PGP and cancer: In resistant cancer cells, P-gp, MRP1, and BCRP can be competitively inhibited by some alkaloids (isoquinoline, protoberberine, quinoline, indole, monoterpeneindole, and steroidal alkaloids), lipophilic terpenoids (mono-, di-, tri-terpenes including saponins), tetraterpenes (including carotenoids) (Eid et al., 2020), and steroids (including cardiac glycosides) (Wink et al., 2012).