Trimethoprim-sulfamethoxazole (TMP-SMX) prophylaxis in HIV-uninfected, exposed (HUE) children variably reduces clinical malaria burden despite antifolate resistance, but data regarding achieved serum levels and adherence are lacking. Serum samples from 70 HUE children aged 3-12 months from Rakai, Uganda, enrolled in an observational study were assayed for random SMX levels using a colorimetric assay. Adherence with TMP-SMX prophylaxis data (yes/no) was also collected. Of 148 visits with concurrent SMX levels available, 56% had self-reported adherence with TMP-SMX therapy. Among these 82 visits, mean (standard deviation) level was 19.78 (19.22) mu g/mL, but 33% had SMX levels below half maximal inhibitory concentrations (IC50) for Plasmodium falciparum with some, but not all, of the reported antifolate resistance mutations reported in Uganda. With TMP-SMX prophylaxis, suboptimal adherence is concerning. Sulfamethoxazole levels below IC50s required to overcome malaria parasites with multiple antifolate resistance mutations may be significant. Further study of TMP-SMX in this context is needed.
