Tuberculosis: New Drug Discovery Pipelines

Table 3:  Antitubercular Drugs in Pipeline.

Moxifloxacin & Gatifloxacin [32] Being in Phase III clinical trials, both moxifloxacin and gatifloxacin are the most advanced compounds under development for TB. They are both 8-methoxyquinolones originally developed as broad-spectrum antibiotics and now being repurposed as anti-TB agents. Fluoroquinolones target gyrase and topoisomerase IV in most bacteria but in Mtb it is assumed they solely target gyrase since there is no evidence of topoisomerase IV present in Mtb. Other quinolones, such as ciprofloxacin and ofloxacin, have been used as second-line TB drugs, but moxifloxacin and gatifloxacin are more potent in vitro than these older quinolones. Currently they both are in Phase III clinical trials. Moxifloxacin is being investigated by a large number of organizations while Gatifloxacin is sponsored by Oflotub consortium. The potential adverse effects that have been reported for these drugs are dysglycemia with Gatifloxacin and QT prolongation with Moxifloxacin. In some regions of the world, there is a fairly high incidence of quinolone resistance, (e.g, in India), and use of either of these compounds in TB regimens might prove to be limited in such circumstances. DC-159a a 8-methoxyfluoroquinolone, possessing three chiral centers, was originally developed as a broad-spectrum antibiotic. However, its potential use in TB patients was investigated by Daiichi-Sankyo. It was shown to be fourfold more potent in vitro than moxifloxacin and gatifloxacin against Mtb. In mouse efficacy studies, it was shown to be more potent than moxifloxacin in the rapidly growing phase as well as in the non- (or slow-) replicating phase. The results seem to warrant its study as part of drug combinations in animal models of TB [33]. The mechanism of action of DC-159a is still under investigation, but as other quinolone derivatives, DC-159a probably affects GyrA activity, which plays important roles in DNA replication. This compound is still in the preclinical development stage [34].

Diarylquinolines

Bedaquiline: (Brand name Sirturo, also known as TMC207). Bedaquiline, the first new TB drug from a new drug class to receive approval in over four decades, has advanced little since its FDA approval in 2012. Bedaquiline (TMC207) is a lead compound in the diarylquinoline series originally discovered by scientists at Janssen Pharmaceutica and is currently undergoing Phase II clinical development for MDR-TB indications. TMC207 was discovered via whole-cell assay screening of approximately 70,000 compounds from Johnson & Johnson’s (Janssen’s parent company) corporate library against the surrogate organism, Mycobacterium smegmatis and it is currently clinically developed by Tibotec, a subsidiary of Johnson & Johnson in collaboration with the TB alliance [35]. TMC207 inhibits the proton transfer chain of the mycobacterial ATP synthase [36,37]. TMC207 is not active on human mitochondrial ATP synthase [38]. Its mode of action is unique i.e it inhibits Mtb ATP synthesis, by interacting with subunit c of its ATP synthase. Subunit c forms an oligomeric structure AtpE of the transmembrane portion of ATP synthase (F₀). This compound is effective against dormant Mtb organisms, even though ATP biosynthesis is significantly down-regulated in these bacteria.

Oxazolidinones: As additional drugs are urgently needed to accompany bedaquiline and delamanid, researchers and clinicians are increasingly interested in three drugs: linezolid—a drug approved for other bacterial infections and used off-label to treat difficult cases of drug-resistant TB and its new chemical relatives, sutezolid and AZD5847. Yet data to support linezolid’s clinical efficacy and safety remain limited. The oxazolidinones are a promising new class of synthetic antimicrobial agents with a unique mechanism of action in inhibiting protein synthesis. They display bacteriostatic activity against many important human pathogens including drug-resistant microbes.

