2.1.2. Metformin

One of the most promising drugs that promotes autophagy is currently used for treatment of type 2 diabetes: metformin [24]. This compound is characterized by a good safety profile, activates 5- -adenosine monophosphate-activated protein kinase (AMPK), induces production of mitochondrial reactive oxygen species (mROS), which are deleterious to *M. tuberculosis* and was found to reduce the severity of TB disease in humans [25]. Despite these promising data, combination therapies which involved metformin in mice had contradictory results. While in one case metformin enhanced the activity of isoniazid and ethionamide [25], in another one it did not improve the efficacy of the combined first-line drugs [26]. Recent retrospective studies reported a protective effect for metformin against reactivation of latent TB in diabetic patients [27–29]. Phase II clinical trials have been initiated (Table 1).

Activity of the major virulence factor ESX-1 can be blocked by compounds BBH7 and BTP15. While BBH7 hinders the secretion mechanism by inducing zinc stress, BTP15 was shown to act by downregulating expression of the *espA-espC-espD* operon upon interaction with MprB. The secreted proteins MptpB, SapM and Zmp1, which prevent phagosomal maturation, can be directly targeted extracellularly by their respective inhibitors. The dedicated secretion systems for these three proteins are not described yet. Gene expression of PhoP-dependent virulence genes can be controlled by PhoP inhibitors which prevent binding of PhoP to specific promoter regions, thus affecting transcription.

## 2.1.3. Imatinib and Other Tyrosine Kinase Inhibitors

A key feature of *M. tuberculosis* is represented by its ability to inhibit phago-lysosome fusion, and thus potentially limit the efficacy of the autophagy process, thanks to the presence of specific virulence factors like lipoarabinomannan in the cell wall [30], the ESX-1 secretion system [31,32] and other key components such as the Eis protein which modulates autophagy and inflammation and suppresses host innate immune responses [33]. This inhibitory effect can be overcome by tyrosine kinase inhibitors, such as imatinib and the second-generation inhibitors nilotinib and dasatinib, which target the BCR-ABL fusion protein and are used for treating chronic myeloid leukemia [34]. Different studies explored the effect of imatinib on *M. tuberculosis*-infected macrophages and revealed that it increases acidification of lysosomes thereby halting bacterial multiplication [35]. Moreover, imatinib was shown to reduce the number of granulomatous lesions in mice and to act synergistically with first-line anti-TB drug rifampicin [36].

#### 2.1.4. Statins

Statins, i.e., agents that lower cholesterol through inhibition of the biosynthetic pathway, also impact autophagy [37]. Given the relevance of cholesterol in *M. tuberculosis* persistence [38], statins have received considerable attention. Indeed, in addition to their cholesterol-lowering effect, statins decrease lipid body biogenesis and limit *M. tuberculosis* survival [39]. Additionally, it was discovered that atorvarstatin potentiates the effect of rifampin in *M. leprae* infection of the mouse footpad [40]. These substances are currently investigated in clinical trials (Table 1).
