**1. The Added Value of Promiscuous Targets for Antitubercular Drug Discovery**

One of the biggest problems in fighting *Mycobacterium tuberculosis* (Mtb), the etiologic agent of tuberculosis (TB), is the development and spread of multi-drug resistant strains [1]. Therefore, there is an urgent need to identify novel druggable targets for the development of more efficient anti-TB agents [2,3]. In this context, the medicinal chemistry efforts made in the last years led to the discovery of new antimycobacterial compounds, and the identification of novel targets [3–5].

High-throughput screenings (HTS), based on Mtb whole cells, were developed to identify hit compounds with potent inhibitory activity, and consequently, new targets emerged. The HTS had the merit of fueling the scarce TB drug development pipeline, since many molecules under preclinical and clinical development came out from this approach. Indeed, the first new TB drug, Bedaquiline, approved by the Food and Drug Administration (FDA) since 1971, was discovered in a whole-cell HTS campaign [6], and the same origin had both Delamanid [7] and Pretomanid [8], other two drugs recently approved for multidrug resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB) treatment [9,10].

Curiously, an unforeseen outcome of many phenotypic screens against Mtb is the finding that the same targets have been frequently found in many different screening assays, despite the use of different compound libraries [11]. These targets were named "promiscuous" targets, for their nonspecific susceptibility to being inhibited by different scaffolds [2,12]. Although this term was initially given with a negative connotation, the value of these potential pharmacological targets has been reevaluated. Indeed, the high vulnerability of these essential targets, reflecting the biggest vulnerabilities of Mtb, can provide new opportunities to be explored for the development of TB drugs [12]. Among them, there are DprE1, MmpL3, QcrB and Psk13 [3,13].

DprE1 is an enzyme that works in concert with DprE2 to synthesize the unique arabinose precursor for lipoarabinomannan and arabinogalactan, essential building blocks of the mycobacterial cell-wall [14]. To date, more than 15 pharmacophores were found to inhibit DprE1 activity.

MmpL3 is the only Mtb transporter of trehalose monomycolate, required for the formation of the mycolic acid layer of the cell wall [15], and has been found to be affected by several molecules. Recently, direct inhibition of MmpL3 by BM212, the first compound found to hit MmpL3, was shown using spheroplasts [15], while the dissipation of proton motive force is the proposed mechanism for the other molecules [16].

QcrB encodes the *b* subunit of ubiquinol cytochrome C reductase, involved in the electron transport. Among the other compounds, Q203 is a promising clinical candidate, which blocks Mtb cell growth by targeting the respiratory cytochrome bc1 complex [17,18].

Finally, polyketide synthase Psk13, essential for the formation of the mycolic acid precursors, has been targeted by thiophenes and benzofuran containing molecules [19,20].

It is noteworthy that several research groups independently identified these promiscuous targets. These proteins are embedded in pathways that have key roles for Mtb growth and survival under the screening conditions, but even more important during infection. Their essentiality represents an Mtb Achilles' heel that is important to exploit for the development of new effective drug regimens able to shorten the TB treatment. Interestingly, all these promiscuous targets are localized within the cell envelope, emphasizing that the cell wall still represents a fruitful source of drug targets.

In recent years, several innovative strategies have been implemented to avoid the rediscovery of promiscuous targets [17,21–23]. Nevertheless, it is noteworthy that promiscuous targets proved to be successful in medicinal chemistry: four inhibitors Q203 (QcrB), SQ-109 (MmpL3), Macozinone (MCZ) and TBA-7371 (DprE1) entered clinical trials [24–29]. Thus, in parallel, a rational exploitation of already available promiscuous targets could lead to optimized scaffolds that retain the ability to attack the most vulnerable weakness of tubercle bacilli.
