Safety and Pharmacokinetics Evaluation of Fostemsavir + (OBT) in HIV-1 Infected Children and Adolescents Who Are Failing Their cART and Have Dual- or Triple-class Antiretroviral Resistance

Inclusion Criteria: * Male and female HIV-1 infected paediatric participants from 6 years old and weighing at least 20 kg to less than 18 years of age. * Antiretroviral-experienced with documented historical or baseline resistance to one or more agents in at least two classes. All resistance has to be properly documented. * Failing current antiretroviral regimen with a confirmed plasma HIV-1 RNA ≥ 1000 c/mL (first value from Investigator within 6 months of screening visit, with the second value obtained from Screening labs, without a decline greater than 1 log10, and no value \<1000 in between). * Documented resistance to at least one component of the current failing regimen per screening resistance testing. * Must have at least 1 fully active and available agent in 2 or more ARV classes, based on current and/or documented historical resistance testing, taking into account tolerability, and other safety concerns. At least two fully active agents must be a part of the initial OBT to be paired with FTR. * Girls who have reached menarche must have a negative pregnancy test at screening, not be breastfeeding, and be willing to adhere to effective methods of contraception if sexually active. All participants (male or female) have to agree with recommendations for effective contraception. Exclusion Criteria: Medical History and Concurrent Diseases: * Unable to comply with dosing requirements (to swallow solid pharmaceutical form of the investigational medicinal product) * Unable to comply with study visits * Presence of a malabsorption syndrome or other gastrointestinal dysfunction which might interfere with drug absorption or render the participant unable to take oral medication. * Any clinical condition (including but not limited to recreational drug use) or prior therapy that, in the opinion of the Investigator, would make the participant unsuitable for the study * Pregnancy and breastfeeding Physical and Laboratory Test Findings: * Chronic untreated Hepatitis B virus (HBV) (however, participants with chronic treated HBV or spontaneously remitted HBV are eligible) * HIV-2 infection * Alanine aminotransferase (ALT) ≥5 times the upper limit of normal (ULN), OR ALT ≥3xULN and bilirubin ≥1.5xULN (with\>35% direct bilirubin) * History of unstable liver disease, decompensated cirrhosis, or known biliary disorder * History of congestive heart failure, or congenital/acquired prolonged QT syndrome/other cardiac diseases predisposing to prolonged QTc * Hemoglobin \< 8.0 g/dL * Platelets \< 50,000 cells/mm3 * Confirmed QTcF value \> 450 msec, regardless of sex, at Screening or Day 1 * Current (defined as taking the medication within 14 days of Day 1) or anticipated treatment with medication considered prohibited or restricted as per Appendix II. Certain medication will be carefully evaluated as acceptable, see Appendix II. * Participation in an experimental drug and/or HIV-1 vaccine trial(s) within the previous 30 days * Child in governmental care, e.g. child is a ward of the state. Note: This criterion does not apply if the child is officially adopted by a family/guardian.