PGP and neoplasm: Currently, there are three lines of research to improve the bioavailability and reduce the side effects of PTX, including the development of PTX conjugates or prodrugs to overcome the limitation of aqueous solubility, the use of PTX in combination with P-gp substrates or inhibitors to improve bioavailability, and the use of targeted and nano-sized delivery systems to deliver PTX directly to the tumor site [4].