Dose-Response Relationship, DrugRitonavirTime FactorsLopinavirHIV Protease InhibitorsPyrimidinonesInfant, PrematureInfant, NewbornSaquinavirAnti-HIV AgentsIndinavirInfant, Premature, DiseasesInfant CareDrug InteractionsInfant FoodArea Under CurveHIV InfectionsInfant FormulaAdministration, OralDrug Therapy, CombinationDose-Response Relationship, RadiationDrug Administration ScheduleInfant BehaviorCarbamatesNelfinavirDouble-Blind MethodInfant MortalityInfant, Newborn, DiseasesSudden Infant DeathSulfonamidesOxytropisCross-Over StudiesRadiation DosageInfant Nutritional Physiological PhenomenaPyridinesTreatment OutcomeCytochrome P-450 CYP3ABenzoxazinesBiological AvailabilityPregnancyInfant, Very Low Birth WeightDrug CombinationsHalf-LifeRadiometryLethal Dose 50Infant, Low Birth Weight