Disopyramide and its N-monodesalkyl metabolite in breast milk

The plasma and breast milk were sampled from a woman who was breastfeeding whilst taking disopyramide (200 mg three times daily). Paired samples taken on the fifth to eighth day of treatment showed that disopyramide was present in breast milk in a similar concentration to plasma (mean +/- s.d. milk; plasma ratio 0.9 +/- 0.17). The estimated dose likely to be ingested by an infant is less than 2 mg kg-1 day-1. The active N-monodesalkyl metabolite of disopyramide (NMD) although present in plasma in much smaller concentrations than the parent compound, was excreted in breast milk (mean +/- s.d. milk: plasma ratio 5.6 +/2.9) in concentrations similar to those of disopyramide. The pharmacological and toxicological properties of the disopyramide metabolite need to be considered when assessing likely effects on the infant. No adverse effects were noted in the infant in this case. Maternal plasma and breast milk were sampled again along with infant plasma after 28 days. Disopyramide and NMD were undetectable in the infant's serum. No evidence was found to indicate that the concentrations of disopyramide or NMD in breast milk might be sufficient to pose a definite risk to the infant. Whenever disopyramide is prescribed in a breast feeding mother, close observation of the baby and measurement of both disopyramide and its active metabolite NMD in breast milk or infant plasma is recommended, pending further investigation.