Rat-Specific Expression of Placental MATE1 Contributes to Species Difference in Fetal Transfer of Metformin and 1-Methyl-4-Phenylpyridinium

Metformin readily crosses the human placenta. However, the fetal-to-maternal plasma concentration ratio (F/M ratio) of metformin has been reported to be as low as 0.057 in rats. This study investigated the underlying mechanisms of these interspecies differences by examining the expression and function of candidate transporters, organic cation transporter 3 (OCT3/SLC22A3) and multidrug and toxin extrusion protein 1 (MATE1/SLC47A1), in the placenta across species. Expression analysis at gene and protein levels using RNA-sequencing and absolute quantification by LC-MS/MS revealed minimal species differences in placental OCT3 expression (less than 4-fold). In contrast, MATE1 showed almost exclusive expression in the rat placental labyrinth, but minimal or undetectable levels were observed in samples prepared from human placental villi and mouse placental labyrinth. To evaluate the function of the placental MATE1, we compared the placental transfer of two dual substrates of MATE1 and OCT3, metformin and 1-methyl-4-phenylpyridinium (MPP⁺), in pregnant rats and mice in the presence and absence of pyrimethamine, a MATE1 inhibitor. The pyrimethamine was dosed to reach the plasma concentration, which specifically inhibits MATE1 based on the measured IC₅₀ of pyrimethamine to rat MATE1 and OCT3, which were 0.015 and 100 μM, respectively. The F/M ratios of metformin and MPP⁺ in the absence of pyrimethamine were half or less in rats compared to mice. Notably, preadministration of pyrimethamine increased the F/M ratios of metformin and MPP⁺ by approximately 50% in rats but not mice. These findings demonstrate that functional placental MATE1 expression is specific to rats, which, in part, contributes to the low fetal transfer of cationic compounds compared with other species. This is of significant concern, as it suggests that nonclinical developmental and reproductive toxicity studies of MATE1 substrate drugs in rats may underestimate fetal exposure and toxicity when extrapolated to humans.