In vitro activities of the novel bicyclolides modithromycin (EDP-420, EP-013420, S-013420) and EDP-322 against MDR clinical Neisseria gonorrhoeae isolates and international reference strains

OBJECTIVES: New antimicrobials are essential to prevent gonorrhoea becoming an untreatable infection. Herein, the in vitro activities of the novel bicyclolides modithromycin (EDP-420, EP-013420, S-013420) and EDP-322 against Neisseria gonorrhoeae strains were investigated and compared with antimicrobials currently or previously recommended for treatment of gonorrhoea. METHODS: MICs (mg/L) were determined using an agar dilution method (modithromycin and EDP-322) or Etest (seven antimicrobials) for a large geographically, temporally and genetically diverse collection of clinical N. gonorrhoeae isolates (n = 225) and international reference strains (n = 29), including diverse MDR and XDR isolates. RESULTS: The MIC range, modal MIC, MIC50 and MIC90 of modithromycin and EDP-322 were 0.004-256, 0.25, 0.25 and 1 mg/L and 0.008-16, 0.5, 0.5 and 1 mg/L, respectively. The activities of modithromycin and EDP-322 were mainly superior to those of azithromycin and additional antimicrobials investigated. In general, there was no cross-resistance with other antimicrobials. CONCLUSIONS: Modithromycin and EDP-322 exhibited high levels of in vitro activities against N. gonorrhoeae, including isolates resistant to azithromycin, cefixime, ceftriaxone, spectinomycin, ampicillin, tetracycline and ciprofloxacin. However, some cross-resistance with high-level azithromycin resistance (MIC = 4096 mg/L) was observed. Modithromycin and EDP-322 could be effective options for treatment of gonorrhoea, particularly for cases resistant to extended-spectrum cephalosporins and as a part of an antimicrobial combination therapy regimen. Nevertheless, it is important to detail the in vitro selection, in vivo emergence and mechanisms of resistance, pharmacokinetics/pharmacodynamics in humans and optimal dosing, and perform appropriate randomized controlled clinical trials.