Fluoroquinolones are a class of antibiotics used clinically to treat a wide array of bacterial infections. These therapeutics act by targeting a bacterial enzyme required for cell viability, bacterial type-II topoisomerases. Fluoroquinolones act by forming a ternary complex with bacterial type II topoisomerases and cleaved DNA; religation of DNA is subsequently blocked, therefore leading to bacterial cell death. In ternary complex the keto-acid moiety of the fluoroquinolone is complexed with a divalent magnesium ion, forming a drug-magnesium-water bridge to a serine and an aspartate (or glutamate) residue on helix-4 of the topoisomerase enzyme. A major issue with fluoroquinolones is the rise in bacterial resistance. Resistance arises through substitutions of the serine or aspartate/glutamate residue, therefore preventing formation of the magnesium-water bridge and dramatically diminishing the overall antibiotic activity of the fluoroquinolone.
Quinazoline-2,4-diones are structurally similar to fluoroquinolones; diones also form a ternary complex similar to fluoroquinolones, however, these complexes are less active due to lack of a potent magnesium-water bridge interaction in helix-4. While quinazoline-2,4-diones are therefore less potent antibiotics, their non-reliance on the magnesium water bridge generally affords equipotent activity with wild-type and fluoroquinolone-resistant strains of bacteria.
The first objective of this work was to probe the helix-4 interaction of the bacterial type-II topoisomerase by quinazoline-2,4-dione modification, specifically at the N3 and C4 positions of the quinazoline-2,4-dione scaffold to afford potentially new binding contacts. These modified quinazoline-2,4-diones will provide deeper understanding of the helix-4 interaction and potentially afford potent novel quinazoline-2,4-dione scaffolds, against both wild-type and resistant bacteria, for iterative drug design.
Metabolism is one of the primary sources of detoxification, inactivation, and clearance of drugs from the body and is a critical consideration for all early stage therapeutic development. Clinically used fluoroquinolones, i.e. Moxifloxacin and Ciprofloxacin, historically are metabolically stable, and are not known to be metabolized by Phase I and/or Phase II drug metabolizing enzymes. However, major modifications to the Moxifloxacin and Ciprofloxacin scaffolds, due to the development of next generation antibiotics, may display different metabolic stability profiles. Moreover, metabolism of quinazoline-2,4-diones, developed for fluoroquinolone-resistant bacteria, is not extensively studied and may be subject to different metabolic liabilities that may render the quinazoline-2,4-dione an ineffective potential antibiotic.
The second objective of this work was to determine the in vitro Phase I and Phase II metabolic stabilities of fluoroquinolone and quinazoline-2,4-dione scaffolds to determine any structural features that render the potential therapeutic a metabolic liability.
The results from these two objectives have led to the discovery of a novel bacterial type-II topoisomerase catalytic inhibitor and the acquisition of initial metabolic stability data of fluoroquinolone and quinazoline-2,4-dione scaffolds. These findings further promote research into quinazoline-2,4-diones as bacterial topoisomerase targets, and provide metabolic considerations for both fluoroquinolone and quinazoline-2,4-dione therapeutic development, which is severely underrepresented in the field of quinolone antibiotics.