeding following mucosal biopsy 10. By participant report at screening, use of systemic immunomodulatory medications, rectally administered medications, rectally administered products (including condoms) containing N-9, or any investigational products within the 4 weeks prior to the Enrollment/Baseline Evaluation Visit and throughout study participation 11. History of recurrent urticaria 12. Any other condition or prior therapy that, in the opinion of the investigator, would preclude informed consent, make study participation unsafe, make the individual unsuitable for the study or unable to comply with the study requirements. Such conditions may include, but are not limited to, current or recent history of severe, progressive, or uncontrolled substance abuse, or renal, hepatic, hematological, gastrointestinal, endocrine, pulmonary, neurological, or cerebral disease In addition to the criteria listed above, female participants will be excluded if they meet any of the following criteria: 13. Pregnant at the Enrollment/Baseline Visit 14. Breastfeeding at screening or intend to breastfeed during study participation per participant report.