Breastfeeding is important in infant rearing. To support safe breastfeeding in a patient using lamotrigine, the transfer of lamotrigine to the infant via breast milk was evaluated in two couples of Japanese mother and infant. The infants took their mothers' milk expressed a day ahead. The plasma and breast milk concentrations of lamotrigine at 4 (or 5) and 89 (or 90) days after the birth were measured by HPLC. The lamotrigine concentrations of the two mothers were 12.63 and 6.21 µg/mL in plasma, 11.50 and 4.13 µg/mL in milk at Day 4. Milk/Plasma ratios (M/P) were 0.91 and 0.67, and the estimated Relative Infant Dose (RID, at Day 5) was calculated to be 12-29％. At 89 and 90 days after birth, the milk concentrations and RID, respectively, were similar to those at 4 days, which indicates that the transfer of lamotrigine into milk is high in Japanese mothers. The infant plasma concentrations of lamotrigine at 5 and 90 days were between 2.8 and 6.7 µg/mL, which was within the therapeutic range in children with epilepsy. Typical adverse effects were not observed in these infants under meticulous surveillance. In conclusion, pharmaceutical healthcare is necessary for lactating mothers using lamotrigine and their infants.
