**3. Globally situation of tuberculosis**

According to the report of WHO, a sum of 1.4 million individuals passed on from TB in 2019 (counting 208,000 individuals with HIV). Around the world, TB is one of the top 10 reasons for death and the main source from a solitary irresistible specialist (above HIV/AIDS). In 2019, an expected 10 million individuals became sick with tuberculosis (TB) around the world. 5.6 million men, 3.2 million ladies and 1.2 million youngsters. In 2019, 1.2 million kids became sick with TB worldwide. The youngster and juvenile TB is frequently ignored by wellbeing suppliers and can be hard to analyze and treat. In 2019, the 30 high TB trouble nations represented 87% of new TB cases. Eight nations represent 66% of the aggregate, with India driving the tally, trailed by Indonesia, China, the Philippines, Pakistan, Nigeria, Bangladesh and South Africa. Multidrug-safe TB (MDR-TB) stays a general wellbeing emergency and a wellbeing security danger. A worldwide all out of 206 030 individuals with multidrug-or rifampicin-safe TB (MDR/RR-TB) were identified and told in 2019, a 10% expansion

from 186 883 out of 2018. Internationally, the TB rate is falling at about 2% each year and somewhere in the range of 2015 and 2019, the combined decrease was 9%. This was not exactly most of the way to the End TB Strategy achievement of a 20% decrease somewhere in the range of 2015 and 2020. An expected 60 million lives were saved through TB analysis and treatment somewhere in the range of 2000 and 2019. Finishing the TB plague by 2030 is among the wellbeing focuses of the United Nations Sustainable Development Goals (SDGs). Tuberculosis generally influences grown-ups in their most gainful years. Nonetheless, all age bunches are in danger. More than 95% of cases and passings are in non-industrial nations. Multidrug-resistant tuberculosis (MDR-TB) is a type of TB brought about by microbes that do not react to isoniazid and rifampicin, the 2 best first-line hostile to TB drugs. MDR-TB is treatable and reparable by utilizing second-line drugs. Nonetheless, second-line treatment choices are restricted and require broad chemotherapy (as long as 2 years of treatment) with meds that are costly and poisonous.

Sometimes, more serious medication opposition can create. TB brought about by microbes that do not react to the best second-line hostile to TB medications can leave patients with no further treatment alternatives.

In 2019, MDR-TB stays a general wellbeing emergency and a wellbeing security danger. A worldwide total of 206 030 individuals with multidrug-or rifampicin-safe TB (MDR/RR-TB) were identified and advised in 2019, a 10% increment from 186 883 out of 2018. About portion of the worldwide weight of MDR-TB is in 3 nations – India, China and the Russian Federation.

Around the world, just 57% of MDR-TB patients are presently effectively treated. In 2020, WHO suggested another more limited (9–11 months) and completely oral routine for patients with MDB-TB. This exploration has shown that patients think that it's simpler to finish the routine, contrasted and the more drawnout regimens that last as long as 20 months. Protection from fluoroquinolones ought to be rejected preceding the commencement of treatment with this routine.

As per WHO rules, the discovery of MDR/RR-TB requires the bacteriological affirmation of TB and testing for drug obstruction utilizing quick sub-atomic tests, culture strategies or sequencing advancements. Treatment requires a course of second-line drugs for at any rate 9 months and as long as 20 months, upheld by advising and checking for unfavorable occasions. WHO prescribes extended admittance to every single oral routine. Before the finish of 2019, 89 nations began utilizing more limited MDR-TB regimens and 109 had imported or begun utilizing bedaquiline, with an end goal to improve the viability of MDR-TB treatment.

#### **4. The course of events in** *Mycobacterium tuberculosis*

*Mycobacterium tuberculosis* basically passes through the 5 stages during its life cycle. At the first stage, the bacteria are inhaled through the air and typically engulfed by alveolar macrophages, further proceed to the symbiosis stage and causing the caseous necrosis in later stages. Eventually spread to other cells and causing rapid spread of diseases. The whole cycle is presented in detail in **Figure 1** and as a flow chart in **Figure 2**. The Mycobacterium gets entry into the lungs and resides in the alveoli of the lungs while it begins its primary infection. If the immune system fails to eliminate it then there are three cases observed with the mycobacterium in the alveoli. The first case could be the elimination phase, in which the immune system completely eliminates the infection. The next one retention phase where the immune system suppresses the infection but the bacteria remain viable and, in this case, the infection is known as Latent Tuberculosis which is the most asymptomatic Tuberculosis. And the third phase may involve Active infection, which makes the

**13**

[1, 2, 7, 22].

**Figure 2.**

**Figure 1.**

*life-cycle-of-organism).*

Tuberculosis [19–21].

mycobacterium capable of evades the immune response and separates the infection in the lung tissue and at this point of active infection it is known as Active

*Flow chart presentation of life cycle of Mycobacterium tuberculosis.*

*Life cycle of Mycobacterium tuberculosis. This presentation is influenced with the figure available at online resource on study of the tuberculosis. (https://sites.google.com/site/mycobacteriumtbstudy/home/*

*M. tuberculosis* has 5 stages in its life cycle as mentioned in **Figure 2** as flow chart

*Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

*Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

#### **Figure 1.**

*Molecular Epidemiology Study of Mycobacterium Tuberculosis Complex*

meds that are costly and poisonous.