Linezolid [39-42] was approved in 2000 to treat drug-resistant, gram-positive bacteria. It was introduced in the USA. It represents the oxazolidinone class of antibiotics, is one of only two new antibiotics introduced into the market in the past 40 years (the other being daptomycin, representing lipopeptide antibiotics), with the main target pathogens being Gram-positive bacteria including MRSA and vancomycin-resistant Enterococci). This compound has MIC of 0.3 to 1.25 mg/mL against Mtb. It has also been shown that linezolid is active against Mtb in vitro and in animal models. Its target is the Mtb ribosome and it exhibits potent activity against MDR-TB clinical isolates. It binds to the 23S RNA in the 50S ribosomal subunit and limits the growth of bacteria by disrupting its production of proteins in the first step of the synthesis by inhibiting formation of the initiation complex. Linezolid has been used for MDR-TB (and XDR-TB) and additional Phase IIb clinical trials are completed. Linezolid treatment course lasts upto 28 days.

PNU-100480 (Sutezolid) by Pfizer, was one of the earliest oxazolidinone antibiotics synthesized, exhibiting increased antimycobacterial activity compared with linezolid, but its potential as anti-TB agent has been recognized and pursued recently. In vitro and mouse models suggest that sutezolid may be more active than linezolid against TB. As linezolid has serious side effects, including optic and peripheral neuropathy and anemia, safer oxazolidinones are needed. The use of linezolid is limited by adverse effects that occur with long term administrations. Therefore, new analogues showing identical or better in vivo activities and a better therapeutic index would be useful. The development of PNU-100480 (Sutezolid), a close structural analogue of linezolid was initiated [43]. Sutezolid is of great interest to the TB research community, Pfizer has only completed two phase I and one phase IIa clinical trials, in addition to preclinical work.

AZD5847 [44,45] by Astra Zeneca, was originally intended as a broad-spectrum antibiotic, but has now been repurposed as an anti- TB agent. It is well recognized that with linezolid, treatment periods longer than 14 days may result in hematological adverse effects and since treatment for TB is considerably longer than 14 days, the degree and severity of this off-target activity with the next-generation oxazolidinone agents will be the key for their development. The compound has completed phase IIa trials is likely to be advancing to next phase.

Nitroimidazoles

Delamanid (Brand name Deltyba, formely known as OPC- 67683) an Otsuka’s compound, a nitroimidazole, shows great promise in treating MDR-TB. Delamanid became the second new drug to receive regulatory approval to treat MDR-TB in Europe in 2014. Delamanid (OPC-67683) is currently under development by Otsuka Pharmaceutical Company [46]. This compound contains a fused oxazoline ring instead of an oxazine ring. After treatment with M. bovis, des-nitro-imidazooxazole was obtained [47]. It inhibits methoxy-mycolic and keto-mycolic acid biosyntheses with corresponding IC₅₀values equal to 0.036 mg/mL and 0.021 mg/mL. A large number of analogues have been prepared by varying the tail portion of the molecule. OPC-67683 is 4 to 16 times more potent than another analog, PA-824 in vitro and it does not show cross-resistance with other first-line TB drugs. It is currently in Phase III clinical trials for MDR-TB patients. No data exist on whether bedaquiline and delamanid can be used together safely to improve TB treatment regimens. Delamanid appears generally safe, although it does cause mild-to-moderate QT prolongation. Delamanid is being tested for administration twice daily at 100 mg for the first two months of treatment, and once daily at 200 mg for the following four months. Delamanid’s pediatric formulation of small, dissolvable tablets is complete and a pediatric study has begun in the Philippines.

PA-824 like delamanid is a nitroimidazole. The TB Alliance is developing PA-824 for both drug-sensitive and drug-resistant TB in its novel combination studies [48]. It is currently in Phase III clinical trials conducted by the TB Alliance and following 14-day dosing in drug sensitive TB patients, PA-824 (at dose levels of 200, 600, 1000 and 1200 mg/day) demonstrated bactericidal activity similar to that demonstrated by the standard first-line drug combination. In terms of SAR, the nitro group is essential for anti-TB activity. In addition to the oxazine ring, other six-membered heterocycles were prepared but the anti-TB activity remains essentially only with an oxazine ring and, to a lesser degree, with a thiazine ring. A diverse range of ‘tail’ variations have been synthesized and some of them exhibit increased in vitro and in vivo potency, and are currently in the preclinical stage of development. Two-electron reduction actually takes place not at the nitro group but at the imidazole ring and it has been hypothesized that the resulting intermediates generate reactive nitrogen species, including nitric oxide [49,50]. Under anaerobic conditions, a correlation between the amount of des-nitro intermediates and cell killing suggests this as the bactericidal mechanism under these conditions. However, under aerobic conditions the exact mechanism is not known although inhibition of mycolic acid biosynthesis may be involved. The apparently multiple mode of action for this class of compounds render optimization of their anti- TB activity rather challenging. At present, a combination therapy including PA-824, moxifloxacin and pyrazinamide is being evaluated in TB patients [51,52].

