Responding to the challenge of untreatable gonorrhea: ETX0914, a first-in-class agent with a distinct mechanism-of-action against bacterial Type II topoisomerases

Abstract

With the diminishing effectiveness of current antibacterial therapies, it is critically important to discover agents that operate by a mechanism that circumvents existing resistance. ETX0914, the first of a new class of antibacterial agent targeted for the treatment of gonorrhea, operates by a novel mode-of-inhibition against bacterial type II topoisomerases. Incorporating an oxazolidinone on the scaffold mitigated toxicological issues often seen with topoisomerase inhibitors. Organisms resistant to other topoisomerase inhibitors were not cross-resistant with ETX0914 nor were spontaneous resistant mutants to ETX0914 cross-resistant with other topoisomerase inhibitor classes, including the widely used fluoroquinolone class. Preclinical evaluation of ETX0914 pharmacokinetics and pharmacodynamics showed distribution into vascular tissues and efficacy in a murine Staphylococcus aureus infection model that served as a surrogate for predicting efficacious exposures for the treatment of Neisseria gonorrhoeae infections. A wide safety margin to the efficacious exposure in toxicological evaluations supported progression to Phase 1. Dosing ETX0914 in human volunteers showed sufficient exposure and minimal adverse effects to expect a highly efficacious anti-gonorrhea therapy.

Figure 1. Structures of previously disclosed antibacterial inhibitors of DNA gyrase/Topo IV.

The four compounds are representatives of four drug classes that operate by orthogonal modes-of-inhibition. ETX0914 is spiropyrimidinetrione with noteworthy activity versus Staphylococci and Streptococci and fastidious Gram-negative pathogens including N. gonorrhoeae. Ciprofloxacin 2 is one of dozens of fluoroquinolones that have been introduced into the clinic. NBTI 3 is a representative from an aminopiperidine class of agents with three reported compounds having progressed to human clinical trials, only one of which is known have progressed to patient trials. Novobiocin 4 is the only commercialized example of a broad class of DNA gyrase/Topo IV ATP competitive inhibitors.

Figure 2. MIC, PK and key in vitro safety data are shown for five spiropyrimidinetriones variably substituted on the benzisoxazole 3-position.

Structure-activity correlations show that the pyrimidinetrione and morpholine moieties cannot be varied without diminishing antibacterial potency or compromising co-dependent physical properties and PPB. Importantly, ETX0914 did not show a signal for genotoxicity or bone marrow toxicity at the highest concentrations tested. Three fluoroquinolones are included as comparators.

Figure 3. Orientation of topoisomerase resistance determinants and mutable sites to inhibitors of bacterial type II topoisomerases.

(a) ‘Overhead’ view of composite DNA gyrase/Topo IV structure showing binding sites for ciprofloxacin 2 (2 molecules in yellow), NBTI 3 (green) and novobiocin 4 (2 molecules in magenta). The structural information was generated by overlaying pdb files of S. aureus GyrB bound to novobiocin (4URO), a three-gate structure of S. pneumoniae TopoIV (4I3H), a S. aureus structure with NBTI (2XCS) and a S. aureus structure with ciprofloxacin (2XCT). The region of the binding pocket used for expansion in (b) has been boxed. (b) Expansion of the DNA cleavage domain (2A) showing: ciprofloxacin (yellow sticks), the associated Mg²⁺ (yellow sphere), and resistance determinants GyrA: S84, S85 and E88 (yellow sticks); NBTI 3 (two orientations of the molecule, green sticks) and resistance determinant GyrA: M121 (green sticks); ETX0914 mutable sites: S442, D437, R458 and N475 (orange sticks). Resistance determinants and mutable sites are numbered by the S. aureus DNA gyrase sequence. The novobiocin T172 mutable sites are not shown.

Figure 4. Relationship between AUC/MIC

(a) C_max/MIC (b) and %T > MIC (c) of ETX0914 against S. aureus ARC516 response in the hollow fiber infection model. R² and weighted sum of the squared residual (WSSR) shown to indicate goodness of fit. Analysis of these data indicate that PK/PD index AUC/MIC is highly correlated to the efficacy of ETX0914.

Figure 5. Margins to AUC and C_max in humans for ETX0914.

(a) Human AUC_0-24h (±SD) versus dose. The efficacious AUC (horizontal green line) was exceeded at doses >800 mg. The margins to the NOAEL in rat (horizontal magenta line) and dog (horizontal red line) were 6.4- and 4.8-fold, respectively, at the highest 4000 mg dose. (b) Human C_max (±SD) versus dose. The margins to the NOAEL in rat (horizontal magenta line) and dog (horizontal red line) were 4.9- and 3.7-fold, respectively, at the highest 4000 mg dose.