Gonorrhoea is a global health concern highlighted by the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) as a bacterial infection with few remaining treatment options [1,2]. The development of antimicrobial resistance among Neisseria gonorrhoeae has required frequent changes in therapeutic management from the early use of sulfonamides to third-generation cephalosporins [3]. Most countries currently recommend a combination of ceftriaxone and azithromycin to treat N. gonorrhoeae infections [4–6]. Combination drug therapy involving two distinct antimicrobial classes with different mechanisms of action has been recommended in an attempt to slow down the progression of resistance to cephalosporins [4]. However, there continues to be an upward trend in the minimum inhibitory concentrations (MICs) of cephalosporins, particularly cefixime, among N. gonorrhoeae isolates collected and tested in countries with established antimicrobial resistance surveillance systems [7,8]. The likelihood that chemotherapeutic options currently recommended and used to treat gonorrhoea will become less effective over time has heightened the necessity to identify new antimicrobial agents for treatment.

Existing antimicrobial agents that inhibit the growth of N. gonorrhoeae impair bacterial DNA replication, protein synthesis or bacterial cell wall development. Ciprofloxacin had been recommended for the treatment of N. gonorrhoeae in the USA from 1993 to 2004, but the gradual emergence and spread of resistant strains facilitated its removal as a primary therapeutic option [9]. Strains acquired resistance to ciprofloxacin through mutations in the gyrA and parC genes that encode DNA gyrase and DNA topoisomerase, respectively, which are enzymes critical for DNA replication and decatenation [10]. A novel spiropyrimidinetrione, ETX0914 (Entasis Therapeutics, Waltham, MA), previously referred to as AZD0914, inhibits bacterial DNA biosynthesis through inhibition of type II topoisomerases, which includes DNA gyrase and topoisomerase IV[11]. The compound stabilises the cleaved covalent of DNA gyrase and prevents religation of DNA bound to the topoisomerase tetramer [12]. Although ETX0914 and ciprofloxacin both target DNA gyrase, the distinct binding modes are functionally sufficient for ETX0914 to inhibit the growth of ciprofloxacin-resistant strains [11,13].

In this study, the in vitro activity of ETX0914 against a well characterised panel of N. gonorrhoeae isolates collected from contemporary clinical infections in the USA was examined.

All isolates tested had relatively low MICs to ETX0914 and a wide range of values to other antimicrobial agents (Table 1). The highest ETX0914 MIC was 0.25 μg/mL, which was less than the highest recorded for ceftriaxone and ciprofloxacin. Eleven isolates had ceftriaxone MICs of 0.125 μg/mL and all had an ETX0914 MIC of0.06 μg/mL. One isolate with a ceftriaxone MIC of 0.5 μg/mL had a corresponding ETX0914 MIC of 0.008 μg/mL. The MIC₉₀ value for ETX0914 was 0.125 μg/mL, which was comparable with ceftriaxone and cefixime at 0.125 μg/mL and 0.25 μg/mL, respectively. In contrast, the MIC₉₀ values for azithromycin, ciprofloxacin, penicillin and tetracycline were considerably higher. The MIC₅₀ and MIC₉₀ values for azithromycin were 1 μg/mL and 8 μg/mL, respectively. One isolate with an azithromycin MIC of 256 μg/mL had an ETX0914 MIC of0.06 μg/mL. For ciprofloxacin, both the MIC₅₀ and MIC₉₀ values were 16 μg/mL. All quality control strains were within range for ciprofloxacin and had reproducible results.

Comparing the modal MIC values between isolates of various MIC groupings (i.e. resistant versus susceptible, and high versus low MIC) revealed a consistent modal MIC of 0.06 μg/mL for ETX0914 regardless of the modal MIC for any other antimicrobial agent (Table 2). The ETX0914 modal value was one dilution lower than the ceftriaxone modal of 0.125 μg/mL for the high ceftriaxone MIC grouping. There were 70 ciprofloxacin-resistant isolates (MIC ≥ 1.0 μg/mL) (ciprofloxacin-resistant MIC range, 2–32 μg/mL; modal MIC 16 μg/mL); the ETX0914 modal value was 0.06 μg/mL when tested with these ciprofloxacin-resistant isolates.

The continual progress towards antimicrobial resistance amongN. gonorrhoeae challenges healthcare providers faced with treating this highly prevalent global STD [1,2]. A total of 350,062 gonorrhoea cases in the USA were reported to the CDC in 2014 [16]. The rate of reported cases in the USA increased from 105.3 per 100,000 in 2013 to 110.7 per 100,000 in 2014. Currently recommended combination therapy with ceftriaxone and azithromycin remains effective in treating gonorrhoea, although MICs have increased slightly [17]. Furthermore, sporadic cases of failed therapy have been reported in various countries [18]. If untreated, N. gonorrhoeae infections can progress to serious sequelae such as pelvic inflammatory disease, urethritis and disseminated gonococcal infection.

The data in this report strongly suggest that ETX0914 is highly effective at inhibiting the in vitro growth of N. gonorrhoeae. The panel of isolates tested against ETX0914 was selected based on multidrug resistance and/or decreased susceptibility to contemporary therapeutic options. The MIC range for ETX0914 remained low irrespective of MICs for all other tested antimicrobial agents. Ceftriaxone, considered as one of the final treatment choices for gonorrhoea, had MICs that were similar to ETX0914. In addition to lower MICs, ETX0914 is administered orally, which compares favourably with the need for intramuscular injection of ceftriaxone. A previous study reported that ETX0914 was effective against N. gonorrhoeae growth using a panel of European isolates [19]. Collectively, ETX0914 may be an international option for the treatment of gonorrhoea and is currently in a phase 2 clinical trial.

The hallmark of resistance, genetic alteration through spontaneous mutations or acquired genetic material, implies that any organism capable of mutating will selectively adapt to render chemotherapeutic agents less effective. Recent recommendations for treating gonorrhoea with combination therapy involving antimicrobial agents with different mechanisms of action are based on the low likelihood that N. gonorrhoeae would develop simultaneous resistance [4]. The data in the current study demonstrated ETX0914 MICs that were similar to current therapeutic options for gonorrhoea. Although ETX0914 has a similar mechanism of action as ciprofloxacin in targeting DNA gyrase, the binding modes are sufficiently distinct [11] that ciprofloxacin-resistant isolates are highly susceptible to ETX0914. ETX0914 shows promise for treatment of gonococcal infections, including ciprofloxacin-resistant infections, and particularly as part of combination therapy with a second antimicrobial agent [19,20].

Research and discovery of new antimicrobial agents for the treatment of gonorrhoea are critical for the prevention of potentially life-altering disease. ETX0914 has excellent activity against a multidrug-resistant panel of N. gonorrhoeae and is currently being further evaluated in a clinical trial for the treatment of gonorrhoea.