India, China and the Russian Federation.

leave patients with no further treatment alternatives.

from 186 883 out of 2018. Internationally, the TB rate is falling at about 2% each year and somewhere in the range of 2015 and 2019, the combined decrease was 9%. This was not exactly most of the way to the End TB Strategy achievement of a 20% decrease somewhere in the range of 2015 and 2020. An expected 60 million lives were saved through TB analysis and treatment somewhere in the range of 2000 and 2019. Finishing the TB plague by 2030 is among the wellbeing focuses of the United Nations Sustainable Development Goals (SDGs). Tuberculosis generally influences grown-ups in their most gainful years. Nonetheless, all age bunches are in danger. More than 95% of cases and passings are in non-industrial nations. Multidrug-resistant tuberculosis (MDR-TB) is a type of TB brought about by microbes that do not react to isoniazid and rifampicin, the 2 best first-line hostile to TB drugs. MDR-TB is treatable and reparable by utilizing second-line drugs. Nonetheless, second-line treatment choices are restricted and require broad chemotherapy (as long as 2 years of treatment) with

Sometimes, more serious medication opposition can create. TB brought about by microbes that do not react to the best second-line hostile to TB medications can

In 2019, MDR-TB stays a general wellbeing emergency and a wellbeing security danger. A worldwide total of 206 030 individuals with multidrug-or rifampicin-safe TB (MDR/RR-TB) were identified and advised in 2019, a 10% increment from 186 883 out of 2018. About portion of the worldwide weight of MDR-TB is in 3 nations –

Around the world, just 57% of MDR-TB patients are presently effectively treated. In 2020, WHO suggested another more limited (9–11 months) and completely oral routine for patients with MDB-TB. This exploration has shown that patients think that it's simpler to finish the routine, contrasted and the more drawnout regimens that last as long as 20 months. Protection from fluoroquinolones ought

to be rejected preceding the commencement of treatment with this routine.

**4. The course of events in** *Mycobacterium tuberculosis*

As per WHO rules, the discovery of MDR/RR-TB requires the bacteriological affirmation of TB and testing for drug obstruction utilizing quick sub-atomic tests, culture strategies or sequencing advancements. Treatment requires a course of second-line drugs for at any rate 9 months and as long as 20 months, upheld by advising and checking for unfavorable occasions. WHO prescribes extended admittance to every single oral routine. Before the finish of 2019, 89 nations began utilizing more limited MDR-TB regimens and 109 had imported or begun utilizing bedaquiline, with an end goal to improve the viability of MDR-TB treatment.

*Mycobacterium tuberculosis* basically passes through the 5 stages during its life cycle. At the first stage, the bacteria are inhaled through the air and typically engulfed by alveolar macrophages, further proceed to the symbiosis stage and causing the caseous necrosis in later stages. Eventually spread to other cells and causing rapid spread of diseases. The whole cycle is presented in detail in **Figure 1** and as a flow chart in **Figure 2**. The Mycobacterium gets entry into the lungs and resides in the alveoli of the lungs while it begins its primary infection. If the immune system fails to eliminate it then there are three cases observed with the mycobacterium in the alveoli. The first case could be the elimination phase, in which the immune system completely eliminates the infection. The next one retention phase where the immune system suppresses the infection but the bacteria remain viable and, in this case, the infection is known as Latent Tuberculosis which is the most asymptomatic Tuberculosis. And the third phase may involve Active infection, which makes the

**12**

*Life cycle of Mycobacterium tuberculosis. This presentation is influenced with the figure available at online resource on study of the tuberculosis. (https://sites.google.com/site/mycobacteriumtbstudy/home/ life-cycle-of-organism).*

#### **Figure 2.**

*Flow chart presentation of life cycle of Mycobacterium tuberculosis.*

mycobacterium capable of evades the immune response and separates the infection in the lung tissue and at this point of active infection it is known as Active Tuberculosis [19–21].

*M. tuberculosis* has 5 stages in its life cycle as mentioned in **Figure 2** as flow chart [1, 2, 7, 22].

**Figure 3.**

*Transmission of Mycobacterium tuberculosis. The representation is influenced with figure available in online resource. (https://www.istockphoto.com/in/vector/tuberculosis-life-cycle-of-mycobacterium-tuberculos -gm1200338165-343779875).*

## **5. Pathogenesis and transmission of** *Mycobacterium tuberculosis*

If somebody has active lung disease with TB they will cough and, in the cough, there would be infected droplets carrying the bacteria that could be inhaled by somebody else [8, 15]. Once the bacteria is inhaled it goes into the lungs and then it invades the normal mechanism for protecting lungs against bacterial infection which are the alveolar macrophages. It actively seeks out and invades these macrophages because it can prevent the normal macrophage killing mechanism. So, it diverts the normal figure lysosome pathways and that allows it to survive in the macrophage and it can be latent in that macrophage for decades [3]. Also, Macrophages because they move will allow the bacterium to spread bull RAC across the body and this is one of the reasons why sites of immune functions such as the lymph nodes often get infected with Tuberculosis and long-term persistence within the macrophages is led to latent diseases [18]. Besides, there is a certain inflammatory response to this infection which causes a very distinctive histologic appearance called granulomas and that is one of the hallmarks of Tuberculosis infection. Our closest infection is the presence of granulomas in the infected tissue [5, 6]. This transmission process is represented in **Figure 3**.

#### **6. Mechanism of drug-resistant TB**

This has been observed that various mechanism of drug resistance in M. tuberculosis is involved.

#### **6.1 Presence of cell wall**

The basic property leading to passive resistance to antibiotics in M. tuberculosis is because of its impervious cell wall [23]. The hydrophilic layer of arabinogalactan ensures the impervious nature of the cell wall to the surrounding hydrophobic substances. This layer is also present in hydrophobic mycolic acids which significantly

**15**

*Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

**6.2 Slow metabolism mechanism**

decline in the metabolic processes of M. tuberculosis.

and thus helps in specializing them for drug resistance roles [28].

between each drug against tuberculosis and its specified target.

of resistance cases reported in Mycobacterial tuberculosis.

with a low affinity for isoniazid adduct [37].

