Radioimmunoassay for the novel platelet activating factor receptor antagonist E5880

A direct radioimmunoassay for E5880, 1-ethyl-2-[[N-(2-methoxybenzoyl)-N-[[(2R)-2-methoxy-3-[[[4- [(octadecylcarbamoyl)- oxy]piperidino]carbonyl]oxy]propoxy]carbonyl]amino]methyl] pyridinium chloride, a novel analogue-type antagonist of platelet activating factor (PAF), was developed. In this procedure, [3H]E5880 was used as the radioligand, and the antiserum was obtained from rabbits immunized with hapten covalently bound to bovine serum albumin. The hapten represents a structural analogue of E5880, with a carboxyl group on the terminal carbon of the 3-position side chain. A metabolite of E5880, deacyl-E5880, cross-reacted weakly (1.8%) with this antiserum. The assay buffer for the radioimmunoassay consisted of PBS, pH 6.5, containing 1% BSA to prevent the degradation of E5880 in aqueous solution and its adsorption to the tube. The detection limit of the assay was 200 pg/mL when a 0.1-mL plasma sample was used. The radioimmunoassay was used for the direct analysis of E5880 in dog plasma. The validity of the radioimmunoassay in dog plasma was demonstrated by comparative analysis of a number of samples by HPLC (r = 0.995, slope = 0.9425). The radioimmunoassay was also used to determine the pharmacokinetics of E5880 in the dog. After the intravenous administration of E5880 (0.2 mg/kg), plasma levels declined biexponentially. The initial plasma half-life, including the distribution phase, was 0.26 h, and the plasma half-life of elimination was 9.96 h.