Tuberculosis drug discovery in the post-post-genomic era

Abstract

The expectation that genomics would result in new therapeutic interventions for infectious diseases remains unfulfilled. In the post-genomic era, the decade immediately following the availability of the genome sequence of Mycobacterium tuberculosis, tuberculosis (TB) drug discovery relied heavily on the target-based approach but this proved unsuccessful leading to a return to whole cell screening. Genomics underpinned screening by providing knowledge and many enabling technologies, most importantly whole genome resequencing to find resistance mutations and targets, and this resulted in a selection of leads and new TB drug candidates that are reviewed here. Unexpectedly, many new targets were found to be 'promiscuous' as they were inhibited by a variety of different compounds. In the post-post-genomics era, more advanced technologies have been implemented and these include high-content screening, screening for inhibitors of latency, the use of conditional knock-down mutants for validated targets and siRNA screens. In addition, immunomodulation and pharmacological manipulation of host functions are being explored in an attempt to widen our therapeutic options.

Figure 1

The figure displays the two main distinct methodologies in the search for new TB drugs. It is noteworthy that a target identified after a chemical screen (A) can enter the target-to-drug pipeline in enzyme-based screening (B). MIC, minimal inhibitory concentration; MBC, minimal bactericidal concentration; IC₅₀, half maximal inhibitory concentration.

Figure 2

Mtb infection leads to the phosphorylation of AKT1 and upregulation of the Src-kinase which inhibits phago-lysosomal fusion. Subsequently, the AKT-inhibitor H-89 as well as the Src-kinase inhibitor PP2 promotes fusion of the Mtb phagosome with the lysosome. In addition, infected macrophages release TGF-β resulting in decreased levels of nitric oxide synthase (iNOS) most likely through phosphorylation of the TGF-β receptor (TGFβR). Blockage of this receptor, for example with the compound D4476, releases nitric oxide leading to enhanced clearance of the bacteria. Several pathogens (e.g. Shigella spp, Salmonella spp, Yersinia spp) are known to use the Abl-tyrosine-kinase to dampen a successful antimicrobial host response. In Mtb infected macrophages, blockage of this kinase with imatinib promotes acidification of the lysosome through up-regulation of the proton pumping enzyme vacuolar-type H⁺-adenosine triphosphatase (vATPase) resulting in improved intracellular killing of Mtb. Infected macrophages release the pro-inflammatory cytokine TNF-α. Though this cytokine promotes an antimycobacterial macrophage response, it also provokes a stress response in the bacteria with upregulation of the Dos-regulon facilitating the transition to metabolically inactive bacteria and persistence. Blockage of TNF-α by the compound CC-3052 alleviates this stress and may render the bacterial cells more susceptible to antimycobacterials such as isoniazid.