**2. Host Directed Therapy**

## *2.1. Promoting Phagosome Maturation and Enhancing Autophagy*

Autophagy is a natural process which protects cells against unfolded proteins and potentially dangerous aggregates, viral and bacterial infections. By means of autophagy, macrophages deliver toxic macromolecules, organelles and phagocytosed pathogens to lysosomes for degradation [7]. Modulating autophagy in order to promote bacterial killing represents one example of HDT against TB infection. This statement is supported by studies conducted by Gutierrez and co-workers who reported that stimulation of autophagy in macrophages promotes phago-lysosome maturation and impacts mycobacterial survival [8]. More recent investigations revealed increased susceptibility to TB in mice defective in autophagy pathways [9] and a positive interplay between autophagy and interferon-gamma (IFN-γ) in TB patients [10].

#### 2.1.1. mTOR Inhibition

The best described autophagy inducer is rapamycin (sirolimus), used in patients who underwent organ transplantation [11]. Rapamycin, a macrolide produced by *Streptomyces hygroscopicus* and originally discovered on Easter Island (Rapa Nui for the inhabitants, hence the name given to the compound), inhibits TOR, the Target Of Rapamycin [12]. The mammalian Target of Rapamycin mTOR is a negative regulator of autophagy [13]. Its clinical use in infectious diseases is restricted due to its broadly immunosuppressive effects. In addition, rapamycin is metabolized by CYP3A4 [14], a hepatic enzyme induced by the first-line TB drug rifampicin, thus hampering its exploitation as HDT in TB patients. However, other molecules capable of inducing autophagy have been discovered and are now under development. Among these, vadimezan [15], Tat-beclin 1 fusion peptide [16], the calcium-channel blocker verapamil [17] and the rapamycin analogue everolimus [18]. In particular, verapamil was shown to be efficacious when combined with rifampicin and with the recently approved medication bedaquiline in mouse models of infection, where it increased the bioavailability of the antibiotic [19–21]. On the other hand, everolimus, an anti-cancer agent, may be repurposed as an anti-TB HDT therapeutic capable of inhibiting mTOR although, as with rapamycin, immunosuppression [22] and toxicity [23] may represent issues in the clinical development as an anti-TB drug.
