Drug Repurposing against the PurS Subunit of Phosphoribosylformylglycinamidine Synthase in Methicillin- and Oxacillin-Resistant Staphylococcus Aureus: In vitro and In silico Studies.

INTRODUCTION

Staphylococcus aureus is the causative agent of mild to severe human infections. The exponential increase in resistance to available antibiotics has created the need to identify new drug targets and develop new drug candidates to combat bacterial infections. The PurS subunit of phosphoribosylformylglycinamidine synthase (synonym FGAM synthase) catalyzes the 4th step of the de novo purine biosynthesis in bacteria, including S. aureus. Purine nucleotides are essential in the bacterial life cycle and are required for growth and survival. Inhibition or disruption of FGAM synthase adversely affects the bacterial life cycle.

METHODS

This study reports cloning, expression, and purification (yield 16.08 mg/mL) of the PurS subunit of FGAM synthase from methicillin- and oxacillin-resistant Staphylococcus aureus, followed by interaction analysis of the PurS subunit with 50 US-FDAapproved drugs, using Saturation Transfer Difference (STD) NMR spectroscopy.

RESULTS

Among these drugs, tramadol HCl (1), nicotinamide (2), amoxicillin trihydrate (3), hydroxychloroquine sulphate (4), and phenylephrine HCl (5) showed interactions with the PurS subunit of FGAM synthase, using STD-NMR spectroscopy. All 50 drugs were also studied for their effects on thermal destabilization of the PurS subunit using differential scanning fluorimetry; among them, only nicotinamide (2) and drotaverine HCl (6) destabilized the PurS subunit of FGAM synthase. However, drug 6 interacted with the PurS subunit either irreversibly or with strong binding affinity, as no interactions in STD-NMR were observed. Therefore, drugs 1‒5, which showed interactions in STD-NMR, were further computationally studied, revealing comparable docking scores (-2.4 to 0.21) for complexes 1‒5 (PurS subunit, and drugs 1‒5), with substantial binding energies, supporting the experimental findings.

DISCUSSION: The current study focuses on FGAM synthase, a potential drug target in S. aureus. This is the first report on the identification of drugs that bind the PurS subunit of FGAM synthase using STD-NMR, DSF, and molecular docking. Drugs 1‒6 can be considered potential hits for further studies for drug discovery against infections caused by methicillin- and oxacillin-resistant S. aureus.

CONCLUSION

These five hits merit further biochemical and mechanistic studies to validate their therapeutic potential against methicillin- and oxacillin-resistant S. aureus infections.