Genetic variants of organic cation transporter 2 (OCT2) significantly reduce metformin uptake in oocytes

Abstract

1. The authors sought to evaluate the contribution of organic cation transporters (OCTs) to the renal tubular transport of metformin using LLC-PK1 cells as an in vitro model for the renal proximal tubule, and to investigate the effects of three non-synonymous genetic variants of OCT2 on the transport activity of metformin in vitro using an oocyte over-expression system.

2. The basolateral-to-apical transport of metformin was significantly greater than the apical-to-basolateral transport and showed concentration dependency with the kinetic parameters: maximum transport rate (V_max), 922 pmol min⁻¹ per 5 × 10⁵ cells; Michaelis–Menten constant (K_m), 393 µM; intrinsic clearance (CL_int), 2.35 µl min⁻¹ per 5 × 10⁵ cells; and diffusion constant (K_d), 0.33 µl min⁻¹ per 5 × 10⁵ cells. The basolateral-to-apical transport of metformin was inhibited by phenoxybenzamine, an inhibitor of OCTs, but not by cyclosporine A, MK571, or fumitremorgin C, which are inhibitors of P-glycoprotein, multidrug resistance proteins (MRPs), and breast cancer resistance protein (BCRP), respectively, suggesting that OCTs play a role in renal tubular secretion of metformin.

3. Metformin uptake was much greater in oocytes expressing OCT2-wild type (OCT2-WT) than OCT1-WT compared with uptake in water-injected oocytes. Uptake was significantly decreased in oocytes expressing OCT2-T199I, -T201M, and -A270S compared with that in OCT2-WT, suggesting that metformin is a better substrate for OCT2 than for OCT1 and that the amino acid-substituted variants of OCT2 cause a functional decrease in metformin uptake.

4. In conclusion, the genetic variants of OCT2 (OCT2-T199I, -T201M, and -A270S) decreased the transport activity of metformin and thus may contribute to the inter-individual variation in metformin disposition as OCT2 plays a pivotal role in renal excretion, the major disposition route of metformin.

Keywords::

• Metformin
• organic cation transporter 2 (OCT2)
• genetic variants
• transport activity

Acknowledgements

This work was supported by a grant from the Korea Health 21 R&D Project, Ministry of Health & Welfare, Korea (Grant No. A030001) and by the 2007 Inje University Research Grant.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.