| This dissertation work focused on two critical determinants of infant exposure risk to drugs present in breast milk, maturation of infant systemic clearance mechanisms and active transport across the lactating mammary epithelium. A tentative general mathematical model describing the maturation of hepatic cytochrome P450-mediated clearance and renal clearance due to glomerular filtration was developed. In vitro-in vivo extrapolation of enzyme activity data scaled relative to functional enzyme content and probe substrate data for glomerular filtration served as the fundamental premise of the model. The model performed reasonably well for CYP1A2, CYP3A4 and glomerular filtration, but CYP2D6 and CYP2C gave poorer predictions. The lactating rat model may predict antiviral drug transfer characteristics in human milk, since the milk distribution characteristics of acyclovir, ganciclovir and zidovudine were similar between the rat and human. However, high dose infusion of specific transporter inhibitors in the lactating rat model failed to identify the active transport mechanism responsible for cimetidine and nitrofiirantoin accumulation in milk. In contrast, quantitative reverse transcription-polymerase chain reaction identified candidate transport proteins involved in drug accumulation into milk. Lactating mammary epithelial cells express OCT1, OCT3, OCTN1, OCTN2, OATP-A, OATP-B, OATP-D, OATP-E, MRP1, MRP2, MRP5, MDR1, CNT1, CNT3, ENT1, ENT3, NBT1, PEPT1 and PEPT2 at levels comparable to liver or kidney, but do not express OCT2, OAT1–4, OATP-C, MRP3, MRP4, CNT2, ENT2 and NBT2. Lactation caused a significant up-regulation of OCTN1, PEPT2, CNT1, ENT3, OCT1, OCTN1, PEPT2, CNT1, CNT3 and ENT3, and down-regulation of MDR1 and OCTN2. These data suggest a role for each expressed transporter in drug disposition at the lactating mammary epithelium. The functional characterization of expressed transporter genes at the mammary epithelium required development of a human mammary epithelial cell line with features consistent with lactation. Addition of lactogenic hormones and laminin resulted in an upregulation of the differentiation markers, β-casein and α-lactalbumin, in 184B5 and MCF 10A cell lines. Lack of reproducibility prohibited further development of the cell culture model. In summary, this work has advanced our understanding of the determinants of infant drug exposure via the breast milk. |
