Development and quality control of a highly sensitive radioimmunoassay for alinidine

A new precise and sensitive radioimmunoassay (RIA) for alinidine (N-allyl-clonidine) has been developed. Synthesis and analysis of the hapten (4-carboxy-alinidine = STH 2329), as well as the production of the antibody in rabbits, are described in detail. At a final dilution of 1 : 1000, the resulting immune serum binds 50% of a tritiated alinidine standard (50 pg). The detection limit of the present RIA for alinidine is 50 pg/ml plasma. The intra-assay coefficient of variance (VC) is lower than 4% (N = 10) for any standard concentration; the inter-assay VC does not exceed 8.7%. There is no cross reactivity of any alinidine metabolite or congener with the antibody. The low detection limit of the assay, 10(-3) of therapeutically relevant alinidine blood levels, brings about several analytical advantages, which are discussed in detail. Quality control tests were performed in comparison with two reference methods (HPLC). Concerning assay sensitivity, specificity, reliability, and expenditure in costs or sample volumes, the RIA turned out to be the optimal method for routine analysis of alinidine in biological fluids. An example for practical use of the assay is given, evaluating the pharmacokinetics of alinidine in beagle dogs. From the accumulated renally-excreted total radioactivities, the enteral absorption of the drug was calculated (91%); the bioavailability of orally administered alinidine was derived from the corresponding areas under the blood plasma concentration curves of the radioimmunologically-evaluated parent compound (72%).