The fluorinated quinolones

Following the discovery of nalidixic acid in 1962, numerous structural modifications have been made to the quinolone nucleus to increase antimicrobial activity and improve pharmacokinetic performance. A major advance occurred during the 1980's with the discovery that a fluorine at position-6 conferred broad and potent antimicrobial activity, (e.g. norfloxacin) but still with relatively less activity for Gram-positive and antiaerobic organisms than Gram-negative bacteria. Subsequent developments produced quinolones with further improvements, predominantly in either solubility (e.g. ofloxacin), antimicrobial activity (e.g. ciprofloxacin) or prolonged serum half-life (e.g. pefloxacin). Recent modifications have attempted to achieve an optimal blend of favourable properties together with minimal potential for undesirable side-effects. The mode of action of quinolones is by blocking of the bacterial enzyme gyrase. This enzyme is responsible for the coiling and supercoiling of the DNA within the cell. When this enzyme is inhibited, DNA transcription, which results in protein synthesis, and DNA replication, which results in cell division, are inhibited. Improvements in antimicrobial activity combined with adequate blood and tissue concentrations do offer expectancy of enhanced therapeutic efficacy for new derivatives in those infections by organisms which are 'marginally' sensitive to currently used quinolones. The possibility of resistance emerging in these organisms during treatment should also be reduced.