Radioreceptor assay for a new antiallergic agent, 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)benzimidazole difumarate (KB-2413), in plasma.

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Abstract
A radioreceptor assay (RRA) for the quantiutative determination of a new antiallergic agent, 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)benzimidazole difumarate (KB-2413), has been developed. This RRA method, based upon competitive binding of [3H]mepyramine and KB-2413 to histamine H1 receptors in guinea pig cerebellum, allows the simple, sensitive and reproducible determination of KB-2413 in plasma. The determination limit was 0.1 ng (as free base)/ml and the standard curve was linear over the range of 0.1 to 5.0 ng/ml. The intra- and inter-assay coefficients of variation for the determination of KB-2413 were 5.2-11.1% and 3.4-5.6%, respectively. The active metabolites, 1-(2-ethoxyethyl)-2-(1-homopiperazinyl)benzimidazole and 1-(2-ethoxyethyl)-5-hydroxy-2-(4-methyl-1-homopiperazinyl)benzimidazole, cross-reacted to the extents of 28.6 and 21.1%, respectively. However, plasma levels obtained by the RRA method and the gas chromatographic method for the intact drug were well correlated in guinea pigs after oral administration of KB-2413.