**6.3 Possession of numerous efflux pumps**

**6.4 Mutation in genetic materials**

prevents the entry of hydrophilic molecules [24]. This impervious nature of the cell wall results in the deposition of antibiotics throughout the cell wall, the accumulated antibiotics near the cell wall are removed steadily by the release of enzyme & with the involvement of several cellular components [25]. It is demonstrated that β-lactams, which act as inhibitors to the inclusion of peptidoglycan (responsible for maintaining the rigidity of the cell wall) into the cell wall, are degraded by the mycobacteria due to the presence of β-lactamases, which are the enzyme responsible for degradation of β-lactam antibiotics. Danilchanka et al. [24], reported the presence of CpnT channel protein in the outer membrane of both M. tuberculosis and *M. bovis*, which plays a dual role in nutrient absorption and selective sensitivity to antibacterial agents.

Bacteria that have long-generation time & undergo metabolic processes with a slower rate are estimated to be challenging targets for most of the antibiotics i.e., bacteria that are metabolically active and rapidly replicating act as a good target for antibiotics [26]. However, in M. tuberculosis, it is still unclear whether the long generation time confirms its resistance to drugs. However, it is been reported that the slow growth rate of M. tuberculosis plays a crucial role in drug resistance. For example, antibiotics such as carbapenems lose their activity comparatively at a faster rate than the growth rate of M. tuberculosis [27]. It is seen that certain specific genes which are involved in the production of triacyl-glycerol permit the growth of M. tuberculosis even in oxygen-deprived conditions. Triacylglycerol

These protein channels play a vital role in the regulation of normal metabolism and the physiology of the organism such as toxins, signaling molecules through the cell wall, residues, and nutrient transport [28]. Efflux pumps have shown adaptation to drug resistance in M. tuberculosis. Multi-drug efflux pumps serve as an outlet for cell antibiotics and usually pass through both the inner and outer membranes of the cell [29]. Regulatory protein systems are present in Drug-efflux proteins which are responsible for controlling the expression of the efflux pump

It has been shown that the acquisition of antibiotics resistance in M. tuberculosis is the result of spontaneous mutation in several chromosomal genes. This frequent mutation has been found to cause a deliberate alteration to the required interaction

*M. tuberculosis* shows resistance to rifampicin due to mutation in rpoB of RNA polymerase, decelerating its affinity for rifampicin [30]. It has been identified in certain studies that specific codons can cause resistance to rifampicin only with the onset of mutation in them [31, 32]. Resistance to pyrazinamide is due to mutation in the pncA gene [33, 34]. The mutations in pncA gene account for the large number

The mode of action of isoniazid resistance is complex and remains unclear, however, most strains of Mtb resistant to isoniazid are associated with a mutation in KatG and inhA [35, 36]. S315T of KatG mutation is more common in isoniazidresistant strains. Mutation at this phase results in the formation of isoniazid product *Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

*Molecular Epidemiology Study of Mycobacterium Tuberculosis Complex*

**5. Pathogenesis and transmission of** *Mycobacterium tuberculosis*

*Transmission of Mycobacterium tuberculosis. The representation is influenced with figure available in online resource. (https://www.istockphoto.com/in/vector/tuberculosis-life-cycle-of-mycobacterium-tuberculos*

transmission process is represented in **Figure 3**.

**6. Mechanism of drug-resistant TB**

culosis is involved.

**Figure 3.**

*-gm1200338165-343779875).*

**6.1 Presence of cell wall**

If somebody has active lung disease with TB they will cough and, in the cough, there would be infected droplets carrying the bacteria that could be inhaled by somebody else [8, 15]. Once the bacteria is inhaled it goes into the lungs and then it invades the normal mechanism for protecting lungs against bacterial infection which are the alveolar macrophages. It actively seeks out and invades these macrophages because it can prevent the normal macrophage killing mechanism. So, it diverts the normal figure lysosome pathways and that allows it to survive in the macrophage and it can be latent in that macrophage for decades [3]. Also, Macrophages because they move will allow the bacterium to spread bull RAC across the body and this is one of the reasons why sites of immune functions such as the lymph nodes often get infected with Tuberculosis and long-term persistence within the macrophages is led to latent diseases [18]. Besides, there is a certain inflammatory response to this infection which causes a very distinctive histologic appearance called granulomas and that is one of the hallmarks of Tuberculosis infection. Our closest infection is the presence of granulomas in the infected tissue [5, 6]. This

This has been observed that various mechanism of drug resistance in M. tuber-

The basic property leading to passive resistance to antibiotics in M. tuberculosis is because of its impervious cell wall [23]. The hydrophilic layer of arabinogalactan ensures the impervious nature of the cell wall to the surrounding hydrophobic substances. This layer is also present in hydrophobic mycolic acids which significantly

**14**

prevents the entry of hydrophilic molecules [24]. This impervious nature of the cell wall results in the deposition of antibiotics throughout the cell wall, the accumulated antibiotics near the cell wall are removed steadily by the release of enzyme & with the involvement of several cellular components [25]. It is demonstrated that β-lactams, which act as inhibitors to the inclusion of peptidoglycan (responsible for maintaining the rigidity of the cell wall) into the cell wall, are degraded by the mycobacteria due to the presence of β-lactamases, which are the enzyme responsible for degradation of β-lactam antibiotics. Danilchanka et al. [24], reported the presence of CpnT channel protein in the outer membrane of both M. tuberculosis and *M. bovis*, which plays a dual role in nutrient absorption and selective sensitivity to antibacterial agents.