Ethylenediamines (Ethambutol analogues)

Ethambutol, one of the main drugs used in TB-treatment regimens presumably, interferes with construction of the arabinogalactan layer of the mycobacterial cell wall. It’s simple and symmetrical chemical structure offers an attractive and symmetrical N,N’ -disubstituted ethylenediamine scaffold for parallel and combinational high throughput synthesis of many of its analogs [53].

SQ109 is an ethylenediamine analogue of ethambutol prepared through combinatorial chemistry. SQ109 appears to act by inhibiting the MmpL3 protein where as ethambutol is known to inhibit the synthesis of arabinogalactan, which is a component of the Mtb cell wall. Though, SQ109 is implicated in affecting cell wall biosynthesis, but it is also active against ethambutol-resistant strains and its precise mode of action has not been elucidated and is potentially novel. SQ109 is shown to over produce the ATP-dependent DNA/RNA helicase and to reduce the production of the β-ketoacyl-acyl carrier protein synthase which may explain its action on mycobacterial cell wall synthesis. It produces a remarkable synergy in vitro and in animal models when combined with rifampin, isoniazid, or TMC207 [54–57]. Relatively low bioavailability might be a problem [58], but Phase IIb/ Phase III studies have been completed, successfully.

Benzothiazinones

BTZ043 is a new class of sulphur containing heterocycle belonging to the benzothiazinones (BTZ) which has been recently described as a potent antimycobacterial agent [59]. The structure activity relationships study showed that sulphur atom and one or two nitro groups on the aromatic structure are required to inhibit bacterial growth in vitro. Compound, BTZ038, has been found to be the most active of the series. Although still in the preclinical stage, BZT043 is an extremely potent new class of antimycobacterial agent (MIC against Mtb = 0.004 μg/ ml) [60]. In vivo this compound at 37.5 mg/kg reduced the bacterial burden by 1 and 2 logs in the lungs and spleen, respectively. Subsequently by sequencing the mutant strains, its target was determined to be the enzyme decaprenylphosphoryl-β- D-ribose 2’’-epimerase (DprE1), which is involved in the biosynthesis of cell-wall component, arabinogalactan. The proposed mechanism involves reduction of the nitro group to a nitroso residue, which reacts with the cysteine group of DprE1.

Future Directions

As many second-line anti-TB drugs show adverse effects (Table 2), an improved TB vaccine regimen is essential to achieving effective global TB control. A number of vaccines are now in clinical trials. However, their further progression to advanced stages of these trials also depends upon the further development of field sites. According to a track report by WHO, globally by 2012, the TB mortality rate had been reduced by 45% since 1990. The target to reduce deaths globally by 50% by 2015 is within reach. Five priority actions required to accelerate progress towards 2015 targets are;

• Reach the missed cases
• Address MDR-TB as a public health crisis
• Accelerate the response to TB/HIV
• Increase financing to close all resource gaps
• Ensure rapid uptake of innovations

Conclusion

The control of TB in humans is heavily reliant on short course chemotherapy. However, this approach is increasingly challenged by widespread multi- and extensively drug resistant strains of Mtb. New druggable targets and novel leads are required for TB drug discovery to develop compounds with greater potency that is less prone to acquired drug resistance. TB drug development has undoubtedly advanced, but progress is slow, the number of new compounds limited, knowledge insufficient to dramatically improve cure rates, reduce treatment duration and make treatment more tolerable. This review highlights mycobacterial pathogenesis, the problem of resistant forms of the disease, current anti-TB drug research and development pipeline.

References