Chalcogenopyrylium dyes as inhibitors/modulators of P-glycoprotein in multidrug-resistant cells Original Research Article

A series of chalcogenopyrylium dyes were evaluated as modulators/inhibitors of P-glycoprotein (Pgp). Their ability to inhibit verapamil (VER)-dependent ATPase activity (IC50 values) in lipid-activated, mouse Cys-less mdr3 Pgp was determined. Their ability to promote calcein-AM (CAM) uptake in MDCKII-MDR1 cells and their capacity to be transported by Pgp in monolayers of MDCKII-MDR1 cells were also evaluated. The chalcogenopyrylium dyes promoted CAM uptake with values of EC50 between 5 × 10−6 and 3.5 × 10−5 M and 7 of the 9 dyes examined in transport studies were substrates for Pgp with efflux ratios (PBA/AB) between 14 and 390. Binding of three compounds (1-S, 3-S, and 4-S) to Pgp was also assessed by fluorescence. These three thiopyrylium dyes showed increased fluorescence upon binding to Pgp, giving apparent binding constants, Kapp, on the order of 10−7 to 10−6 M. Compound 8-Te was particularly intriguing since it appeared to influence Pgp at low micromolar concentrations as evidenced by its influence on VER-stimulated ATPase activity (IC50 of 1.2 × 10−6 M), CAM uptake (EC50 of 5.4 × 10−6 M), as well as [3H]-vinblastine transport by Pgp in cells (IC50 of 4.3 × 10−6 M) and within inside-out membrane vesicles (IC50 of 9.6 × 10−6 M). Yet, Pgp did not influence the distribution of 8-Te in MDCKII-MDR1 monolayers suggesting that 8-Te may bind to an allosteric site.