Increasing costs, much time consumption and high risk of failure associated with the process of de novo development of new anticancer drugs have prompted the pharmaceutical industry to seek alternative strategies that may facilitate and accelerate the whole process. In particular, the repurposing strategy, known also as repositioning or reprofiling strategy, is a potential source of new treatment options for cancer patients with high unmet medical needs. However, it should be noted that the repurposing strategy, being still a new trend in drug development, should only complement the process of discovering new anticancer drugs, and should not be its alternative. The best repurposable oncological drug candidates are the agents whose original patent protection has already expired, and for which there is a possibility to create a formulation enabling, together with a new therapeutic indication, new patent protection. In this review article we discuss the advantages of the repurposing strategy, and provide an overview of a number of promising candidates, such as artesunate, aspirin, cimetidine, doxycycline, ivermectin, metformin, rapamycin (sirolimus), and thalidomide, that have the potential to be repurposed as anticancer drugs both in cancer prevention and therapy. In addition, we highlight some of the studies regarding the signalling pathways and molecular targets altered by these drugs, and describe the biological mechanisms underlying their anticancer effects.
