Radioimmunoassay method for DX-9065a, an anticoagulant agent. Development, evaluation and application to human plasma

A simple and sensitive radioimmunoassay (RIA) method was developed for determination of DX-9065a, (+)-(2S)-2[4-[[(3S)-1-acetimidoyl-3-pyrrolidinyl] oxy]phenyl]-3-[7-amidino-2-naphthyl]propanoic acid hydrochloride pentahydrate, a newly synthesized anticoagulant agent. Immunogens were prepared by condensation of a hapten with bovine serum albumin via a carboxyl group. Antisera was obtained by immunization of five rabbits with immunogen. High-titer antisera was obtained from 2 rabbits immunized with immunogen. The sensitivity of this newly developed RIA method was 100-fold greater than that of a previously used conventional HPLC method. This method was validated for determination of human plasma samples in clinical trials. The cross-reactivities of employed antisera with three steroisomers (2R3R-, 2R3S- and 2S3R forms) were 0.7, 20.2 and 43.9% respectively. The effect of cross-reactivity of postulated stereoisomers in clinical samples was evaluated by a parallelism study using human plasma samples obtained after oral administration of the drug to healthy Japanese volunteers. Results showed no effect on measured concentration. From these data, this method showed suitable accuracy and precision for the pharmacokinetic evaluation of DX-9065a in clinical study. The method was applied to plasma samples obtained from a healthy Japanese volunteer who had orally received 12.85 mg (10 mg as DX-9065) of the drug. The maximum plasma concentration measured was 6.2 ng ml-1 1 h after administration.