P-gp can interact with hundreds of substrates, which are diverse in chemical structure and action, such as those found in natural products, anti-cancer drugs, cardiac glycosides, calcium channel blockers, analgesics, immunosuppressive agents, anthelminthic, HIV protease inhibitors (HPI), antibiotics, H2-receptor antagonists, fluorescent dyes, steroids, and peptides (linear and cyclic) (Amin, 2013). Here, PGP is linked to cancer.