Tenofovir Disoproxil Fumarate to Treat Pediatric HIV

This study will test the safety, side effects and antiviral activity of different doses of tenofovir DF in children and adolescents with human immunodeficiency virus (HIV) infection. Tenofovir DF belongs to a group of drugs called nucleotide analog reverse transcriptase inhibitors. These drugs prevent the virus from replicating (making more copies of itself). HIV becomes resistant to many drugs used to fight the virus and these drugs become ineffective. In laboratory tests, tenofovir DF has remained effective against HIV longer than other anti-HIV medicines, and when resistance does develop, the virus may still be sensitive to other drugs. HIV-infected children between the ages of 4 and 18 years who weigh at least 10 kg (22 pounds) may be eligible for this study. They must be able to receive antiretroviral therapy and have completed at least two previous antiretroviral courses of treatment without benefit. Upon entering the study, participants will have physical, eye and neuropsychiatric examinations, blood tests, including tests to determine what anti-HIV drugs the patient is resistant to, an echocardiogram (echo), electrocardiogram (EKG), chest X-ray, head CT scan, skin tests, and special tests to examine the bones. These physical exams and tests will be repeated throughout the study to determine changes in health. Participants will continue their current anti-HIV therapy for 2 weeks and then stop all medicines for a 1-week 'washout' period. After the washout period, patients will begin taking tenofovir DF. For the first 2 days on the drug, a small blood sample (1/2 teaspoon) will be collected 11 times over a 48-hour period through. A heparin lock (a tube kept in place in a vein) may be put in place to avoid multiple needle sticks. Blood samples will be collected for another 4 days to measure how well tenofovir DF alone works against HIV before other drugs are added to the treatment regimen. After these first 6 days, at least two other anti-HIV drugs will be a