Drug repositioning involves the investigation of existing drugs for new therapeutic purposes. A number of successes have been achieved, the foremost including sildenafil for erectile dysfunction and pulmonary hypertension and thalidomide for leprosy and multiple myeloma. Clinical trials have been performed on posaconazole and ravuconazole for Chagas disease. Other antifungal agentsclotrimazole and ketoconazole have been investigated for anti-trypanosome therapy. Drug repositioning is a "universal strategy" for neglected diseases due to 1) reduced number of required clinical trial steps could reduce the time and costs for the medicine to reach market, 2) existing pharmaceutical supply chains could facilitate "formulation and distribution" of the drug, 3) known possibility of combining with other drugs could allow more effective treatment, 4) the repositioning could facilitate the discovery of "new mechanisms of action for old drugs and new classes of medicines", 5) the removal of “activation barriers” of early research stages can enable the project to advance rapidly into disease-oriented research. Often considered as a serendipitous approach, where repurposable drugs are discovered by chance, drug repurposing has heavily benefited from advances in human genomics and network biology. It is now possible to identify serious repurposing candidates by finding genes involved in a specific disease and checking if they interact, in the cell, with other genes which are targets of known drugs. It was shown that drugs against targets supported by human genetics are twice as likely to succeed than overall drugs in the pharmaceutical pipeline. However, there are also a number of downsides to drug repositioning. Firstly, the dosage required for the treatment of novel disease usually differs from that of its original target disease, and if this happens, the discovery team will have to begin from Phase I clinical trials, which effectively strips drug repositioning of its advantages of over de novodrug discovery. Secondly, the finding of new formulation and distribution mechanisms of existing drugs to the novel-disease-affected areas rarely includes the efforts of "pharmaceutical and toxicological" scientists. Thirdly, patent right issues can be very complicated for drug repurposing due to the lack of experts in the legal area of drug repositioning, the disclosure of repositioning online or via publications, and the extent of the novelty of the new drug purpose.
