Lopinavir/ritonavir (LPV/r) is an effective and commonly used protease inhibitor in HIV-infected adolescents. Previous data showed high plasma concentrations of LPV in Thai patients. This study determined the pharmacokinetic (PK) parameters of a low-dose LPV/r tablet (70% of standard dose) in HIV-infected Thai adolescents. Methods: A total of 24 adolescents on LPV/r-containing HAART regimens with HIV RNA<50 copies/ml were included. Standard and low doses for two different weight bands using

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... r 100/25 mg tablet formulation twice daily were prescribed as follows: 3 and 2 tablets for adolescents weighing 25-35 kg, and 4 and 3 tablets for those weighing >35 kg, respectively. On the fourth week of treatment, PK was performed for all doses at 0 (predose), 2, 4, 6, 8, 10 and 12 h. LPV and ritonavir concentrations were measured using the HPLC method. Results: The median (IQR) age was 13.5 (12-15) years. The median LPV doses of standard and low doses were 290 and 208 mg/m 2 . The mean (sd) area under the concentration-time curve at 0-12 h, maximum concentration and plasma concentration at 12 h for the stand- ard dose were 97.6 (25.7) mg•h/l, 11.1 (2.6) mg/l and 4.1 (2.0) mg/l, and for the low dose were 87.4 (29.0) mg•h/l, 11.0 (3.1) mg/l and 3.2 (1.9) mg/l, respectively. No significant differences were detected between the groups. One child had plasma concentration at 12 h <1.0 mg/l while on low-dose LPV/r but HIV RNA was undetectable. Conclusions: The low-dose LPV/r tablet provides adequate PK parameters in HIV-infected Thai adolescents. A randomized study to assess the efficacy of low and standard doses of LPV/r among Thai HIV-infected adolescents should be explored.