#### **6.2 Slow metabolism mechanism**

Bacteria that have long-generation time & undergo metabolic processes with a slower rate are estimated to be challenging targets for most of the antibiotics i.e., bacteria that are metabolically active and rapidly replicating act as a good target for antibiotics [26]. However, in M. tuberculosis, it is still unclear whether the long generation time confirms its resistance to drugs. However, it is been reported that the slow growth rate of M. tuberculosis plays a crucial role in drug resistance. For example, antibiotics such as carbapenems lose their activity comparatively at a faster rate than the growth rate of M. tuberculosis [27]. It is seen that certain specific genes which are involved in the production of triacyl-glycerol permit the growth of M. tuberculosis even in oxygen-deprived conditions. Triacylglycerol decline in the metabolic processes of M. tuberculosis.

#### **6.3 Possession of numerous efflux pumps**

These protein channels play a vital role in the regulation of normal metabolism and the physiology of the organism such as toxins, signaling molecules through the cell wall, residues, and nutrient transport [28]. Efflux pumps have shown adaptation to drug resistance in M. tuberculosis. Multi-drug efflux pumps serve as an outlet for cell antibiotics and usually pass through both the inner and outer membranes of the cell [29]. Regulatory protein systems are present in Drug-efflux proteins which are responsible for controlling the expression of the efflux pump and thus helps in specializing them for drug resistance roles [28].

#### **6.4 Mutation in genetic materials**

It has been shown that the acquisition of antibiotics resistance in M. tuberculosis is the result of spontaneous mutation in several chromosomal genes. This frequent mutation has been found to cause a deliberate alteration to the required interaction between each drug against tuberculosis and its specified target.

*M. tuberculosis* shows resistance to rifampicin due to mutation in rpoB of RNA polymerase, decelerating its affinity for rifampicin [30]. It has been identified in certain studies that specific codons can cause resistance to rifampicin only with the onset of mutation in them [31, 32]. Resistance to pyrazinamide is due to mutation in the pncA gene [33, 34]. The mutations in pncA gene account for the large number of resistance cases reported in Mycobacterial tuberculosis.

The mode of action of isoniazid resistance is complex and remains unclear, however, most strains of Mtb resistant to isoniazid are associated with a mutation in KatG and inhA [35, 36]. S315T of KatG mutation is more common in isoniazidresistant strains. Mutation at this phase results in the formation of isoniazid product with a low affinity for isoniazid adduct [37].

Mutations in embB497 and embB406, codon 306 in embB and Polymorphism in embA, embC, are all involved in ethambutol resistance [38]. In 2013, Safi et al. proposed that the mutation in ubiA (Rv3806c) showed a high level of ethambutol resistance [39]. Some investigators have reported that the mutations in tlyA gene play a vital role in the resistance of Viomycin and Capreomycin [40, 41].

### **7. Extrapulmonary tuberculosis (EPTB)**

TB as a rule influences the lungs, however, it can likewise influence different pieces of the body, like the brain, the kidneys, or the spine. An individual with TB can pass on if they do not get treatment. TB influencing any piece of the body other than lung parenchyma including different structure inside the chest like the pleura, pericardium and perihilar lymph hubs, alluded as extra aspiratory tuberculosis. EPTB incorporates tuberculosis meningitis, stomach tuberculosis (for the most part with ascites), skeletal tuberculosis, Pott's infection (spine), scrofula (lymphadenitis), and genitourinary (renal) tuberculosis. Scattered, or miliary tuberculosis regularly incorporates aspiratory and extrapulmonary locales. It is assessed that extrapulmonary tuberculosis (EPTB) represents 15–25% of all instances of TB. HIV patients, particularly with low CD4 tallies, have higher paces of EPTB. Youngsters are bound to have skeletal TB than grown-ups [42]. Approximately 10% of all TB cases have both pulmonary and extrapulmonary TB, and an additional 20% have EPTB without pulmonary involvement [2, 43].

#### **8. Major limitations and considerations to work with** *M. Tuberculosis*

*Mycobacterium tuberculosis* is a gradually developing bacteria which must be handled cautiously under exacting containment to minimize the hazard to research centre individual [4]. The bacterium can reproduce inside the macrophage and kill the immune cell. Another limitation presented by the bacteria in the innovative work of new drugs is the idea of its cell wall which is wealthy in lipids and ultimately makes the development of homogenous and single-cell culture and troublesome [2]. *M. tuberculosis* can evade the immune response and recreate inside macrophages coming about because of several bacterial variables which along these lines can modulate the immune reaction [4, 5]. Although *M. tuberculosis* is Grampositive bacteria its cell wall resembles the external membrane of Gram-negative bacteria since it is composed of an asymmetric bi-layer containing particular mycolic acids, along with glyco-lipids, lipo-glycans, and proteins [3, 9]. Therefore, novel drugs with viability and quicker acting mechanism which can most likely work in the shorter-term and along these lines give better outcomes in the treatment are desperately required [7].

### **9. Possible opinion regarding the challenges of new drug discovery for tuberculosis**

Besides, the development of XDR strains of M. tuberculosis, 5.4% of MDR-TB cases are discovered to be XDR-TB (World Health Organization, 2010, Ref. [3]). Multidrug and Extensive Drug-Resistant Tuberculosis: 2010 Global Report on Surveillance and Response (World Health Organization, 2010, Ref. [4]) is testing TB treatment programs in a few nations and even raises the chance of a re-visitation of a circumstance much the same as the pre-anti-microbial TB time [1]. As

**17**

conform to this list.

*Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

the development of obstruction.

properties [8].

of now, MDR-TB is treated by a blend of eight to ten medications with treatments enduring up to 18 two years; just four of these medications were really evolved to treat TB5. Such imperfect treatment prompts practically 30% of MDR-TB patients to encounter treatment disappointment [44]. The treatment alternatives for XDR-TB are exceptionally restricted as XDR-TB bacilli are safe not exclusively to isoniazid and rifampicin, yet in addition to fluoroquinolones and injectables, for example, aminoglycosides. Furthermore, there are not kidding results with most MDR-TB and XDR-TB drugs, incorporating nephrotoxicity and ototoxicity with aminoglycosides, hepatotoxicity with ethionamide and dysglycaemia with gatifloxacin [45]. In this manner, the current circumstance requires the prompt distinguishing proof of new frameworks that can address arising opposition and furthermore requests the direct of suitable clinical preliminaries as verifiably not very many clinical examinations have been performed to assess the adequacy of medications in MDR-TB or XDR-TB patient gatherings. Improving the diagnostics with more extensive inclusion of medication vulnerability testing will likewise assist with tending to the high mortality of MDR/XDR-TB and control

Critical difficulties exist in TB drug revelation because of the idea of the causative bacterium. The absence of prescient models for compound section into mycobacteria is likewise a restricting variable since the direct trial proof is arduous to get. Creating essential guidelines around compound passage and efflux could help with improving hits from biochemical screens which need entire cell action, just as adjusting the synthetic properties needed for great pharmacokinetic

The present routine of medication for drug-sensitive Tuberculosis treatment was set up during the 1980s. This treatment process encompasses four levels of medications, isonicotinic acid hydrazide, rifampin, Ethambutol dihydrochloride and Pyrazinoic acid amide for six months of treatment (**Table 1**). The essential focus of Tuberculosis drugs is cell wall biogenesis, deoxyribonucleotide replication,

Treatment of drug-resistant or multidrug-resistant (MDR) tuberculosis is substantially further unpredictable [8]. The success of the treatment process relies upon the patient record and drug affectability. MDR-Tuberculosis needs therapy for a long time with a combination of 5 other medications. These second-line drugs will in general be progressively costly and incorporate Sirturo, 2-ethylthioisonicotinamide, Seromycin, Moxifloxacino, and Streptomycine, just like cutting edge medications rifampin systemic and Myambutol [5, 46]. For MDR tuberculosis therapy, we need to go through at least 6 months long treatment process including various vaccinations. Some have been observed to show adverse effects like heart

**11. Drug combination trials and standardization of TB regimens**

The WHO-recommended formulations of anti-TB drugs and fixed-dose combinations (FDCs) of drugs appear in the *WHO Model List of Essential Medicines* (available at www.who.int/medicines/publications/essentialmedicines/en). The formulations and combinations of anti-TB drugs available in each country should

**10. Existing and upcoming tuberculosis drug regime**

ribonucleotide transcription, and protein synthesis [15, 46].

electrophysiology dysfunction and ototoxicity [10, 13].

*Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

*Molecular Epidemiology Study of Mycobacterium Tuberculosis Complex*

**7. Extrapulmonary tuberculosis (EPTB)**

EPTB without pulmonary involvement [2, 43].

ment are desperately required [7].

**for tuberculosis**

Mutations in embB497 and embB406, codon 306 in embB and Polymorphism in embA, embC, are all involved in ethambutol resistance [38]. In 2013, Safi et al. proposed that the mutation in ubiA (Rv3806c) showed a high level of ethambutol resistance [39]. Some investigators have reported that the mutations in tlyA gene

TB as a rule influences the lungs, however, it can likewise influence different pieces of the body, like the brain, the kidneys, or the spine. An individual with TB can pass on if they do not get treatment. TB influencing any piece of the body other than lung parenchyma including different structure inside the chest like the pleura, pericardium and perihilar lymph hubs, alluded as extra aspiratory tuberculosis. EPTB incorporates tuberculosis meningitis, stomach tuberculosis (for the most part with ascites), skeletal tuberculosis, Pott's infection (spine), scrofula (lymphadenitis), and genitourinary (renal) tuberculosis. Scattered, or miliary tuberculosis regularly incorporates aspiratory and extrapulmonary locales. It is assessed that extrapulmonary tuberculosis (EPTB) represents 15–25% of all instances of TB. HIV patients, particularly with low CD4 tallies, have higher paces of EPTB. Youngsters are bound to have skeletal TB than grown-ups [42]. Approximately 10% of all TB cases have both pulmonary and extrapulmonary TB, and an additional 20% have

**8. Major limitations and considerations to work with** *M. Tuberculosis*

**9. Possible opinion regarding the challenges of new drug discovery** 

Besides, the development of XDR strains of M. tuberculosis, 5.4% of MDR-TB cases are discovered to be XDR-TB (World Health Organization, 2010, Ref. [3]). Multidrug and Extensive Drug-Resistant Tuberculosis: 2010 Global Report on Surveillance and Response (World Health Organization, 2010, Ref. [4]) is testing TB treatment programs in a few nations and even raises the chance of a re-visitation of a circumstance much the same as the pre-anti-microbial TB time [1]. As

*Mycobacterium tuberculosis* is a gradually developing bacteria which must be handled cautiously under exacting containment to minimize the hazard to research centre individual [4]. The bacterium can reproduce inside the macrophage and kill the immune cell. Another limitation presented by the bacteria in the innovative work of new drugs is the idea of its cell wall which is wealthy in lipids and ultimately makes the development of homogenous and single-cell culture and troublesome [2]. *M. tuberculosis* can evade the immune response and recreate inside macrophages coming about because of several bacterial variables which along these lines can modulate the immune reaction [4, 5]. Although *M. tuberculosis* is Grampositive bacteria its cell wall resembles the external membrane of Gram-negative bacteria since it is composed of an asymmetric bi-layer containing particular mycolic acids, along with glyco-lipids, lipo-glycans, and proteins [3, 9]. Therefore, novel drugs with viability and quicker acting mechanism which can most likely work in the shorter-term and along these lines give better outcomes in the treat-

play a vital role in the resistance of Viomycin and Capreomycin [40, 41].

