P-glycoprotein is a transport protein which represents the body’s effort to influence absorption and distribution (and elimination). It actively transports xenobiotics out of cells or across membranes. It is the product of the MDR-1 gene first studied as the Multiple Drug Resistance phenotype associated with cancer treatment. P-glycoprotein reduces intestinal drug absorption from the lumen of the small intestine and enhances drug elimination through the liver and kidney.
It also plays a role reducing the exposure to toxins at the blood-brain barrier, testes and placenta. Genetic polymorphism of MDR-1 gene causes variable expression of the P-glycoprotein pump function. Increased function can lead to drug resistance and decreased function can lead to toxicities.