The Role of Organic Cation Transporter 3 in Metformin Disposition during Pregnancy

Abstract

Metformin, a front line drug for type 2 diabetes mellitus, has been recently introduced as an alternative therapy for insulin in treatment of gestational diabetes mellitus. Despite clinical enthusiasm for the drug, there are still concerns about the effects of metformin on maternal and neonatal outcomes. A clinical study conducted by Obstetric-fetal Pharmacology Research Network showed that metformin renal clearance was significantly increased during pregnancy. Moreover, metformin readily crosses the placental barrier and its concentration in fetal circulation can be as high as maternal concentrations. The overall goal of this dissertation is to understand mechanisms involved in maternal and fetal disposition of metformin. The studies were designed to 1) determine the mechanism of altered maternal metformin disposition during pregnancy; 2) elucidate the role of OCT3 in fetal exposure to metformin and in tissue distribution of metformin in both pregnant and non-pregnant states. To evaluate the effect of pregnancy on expression of polyspecific organic cation transporters, we measured mRNA and protein expression of major organic cation transporters in kidney, liver and placenta in pregnant mice and in human placentas. The overall effect of pregnancy on renal and hepatic expression of organic cation transporters was moderate, suggesting that increased renal clearance of metformin in pregnant women might be due to other pregnancy-associated changes. OCT3/Oct3 were highly expressed in human and mouse placentas. Interestingly, the expression of mouse placental Oct3 significantly increased from mid gestational stage to term whereas OCT3 in human placenta only showed a moderate increase in mid-to-late pregnancy. The role of OCT3/Oct3 in metformin tissue specific distribution was determined by performing an in vivo study in Oct3 gene deletion mice. The in vivo study results suggest that Oct3 profoundly affects metformin plasma kinetics and distribution in peripheral tissues including salivary gland, heart and skeletal muscle, which highly express OCT3/Oct3. Our findings demonstrate that OCT3/Oct3 is responsible for high accumulation of metformin in the salivary glands, which potentially accounts for the taste disturbance caused by metformin in type 2 diabetic patients taking this drug. In order to determine the role of OCT3 in fetal exposure to metformin, immunolocalization study in human term placenta and in vivo study in pregnant Oct3-/- mice were carried out. OCT3 was concentrated to the basal membrane of syncytiotrophoblast in human placenta. Pregnant Oct3-/- mice showed reduced fetal exposure to metformin, suggesting that OCT3/Oct3 facilitates transport of metformin from the placenta to the fetal compartment. In summary, findings from this dissertation research have greatly contributed our understanding of the mechanisms involved in maternal and fetal disposition of metformin. Importantly, our studies uncovered a novel molecular mechanism underlying taste disturbance caused by metformin and identified OCT3 as an important determinant of fetal exposure to metformin.