Terfenadine pharmacokinetics in breast milk in lactating women.

The excretion of terfenadine into breast milk has not been reported previously. Disposition of terfenadine was prospectively studied in four healthy lactating mothers (age, 33 +/- 4 years). Subjects received 60 mg terfenadine every 12 hours over a period of 48 hours to achieve steady-state milk and plasma concentrations. Milk and plasma samples were collected at 1/2, 1, 1 1/2, 2, 3, 4, 6, 8, 12, 24, and 30 hours after the last dose. Terfenadine and its active metabolite milk and plasma concentrations were quantitated by HPLC. Terfenadine was not detected in milk or plasma. Mean +/- SD active metabolite data for milk and plasma are as follows: Cmax (ng/ml), 41.0 +/- 16.4 for milk, 309.0 +/- 120.5 for plasma; tmax (hours), 4.3 +/- 2.4 for milk, 3.9 +/- 3.0 for plasma; t1/2 beta (hours), 14.2 +/- 5.4 for milk, 11.7 +/- 6.4 for plasma; AUC(0-12) (ng.hr/ml) 320.4 +/- 99.8 for milk, 1590.0 +/- 300.4 for plasma. Metabolite milk/plasmaAUC(0-12) ratios ranged from 0.12 to 0.28 (mean, 0.21 +/- 0.07). Newborn dosage estimates based on the highest measured concentration of terfenadine metabolite in milk suggests the maximum level of newborn exposure would not exceed 0.45% of the recommended maternal weight-corrected dose. Estimated amounts consumed by the neonate after the mother is given the recommended dose of the drug are not likely to result in plasma levels producing untoward effects.