The trial will evaluate the pharmacokinetics, safety, efficacy and acceptability of twice- and once-daily dosing of lopinavir/ritonavir tablets (Kaletra) dosed by weight in HIV-1 infected children who are currently taking lopinavir/ritonavir as part of their combination antiretroviral therapy and who are currently achieving virological suppression (<50 copies/ml). Specifically:

• To confirm weight-based dosing recommendations by evaluating the pharmacokinetics of twice-daily lopinavir/ritonavir half strength formulation tablets dosed on body weight and comparing to historical adult and paediatric data of pharmacokinetics of lopinavir/ritonavir soft gel capsules and oral solution respectively (1, 2).
• To compare the pharmacokinetics of twice-daily lopinavir/ritonavir tablets with once-daily dosing in the same children.
• To evaluate whether once-daily dosing of lopinavir/ritonavir is comparable to twice-daily dosing in terms of virological suppression at 48 weeks. Adherence and acceptability will also be compared.

Inclusion Criteria:

• aged <18 years (up to 18th birthday) with confirmed HIV-1 infection
• weight ≥15 kg
• able to swallow tablets
• stable (i.e. CD4 not declining) on a combination antiretroviral regimen that has included lopinavir/ritonavir for at least 24 weeks
• taking lopinavir/ritonavir dosed twice-daily and be willing at the screening visit to change to tablet formulation (if not currently taking tablets) and to change the lopinavir/ritonavir dose to follow the recommended FDA dosing plan based on body weight bands as necessary (see 7.2.2); if participating in the PK study*, be willing at the screening visit to change to lopinavir/ritonavir half strength formulation tablets (100/25mg) only, dosed twice-daily and change the lopinavir/ritonavir dose to follow the recommended FDA dosing plan based on body weight bands as necessary (see 7.2.1)
• viral suppression (HIV-1 RNA <50 copies/ml) for at least the prior 24 weeks (minimum of 2 measurements).
• children and caregivers willing to participate in the PK study if they are among a minimum of the first 16 children enrolled in each body weight band in the trial, including a second PK assessment if randomised to switch to once-daily lopinavir/ritonavir.
• parents/carers and children, where applicable, give informed written consent

Exclusion Criteria:

• children on an antiretroviral regimen that includes a non-nucleoside reverse transcriptase inhibitor (NNRTI), fosamprenavir or nelfinavir
• children who have previously failed virologically on a protease inhibitor (PI) containing regimen (where virological failure is defined as two successive HIV-1 RNA results>1000 copies/ml (confirmed) more than 24 weeks after starting highly active antiretroviral therapy (HAART), i.e changes for toxicity are not counted as failure)
• acute illness
• abnormal renal or liver function (grade 3 or above)
• receiving concomitant therapy except for prophylaxis; Some treatments may be allowed, but must first be discussed with a trial medical expert
• pregnancy or risk of pregnancy in females of child bearing potential