Proctor William R, Bourdet David L, Thakker Dhiren R
Division of Molecular Pharmaceutics, Eshelman School of Pharmacy, the University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA.
Drug Metab Dispos. 2008 Aug;36(8):1650-8. doi: 10.1124/dmd.107.020180. Epub 2008 May 5.
The purpose of the study was to elucidate mechanisms of metformin absorptive transport to explain the dose-dependent absorption observed in humans. Apical (AP) and basolateral (BL) uptake and efflux as well as AP to BL (absorptive) transport across Caco-2 cell monolayers were evaluated over a range of concentrations. Transport was concentration-dependent and consisted of saturable and nonsaturable components (K(m) approximately 0.05 mM, J(max) approximately 1.0 pmol min(-1) cm(-2), and K(d, transport) approximately 10 nl min(-1) cm(-2)). AP uptake data also revealed the presence of saturable and nonsaturable components (K(m) approximately 0.9 mM, V(max) approximately 330 pmol min(-1) mg of protein(-1), and K(d, uptake) approximately 0.04 microl min(-1) mg of protein(-1)). BL efflux was rate-limiting to transcellular transport of metformin; AP efflux was 7-fold greater than BL efflux and was not inhibited by N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GW918), a P-glycoprotein inhibitor. AP efflux was trans-stimulated by metformin and prototypical substrates of organic cation transporters, suggesting that a cation-specific bidirectional transport mechanism mediated the AP efflux of metformin. BL efflux of intracellular metformin was much less efficient in comparison with the overall transport, with BL efflux clearance accounting for approximately 7 and approximately 13% of the overall transport clearance at 0.05 and 10 mM metformin concentrations, respectively. Kinetic modeling of cellular accumulation and transport processes supports the finding that transport occurs almost exclusively via the paracellular route (approximately 90%) and that the paracellular transport is saturable. This report provides strong evidence for a saturable mechanism in the paracellular space and provides insight into possible mechanisms for the dose dependence of metformin absorption in vivo.
本研究的目的是阐明二甲双胍吸收转运的机制，以解释在人体中观察到的剂量依赖性吸收。在一系列浓度范围内，评估了顶侧（AP）和基底外侧（BL）的摄取和流出以及二甲双胍跨Caco-2细胞单层的从AP到BL（吸收性）的转运。转运具有浓度依赖性，由可饱和和不可饱和成分组成（米氏常数（K(m)）约为0.05 mM，最大转运速率（J(max)）约为1.0 pmol·min⁻¹·cm⁻²，转运解离常数（K(d, transport)）约为10 nl·min⁻¹·cm⁻²）。AP摄取数据还显示存在可饱和和不可饱和成分（K(m)约为0.9 mM，最大摄取速率（V(max)）约为330 pmol·min⁻¹·mg蛋白⁻¹，摄取解离常数（K(d, uptake)）约为0.04 μl·min⁻¹·mg蛋白⁻¹）。BL流出是二甲双胍跨细胞转运的限速步骤；AP流出比BL流出大7倍，且不受P-糖蛋白抑制剂N-(4-[2-(1,2,3,4-四氢-6,7-二甲氧基-2-异喹啉基)乙基]-苯基)-9,10-二氢-5-甲氧基-9-氧代-4-吖啶甲酰胺（GW918）抑制。AP流出受到二甲双胍和有机阳离子转运体的典型底物的反式刺激，表明一种阳离子特异性双向转运机制介导了二甲双胍的AP流出。与整体转运相比，细胞内二甲双胍的BL流出效率要低得多，在二甲双胍浓度为0.05和10 mM时，BL流出清除率分别约占整体转运清除率的7%和约13%。细胞积累和转运过程的动力学模型支持以下发现：转运几乎完全通过细胞旁途径发生（约90%），且细胞旁转运是可饱和的。本报告为细胞旁空间中的可饱和机制提供了有力证据，并深入探讨了二甲双胍体内吸收剂量依赖性的可能机制。
