Several studies revealed that PPIs inhibit not only the H+/K+ ATPase in gastric parietal cells, but also the vacuolar H+ ATPase (V-ATPase) overexpressed in tumor cells, the V-ATPase is an ATP-dependent proton pump that transports H+ across both intracellular and plasma membranes to regulate intracellular and extracellular pH.
Co-administration or pretreatment with PPIs could inhibit H+ transport via the V-ATPase in tumor cells, resulting in lower extracellular acidification and intracellular acidic vesicles which increase the sensitivity of the tumor cells to the anticancer agents. Moreover, the low extracellular pH induces an increased activity of drug efflux pumps P-glycoprotein (P-gp), which is closely associated with multi-drug resistance (MDR) of tumors. As a consequence, there remains a lower concentration of chemotherapeutic drugs in tumor cells and reduced cytotoxic efficacy, thus restoring the normal extracellular PH will decrease MDR.
