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

The primary objective of this study is to evaluate the efficacy of elvitegravir/cobicistat/emtricitabine/tenofovir alafenamide (E/C/F/TAF) fixed dose combination (FDC) plus darunavir (DRV) relative to current antiretroviral regimens (ARV) in virologically suppressed, HIV-1 positive participants with HIV-1 RNA \<50 copies/mL at Week 24.

This study consists of 48 weeks of open-label phase followed by an optional Extension Phase in which all the participants will receive E/C/F/TAF+DRV.


Eligibility Criteria:
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.