Prospects for Aminoacyl-tRNA Synthetase Inhibitors as New Antimicrobial Agents

The dramatic rise in the prevalence of antibiotic resistance among bacteria currently poses a serious threat to public health worldwide. Of particular concern are infections caused by methicillin-resistant Staphylococcus aureus (MRSA), penicillin-resistant Streptococcus pneumoniae, vancomycin-resistant Enterococcus (62), and Mycobacterium tuberculosis (60), since many of these organisms are resistant to several classes of established antibiotics (60, 62). This situation is driving the search for novel antibacterial agents that inhibit targets that are essential to bacteria and that are not affected by mechanisms of resistance to current chemotherapeutic agents (18, 19). In this regard, the aminoacyl-tRNA synthetase (AaRS) enzymes have been a focus of recent research for antibacterial drug discovery. These enzymes play crucial roles in protein biosynthesis by catalyzing the synthesis of aminoacyl-tRNAs (aa-rRNA). Once these enzymes are inhibited, protein biosynthesis is halted, which in turn results in the attenuation of bacterial growth under both in vitro and infectious conditions (80). Consequently, these enzymes are interesting antibacterial drug targets. An important example of the clinical application of an AaRS inhibitor is provided by the antibiotic mupirocin (marketed as Bactroban), which selectively inactivates bacterial isoleucyl-tRNA synthetase (IleRS). This product is currently the world's most widely used topical antibiotic for the control of MRSA (12). In recent years, reports of compounds (both natural and synthetic) which inhibit AaRS have increased. While many of these new inhibitors also affect the counterpart human enzymes, there has been some success in identifying molecules that are specific for bacterial AaRS enzymes and that also exhibit antibacterial activities against several species in experimental infections (37, 54, 71, 75). However, mupirocin is currently the only clinically available AaRS inhibitor and therefore acts as the paradigm for the prospective clinical development and deployment of future AaRS inhibitors. Several suggestions for the prioritization of targets in the current process of antibiotic discovery and development have been made (63). The design of antibiotics which minimize the potential for the subsequent emergence of resistance is paramount (23, 32, 33, 63, 78). An opportunity to minimize drug-resistant organisms occurs when resistance emergence is accompanied by substantial reductions in the biological fitness of bacteria (3, 27, 59). Consequently, it is perceived that unfit drug-resistant mutants would be unable to survive upon withdrawal of an antibacterial drug and that policies involving antibiotic cycling would provide a means of eliminating drug-resistant mutants from natural populations (3, 27, 51, 58, 59). However, drug targets for which this approach may be appropriate have not yet been clearly identified, since in most cases compensatory evolution restores the fitness of drug-resistant mutants without the concomitant loss of resistance (2, 3). Nevertheless, recent novel findings in our laboratory suggest that AaRS could represent therapeutic targets where the loss of fitness associated with resistance could be compatible with antimicrobial restriction policies to eliminate resistance (46, 48). Another opportunity to search for new drugs with reduced potential to select drug-resistant variants concerns the identification of novel agents with the capacity for multiple-target inhibition (23, 63, 76). Simultaneous inhibition of more than one molecular target renders the emergence of resistance less likely because mutations are required in all targets to confer resistance to the drug (23). The β-lactam antibiotics provide an excellent example to support this contention (76), since they are multisite inhibitors which target bacterial penicillin binding proteins (PBPs). Resistance to β-lactam antibiotics rarely results from target site modification and is more commonly due to expression of β-lactamases, efflux mechanisms, or β-lactam-resistant PBPs acquired by horizontal gene transfer (76). Approaches that have been considered or that are currently under investigation include dual or multitargeting of DNA topoisomerase IV and DNA gyrase (78); cell wall biosynthetic ATP-dependent amino acid ligases (33); MurA and AroA (32); DNA gyrase and dihydropteroate synthetase (1); and DNA gyrase, DNA topoisomerase IV, and rRNA (43). Due to the existence of homologous sequences in phylogenetically related synthetases (Table ​(Table1),1), there is an important opportunity to develop single molecules which simultaneously inhibit multiple AaRS enzymes. These molecules could prove essential in limiting the emergence of both chromosomally and horizontally acquired forms of resistance to AaRS inhibitors. TABLE 1. Structural classification of aminoacyl-tRNA synthetase (49) In this minireview, we examine the prospects for the development of several recently reported AaRS inhibitors as chemo-therapeutic agents. The chemistry of these inhibitors will not be described in detail, as comprehensive reviews of these compounds from chemical perspectives have already been published (54, 71). Consequently, we examine the modes of action and potential mechanisms of clinical resistance to these compounds. We also discuss ways in which novel strategies aimed at reducing the emergence of resistance to these compounds can be exploited.