**16**

of now, MDR-TB is treated by a blend of eight to ten medications with treatments enduring up to 18 two years; just four of these medications were really evolved to treat TB5. Such imperfect treatment prompts practically 30% of MDR-TB patients to encounter treatment disappointment [44]. The treatment alternatives for XDR-TB are exceptionally restricted as XDR-TB bacilli are safe not exclusively to isoniazid and rifampicin, yet in addition to fluoroquinolones and injectables, for example, aminoglycosides. Furthermore, there are not kidding results with most MDR-TB and XDR-TB drugs, incorporating nephrotoxicity and ototoxicity with aminoglycosides, hepatotoxicity with ethionamide and dysglycaemia with gatifloxacin [45]. In this manner, the current circumstance requires the prompt distinguishing proof of new frameworks that can address arising opposition and furthermore requests the direct of suitable clinical preliminaries as verifiably not very many clinical examinations have been performed to assess the adequacy of medications in MDR-TB or XDR-TB patient gatherings. Improving the diagnostics with more extensive inclusion of medication vulnerability testing will likewise assist with tending to the high mortality of MDR/XDR-TB and control the development of obstruction.

Critical difficulties exist in TB drug revelation because of the idea of the causative bacterium. The absence of prescient models for compound section into mycobacteria is likewise a restricting variable since the direct trial proof is arduous to get. Creating essential guidelines around compound passage and efflux could help with improving hits from biochemical screens which need entire cell action, just as adjusting the synthetic properties needed for great pharmacokinetic properties [8].

### **10. Existing and upcoming tuberculosis drug regime**

The present routine of medication for drug-sensitive Tuberculosis treatment was set up during the 1980s. This treatment process encompasses four levels of medications, isonicotinic acid hydrazide, rifampin, Ethambutol dihydrochloride and Pyrazinoic acid amide for six months of treatment (**Table 1**). The essential focus of Tuberculosis drugs is cell wall biogenesis, deoxyribonucleotide replication, ribonucleotide transcription, and protein synthesis [15, 46].

Treatment of drug-resistant or multidrug-resistant (MDR) tuberculosis is substantially further unpredictable [8]. The success of the treatment process relies upon the patient record and drug affectability. MDR-Tuberculosis needs therapy for a long time with a combination of 5 other medications. These second-line drugs will in general be progressively costly and incorporate Sirturo, 2-ethylthioisonicotinamide, Seromycin, Moxifloxacino, and Streptomycine, just like cutting edge medications rifampin systemic and Myambutol [5, 46]. For MDR tuberculosis therapy, we need to go through at least 6 months long treatment process including various vaccinations. Some have been observed to show adverse effects like heart electrophysiology dysfunction and ototoxicity [10, 13].

#### **11. Drug combination trials and standardization of TB regimens**

The WHO-recommended formulations of anti-TB drugs and fixed-dose combinations (FDCs) of drugs appear in the *WHO Model List of Essential Medicines* (available at www.who.int/medicines/publications/essentialmedicines/en). The formulations and combinations of anti-TB drugs available in each country should conform to this list.


#### **Table 1.**

*Current drugs and their property.*

Normalized treatment implies that all patients in a characterized bunch get a similar treatment routine. Standard regimens have the accompanying benefits over the individualized solution of medications:


### **12. Pharmaco-kinetic and pharmaco-dynamic contemplations for tuberculosis medications**

Pharmacokinetic (PK) and pharmacodynamics (PD) properties of a medicinal drug play a substantial role to propose its feasibility for medicinal purpose *In vivo* [47, 48]. Along with the PK/PD of any anti-tubercular drugs, medication also considers other factors like comorbid conditions, safety profile, oral bioavailability and metabolic strength [4, 10]. Oral administration is mostly preferred for advanced Tuberculosis medication whereas, oral bioavailability is critical to treat Tuberculosis [4, 46]. Solubility and gastrointestinal permeability are the two major factors that affect oral bioavailability. At present. Generally, the bioavailability of tablets Tuberculosis ranges from 40–90% and new drugs must show such property of bioavailability [2, 7]. The smaller successive dosing of drugs is suggested to improve the adhesion and recommend to have daily doses. An ideal TB medicine must transmit to the lungs, the site of the primary infection, and should have the

**19**

**14.1 SQ109**

*Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

treatment [5, 6].

