• Termination from treatment due to suspected drug reaction attributable to the study medication [ Time Frame: throughout study ] [ Designated as safety issue: Yes ]
  
• Grade 3 or 4 adverse events [ Time Frame: throughout study ] [ Designated as safety issue: Yes ]
  
• Pharmacokinetic parameters [ Time Frame: throughout study ] [ Designated as safety issue: No ]
  

• HIV viral load [ Time Frame: throughout study ] [ Designated as safety issue: No ]
  
• CD4 count and percentage [ Time Frame: throughout study ] [ Designated as safety issue: No ]
  
• Genotypic and phenotypic resistance measures [ Time Frame: Weeks 12, 24, 36, and 48 ] [ Designated as safety issue: No ]
  

Integrase is one of three enzymes necessary for HIV replication. Integrase allows for the integration of HIV DNA into the human genome. Currently, there are no FDA approved drugs that prevent integrase from working properly. Raltegravir is a strong and selective inhibitor of HIV integrase. In adults, raltegravir has shown significant antiretroviral activity in clinical trials and is well tolerated. However, there is no data on the drug's safety and effectiveness in children and adolescents. The purpose of this study is to determine the safety and effectiveness of raltegravir in treatment-experienced, HIV infected children and adolescents.

This study will last at least 2 years and will take place in two stages. Participants will be stratified by age and will be assigned to one of six cohorts. Participants in Cohort 1 will be between the ages of 12 and 18 and will receive raltegravir tablets. Participants in Cohort 2A will be between ages 6 and 11 and will receive raltegravir tablets. Participants in Cohort 2B will be between the ages 6 and 11 and will receive chewable raltegravir tablets. Participants in Cohort 3 will be between the ages 2 and 5 and will receive a chewable raltegravir tablets. Participants in Cohort 4 will be between the ages of 6 and 23 months and will receive an age appropriate formulation that is currently in development. Participants in Cohort 5 will be between the ages of 4 weeks and 5 months and will receive an age appropriate formulation that is currently in development.

Enrollment for Stage I of this study will begin with Cohort 1 and progress to the younger cohorts once preliminary dosage has been determined and safety data have been reviewed. When this information has been determined for Cohort 1, Cohorts 2A and 2B will begin enrollment. Once safety and dose data for these cohorts have been reviewed, enrollment into Cohort 3 will begin. Once safety and dose data for Cohort 3 has been reviewed, enrollment into Cohorts 4 and 5 will begin.

Stage I will begin with enrollment of 4 participants into Cohort 1. On Days 5 to 12, an intensive pharmacokinetic (PK) evaluation will be performed. If these PK data are acceptable and safety data at Week 4 are acceptable, Cohort 1 will open to a full enrollment of 10 participants. The purpose of Stage II is to determine long-term safety of raltegravir once a safe and effective dose has been determined. During Stage II of this study, participants will take raltegravir at the dosage determined as safe and effective by the Stage I data. Ten additional participants will be enrolled into each cohort. The remaining 50 to 70 additional participants will be enrolled into Stage II without restriction to age.

Stage I participants who have not had individual dose adjustments due to extreme PK values will have their raltegravir dose changed to the selected Stage II dose. Participants will continue to follow the same visit schedule after the dose modification, with an additional safety visit 4 weeks after the dose modification. For example, if a subject's dose is modified to the Stage II selected dose at Week 24 visit, a safety visit will be conducted at week 28 and then continue with the next scheduled visit for that subject at week 36.

There will be about 16 study visits for this study split up into stages. At each visit, a physical exam, blood collection and determinations of treatment adherence will occur. At some visits, urine collection and Tanner staging will also occur. Cohorts 2B and 3 will undergo a taste evaluation at one of two visits. Participants may be re-registered into the same cohort if a dose change is recommended.