The primary objectives of this study are to evaluate the steady-state pharmacokinetics (PK) and confirm the dose of the elvitegravir/cobicistat/emtricitabine/tenofovir disoproxil fumarate (EVG/COBI/FTC/TDF) single-tablet regimen (STR) (Part A) and to evaluate the safety and tolerability of EVG/COBI/FTC/TDF STR through Week 48 (Part B) in HIV-1 infected, antiretroviral (ARV) treatment-naive adolescents.

A total of 50 adolescent participants (12 to < 18 years of age) will be enrolled to receive EVG/COBI/FTC/TDF as follows:

• Part A: Twelve to 16 eligible participants will be enrolled to evaluate steady-state PK, and confirm the dose, with the intent to enroll at least 4 participants 12 to < 15 and at least 4 participants 15 to < 18 years of age.
• Part B: Following confirmation of EVG exposure in at least 12 participants from Part A, 34 to 38 participants in addition to those enrolled in Part A will be enrolled to evaluate the safety, tolerability, and antiviral activity of EVG/COBI/FTC/TDF STR.

Key Inclusion Criteria:

• 12 years to < 18 years of age at baseline
• Able to give written assent prior to any screening evaluations
• Parent or guardian able to give written informed consent prior to any screening evaluations and willing to comply with study requirements
• Plasma HIV-1 RNA levels of ≥ 1,000 copies/mL
• CD4+ cell count > 100 cells/µL
• Weight ≥ 35 kg (77 lbs)
• Screening genotype report must show sensitivity to FTC and TDF
• Able to swallow oral tablets
• Adequate renal function
• Clinically normal ECG
• Documented screening for active pulmonary tuberculosis per local standard of care within 6 months of a screening visit
• Hepatic transaminases ≤ 5 x upper limit of normal
• Total bilirubin ≤ 1.5 mg/dL, or normal direct bilirubin
• Individuals with a positive Hepatitis B surface antigen screening test can participate in the study, providing that alternate therapy (other than TDF) for chronic Hepatitis B infection is available as a part of local standard of care
• Adequate hematologic function
• Negative serum pregnancy test for all females
• Males and females of childbearing potential must agree to utilize highly effective contraception methods while on study treatment or agree to abstain from heterosexual intercourse throughout the study period and for 30 days following the last dose of study drug
• Males must agree to utilize a highly effective method of contraception during heterosexual intercourse throughout the study period and for 30 days following discontinuation of investigational medicinal product
• Must be willing and able to comply with all study requirements
• Life expectancy ≥ 1 year

Key Exclusion Criteria:

• A new AIDS-defining condition diagnosed within the 30 days prior to screening
• Prior treatment with any approved or investigational or experimental anti HIV-1 drug for any length of time (other than that given for prevention of mother-to-child transmission)
• Evidence of active pulmonary or extra-pulmonary tuberculosis disease within 3 months of the screening visit
• Anticipated to require rifamycin treatment for mycobacterial infection while participating in the study. Note: prophylactic Isoniazid (INH) therapy for latent tuberculosis (TB) treatment is allowed.
• Individuals experiencing decompensated cirrhosis
• Pregnant or lactating females
• Have any serious or active medical or psychiatric illness which would interfere with treatment, assessment, or compliance with the protocol. This would include uncontrolled renal, cardiac, hematological, hepatic, pulmonary, endocrine, central nervous, gastrointestinal, vascular, metabolic, immunodeficiency disorders, active infection, or malignancy that are clinically significant or requiring treatment within 30 days prior to the study dosing.
• Current alcohol or substance abuse that will potentially interfere with compliance
• Have history of significant drug sensitivity or drug allergy
• Known hypersensitivity to the study drugs, the metabolites or formulation excipients
• Have been treated with immunosuppressant therapies or chemotherapeutic agents within 3 months of study screening or expected to receive these agents during the study
• A history of malignancy within the past 5 years or ongoing malignancy other than cutaneous Kaposi's sarcoma, basal cell carcinoma, or resected, non-invasive cutaneous squamous carcinoma
• Have previously participated in an investigational trial involving administration of any investigational agent within 30 days prior to the study dosing
• Participation in any other clinical trial without prior approval from sponsor is prohibited while participating in this trial
• Receiving ongoing therapy with any disallowed medications, including drugs not to be used with EVG, COBI, FTC, TDF or individuals with any known allergies to the excipients of EVG/COBI/FTC/TDF STR tablets

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