Mechanisms Underlying Saturable Intestinal Absorption of Metformin

Abstract

ABSTRACT: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 ϳ 0.05 mM, J max ϳ 1.0 pmol min ؊1 cm ؊2, and K d, transport ϳ 10 nl min ؊1 cm ؊2 ). AP uptake data also revealed the presence of saturable and nonsaturable components (K m ϳ 0.9 mM, V max ϳ 330 pmol min ؊1 mg of protein ؊1, and K d, uptake ϳ 0.04 l 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 ϳ7 and ϳ13% 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 (ϳ90%) 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.

Transporters Involved in Metformin Pharmacokinetics and Treatment Response

“…Though bioavailability for most drugs is determined by absorption and metabolism in the gastrointestinal tract as well as hepatic metabolism, because metformin is not metabolized its bioavailability is determined primarily by intestinal absorption. These data, which suggest that metformin absorption is mediated by a saturable absorption process 9,14,33 , are consistent with a major role of intestinal influx transporters in the oral absorption of metformin.…”

Four Cation-Selective Transporters Contribute to Apical Uptake and Accumulation of Metformin in Caco-2 Cell Monolayers

“…The cells were passaged following 90% confluence using trypsin-EDTA, and plated at a 1:20 ratio in 75 cm 2 T-flasks. Caco-2 cells were cultured as described previously (Proctor et al, 2008). Briefly, the cells (passage numbers 30-40) were seeded at a density of 60,000 cells/cm 2 on polycarbonate membranes of Transwells (Corning Inc., Tewksbury, MA).…”

“…We have investigated the mechanism of intestinal absorption of metformin in the well established model of human intestinal epithelium, the Caco-2 cell monolayers (Proctor et al, 2008). The study showed that metformin was taken up efficiently across the apical (AP) membrane of Caco-2 cells by bidirectional cation-selective transporter(s) during absorptive transport.…”

“…However, efflux of this drug across the basolateral (BL) membrane of Caco-2 cell monolayers was inefficient, resulting in accumulation in Caco-2 cells. We have proposed that after an oral dose, metformin is taken up by AP transporters into enterocytes, accumulated in the intestinal epithelium, and cycled between the enterocytes and intestinal lumen via AP uptake and efflux transporters; thus, availing multiple opportunities for efficient absorption via the paracellular route (Proctor et al, 2008).…”

Statistical Comparison of Dissolution Profiles to Predict the Bioequivalence of Extended Release Formulations

“…It has been stated that ER formulations can produce flip flop-like kinetics when the apparent k a <k el (k el /k a ratio >1) and could lead to miscalculations of the PK parameters (53). For pramipexole, we observed reductions in the C max BE space for values of k a ≤0.4 h −1 (k el /k a ratio=0.2).…”