Characterization of the novel DNA gyrase inhibitor AZD0914: low resistance potential and lack of cross-resistance in Neisseria gonorrhoeae

Abstract

The unmet medical need for novel intervention strategies to treat Neisseria gonorrhoeae infections is significant and increasing, as rapidly emerging resistance in this pathogen is threatening to eliminate the currently available treatment options. AZD0914 is a novel bacterial gyrase inhibitor that possesses potent in vitro activities against isolates with high-level resistance to ciprofloxacin and extended-spectrum cephalosporins, and it is currently in clinical development for the treatment of N. gonorrhoeae infections. The propensity to develop resistance against AZD0914 was examined in N. gonorrhoeae and found to be extremely low, a finding supported by similar studies with Staphylococcus aureus. The genetic characterization of both first-step and second-step mutants that exhibited decreased susceptibilities to AZD0914 identified substitutions in the conserved GyrB TOPRIM domain, confirming DNA gyrase as the primary target of AZD0914 and providing differentiation from fluoroquinolones. The analysis of available bacterial gyrase and topoisomerase IV structures, including those bound to fluoroquinolone and nonfluoroquinolone inhibitors, has allowed the rationalization of the lack of cross-resistance that AZD0914 shares with fluoroquinolones. Microbiological susceptibility data also indicate that the topoisomerase inhibition mechanisms are subtly different between N. gonorrhoeae and other bacterial species. Taken together, these data support the progression of AZD0914 as a novel treatment option for the oral treatment of N. gonorrhoeae infections.

FIG 1

Chemical structure of the spiropyrimidinetrione AZD0914.

FIG 2

Alignment of type II topoisomerase subunits. (A) The relevant section of the GyrB protein from N. gonorrhoeae is aligned with GyrB and ParE from other key representative Gram-positive and Gram-negative pathogens. The residues in common are boxed. The locations of the mutations affecting AZD0914 susceptibility are marked with an arrowhead, and the equivalent residues in the other proteins are highlighted in green. (B) Alignment of the N. gonorrhoeae subunits from DNA gyrase (GyrA) and topoisomerase IV (ParC) with other A subunits from key pathogens. The residues in common are boxed, and the key residues known to confer fluoroquinolone resistance are highlighted in blue and indicated with an arrowhead.

FIG 3

Modeling of mutations on topoisomerase structures. Key catalytic residues and the residues resulting in AZD0914 resistance are labeled using N. gonorrhoeae numbering, and residues that vary in the displayed sequence alignment are indicated with an asterisk. (A) Overlay showing the similarity of topoisomerases from Gram-positive and Gram-negative pathogens bound with fluoroquinolones. The S. pneumoniae topoisomerase IV bound with levofloxacin (yellow sticks) is overlaid with the A. baumannii gyrase bound with moxifloxacin (blue sticks). The GyrB monomers are shown in blue, the GyrA monomers are shown in cyan, and the bound DNA strands are shown in orange. The Mg²⁺ molecules are shown as green spheres. (B) Overlay showing S. aureus gyrase bound with ciprofloxacin (yellow sticks) with S. pneumoniae topoisomerase IV bound with the pyrimidinedione PD 0305970 (green sticks). The Mg²⁺ molecule is shown as a purple sphere. (C) Schematic representation of the binding pocket interactions with DNA gyrase. The interactions with a fluoroquinolone (I) and quinazolinedione (II) are based on the available crystal structures, while the proposed interactions with AZD0914 (III) are based on the observed resistance data. The GyrA and GyrB residues using N. gonorrhoeae numbering are labeled. The Mg²⁺ essential for fluoroquinolone binding is shown as a green circle.