**13. Target identification**

methodologies are as follows:

ability to infiltrate the granuloma to reach, such as intracellular and extracellular bacilli in the centre of hypoxia and undoubtedly necrotic region [9]. Preferably, the adhesion of drugs compounds in the target tissue must be maintained at a chosen site at minimal inhibitory conditions [49, 50]. This approach is used to avoid the phenomenon of drug binding to plasma protein, inhibition of tissue diffusion and improving the half-life of medicine. Lipophilic drugs have a major portion in antitubercular drugs. PK/PD and mode of action determines the dose of drugs for the

In terms of drug safety, an ideal drug for Tuberculosis should not show any acute toxicity or long duration for the treatment [47, 51]. Because of the global nature of Tuberculosis therapy, an excellent drug must not show drug–drug interaction with

With the entire genomic sequence available for *Mycobacterium tuberculosis*, the potentiality to explore new targets for the development of antibiotic throughout the *M. tuberculosis* genome became convenient [9, 10]. Novel chemical entities & targets are expected to avoid resistance to existing drugs and therefore improve current treatments. An ideal target for the development of antibiotic must necessarily

Genetic screens trials are the preliminary step in manifesting which genetic products might be targeted at chemotherapy against tuberculosis. Howsoever, all the necessary genes are not equally vulnerable to pharmacological action [20]. Besides, the target should also be available for competitive or chemical inhibition. That is, the target must have the ability to bind with another molecule rather than its substrate [10, 52, 53]. The inhibition or initiation of the protein function with a possible concentration of the low molecular weight compounds results in cellular breakdowns, such as cell death leading to apoptosis or attenuated growth [14, 46]. Besides being susceptible to chemical inhibition, an anti-target screen inhibitor should also produce drug-like compounds with specificity to affect target function

Various strategies have been developed by researchers and investigators and they proposed combined drugs for clinical trials after screening. All these drugs have a specific mode of actions but at the same time, they also showed some side effect which is a challenging task for investigators (**Table 2**). Currently, about 7 new combinations of drugs are under clinical trials. These lead combinations have been recognized by several methods and differential screening [10]. Few screening

A combinatorial library entirely based on 1,2-ethylenediamines such as Ethambutol was examined on two high-throughput in-vitro analysis. The first evaluation involves dilution of bouillon to calculate minimal inhibitory concentration (MIC) contrary to *Mycobacterium tuberculosis* [55]. The subsequent measurement is based on iniBAC promoter, inhibition of cell wall and bioluminescent assays for high-throughput screening [56]. SQ109 was determined on this screen. But the

other chemically or biologically active TB drugs within the regime [7, 22].

be in vivo, vulnerable to medicines and drug-effective [6].

in the absence of interference with any host orthologs [5, 54].

**14. Current status of tuberculosis drug discovery**

*Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

*Molecular Epidemiology Study of Mycobacterium Tuberculosis Complex*

Kills more than 90% of bacilli in first

Minimizes the emergence of drug

Isoniazid (H) Bactericidal after 24 hrs with a high potency.

Rifampicin (R) Bactericidal within 1 hrs with high potency.

Ethambutol (E) Bacteriostatic with a low potency.

resistance

Pyrazinamide (Z)

Streptomycin (S)

*Current drugs and their property.*

**Table 1.**

few days of treatment.

**Drug Drug property Acting pH Site of action**

Both alkaline and acidic medium

Both alkaline and acidic medium

Both alkaline and acidic medium

medium

Bactericidal with a low potency Acidic medium Intracellular bacilli

Both intracellular and extracellular

Both intracellular and extracellular

Both intracellular and extracellular

Extracellular bacilli

Normalized treatment implies that all patients in a characterized bunch get a similar treatment routine. Standard regimens have the accompanying benefits over

Bactericidal with a low potency Alkaline

i. errors in remedy – and in this way the danger of advancement of medication

ii. estimating drug needs, buying, circulation and checking are encouraged;

v.maintaining a regular drug supply when patients move to start with one

Pharmacokinetic (PK) and pharmacodynamics (PD) properties of a medicinal drug play a substantial role to propose its feasibility for medicinal purpose *In vivo* [47, 48]. Along with the PK/PD of any anti-tubercular drugs, medication also considers other factors like comorbid conditions, safety profile, oral bioavailability and metabolic strength [4, 10]. Oral administration is mostly preferred for advanced Tuberculosis medication whereas, oral bioavailability is critical to treat Tuberculosis [4, 46]. Solubility and gastrointestinal permeability are the two major factors that affect oral bioavailability. At present. Generally, the bioavailability of tablets Tuberculosis ranges from 40–90% and new drugs must show such property of bioavailability [2, 7]. The smaller successive dosing of drugs is suggested to improve the adhesion and recommend to have daily doses. An ideal TB medicine must transmit to the lungs, the site of the primary infection, and should have the

the individualized solution of medications:

opposition – are decreased;

iii. staff preparing is encouraged;

**for tuberculosis medications**

region then onto the next is made simpler;

vi.outcome assessment is helpful and results are tantamount.

**12. Pharmaco-kinetic and pharmaco-dynamic contemplations** 

iv. costs are decreased;

**18**

ability to infiltrate the granuloma to reach, such as intracellular and extracellular bacilli in the centre of hypoxia and undoubtedly necrotic region [9]. Preferably, the adhesion of drugs compounds in the target tissue must be maintained at a chosen site at minimal inhibitory conditions [49, 50]. This approach is used to avoid the phenomenon of drug binding to plasma protein, inhibition of tissue diffusion and improving the half-life of medicine. Lipophilic drugs have a major portion in antitubercular drugs. PK/PD and mode of action determines the dose of drugs for the treatment [5, 6].

In terms of drug safety, an ideal drug for Tuberculosis should not show any acute toxicity or long duration for the treatment [47, 51]. Because of the global nature of Tuberculosis therapy, an excellent drug must not show drug–drug interaction with other chemically or biologically active TB drugs within the regime [7, 22].

### **13. Target identification**

With the entire genomic sequence available for *Mycobacterium tuberculosis*, the potentiality to explore new targets for the development of antibiotic throughout the *M. tuberculosis* genome became convenient [9, 10]. Novel chemical entities & targets are expected to avoid resistance to existing drugs and therefore improve current treatments. An ideal target for the development of antibiotic must necessarily be in vivo, vulnerable to medicines and drug-effective [6].

