- Part A: Participants will be enrolled sequentially by age cohort to evaluate the steady state PK and confirm dose of ATV/co and DRV/co. Following screening, enrolled participants will continue their suppressive regimen of either ATV/r or DRV/r once-daily plus their BR. On Day 1, participants will discontinue ritonavir and initiate once daily cobicistat (COBI) to be taken with their ATV or DRV plus their BR. All participants enrolled in Part A will participate in an intensive PK evaluation of COBI and ATV or DRV on Day 10. Following completion of the intensive PK visit, participants will continue to receive COBI coadministered with DRV or ATV each with a BR and return for scheduled study visits.