**3.2 Approaches of drug repurposing in TB**

Host-directed approaches/therapy: host-directed therapy (HDT) is used to target pathogen-exploited pathways in the host. This therapy makes use of repurposed drugs, antibodies, vitamins, small molecules, as adjuvants to support the conventional treatment. Pulmonary diseases, involving uncontrolled healing mediated by profibrotic cytokines, are considered as autoimmune diseases. Such pulmonary pathologies usually do not respond to the standard anti-inflammatory agents. TB also represents this kind of pathophysiology. Interferon-γ, an adjunct, is delivered subcutaneously for chronic granulomatous disease and osteopetrosis. Interferon-γ stimulates macrophage function and inhibits fibrotic pathways. Interferon-γ has been repurposed as an inhaled aerosol, targeting directly to the lung so, to treat many diseases exaggerated by dysregulated immunity like TB. Inhalation of

interferon-γ has been studied as potent antitubercular adjuvant in a clinical trial against MDR-TB by Condos *et al* and has been found effective [47]. Elevated levels of *IRF-1*, *IRF-9*, and *STAT1,* from lung segments in BAL cells, were visualized when in other trial co-administration of anti-TB drugs and IFN-γ were given to TB patients. IFN-γ provided potential to be used as an adjuvant therapy as it energetically stimulated gene expression and signal transduction in alveolar macrophages of TB patients. In addition to a chemotherapeutic cocktail, IFN-γ has been evaluated as an adjuvant therapy *via* other approaches. An intramuscular injection of IFN-γ as an adjuvant chemotherapy for a time period of 6 months led to the reduction of lesion sizes, cultures, negative sputum smears, and increased body mass index [48].

### *3.2.1 Pathogen-directed approaches*

Growth of heterogeneous *M.tb* populations during infection is an important factor for antibiotic tolerance. Inside phagolysomes, acidification alters the redox physiology of *M.tb*, which alters the bug to replicate into population of drug-tolerant strains. The mechanism behind this tolerance has been elucidated with RNA sequencing of redoxaltered *M.tb* population; and involvement of iron-sulfur (Fe-S) cluster biogenesis, hydrogen sulfide (H2S) gas, and drug efflux pumps. Chloroquine (CQ ), an antimalarial drug inhibited phagosomal acidification, improved lung pathology and reduced post-chemotherapeutic relapse in experimental animal models. The pharmacological parameters of CQ did not show any significant drug-drug interaction with first-line anti-TB drugs upon co-administration in mice. A link between phagosomal pH, redox metabolism, and drug tolerance in replicating *M.tb* is suggestive of repositioning potential of CQ against TB and a relapse-free cure [49]. One of the determinants of *M.tb* virulence is protein phosphorylation. Unique tyrosine-specific kinase, protein tyrosine kinase A (PtkA), present in the *M.tb* genome phosphorylates protein tyrosine phosphatase A (MptpA) and increases PtpA activity and pathogenicity. Several proteins including the cyclophilins are essential for biofilm generation. *M.tb* cyclophilin peptidyl-prolyl isomerase (PpiB), interaction cyclosporine-A, and acarbose (US FDAapproved drugs) were predicted by *in silico* docking studies. Further surface plasmon resonance (SPR) spectroscopy was used to confirm the inhibition in growth of *M.tb*. Gallium nanoparticle (GaNP) reported to have bactericidal effect, when used with Cyclosporine—additionally disrupted *M.tb* H37Rv biofilm formation. Co-culturing *M.tb* in their presence resulted in significant (2–4-fold) decrease in dosage of antitubercular drugs such as isoniazid and ethambutol [50]. Targeting MurB and MurE enzymes involved in the muramic acid synthesis pathway (Mur Pathway) in *M.tb* has been studied and FDA-approved drugs from two repositories, that is, Drug Bank (1932 drugs) and e-LEA3D (1852 drugs), have been screened against these proteins. Binding-free energy and hydrogen bonding interactions have been seen to effect the stability of interactions among drugs and drug sites. Sulfadoxine (−7.3 kcal/mol) and pyrimethamine (−7.8 kcal/mol) showed stable interaction with MurB. Lifitegrast (−10.5 kcal/mol) and sildenafil (−9.1 kcal/mol) showed most reliable interaction with MurE. Hence, these characteristics of drugs for repurposing are supposed to be further explored to achieve efficient repurposing of the drugs [51].

#### *3.2.2* In silico *approach*

Several computational approaches have been developed to discover new repurposing opportunities and integration of these approaches can help rediscovering drugs with more chances of success as prediction of new drug-target interaction, target-disease, and drug-disease associations can be done more rationally. Based on systemic data analysis of host, pathogen, or drug which

include signature *matching* gene expression, chemical structure, genotype, or proteomic data or Electronic health records (EHRs) can help to formulate repurposing hypotheses for various drugs [52].
