Efficacy of Tenofovir Alafenamide Versus Placebo Added to a Failing Regimen Followed by Treatment With Elvitegravir/Cobicistat/Emtricitabine/Tenofovir Alafenamide Plus Atazanavir in HIV-1 Positive, Antiretroviral Treatment-Experienced Adults

Key Inclusion Criteria: * Ability to understand and sign a written informed consent form, which must be obtained prior to initiation of study procedures * Currently taking a failing ARV regimen * Plasma HIV-1 RNA ≥ 500 copies/mL but ≤ 100,000 copies/mL at screening * Normal ECG * Estimated glomerular filtration rate (eGFR) ≥ 50 mL/min according to the Cockcroft-Gault formula for creatinine clearance * Alanine aminotransferase (AST)/aspartate aminotransferase (AST) ≤ 5 × the upper limit of the normal range (ULN) * Total bilirubin ≤ 1.5 mg/dL, or normal direct bilirubin * Adequate hematologic function * Serum amylase ≤ 5 × ULN * Females may enter the study if it is confirmed that she is: * Not pregnant or nursing * Of non-childbearing potential (ie, have had a hysterectomy, both ovaries removed, medically documented ovarian failure, or are postmenopausal women \> 54 years of age with cessation \[for ≥ 12 months\] of previously occurring menses), or * Of childbearing potential and agrees to utilize highly effective contraception methods or be non-heterosexually active or practice sexual abstinence from screening throughout the duration of study treatment and for 30 days following study drug dosing * Females who utilize hormonal contraceptive as one of their birth control methods must have used the same method for at least three months prior to study dosing. * Males must agree to utilize a highly effective method of contraception during heterosexual intercourse or be non-heterosexually active, or practice sexual abstinence from first dose throughout the study period and for 30 days following the last study drug dose. * Males must agree to refrain from sperm donation from first dose until at least 30 days after the last study drug dose. Key Exclusion Criteria: * A new AIDS-defining condition diagnosed within the 30 days prior to screening * Hepatitis B surface antigen (HBsAg) positive * Hepatitis C antibody positive (individuals with positive hepatitis C virus (HCV) antibody and without detectable HCV RNA are permitted to enroll) * History of integrase inhibitor use * Screening or historical genotype reports shows Q151M or T69ins or more than 3 TAMs. * Screening or historical genotype report shows resistance to integrase inhibitors * Individuals experiencing decompensated cirrhosis * Current alcohol or substance use * History of malignancy within the past 5 years (prior to screening) or ongoing malignancy other than cutaneous Kaposi's sarcoma (KS), basal cell carcinoma, or resected, non-invasive cutaneous squamous carcinoma. Individuals with cutaneous KS are eligible, but must not have received any systemic therapy for KS within 30 days of Part 1, Day 1 and must not be anticipated to require systemic therapy during the study. * Active, serious infections (other than HIV-1 infection) requiring parenteral antibiotic or antifungal therapy within 30 days prior to Part 1, Day 1 * Any other clinical condition or prior therapy that would make the individual unsuitable for the study or unable to comply with dosing requirements * Participation in any other clinical trial (including observational trials) without prior approval from the sponsor is prohibited while participating in this trial * Receiving ongoing therapy with any disallowed medications, including any drugs not to be used with elvitegravir, cobicistat, emtricitabine and tenofovir alafenamide or known allergies to the excipients of E/C/F/TAF STR Note: Other protocol defined Inclusion/Exclusion criteria may apply.