PGP and cancer: Subsequently, we conducted studies to determine the mechanism(s) by which ribociclib reverses resistance to the P-gp substrates, colchicine and doxorubicin, in human epidermoid carcinoma KB-C2 cells, which have been frequently used to study P-gp-mediated MDR in cancers (Akiyama et al., 1985; Yoshimura et al., 1989).