PGP and cancer: Its action is unavoidable in the blood–placental barrier, blood–brain barrier, and blood–testes barrier, but when the focus is on the therapeutic aspect of the drugs, the absorption of the drugs through the intestinal lumen is slowed due to the expression of the P-gp in the intestinal lumen, although their presence is predicted all over the body; their expression is higher, however, in diseased cells, particularly cancer cells [109].