Phenothiazine Derivatives: The Importance of Stereoisomerism in the Tolerance and Efficacy of Antimicrobials

Stereoisomers are molecules that are identical in atomic constitution and bonding. The biological properties may, however, differ significantly between two enantiomers (individual stereoisomers). JBC 1847, a phenothiazine derivative with strong antimicrobial activity against Gram-positive bacteria, exists in two enantiomers, S and R. Under standard chemical synthesis (S)-and (R)-JBC 1847 will be present in 50/50 amount (racemic). In this study, we have investigated the antimicrobial activity, the in vivo tolerance and therapeutic efficacy of purified (S)-JBC 1847. Compared to JBC 1847 racemic, the antimicrobial activity of (S)-JBC 1847 in vitro was in the same range or slightly increased, while the maximum tolerable concentration in vivo was five times higher for (S)-JBC 1847 (5 mg/kg versus 20 mg/kg bodyweight). Furthermore, the in vivo efficacy of (S)-JBC 1847 in a mouse peritonitis MRSA model was comparable to the activity of vancomycin. In conclusion, the antimicrobial activity and tolerance of a medical stereoisomeric compound may be significantly different using purified enantiomers compared with the racemic state.