Impact on toxin production and cell morphology in Clostridium difficile by ridinilazole (SMT19969), a novel treatment for C. difficile infection

Abstract

Objectives: Ridinilazole (SMT19969) is a narrow-spectrum, non-absorbable antimicrobial with activity against Clostridium difficile undergoing clinical trials. The purpose of this studywas to assess the pharmacological activity of ridinilazole and assess the effects on cell morphology. Methods: Antibiotic killing curveswere performed using the epidemic C. difficile ribotype 027 strain, R20291, using supra-MIC (4× and 40×) and sub-MIC (0.125×, 0.25×and 0.5×) concentrations of ridinilazole. Following exposure, C. difficile cells were collected for cfu counts, toxin A and B production, and morphological changes using scanning electron and fluorescence microscopy. Human intestinal cells (Caco-2) were co-incubated with ridinilazoletreated C. difficile growth medium to determine the effects on host inflammatory response (IL-8). Results: Treatment at supra-MIC concentrations (4×and 40×MIC) of ridinilazole resulted in a significant reduction in vegetative cells over 72 h (4 log difference, P,0.01) compared with controls without inducing spore formation. These results correlated with a 75% decrease in toxin A production (P<0.05) and a 96% decrease in toxin B production (P,0.05). At sub-MIC levels (0.5× MIC), toxin A production was reduced by 91% (P<0.01) and toxin B production was reduced by 100% (P<0.001), which resulted in a 74% reduction in IL-8 release compared with controls (P<0.05). Sub-MIC (0.5×)-treated cells formed filamentous structures ~10-fold longer than control cells. Following fluorescence labelling, the cell septum was not forming in sub- MIC-treated cells, yet the DNA was dividing. Conclusions: Ridinilazole had robust killing effects on C. difficile that significantly reduced toxin production and attenuated the inflammatory response. Ridinilazole also elicited significant cell division effects suggesting a potential mechanism of action.