Abstract

We have tested 33 flavonoids, occurring ubiquitously in foods of plant origin, for their ability to alter the transport of the β-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 V_max of cefixime influx without changing the apparent K_m 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 β-lactam antibiotic cefixime in Caco-2 cells by activation of apical Na⁺/H⁺-exchange and a concomitant increase of the driving force for PEPT1.