Phase 3 Open-Label Study to Evaluate Switching From Optimized Stable Antiretroviral Regimens Containing Darunavir to Elvitegravir/Cobicistat/Emtricitabine/Tenofovir Alafenamide (E/C/F/TAF) Fixed Dose Combination (FDC) Plus Darunavir (DRV) in Treatment Experienced HIV-1 Positive Adults

Key Inclusion Criteria:

• Ability to understand and sign a written informed consent form
• History of at least two prior antiretroviral regimens, and history of resistance to at least two different classes of antiretroviral agents
• Plasma HIV-1 RNA levels < 50 copies/mL at screening. Virologically suppressed on the current antiretroviral regimen containing darunavir 600 mg twice a day or 800 mg once daily continuously for ≥ 4 months preceding the screening visit and have maintained documented undetectable plasma HIV-1 RNA levels (< 50 copies/mL) and must have documentation of genotype/phenotype prior to current regimen which shows no darunavir associated resistance mutation.
• Currently receiving raltegravir, elvitegravir, or dolutegravir (50 mg once daily, but not twice daily), or have never received integrase inhibitor, or have documentation of genotype/phenotype within 12 months prior to current regimen which must show no evidence of resistance to integrase inhibitors
• Normal ECG
• Estimated glomerular filtration rate (eGFR) ≥ 50 mL/min according to the Cockcroft Gault formula for creatinine clearance
• Hepatic transaminases (AST and ALT) ≤ 5 × upper limit of normal (ULN)
• Total bilirubin ≤ 1.5 mg/dL, or normal direct bilirubin
• Adequate hematologic function (absolute neutrophil count ≥ 1,000/mm^3; platelets ≥ 50,000/mm^3; hemoglobin ≥ 8.5 g/dL)
• Serum amylase ≤ 5 × ULN (individuals with serum amylase > 5 × ULN will remain eligible if serum lipase is ≤ 5 × ULN)
• A female individual is eligible to enter the study if it is confirmed that she is:

• Not pregnant or nursing
• Of non-childbearing potential (i.e., women who have had a hysterectomy, have had both ovaries removed or 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 the last study drug dose.
• Female individuals 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.
• Male individuals 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.
• Male individuals 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 (except CD4 cell count and/or percentage criteria)
• Hepatitis B surface antigen (HBsAg) positive
• Individuals receiving drug treatment for Hepatitis C, or individuals who are anticipated to receive treatment for Hepatitis C during the course of the study.
• Must not have Q151M, T69ins, or > 3 thymidine analogue mutations (TAMS) present on documented historic genotype report
• Individuals experiencing decompensated cirrhosis
• Females who are breastfeeding
• Positive serum pregnancy test
• Have an implanted defibrillator or pacemaker
• Current alcohol or substance use that may interfere with individual's study compliance
• A history of malignancy within the past 5 years or ongoing malignancy other than cutaneous Kaposi's sarcoma (KS), basal cell carcinoma, or resected, non-invasive cutaneous squamous carcinoma.
• Active, serious infections (other than HIV-1 infection) requiring parenteral antibiotic or antifungal therapy within 30 days prior to Day 1 visit
• 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
• Individuals receiving ongoing therapy with any of the disallowed medications, including drugs not to be used with elvitegravir, cobicistat, emtricitabine, TAF, or DRV; or individuals with any known allergies to the study drugs.

Note: Other protocol defined Inclusion/Exclusion criteria may apply.