[Effect of pH value and magnesium on the antibacterial activity of quinolone preparations]

The activity of ten different 4-quinolone antibacterial agents against Escherichia coli KL16 was investigated at four pH values between 5.6 and 8.3, which is the pH range of urine. It was found that pH affected the minimum inhibitory concentrations of all the drugs, but the nature of the substituent at the C7 position of each 4-quinolone governed how its activity was altered by the hydrogen ion concentration. When a 4-quinolone possessed a piperazine group at its C7 position its activity became progressively less as the pH fell. However, all drugs that lacked a C7 piperazine, irrespective of the nature of their substituent at this position, exhibited a progressive increase in activity as the pH was reduced. In addition it was found that, excepting cinoxacin, a urinary concentration of magnesium generally caused a further antagonism of the activity of the nine other 4-quinolone antibacterials at all pH values tested.