Drug repurposing for anticancer therapies. A lesson from proton pump inhibitors

Introduction: Worldwide, the annual expenditure on anticancer drugs is grossly calculated to be in the order of US$100 billion, and is expected to escalate up to $150 billion by 2020. It is evident that the vast majority of the most recently devised anticancer drugs are unaffordable in economically developing nations, frequently resulting in subpar therapies. In this complex medical and economic scenario, the repurposing of older drugs for anticancer therapies becomes a necessity. The repurposing of antiacid drugs such as the proton pump inhibitors as antitumoral agents and chemosensitizers is probably one of the most recent and promising phenomenon in oncology. Areas covered: Important research articles and patents focusing on proton pump inhibitors as a potential class of therapeutics, published between the period of 2006–2019, have been covered. This review mainly focuses on the therapeutic applications, as direct anticancer agents as well as modifiers of the tumor microenvironment and modulator of chemoresistance. Expert opinion: PPIs have significant anticancer applications and are proving to be safe, effective and inexpensive. Here the authors review the current knowledge regarding the influence of PPIs on the efficacy and safety of cancer chemotherapeutics through the regulation of targets other than the H⁺/K⁺-ATPase.