MRC Innovation Grant.Multi-Targetting of tRNA synthetases: A paradigmshift in combating AMR

Antimicrobial resistance to existing antibiotics threats future healthcare at multiple levels and has been acknowledge as a worldwide issue with an impact as important as climate change. However, a number of factors has lead to a steady decline in the discovery and development of antimicrobials in the pharmaceutical industry despite the clear clinical need. The remaining pharmaceutical company antibacterial research and development in this area has focused generally in the past on either attempts to discovery new targets for antibiotics or new compounds with limited activity profiles based on existing targets. Generally this approach has met with very limited success which combined with other factors, means we have a decreasing number of drugs to treat bacterial infection leading to a crisis on healthcare.
It is clear that bacteria are adept at the selection of resistance to drugs for single gene targets (i.e. their protein products) and that the successful antimicrobial chemotherapy of the past targets activities where there are multiple essential activities e.g. protein synthesis at the ribosome, the proteins which are responsible for DNA supercoiling in the cell and mechanisms by which cross-linking of the bacterial cell wall is achieved. We have identified another area of bacterial metabolism which has these similar properties and if correctly targeted may provide an avenue for next generation antimicrobial discovery.
Protein synthesis, itself a golden area for past antimicrobial discovery as described above, is reliant upon the delivery of single amino acids to the ribosome which are activated by chemical ligation to transfer (t)RNA molecules. Each amino acid has its own synthetase but some of these enzymes lack the ability to select the correct amino acid initially and are reliant on other systems to produce the correct amino acid-tRNA liganded product. It is this selection procedure, which we can circumvent with potential drugs and thus produce a new line of antimicrobial chemotherapy this is less likely to be overcome by resistance through mutation since these potential drugs would require mutations in multiple genes. Whilst there is a great deal of existing biochemical data to draw upon in this field we lack the three-dimensional structures of some which are required for the next stage of this discovery process. The goal of this project will be to produce these data.

Recent antibacterial R&D to discover new lead compounds has primarily focused on the search for inhibitors of essential single gene targets. However consideration of historically successful antibiotic targets suggest that multi-targeting is more successful and should lead to lower resistance rates as a result of mutation in those targets. Whilst amino acyl tRNA synthetases are essential enzymes in protein synthesis, itself a validated historical target, relatively few successful drugs have been developed. Overlooked in this space thus far is the fact that several synthetases have overlapping amino acid specificity and require editing systems to ensure the fidelity of their amino acyl-tRNA product that is essential for protein synthesis. Work leading to this application demonstrates that a focus on several selected pair of synthetase enzyme may lead to a multi-targeting opportunity that could yield new lead compounds for drug discovery. This is because resistance mutations in multiple genes would be required in a crucial aspect of their function, which would be difficult to achieve.
To explore this approach we have identified a critical lack of X-ray structural information on several key bacterial tRNA synthetases representing Gram-positive and Gram-negative bacteria of significance to combating antimicrobial resistance. Thus our project will focus on obtaining this information in liganded forms to complement the high throughput assay we have already developed suitable for drug discovery. This data will complement the limited information available in the protein databank already and rapidly accelerate in-silico efforts. To this end we have engaged the services of Dr Lloyd Czaplewski as a consultant to this project to provide industrial structure-based drug design and in silico screening antibacterial experience and to help preparation for future applications to MRC, WT and elsewhere as well as to aid project engagement with industry.

Who will benefit from this research?
Currently antimicrobial resistance threatens virtually every aspect of healthcare and especially those surrounding infectious disease and surgical intervention. Since our ability to combat infectious organisms is declining due to lack of investment and innovation in the private sector accompanied with poor prescribing and use of existing drugs leading to resistance, then the discovery of a new avenue of intervention may have profound societal consequences effecting public and third sector.
The provision of the structures identified in this proposal will provide the necessary background information for in silco design of novel duel inhibitor molecules targeting a relatively unexplored aspect of an established target. Through future iterative cycles of combinatorial chemistry these chemical leads may progress to the point where they become useful lead compounds with the properties required to be used as antimicrobial drugs. The provision of the X-ray crystal structures is a key issue in the in silico design process and will be invaluable with future characterization efforts. During this translational phase, commercial, and /or private sector pharma companies will be required to advance the lead compounds requiring expertise, facilities and techniques beyond the academic environment. If successful, this research project may be transformational in the search for novel antimicrobial drugs and could represent a step change in our ability to combat antimicrobial resistance. Thus the effect of this research could be profound, wide ranging in society from individuals undergoing treatment to government policy in the area of infectious bacterial disease.
In the first instance the applicants and workers on this proposal will benefit from first hand experience and knowledge discovery stemming from the research proposal and its outputs. This knowledge transfer extends to other associated with the project including undergraduate and postgraduate students. More widely, the output of our research may have benefit directly to those treated with drugs developed from this programme in the long term as well as contributing to the development of a UK bioscience agenda and infrastructure for UK health, wealth and knowledge based economy.

How will they benefit from this research?
The discovery of new strategy's and targets for antimicrobial drug development could have profound influence on the nations health through the direct application of new drugs stemming from this research to treat infections disease. It is absolutely clear that some infections are becoming difficult, if not impossible to treat leading to increased morbidity and mortality. Hospital acquired infections in particular lead to the premature death of citizens who would otherwise have much to contribute to society. Treating these patients is a significant cost to health care services and places strain on all concerned particularly society supporting the affected individuals, their relatives and wider communities.
Although the development of new antimicrobials takes in excess of 10 years at present and represent multimillion investments for the companies involved, there is a clear unmet medical need for these drugs and new treatments are urgently needed to enter the developmental pipeline. Academic-Industry interaction is a key part of this process, particularly in early stage discovery. This project may be a pivotal step in the process and overcomes a clearly defined scientific barrier in the process and contribute to the (re) generation of antimicrobial drug development in the UK where we have a long history of development.