Design, Synthesis, and Evaluation of Pyrazinoic Acid-Derived Antituberculars for Drug-Resistant Mycobacterium tuberculosis

Abstract

Tuberculosis (TB), an infectious disease caused by the pathogen Mycobacterium tuberculosis (Mtb), is a major cause of suffering worldwide. The impact of this disease has been exacerbated by the ongoing coronavirus pandemic, reversing much recent progress that had increased diagnosis and treatment rates in the preceding decade. In addition to ongoing issues related to public health shortcomings and lack of access to treatment, the emergence and spread of resistant strains is an increasing cause for concern. While the antitubercular pipeline has produced a few new antibiotics in the 21st century, more novel treatments are urgently needed to keep abreast of resistant strains. This dissertation describes efforts to create new therapeutic options for resistant TB, centered around pyrazinoic acid (POA), the active form of pyrazinamide (PZA), an important first-line TB drug.Recently, a growing number of reports have highlighted the promise of β-lactam conjugates in selectively targeting resistant organisms. β-lactams are a widely-employed class of antibiotics that target cell wall biosynthesis. Bacteria can evade this activity through expression of β-lactamases, powerful enzymes that destroy the electrophilic β-lactam warhead. However, researchers have learned to take advantage of this resistance mechanism, designing β-lactam conjugates that release a molecular payload following β-lactamase cleavage. This strategy, referred to here as β-lactamase-mediated fragmentation, is explored in great detail in Chapter 1, including descriptions of its discovery and applications in a variety of fields, including diagnostics, cellular imaging, and antibiotic design. Chapter 2 describes our own work in this space, designing β-lactam conjugates bearing POA as a Mtb-selective warhead. This strategy circumvents the most common resistance mechanism against PZA, imparting activity in an Mtb macrophage infection model (where conventional β-lactams are typically ineffective). We also provide preliminary mechanistic evidence that our conjugates act as codrugs, achieving antibacterial activity through action of the β-lactam scaffold as well as the POA warhead. In Chapter 3, we remove the β-lactam scaffold and focus on POA itself, reporting a series of new analogues featuring substitutions on the pyrazine ring. We identify several analogues with improved activity over POA, and use biochemical techniques to demonstrate they are inhibitors of PanD, a putative target of POA. We use the structure of our most active lead and recent structural insights into PanD to design additional inhibitors with comparable antimycobacterial activity, providing proof-of-concept for future structure-based design of new PanD inhibitors.