## 4 Target-Based Screening  
The target-based lead identification approach was given a major impetus because of the following developments:  
* The availability of the MTB genome sequence in 1998 [51] promised a new era both for studying the 'biology' of the pathogen and investigating novel pathways suitable for drug development. Several publications appeared, proving the essentiality of biochemical targets based on gene knockout studies in vitro, as well as investigations on the survival of the gene knockouts of MTB in the mouse model, confirming the essentiality of a variety of metabolic targets in vivo [52]. * [PERSON] et al. published the 'Lipinski rule of 5' for oral drugs [53]. The poor physiochemical properties of lead compounds vis a vis the 'Lipinski rule of 5' was shown to be the leading reason for the failure of potent compounds in the clinical trials, which was the direct result of poor pharmacokinetics. This led to the understanding of the concept of 'lead-like compounds' [54]. These rules served as guidelines for the selection/prioritization of 'hits' with a higher probability of being converted to drugs with favorable pharmacokinetics. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Approach** & **Target** & **Inhibitor Series** & **Mechanism of Inhibition** & **Reference** \\ \hline Following known & & & & \\ series & GyrB & Pyrrolamides & ATPase inhibitor & [45] \\ \hline Pharmacophore & & & & \\ based library & GyrB & Thiazolopyridine & ATPase inhibitor & [46] \\ \hline Pharmacophore & & & & \\ based library, & GyrB & Thiazolopyridone & ATPase inhibitor & [47] \\ scaffold morphing & & & & \\ \hline High throughput & & & & \\ screening & GyrB & Aminopyrazinamides & ATPase, MTB gyrase specific. & [48] \\ \hline Focused library & & & & \\ screening & GyrB & Aminopiperidine & & \\  & & & & \\ \hline Scaffold hopping & GyrB & Benzimidazolees & & \\  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Gyrase inhibitors identified at AstraZeneca (AZ). * This period also saw an enhanced efficiency in solving crystal structures; the macromolecular structures of several mycobacterial enzymes were solved and became key tools for the rationale design of novel inhibitors. A consortium of structural genomics groups was formed for facilitating the determination and analysis of structures of proteins from MTB [55]. * In addition to these facilitators of drug development, several public-private partnership consortia were also formed, which propelled interactions between the pharmaceutical industry and academic laboratories to engage in the TB drug discovery process. Examples include 'The Action TB program' (1996); the 'Global Alliance for TB' (GATB), which was launched in 2000; and the EU FW6/7 program entitled 'New Medicines for Tuberculosis' (NM4 TB), which was launched in 2005, followed by the 'More Medicines for Tuberculosis' (MM4 TB) program. Several pharmaceutical companies turned to target-based screening to find novel 'lead compounds' with a potential to inhibit the growth of MTB cells [56]. * Examples of efforts at AZI toward finding potent inhibitors by using target-based screening are summarized in Table 2. Though there are several publications on these efforts, it is interesting to view them in the context of the learning and the impact. A few of these programs were also in collaboration between AZI and partners of the NM4 TB and MM4 TB, the EU FW6, and seven programs, as well as with the Global Alliance for TB. The key learning derived from these extensive studies are as follows:  
* The patterns obtained throw important light on both the characteristics of the target and the characteristics of the inhibitor. * The question of target vulnerability: Gene knockout studies for PanK demonstrated the essentiality of the gene product. However, elegant studies with the PanK target showed that modulation of enzyme levels (inferred based on regulating gene expression) altered the sensitivity of the MTB cell to inhibitors of the enzyme [62].