Fluoroquinolone (ciprofloxacin) secretion by human intestinal epithelial (Caco-2) cells

1 Human intestinal epithelial Caco-2 cells were used to investigate the mechanistic basis of transepithelial secretion of the fluoroquinolone antibiotic ciprofloxacin. 2 Net secretion and cellular uptake of ciprofloxacin (at 0.1 mM) were not subject to competitive inhibition by sulphate, thiosulphate, oxalate, succinate and para-amino hippurate, probenecid (10 mM), taurocholate (100 mu M) or bromosulphophthalein (100 mu M). Similarly tetraethylammonium and N-'methylnicotinamide (10 mM) were without effect. 3 Net secretion of ciprofloxacin was inhibited by the organic exchange inhibitor 4,4'-diisothiocyanostilbene-2-2'-disulphonic acid (DIDS, 400 mu M). 4 Net secretion of ciprofloxacin was partially inhibited by 100 mu M verapamil, whilst net secretion of the P-glycoprotein substrate vinblastine was totally abolished under these conditions. Ciprofloxacin secretion was unaltered after preincubation of cells with two anti-P-glycoprotein antibodies (UIC2 and MRK16), which both significantly reduced secretory vinblastine flux (measured in the same cell batch). Ciprofloxacin (3 mM) failed to inhibit vinblastine net secretion in Caco-2 epithelia, and was not itself secreted by the P-glycoprotein expressing and vinblastine secreting dog kidney cell line, MDCK. 5 Net secretion and cellular uptake of ciprofloxacin (at 0.1 mM) were not subject to alterations of either cytosolic or medium pH, or dependent on the presence of medium Na+, Cl- or K+ in the bathing media. 6 The substrate specificity of the ciprofloxacin secretory transport in Caco-2 epithelia is distinct from both the renal organic anion and cation transport. A role for P-glycoprotein in ciprofloxacin secretion may also be excluded. A novel transport mechanism, sensitive to both DIDS and verapamil mediates secretion of ciprofloxacin by human intestinal Caco-2 epithelia.