Hit Generation in TB Drug Discovery: From Genome to Granuloma

Abstract

Current tuberculosis (TB) drug development efforts are not sufficient to end the global TB epidemic. Recent efforts have focused on the development of whole-cell screening assays because biochemical, target-based inhibitor screens during the last two decades have not delivered new TB drugs. Mycobacterium tuberculosis (Mtb), the causative agent of TB, encounters diverse microenvironments and can be found in a variety of metabolic states in the human host. Due to the complexity and heterogeneity of Mtb infection, no single model can fully recapitulate the in vivo conditions in which Mtb is found in TB patients, and there is no single "standard" screening condition to generate hit compounds for TB drug development. However, current screening assays have become more sophisticated as researchers attempt to mirror the complexity of TB disease in the laboratory. In this review, we describe efforts using surrogates and engineered strains of Mtb to focus screens on specific targets. We explain model culture systems ranging from carbon starvation to hypoxia, and combinations thereof, designed to represent the microenvironment which Mtb encounters in the human body. We outline ongoing efforts to model Mtb infection in the lung granuloma. We assess these different models, their ability to generate hit compounds, and needs for further TB drug development, to provide direction for future TB drug discovery.

Figure 1

Diagrams of characteristic granulomas. (A) Cellular granuloma. Macrophages infected with Mtb (red) are at the center. Lipid bodies (white) are within foamy macrophages. (B) Necrotic granuloma. Macrophages have died and released Mtb into the necrotic center, which is hypoxic and filled with lipid caseum.

Figure 2

Chemical structures of front-line TB drugs.

Figure 3

Chemical structures of second-line TB drugs.

Figure 4

Hit compounds (A) identified in target-based whole-cell screens and (B) identified in whole-cell screen under growth culture conditions and discussed in the text. Also see Table 2.

Figure 5

Diagram of an Mtb-infected macrophage within the necrotic core of a granuloma illustrating the intracellular and extracellular microenvironments which Mtb encounters.

Figure 6

Hit compounds identified in whole-cell screens under stress culture conditions and discussed in the text. Also see Table 2.

Figure 7

Hit compound identified in intracellular whole-cell screens against Mtb-infected macrophages and discussed in the text. Also see Table 2.

Figure 8

TB compounds identified in the screens outlined in Table 2 that are currently in clinical trials.