Is repositioning of drugs a viable alternative in the treatment of tuberculosis?

Antimicrobial resistance is a serious problem because of the scarcity of new antibiotics effective against pathogens such as methicillin-resistant Staphylococcus aureus, β-lactamase-producing Gram-negative bacteria and multidrug-resistant Mycobacterium tuberculosis. Extensively drug resistance is particularly worrying in tuberculosis (TB), since the causative bacteria have become resistant to almost all available first- and second-line drugs and resistance is a threat to achieving control of the disease. Development of new drugs is a lengthy and costly endeavour. This is a particular problem for antibiotics, usage of which is likely to be of limited duration, and is even more true of antibiotics whose use is restricted to the treatment of a disease, such as TB, that is considered to be 'poverty related', and for which the return on the investment is seen as non-attractive. In spite of this, there is an emerging pipeline of new drugs under development that hopefully will bring new anti-TB drugs to the market in the near future. The strategy of drug repurposing, finding new uses for existing approved medicines, has seen unexpected success in other medical areas. More than one blockbuster drug has originated from this strategy. And in the field of TB, there have been several examples in recent years of this approach leading to the use of drugs for which there is undeniable evidence of efficacy in the treatment of the disease, the best example being the fluoroquinolones, which were not developed originally to treat TB. This article reviews some examples of repurposing of drugs in the treatment of TB, newer candidates for repurposing for which there is already preliminary evidence of activity and possible new options that merit further investigation.