Active transport of nitrofurantoin into human milk

Study Objective. To determine the extent to which nitrofurantoin is transferred into human milk. Design. Prospective, single-dose pharmacokinetic study. Setting. University-affiliated clinical research center. Patients. Four healthy lactating women 8-26 weeks postpartum. Intervention. All subjects received a single, oral, 100-mg dose of nitrofurantoin macrocrystals with food. Serial serum milk samples were obtained analyzed by high-performance liquid chromatography. Measurements and Main Results. Milk pH, milk fat partitioning, and protein binding in serum and milk were determined. Predicted milk:serum ratio (M:S) was compared with the observed M:S. Nitrofurantoin M:S predicted was 0.28 ± 0.05 whereas M:S observed was 6.21 ± 2.71. Average milk concentration was 1.3 mg/L, and estimated suckling infant dosage was 0.2 mg/kg/day or 6% of maternal dose (mg/kg). Conclusions. Nitrofurantoin is actively transported into human milk, achieving concentrations in milk greatly exceeding those in serum. Concern is warranted for suckling infants younger than 1 month old, or for infants with a high frequency of glucose-6-phosphate dehydrogenase deficiency or sensitivity to nitrofurantoin.