Human P-glycoprotein transports cyclosporin A and FK506

Cyclosporin A, a cyclic undecapeptide, and FK506 are efficient immunosuppressive agents. They also attract attention as effective P- glycoprotein modulators that inhibit P-glycoprotein from binding to anticancer drugs and overcome multidrug resistance. Cyclosporin A itself interacts with a common binding site of P-glycoprotein to which Vinca alkaloids and verapamil bind. We were interested to determine whether cyclosporin A and FK506 are substrates for P-glycoprotein to transport, and we studied their transcellular transport. In LLC-PK₁ cells, derived from porcine kidney proximal tubule and forming a highly polarized epithelium, cyclosporin A was transported in a saturable manner. LLC-GA5-COL300, a transformant cell line derived by transfecting LLC-PK₁ with human MDR1 cDNA isolated from normal adrenal gland, expresses P-glycoprotein specifically on the apical surface and shows a typical multidrug-resistant phenotype. LLC- GA5-COL300 cells showed increased transport of cyclosporin A from the basal to the apical side. Kinetic analysis showed that this transport was a typical saturable transport with the calculated apparent Michaelis constant (K(m)/(app)) and the maximum flux (V(max)) as 8.4 μM and 2.4 nmol/mg protein/h, respectively. LLC-GA5-COL300 also showed increased transport of FK506 from the basal to the apical side. These results indicate that P- glycoprotein transports the immunosuppressive agents cyclosporin A and FK506.