NPC 15669 inhibits the reversed passive Arthus reaction in rats by blocking neutrophil recruitment

NPC 15669, N-carboxy-L-leucine,N-[(2,7-dimethylfluoren-9-yl)methyl]ester, has been shown to inhibit several inflammatory reactions that depend upon recruitment of neutrophils into the primary lesion. In the present study we examined the effects of NPC 15669 in the reversed passive Arthus reaction, an inflammatory reaction occurring in the skin of rats in response to intracutaneous injection of antigen followed by intravenous administration of antibody. In this model, immune complex formation activates complement, resulting in rapid recruitment of neutrophils to the site, which releases free radicals and proteases that damage capillaries, resulting in plasma leak. NPC 15669 inhibited the increased capillary permeability occurring in the reversed passive Arthus reaction in a dose-dependent manner, with an ED50 of 4 mg/kg. The agent similarly inhibited the recruitment of radiolabeled neutrophils as well as the accumulation of myeloperoxidase, a neutrophil marker. NPC 15669 in vitro inhibited the adherence of formyl-L-Met-L-Leu-L-Phe- or human recombinant C5a-activated neutrophils to endothelium, with IC50 values of 15 to 30 microM (ca. 4-9 micrograms/ml). Measurement of plasma NPC 15669 showed that at the ED50 dose, the average circulating concentration of drug was 5 micrograms/ml, consistent with the hypothesis that NPC 15669 exerts its anti-inflammatory effects by inhibiting neutrophil adherence to endothelium and recruitment into the inflammatory lesion.