ReviewMolecular mechanisms of action, resistance, detection to the first-line anti tuberculosis drugs: Rifampicin and pyrazinamide in the post whole genome sequencing era
Introduction
Despite tuberculosis (TB) being a curable disease, it continues to be a major public health problem, with a death toll of 1.8 million and infection rate of 10.4 million people (including 0.4 million people with HIV), according to the latest estimates [1]. Over 95% of TB deaths occur in low- and middle-income countries. Short course chemotherapy (SCC), the back-bone of anti-tubercular chemotherapy, consists of an initial intensive phase of treatment for two-months and a continuation phase of four months. The intensive phase regimen includes rifampicin (RIF), isoniazid (INH) and pyrazinamide (PZA), plus a fourth drug, Ethambutol (EMB). The continuation phase comprises of a choice of several options including INH and RIF for four months, or alternatively, INH and EMB or thiacetazone for six months [2]. The current 6–9 months long treatment schedule has several side effects resulting in poor adherence to therapy that eventually leads to drug-resistance. The evolution and transmission of drug-resistant (DR) Mycobacterium tuberculosis (MTB) threatens the success of TB treatment and control programs. Although tremendous success has been achieved in unraveling the molecular mechanisms of drug resistance, the need for rapid and accurate diagnostics for drug resistance in TB remains a critical factor that is hindering progress towards the millennium development goals for TB control [3]. Recent studies of whole genome sequencing (WGS) of MTB clearly indicate that there is still much to be understood (4). A WGS study from Russia demonstrated that the phenomenon of drug resistance may be more multi-factorial, which may explain discordance between the phenotype and genotype observed in some cases [5]. Elucidation of the mechanisms underlying emergence of drug resistance is crucial for the design of novel strategies to detect and overcome resistance.
Drug resistance in TB is believed to be mediated exclusively by chromosomal mutations, which affect either the drug target itself or bacterial enzymes that activate pro-drugs. Since the early 1990s, many studies have described the genetic mechanisms of drug resistance in MTB and much data has accumulated on the type of mutations found in isolates resistant to specific drugs [6], [7]. However, recent studies including modern methodologies like WGS have provided novel insights into the mechanisms of resistance [4], [5], [8], [9], [10]. Most significant amongst drug resistance is the phenomenon of multidrug resistance (MDR), defined as resistance to the two most potent anti-TB drugs, RIF and INH, and extensive drug resistance (XDR) defined as MDR plus additional resistance to second-line anti-TB drugs such as fluoroquinolones and at least one of the injectable drugs, capreomycin, kanamycin or amikacin. Globally, 480,000 people are estimated to have developed MDR-TB and about 170,000 MDR-TB deaths are estimated to have occurred in 2015 [1]. It is estimated that about 9.5% of MDR-TB cases had XDR-TB [1].
Resistance to RIF is widely considered as surrogate marker for MDR-TB [11]. According to the 2015 estimates, around 100,000 people had developed resistance to RIF [1]. Of note, more than half of all MDR-TB cases were also found to be resistant to PZA. Lately, it has been recognized that PZA resistance has become ubiquitous, with an estimated one in six incident TB cases having PZA resistance. The estimated global burden of new PZA resistant TB cases annually is 1.4 million; about 270,000 MDR-TB patients also have PZA resistance [12]. The present review focuses on the latest developments in our understanding on the molecular mechanisms of resistance to RIF and PZA as well as their detection in the era of WGS. It is alarming to note the rapid evolution of newer mutations associated with RIF resistance (mutations in rpoA and rpoC genes) and PZA resistance (mutations in rpsA, panD, mas and ppsA-E genes) [4], [5], [8], [9], [10], [12], [13], [14], [15], [16], [17]. A thorough understanding of all mutations associated with resistance to the first-line anti-TB drugs and the molecular mechanisms of action is critical for accurate assessment and early detection of resistance, particularly in countries with high prevalence of MDR-TB. This understanding will not only help in the development of simple, rapid and low cost diagnostics for drug resistance, but also throw light on transmission dynamics of MDR-TB in the population.
Section snippets
Rifampicin
RIF, a semisynthetic derivative of rifamycin, was introduced as an anti-TB drug in 1972. It is extremely effective against MTB with a minimum inhibitory concentration (MIC) of 0.1–0.2 μg/ml [18]. Owing to its high sterilizing activity, RIF, in conjunction with INH and PZA, has formed the backbone of short course chemotherapy [19] (Fig. 1).
Pyrazinamide
The use of PZA as an anti-TB drug began in 1980. Ever since, it has remained an important drug in anti-TB therapy, and has played a unique role in shortening treatment duration from a period of 9–12 months to 6 months, as it kills a population of semi-dormant tubercle bacilli residing in an acidic environment that is not suitable for the action of other anti-TB drugs [62].
Summary
The focus of the present review was on the molecular mechanisms of resistance to two important first-line anti-TB drugs, namely, RIF and PZA. With respect to RIF, mutations in rpoB gene are responsible for changes in RpoB activity. Of note, the rpoA and rpoC gene mutations serve as compensatory mutations for mutations in rpoB. Secondly, the significance of ponA1 in RIF resistance has also been recently acknowledged. In the case of PZA, the well-known mechanism of PZA resistance is principally
Conclusions
The publication of the WGS of MTB in 1998 [114] was one of the most significant hallmark in the history of DR-MTB, which has accelerated the pace of detection of drug resistance at the molecular level. This has led to an enormous amount of development and advancement in the field of molecular diagnosis of DR-TB during the last decade. There have several studies on WGS of MTB from different geographical including China, Russia, Pakistan, Myanmar and others [4], [5], [8], [9], [10], [111] in the
Transparency declarations
None to declare.
Author contributions
ANU Conceived and designed the study, searched, extracted and analyzed data and drafted the review manuscript and LEH Revised the manuscript critically.
Acknowledgements
Dr. A. Nusrath Unissa received financial support for post doctoral fellowship No. 3/1/3/PDF(8)/2013 from the Indian Council of Medical Research.
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