Pharmacokinetics of verapamil in lactating rabbits: Prediction of verapamil distribution into rabbit milk

In this work, we have studied the pharmacokinetics and milk penetration of verapamil following intravenous administration in lactating rabbits. Milk-to-serum drug concentration ratios (M/B obs) have been determined using area under the milk and serum concentration–time profiles, and the resulting values have then been compared with those obtained by theoretical classical diffusion milk transfer models that were described by Fleishaker et al. [J. Pharm. Sci. 76 (1987) 189.], Atkinson and Begg [Br. J. Clin. Pharmacol. 25 (1990) 495.], and Stebler and Guentert [Pharm. Res. 9 (1992) 1299.]. The pharmacokinetic profile of verapamil in lactating rabbits following endovenous administration is described in the form of a two-compartment model. Moreover, we detected an important milk transfer after endovenous administration of verapamil in lactating rabbits. M/B obs was near 15. The classical diffusional models mentioned were not able to predict this extensive transfer of verapamil into rabbit milk. However, when the classical Fleishaker equation was modified and a stepwise regression was carried out, we found that the M/B obs value could be predicted using the plasma and milk protein binding.