The way abacavir (ABC) behaves in the body differs between children and adults, but little is known about ABC in adolescents. It is unclear if adult doses of ABC are appropriate for adolescents. The purpose of this study is to determine the blood levels of ABC in HIV infected adolescents who are on ABC-containing regimens.

ABC is approved for the treatment of HIV in adults and children, but it is unclear if currently recommended ABC doses are appropriate for adolescents. Previous data suggest ABC clearance in children is about twice that found in adults, but limited data exist on the pharmacokinetics of ABC in adolescents. This study will evaluate the 8-hour pharmacokinetics of ABC in HIV infected adolescents who are currently on ABC-containing treatment regimens.

There will be two groups in this study. Group 1 participants will be 13 to 17 years old. Group 2 participants will be 18 to 25 years old. All participants will receive a 300 mg dose of ABC as either a single-agent tablet or a combination tablet of ABC, lamivudine, and zidovudine (whichever they are currently taking). Participants will have a medical history assessment and a physical exam at screening and study entry. Participants will also be asked about adherence to their ABC-containing regimen at study entry. During the 8-hour drug level study, blood collection for pharmacokinetic analysis will occur prior to taking ABC, and at 0.5, 1, 2, 3, 4, 6, and 8 hours after taking ABC.

Inclusion Criteria:

• HIV-1 infected
• CD4 count greater than 100 cells/mm3
• Viral load less than 100,000 copies/ml
• Have taken an abacavir-containing regimen for at least 8 weeks prior to study entry
• Weight more than 83 lbs (37.5 kg)
• Ability and willingness to swallow study medications
• Consent of parent or guardian, if applicable

Exclusion Criteria:

• Any Grade 3 or greater toxicity within 14 days prior to study entry
• Participation in PACTG P1018
• CDC Category C opportunistic infections or HIV-1 associated cancer requiring drug therapy at the time of study enrollment
• Treatment with immune modulators, including interleukin-2 or intravenous gamma globulin, within 30 days of study entry
• Received therapeutic vaccines or any HIV-1 vaccine given for primary prevention within 30 days of study entry
• Surgical or medical problem affecting gastrointestinal motility or absorption (e.g., ileus, ulcerative colitis) or liver function
• History of chronic alcohol use
• Any clinically significant disease other than HIV-1 infection that, in the investigator's opinion, would represent an increased risk for the participant or compromise the outcome of the study
• Chemotherapy for active cancer
• Pregnancy or breastfeeding