**Balachandra Bandodkar 1, Radha Krishan Shandil 2, Jagadeesh Bhat <sup>3</sup> and Tanjore S. Balganesh 3,\***


Received: 3 May 2020; Accepted: 11 August 2020; Published: 17 August 2020

**Abstract:** After several years of limited success, an effective regimen for the treatment of both drug-sensitive and multiple-drug-resistant tuberculosis is in place. However, this success is still incomplete, as we need several more novel combinations to treat extensively drug-resistant tuberculosis, as well newer emerging resistance. Additionally, the goal of a shortened therapy continues to evade us. A systematic analysis of the tuberculosis drug discovery approaches employed over the last two decades shows that the lead identification path has been largely influenced by the improved understanding of the biology of the pathogen *Mycobacterium tuberculosis*. Interestingly, the drug discovery efforts can be grouped into a few defined approaches that predominated over a period of time. This review delineates the key drivers during each of these periods. While doing so, the author's experiences at AstraZeneca R&D, Bangalore, India, on the discovery of new antimycobacterial candidate drugs are used to exemplify the concept. Finally, the review also discusses the value of validated targets, promiscuous targets, the current anti-TB pipeline, the gaps in it, and the possible way forward.

**Keywords:** tuberculosis; *Mycobacterium tuberculosis*; drug discovery; drug development; target-based screening; phenotypic screening; antituberculosis agents; antimycobacterial; anti-TB drug pipeline; privileged targets; promiscuous targets; lead generation

Chemotherapy for the treatment of tuberculosis has evolved over a period of several decades, starting from the 1950s. The discovery of drugs with superior effectiveness that are part of the current regimen has been facilitated through the success of several novel approaches and technologies. While the medical need has at times hastened newer approaches to be adopted, the main driver for the improvement has been the increased understanding of the biology of the pathogen, as well as its interaction with the human host. In this review, we try to discuss the discovery of new drugs in groups, with the groups sharing a common key driver that precipitated the changes of the treatment regimen. As 'newer aspects of the biology' of the pathogen became known, they provided opportunities to build novel drug discovery approaches. The paper also uses as examples the approaches, the results, and the learning gathered as a part of the antituberculosis (anti-TB) program at Astra Zeneca, R and D, Bangalore, India (AZI).

Several of the drugs that have been discovered are based on the learning that followed the introduction of each of the new compounds into the drug regimen. The data have several pointers; while each of the successful drugs was discovered building on the then state of the knowledge, the newer drugs themselves also helped in the further understanding of the pathogen biology. This is even reflected in the current set of drugs that are in late-stage development or have been recently introduced into the anti-TB regimen.

The anti-TB drugs currently in use, and those in the late stages of clinical development, can be broadly pooled into the following groups:


#### **1. Serendipitous Drug Discovery: Early Chemotherapy**

The first successful chemotherapy and cure of an infectious disease is indeed the discovery and design of the 'first line' therapy for the treatment of tuberculosis—the design and development of which was completed in the 1960s. Even today, this regimen is the therapy of choice for treating drug-sensitive tuberculosis (DSTB). The drugs in the first-line treatment for DSTB, isoniazid, rifampicin, pyrazinamide, and ethambutol became anti-TB drugs based on their activity on *Mycobacterium tuberculosis* (MTB) cells in vitro, followed by testing in animal models and their rapid introduction into humans [1]. This progression was driven by the medical need, as no chemotherapy existed before these drugs were discovered.

It is interesting to note that two biological observations and a hypothesis on potential 'chemical structures' that may interfere with the observed biological process were the first starting points of anti-TB drug discovery. Aspirin was shown to be a potent stimulator of the TB bacilli's 'oxygen consumption'; analogs of aspirin were then postulated to be inhibitors of this process. This led to the synthesis of a number of aspirin-like structures, of which para-aminosalicylic acid (PAS) became a successful anti-TB drug [1–3]. The second observation was that niacin helped in the recovery of guinea pigs infected with MTB, as well as the observation that niacin helped in faster recovery of TB patients, raising the possibility that niacin was acting as a 'vitamin' [4,5]. Chemical synthesis focused on making derivatives of niacin led to the design of three anti-TB drugs, namely isoniazid (Inh), pyrazinamide (Pza) and ethionamide (Eth) [6]. Two of these are even today the most potent drugs for the treatment of TB. This early period of chemotherapy also included extensive search for natural products with antibacterial activity; Rifamycin and Streptomycin were natural products that showed potent activity against MTB cells and were also introduced into the treatment of TB.

In 1979, Mitchison observed that 10 to 12 drugs were available for the treatment of tuberculosis, which could be classified in terms of their effectiveness [7]. The choice of combinations was dictated by animal toxicity of the individual drugs and in human trials, which assessed time taken to the sputum negative state, cure as reflected by relapse rates, the emergence of resistant strains, and compliance. Once an effective combination had been proven, it became to be referred to as the 'Short Course Chemotherapy' and was adopted systematically all over the world [8]. This was the first successful conquering of an infectious disease. In about 20 years, TB patients went from complete helplessness to an effective cure achieved with drug treatment.

The key learning from this pioneering era was as follows:


The cure was defined as the lack of relapse.

The last two points remain, to date, the biggest challenge in finding and developing novel combinations. The era of 1950 to 1980 had 10 drugs, of which the most efficacious combination of four drugs was identified through rapid testing in humans. In spite of more than two decades of sustained research into the biology of MTB, the traits of a new drug that could contribute to both the 'cure' and the shortening of therapy are still unknown.
