PEPT1-mediated cefixime uptake into human intestinal epithelial cells is increased by Ca2+ channel blockers

Ca2+ channel blockers like nifedipine have been shown to increase the oral bioavailability of beta-lactam antibiotics, such as cefixime, in humans. The molecular mode of action of Ca2+ channel blockers on beta-lactam absorption, however, has not yet been defined. Using the Caco-2 human intestinal epithelial cell line, we assessed whether alterations in intracellular free Ca2+ ion (Ca2+in) concentrations by Ca2+ channel blockers or by Ca2+ ionophores affect [14C]cefixime absorption. Reduction of Ca2+in levels by Ca2+ channel blockers (nifedipine, verapamil, diltiazem, or bepridil) at concentrations of 100 microM led to 35 to 50% increases in the cellular uptake of 1 mM [14C]cefixime. Increases in Ca2+in levels by Ca2+ ionophores, on the other hand, led to 40% reductions in [14C]cefixime absorption. Nifedipine increased the V(max) of cefixime transport by 67%, whereas the K(m) of cefixime transport remained unaffected. By measuring the pH in Caco-2 cells loaded with the pH-sensitive fluorescent dye 2',7'-bis(2-carboxyethyl)-5-(6)-carboxyfluorescein, we show that cefixime transport mediated by the intestinal H+-coupled peptide transporter PEPT1 leads to intracellular acidification. This acid load was reduced by nifedipine, although the Ca2+ channel blocker increased the level of H+ and cefixime cotransport. Increases in Ca2+in levels by ionomycin enhanced the decline in intracellular pH induced by cefixime alone, although ionomycin reduced the level of H+ and cefixime cotransport. In conclusion, our studies demonstrate that alterations of Ca2+in levels, e.g., by Ca2+ channel blockers, affect pH regulatory systems, such as apical Na+ and H+ exchange, and thereby alter the H+ gradient that serves as the driving force for uptake of beta-lactams into intestinal epithelial cells.

Flavonoids with epidermal growth factor-receptor tyrosine kinase inhibitory activity stimulate PEPT1-mediated cefixime uptake into human intestinal epithelial cells

We have tested 33 flavonoids, occurring ubiquitously in foods of plant origin, for their ability to alter the transport of the beta-lactam antibiotic cefixime via the H+-coupled intestinal peptide transporter PEPT1 in the human intestinal epithelial cell line Caco-2. Of the flavonoids tested, quercetin, genistein, naringin, diosmin, acacetin, and chrysin increased uptake of [14C]cefixime dose dependently by up to 60%. All other flavonoids were either without effect or decreased the absorption of cefixime. Quercetin was shown to increase the Vmax of cefixime influx without changing the apparent Km for transport. However, the expected concomitant increase in intracellular acidification due to PEPT1-mediated cefixime/H+-cotransport was less pronounced in the presence of quercetin. This suggested that pH regulatory systems such as apical Na+/H+-exchange could be activated by quercetin and maintain the proton-motive driving force for cefixime uptake. Since quercetin and genistein have been shown to inhibit epidermal growth factor (EGF)-receptor tyrosine kinases, we applied tyrphostin 25 to prove whether such an inhibition could explain the stimulatory effects seen on cefixime uptake. It was found that tyrphostin 25 simulated the effects of quercetin by increasing cefixime absorption due to maintenance of the transmembrane pH gradient. In conclusion, our studies show that flavonoids with EGF-receptor tyrosine kinase inhibitory activities enhance the intestinal absorption of the beta-lactam antibiotic cefixime in Caco-2 cells by activation of apical Na+/H+-exchange and a concomitant increase of the driving force for PEPT1.