PGP and cancer: Other substances, such as taxifolin, sitravatinib, cinobufagin, crown ethers, ascorbic acid, TTM, so-PXA, mPEG glycine-quinidine conjugate, and TiO2 PEG NPs that have been designed to block efflux of drugs outside the cancer cells are known as P-gp inhibitors [18,28,29,30,31,32,33,34,35].