**7.** *In silico* **approach to prioritize drugs for repurposing against TB**

FDA-approved drugs are pharmacists' choice pharmacist for repurposing against TB. Bioinformatic approach is economic, time efficient with better chances of success. About 1554 FDA-approved drugs obtained from DrugBank have been approached for TB therapy using *in silico* method. Serine/threonine-protein kinase, pknB (Rv0014c) of *M.tb* was selected as the drug target and all of the 1554 drugs were subjected to molecular docking with pknB. Rigid docking followed by induced fit docking protocol was employed for prioritization of drugs. Fourteen drugs were prioritized, out of which six are suggested as high-confident drugs toward repurposing for TB. These drugs strongly bound in the active site of the pknB. Atorvastatin was one of the high-confident drugs [68]. It has been reported that a gene ontology-based network containing 26,404 edges, 6630 drug, and 4083 target nodes analyzed using network-based inference (NBI) are used to identify novel drug-target interactions that are further evaluated on basis of a combined evidence approach for identification of potential drug repurposing candidates. Targets are prioritized on basis of known variation in clinical isolates and human homologs, essentiality for *M.tb's* survival and virulence. DTIs were used to identify target pairs against which the predicted drugs could have synergistic bactericidal effect. Enlisted DTIs from RepTB, four TB targets, namely, FolP1 (Dihydropteroate synthase), Tmk (Thymidylate kinase), Dut (Deoxyuridine 5′-triphosphate nucleotidohydrolase), and MenB (1,4-dihydroxy-2-naphthoyl-CoA synthase) have the potential for future drug candidature.

#### **7.1 Potential targets for drug repurposing**

Information about the structure of drug-binding site reveals novel connections between drugs and targets. A correlation between drug-promiscuity and shared binding sites across the drug's multiple targets demonstrates the potential role of structural analyses of shared binding sites in drug repositioning [69]. A dockingbased approach has been employed to screen new novel targets for existing drugs by computationally screening the whole druggable proteome [70]. Target-based screening has revealed potential of anti-Parkinson drugs entacapone and tolcapone against drug-resistant (MDR) and extensively drug-resistant (XDR) TB. The logic

for this activity is based on similarity between the original target COMT and the new target InhA [71]. *M.Tb* phosphoserine phosphatase SerB2 is a promising drug target, being a key essential metabolic enzyme of the pathogen's serine pathway. About one hundred and twenty two compounds from an internal chemolibrary were screened using malachite green-based phosphatase assay and Tri-substituted derivatives were found among the best hits that inhibited SerB2 activity. Their interaction with the enzyme was studied through induced fit docking experiments. Cellular assays showed that the selected compounds also inhibit *M.tb* growth *in vitro*. Those promising results may provide a basis for the development of new antimycobacterial agents targeting SerB2 [24]. Drug efflux is an important resistance mechanism in *M.tb.* Different medications used to treat unrelated human conditions such as psychoses and angina serve to inhibit the multidrug efflux pumps in *M. TB;* this increases the pathogen's susceptibility to other drugs. Thiazolidinedione enhances (a) killing of intracellular pathogen by non-killing macrophages and (b) inhibits the expression of efflux pumps that extrude antibiotics prior to their action. The other targets are based on overexpressed efflux pumps, to make otherwise inefficient antibiotics again effective. Molecules 4-OH-OPB depleted flavinsformed covalent adducts with N-acetyl-cysteine and mycothiol. This molecule killed *M.tb* synergistically with oxidants and other anti-TB drugs. The conditions that block *M.tb's* replication modify OPB and enhance its killing action. Modified OPB kills both replicating and non-replicating *M.tb* and sensitizes to both hostderived and medicinal antimycobacterial agents [72]. Several phosphodiesterase inhibitors have also shown promise as adjuvants for host-directed therapy. All phenothiazines are known to have common function to inhibit the binding of calcium to calcium-dependent proteins of eukaryotic cells [73]. Calcium binding is important for the bacterial phagocytosis [74]. Consequently, inhibition of calcium signaling processes, by phenothiazines, ought to affect processes of phagocytosis [75]. Moreover, the killing activity of neutrophils is dependent upon the retention of calcium [76] and potassium within the phagolysosome [77]. Thus, verapamil, an inhibitor of calcium transport, and ouabain, an inhibitor of potassium transport, promotes the killing of intracellular *M. Tb* by non-killing human macrophages [78]. Thiazolidinediones, otherwise an antidiabetic drug, acts as inhibitor of calcium and potassium transport, hence has repurposing potential to promote bug killing [79]. In a study, TDZ treatment to *M. Tb* infected mouse was successful by inhibiting efflux of calcium and potassium from the phagolysosome as potassium is requisite for the phagolysosomal acidification and degradation of the entrapped pathogen. Thiazolidinediones can be sought as future drugs in TB drug repurposing [77]. Increasing resistance to isoniazid due to prolonged exposure of INH-susceptible *M. Tb* strains to increasing concentrations of INH can be reduced to wild-type INH susceptibility by using inhibitors of efflux pump CPZ and reserpine [80]. RIF-resistant *M. Tb-*infected mice have over expression of an efflux pump upon treatment with RIF, rendering the strain resistant to oxacillin as well [81]. Though phenothiazine inhibits the efflux pump systems of mycobacteria [82], only recently has TDZ been shown to inhibit the expression of genes that code for efflux pumps [83]. Specifically, efflux pumps coded by mmpL7, p55, efpA, mmr, Rv1258c, and Rv2459 [84] have direct effects on the efflux pumps of *M. Tb*. An agent that inhibits an efflux pump system, responsible for its resistance to antibiotics renders that organism again susceptible to the otherwise resistant antibiotics [85]. Consequently, when TDZ inhibits the activity of efflux pumps of MDR mycobacteria, it renders the organism susceptible to the antibiotics to which it was initially resistant as a consequence of their extrusion from the cell [86]. However, with time, accumulation of mutations takes place and, commensurate with this accumulation, the level of expression of the efflux pump decreases to almost that of the wild-type parent [86].

