PGP and cancer: Furthermore, overexpression of P-gp is often associated with multidrug resistance (MDR) in cancer cells, a major obstacle to effective treatment with many chemotherapeutic agents such as paclitaxel and vincristine, which are well characterized P-gp substrates (Gottesman et al., 2002; Eckford and Sharom, 2009; Robey et al., 2018).