Genetic screens trials are the preliminary step in manifesting which genetic products might be targeted at chemotherapy against tuberculosis. Howsoever, all the necessary genes are not equally vulnerable to pharmacological action [20]. Besides, the target should also be available for competitive or chemical inhibition. That is, the target must have the ability to bind with another molecule rather than its substrate [10, 52, 53]. The inhibition or initiation of the protein function with a possible concentration of the low molecular weight compounds results in cellular breakdowns, such as cell death leading to apoptosis or attenuated growth [14, 46]. Besides being susceptible to chemical inhibition, an anti-target screen inhibitor should also produce drug-like compounds with specificity to affect target function in the absence of interference with any host orthologs [5, 54].

### **14. Current status of tuberculosis drug discovery**

Various strategies have been developed by researchers and investigators and they proposed combined drugs for clinical trials after screening. All these drugs have a specific mode of actions but at the same time, they also showed some side effect which is a challenging task for investigators (**Table 2**). Currently, about 7 new combinations of drugs are under clinical trials. These lead combinations have been recognized by several methods and differential screening [10]. Few screening methodologies are as follows:

#### **14.1 SQ109**

A combinatorial library entirely based on 1,2-ethylenediamines such as Ethambutol was examined on two high-throughput in-vitro analysis. The first evaluation involves dilution of bouillon to calculate minimal inhibitory concentration (MIC) contrary to *Mycobacterium tuberculosis* [55]. The subsequent measurement is based on iniBAC promoter, inhibition of cell wall and bioluminescent assays for high-throughput screening [56]. SQ109 was determined on this screen. But the


#### **Table 2.**

*Current mode of therapy and therapeutic drugs for tuberculosis.*

mode of action and efficacy of SQ109 differ widely from ethambutol [57, 58]. SQ109 is bactericidal in nature and works by targeting a transmembrane transport protein MmpL3 which is responsible for transmitting trehalose monomicolate during cell wall synthesis [59, 60]. It acts against extracellular as well as intracellular bacilli and works on acute and chronic mouse models of tuberculosis infection [61]. SQ109 improved the pharmacological efficacy of the present four available first-line drugs against tuberculosis and represents synergy with Sirturo. It is presently under phase 2 clinical studies [5, 15].

#### **14.2 Q203**

It is an amide compound of imidazopyridine and was recognized by the whole-cell screening of infected macrophages [17]. Q203 prevents ATP synthesis via causing an

**21**

*Challenges in Drug Discovery against Tuberculosis DOI: http://dx.doi.org/10.5772/intechopen.97857*

**14.3 TBA-7371**

**14.4 OPC-167832**

**14.5 GSK-070**

tuberculosis infection [10, 63].

**14.6 PBTZ-169 & BTZ-043**

scientific studies [7, 9, 63].

interruption in the electron transport chain and thus also inhibits the cytochrome bc1 complex involved in the electron transport mechanism. Q203 possess an exceptional

A member of a series of 1,4-azaindole which was recognized by a strategy of transformation of scaffolds preceded by a program of optimization of lead of a compound imidazopyridine [62]. TBA-7371 inhibits DprE1 non-covalently, a decaprenyl phosphoryl-β-Dribose2′-epimerase, in cell wall Arabian biosynthetic pathway. TBA-7371 is bactericidal and is working against both acute and chronic mouse models of

It is a derivative of 3,4-dihydrocarostyril. OPC-167832 is bactericidal and works by targeting DprE1, leading to the prevention of mycobacterial infection. It represents improved performance when in combination with delamide. Presently it falls

It targets leucyl-tRNA synthetase and is an oxaborole derivative. Oxaborols block leucyl-t-RNA synthesis and ultimately results in blocking protein synthesis by constructing an adduct with t-RNA. It is active against both acute and chronic

They belong to benzothiazinones and were diagnosed from a broth dilution evaluation in vitro for the detection of antibacterial and antifungal activities. Benzothiazinones basically prevents the formation of arabinose involved in the biosynthesis of cell wall by covalently targeting DprE1. Both PBTZ-169 and BTZ-043 are bactericidal thus prevents bacterial replication and multidrug-resistant tuberculosis infection. They represent almost equal potency against isoniazid and rifampin in the mouse models of recurrent tuberculosis infection. PBTZ-169 is under phase 1

Recent emigration makes Tuberculosis very likely to reactivate. Vitamin D deficiency has the same effect because vitamin D is an immune modulator and deficiency of that weakens the immune system, thus protecting against tuberculosis [3, 9]. Another factor, HIV infection, which is present in 8% of patient cases of tuberculosis and this problem of HIV allowing TB to be reactive and become a problem is actually before the patient has become heavily immune-suppressed [64]. Smoking, diabetes and the elderly are all examples where the immune system has been weakened to a degree and allows the potential infection to take hold and cause a problem [22, 63]. Homelessness drug abuse, alcoholism and other immune suppression steroids after transplantation to mention corrosive tumor necrosis factor treatment, all make an individual more likely to reactivate latent disease, like tuberculosis [6]. The antibiotics being used for the TB treatment have also shown

**15. Risk factors associated with tuberculosis treatments**

Pharmacokinetic profile and prevents bacterial replication [2, 20].

tuberculosis infection. It is under phase 1 clinical studies [3, 46, 57].

under the category of phase 1 clinical studies [10, 15].

interruption in the electron transport chain and thus also inhibits the cytochrome bc1 complex involved in the electron transport mechanism. Q203 possess an exceptional Pharmacokinetic profile and prevents bacterial replication [2, 20].
