The antibiotic combination trimethoprim (TMP)-sulfamethoxazole (SMX) has a broad spectrum of activity and it is used for the treatment of numerous infections, but pediatric pharmacokinetic (PK) data are limited. We previously published population PK (popPK) models of oral TMP-SMX in pediatric patients based on sparse opportunistically collected data (POPS study, Antimicrob Agents Chemother 62:e01813-17, 2017, https://doi.org/10.1128/AAC.01813-17). We performed a separate PK study of oral TMP-SMX in infants and children with more traditional PK sample collection, and independently developed new popPK models of TMP-SMX using this external dataset. The POPS dataset and the External dataset were each used to evaluate both popPK models. The External TMP model had identical model and error structure as the POPS TMP model with typical values for PK parameters within 20%. The External SMX model did not identify the covariates in the POPS SMX model as significant. The External popPK models predicted higher exposures of TMP (median overprediction of 0.13 mg/L for the POPS dataset and 0.061 mg/L for the External dataset) and SMX (median overprediction of 1.7 mg/L and 0.90 mg/L) than the POPS TMP (median underprediction of 0.016 mg/L and 0.39 mg/L) and SMX (median underprediction of 1.2 mg/L and 14 mg/L) models. Nonetheless, both models supported TMP-SMX dose increases in infan ts and young children for more resistant pathogens with a minimum inhibitory concentration of 1 mg/L, although the required dose increase based on the External model is lower.
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