KONCERT A Kaletra ONCE Daily Randomised Trial of the Pharmacokinetics, Safety and Efficacy of Twice-daily Versus Once-daily Lopinavir/Ritonavir Tablets Dosed by Weight as Part of Combination Antiretroviral Therapy in Human Immunodeficiency Virus-1 (HIV-1) Infected Children (PENTA 18)

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.