Evaluation of Long-Acting Lenacapavir for the Treatment of HIV-1 in Treatment-experienced Adolescents and Children

Key Inclusion Criteria: * Body weight at screening ≥ 35 kg. * On a stable failing antiretroviral (ARV) regimen for \> 8 weeks before screening and willing to continue the regimen until Day 1. * Plasma HIV-1 RNA ≥ 400 copies/mL on at least 2 consecutive occasions spanning at least 6 months, including at screening. * Have previously changed their ARV regimen due to treatment failure. * ARV treatment options limited due to resistance, tolerability, contraindications, safety, drug access. * Able and willing to commit to taking LEN in combination with their OBR. * The following laboratory parameters at screening: 1. Estimated glomerular filtration rate (eGFR) ≥ 60 mL/min/1.73 m\^2 using Bedside Schwartz Formula. 2. Absolute neutrophil count \> 0.50 GI/L (\> 500 cells/mm\^3). 3. Hemoglobin ≥ 85 g/L (\> 8.5 g/dL). 4. Platelets ≥ 50 GI/L (≥ 50,000/mm\^3). 5. Hepatic transaminases (aspartate aminotransferase and alanine aminotransferase) ≤ 5 × upper limit of normal. 6. Total bilirubin ≤ 23 μmol/L (≤ 1.5 mg/dL) and direct bilirubin ≤ 7 μmol/L (≤ 0.4 mg/dL). Key Exclusion Criteria: * Life expectancy ≤ 1 year. * An opportunistic illness requiring treatment within the 30 days prior to screening. * Evidence of active pulmonary or extra-pulmonary tuberculosis within 3 months prior to screening. * Hepatitis C virus (HCV) antibody positive with detectable HCV RNA at screening. * Hepatitis B virus (HBV) surface antigen (HBsAg) positive or HBV core antibody (antibody against hepatitis B core antigen (anti-HBc)) positive; if individual is HBsAg negative and anti-HBc positive but HBV DNA undetectable, individual may be enrolled. Note: Other protocol defined Inclusion/Exclusion criteria may apply.