In vitro efficacy of 21 dual antimicrobial combinations comprising novel and currently recommended combinations for treatment of drug resistant gonorrhoea in future era

BACKGROUND Recent WHO guidelines recommend dual therapy with ceftriaxone or cefixime plus azithromycin for gonorrhea. Azithromycin in combination with gentamicin or spectinomycin has been recommended in treatment failure cases. Due to emergence of multi-drug resistant (MDR) and extensively-drug resistant (XDR) Neisseria gonorrhoeae strains, it is important to look for efficacy of these combinations and also of others that might be used in future. Therefore, we aimed to evaluate in vitro synergy of 21 dual combinations including current and alternative WHO recommended treatment regimens and other dual combinations. METHODS AND FINDINGS The potential utility of in-vitro interactions of 21 combinations was investigated against 95 N. gonorrhoeae strains including 79 MDR and one XDR strain collected during March 2013 to July 2017 and fractional inhibitory concentration index (FICI) was calculated. These 21 combinations comprised of two WHO currently recommended (cefixime+azithromycin, ceftriaxone+azithromycin); two WHO recommended in treatment failure cases (azithromycin+gentamicin, spectinomycin+azithromycin) and other 17 combinations. RESULTS FICI of the four WHO recommended antimicrobial combinations were higher (>1.0) than the five novel combinationbreeds (FICI range 0.603-0.951) in the study i.e. gentamicin+ertapenem, moxifloxacin+ertapenem, spectinomycin+ertapenem, azithromycin+ moxifloxacin, cefixime+gentamicin. No antagonistic effect of the above four WHO recommended combinations except spectinomycin+azithromycin (FICI = 4.25) was observed for the XDR strain. Out of above five novel combinations, four combinations produced high synergistic effects in overall 95 strains and also for the XDR strain with FICI of 0.13 to 0.38. Antagonistic effects varying from 3.2 to 12.6% were observed for 10 out of 21 tested combinations (azithromycin in combination with gentamicin and spectinomycin; ceftriaxone with moxifloxacin, gentamicin, spectinomycin and ertapenem; spectinomycin with moxifloxacin and gentamicin; cefixime and gentamicin combination with moxifloxacin). CONCLUSION WHO recommended cefixime+azithromycin, ceftriaxone+azithromycin combinations having no antagonism indicates their continuing clinical utility. Highest antagonism without any synergistic effect for the WHO recommended spectinomycin+azithromycin in treatment failure cases suggests that this combination should be evaluated further both in vitro and in vivo. Highest synergistic or additive effect without any antagonistic effect of the above five novel combinations suggests that these may be recommended for treatment in future.