### *Antituberculosis Drug Repurposing: A New Hope for Tackling Multi-Challenging TB in Timely… DOI: http://dx.doi.org/10.5772/intechopen.101642*

Repurposed drugs with synergistic effects: Synergistic effects of repurposed drugs with other anti-TB drugs for treatment of MDR-TB, XDR-TB, and TDR-TB have been proposed for the future WHO regimen. Clofazimine (CZM) in a combination with moxifloxacin (MOX) and ethambutol (EMB) might be a promising drug regimen for the treatment of MDR-TB [87]. Similarly, *in vitro*, synergistic effect of sulfamethoxazole (SMX) has been reported with rifampin [88]. For the treatment of MDR-TB, pyrazinamide and bedaquiline in combination with CZM have been reported as a best example of synergistic effect [89]. The combinatorial therapy of capreomycin and linezolid showed partial synergistic effect suggestive of increased efficacy against *M.tb* [89]. Synergistic therapy of linezolid and bedaquiline has been suggested for rescuing female XDR-TB patients during pregnancy [90]. Synergistic effect of carbapenems is also known with rifampicin against *M. tb* [91]. Thioridazine (TDZ), a neuroleptic drug in combination with antibiotics, kills extremely drug-resistant *M.tb* (XDR-TB). This combination is not prone to mutations as it does not affect the pathogen directly. With proper precautions and cardiac monitoring prior to and during therapy, TDZ will be essentially safe. Given the serious prognoses associated with MDR/XDR-TB and TDR-TB infections, TDZ provides a suitable alternative to current ineffective therapy. Numerous cephalosporins were synergistic with rifampicin, the cornerstone drug for TB therapy and ethambutol, a first-line anti-TB drug. When used in combination, cephalosporins and rifampicin had 4- to 64-fold more activity than used alone. Clavulanate has also shown key synergistic partner role in triple combinations. Cephalosporins (and other beta-lactams) together with clavulanate reversed the inefficacy of rifampicin in a rifampicin-resistant strain. Cephalosporins also showed synergism with new anti-TB drugs such as bedaquiline and delamanid. More studies will be needed to validate their *in vivo* activities. Additional features like oral bioavailability with good safety profiles and antimycobacterial effects of cephalosporins suggest that they could be promising repurposing agents [92]. The newly synthesized and patented SILA compounds were tested for *in vitro* and *ex vivo* activity against XDR-TB. These compounds had *in vitro* activity against XDR-TB (MIC<3.5 mg/L) could transform non-killing macrophages into effective killers of phagocytosed bacteria, without any cytotoxic activity. Among them, SILA 421 revealed good *in vitro* and *ex vivo* activities without exhibiting any cytotoxic activity; thus, it seems to be a potential candidate to be anti-MDR/XDR-TB drug [93].
