Quantification of the effect of fluconazole and ciprofloxacin on the metabolism of 4-MAA
As shown in Figure 1, the metabolism of 4-MAA is complex. 4-MAA can be converted to 4-AA and 4-FAA, and 4-AA can be metabolized further to 4-AAA. Figure 4 shows the change in the AUC of 4-MAA, 4-AA, 4-AA+4-AAA and 4-AA+4-AAA+4-FAA produced by ciprofloxacin, fluconazole and the combination ciprofloxacin/fluconazole at different time points. The effect of fluconazole and ciprofloxacin on the increase in the AUC of 4-MAA grows with time whereas the reduction in the AUC of 4-AA, 4-AAA and 4-FAA by fluconazole and ciprofloxacin decreases with time. Figure 4 visualizes that the effect of fluconazole on the metabolism of 4-MAA is approximately only one third compared to ciprofloxacin. The addition of the AUC of 4-AAA and 4-FAA to the AUC of 4-AA did not change the extent and the pattern of the inhibition of the metabolite formation. The inhibition of the formation of 4-AA and 4-AA+4-AAA+4-FAA by ciprofloxacin or ciprofloxacin/fluconazole was in the range of 50 to 60% at 6 h and 30 to 40% at 12 h after ingestion of metamizole. The figure also demonstrates that the effects of fluconazole and ciprofloxacin on the increase in the AUC of 4-MAA and on the decrease in the AUCs of 4-AA, 4-FAA and 4-AAA are additive.
An additional possibility to express the inhibition of the metabolism of 4-MAA by ciprofloxacin and fluconazole is by calculating the metabolic ratio, which also considers the increase in the AUC of the substrate (in this case 4-MAA) and not only the decrease in the formation of the metabolites (suppl. Table 2). The reduction in the MR was strongest at 6 h and decreased with time, similar to the effect on the AUC of 4-MAA. The reduction in the MR was more accentuated for ciprofloxacin than fluconazole and strongest for the combination ciprofloxacin/fluconazole. The strongest reduction was observed for the couple 4-FAA/4-MAA at 6 h, reaching 63% for ciprofloxacin, 24% for fluconazole and 79% for the combination ciprofloxacin/fluconazole. At 12 h, the corresponding values were 55%, 22% and 75% for ciprofloxacin, fluconazole and ciprofloxacin/fluconazole, respectively. In comparison, the effect on the couple 4-AAA/4-AA was much weaker, reaching statistical significance only for ciprofloxacin. This reflects the fact the formation of 4-AAA from 4-AA is dependent on NAT2 and not on CYPs [29].