Interaction of Rifalazil with Oxidant-Generating Systems of Human Polymorphonuclear Neutrophils

Abstract

ABSTRACT It is well acknowledged that ansamycins display immunosuppressive and anti-inflammatory properties in vitro and in vivo. Rifalazil, a new ansamycin derivative, has not been studied in the context of inflammation. In particular, there are no data on the possible interference of rifalazil with oxidant production by phagocytes. We have compared the antioxidant properties of rifalazil to those of rifampin, a drug well known in this context, by using cellular and acellular oxidant-generating systems. Oxidant production by polymorphonuclear neutrophils was measured in terms of cytochrome c reduction, lucigenin-amplified chemiluminescence (Lu-ACL), and the 2′,7′-dichlorofluorescin diacetate H 2 (DCFDA-H 2 ) technique (intracellular oxidant production). Rifalazil impaired O 2 − production in a concentration-dependent manner, with 50% inhibitory concentrations (IC 50 ) (concentrations which inhibit 50% of the response) of 5.4 (30 and 60 min of incubation) and 6.4 (30 min) mg/liter, respectively, for phorbol myristate acetate (PMA) and formyl-methionyl-leucyl-phenylalanine (fMLP) stimulation. In agreement with the published fMLP-like activity of rifampin, the inhibitory effect of rifampin was significantly greater for fMLP (IC 50 of 5.6 mg/liter) than for PMA (IC 50 of 58 mg/liter) stimulation. Alteration of intracellular oxidant production was also observed with IC 50 values similar to those obtained by the cytochrome assay. In addition, rifalazil and rifampin (≥25 mg/liter) scavenged O 2 − , as demonstrated by the acellular (hypoxanthine-xanthine oxidase) system. Interference with light detection systems was evidenced for both drugs by Lu-ACL. The clinical relevance of the antioxidant effect of rifalazil demonstrated in vitro, in particular its potential anti-inflammatory activity, requires further investigation.