The goal of this clinical study is to learn more about the study drug, lenacapavir (LEN). The study will assess the safety, tolerability, and efficacy of long-acting LEN when combined with other medicines in adolescents and children living with HIV-1 who weigh at least 35 kg and have been treated before for HIV-1. The study will also see how easy it is for participants to take LEN as injection or an oral pill.

The primary objectives are to evaluate the pharmacokinetics and safety of LEN in combination with optimized background regimen (OBR) in TE pediatric participants with HIV-1.

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.