Radioimmunoassay of guanabenz, an antihypertensive agent

Antibodies with a high specificity and sensitivity for [(2,6-dichlorobenzylidene)amino]guanidine acetate (guanabenz, GB), a potent antihypertensive agent, were produced in rabbits immunized with GB conjugated to bovine serum albumin. GB-specific antibodies were detected 6-8 days after the first booster injection and the titer steadily increased over the initial 17-25 weeks of immunization. A high degree of specificity was demonstrated. No metabolite of GB detected in humans or several animal species cross-reacted, even when present in over 100-fold excess. A rapid and convenient radioimmunoassay procedure for GB was designed using these antibodies. Competition between tritiated GB tracer and unlabeled GB for specific antibody binding sites made feasible measurement of as little as 30 pg GB. GB was absorbed rapidly from the gastrointestinal tract and reached a maximum level in the plasma (2.50 +/- 0.45 ng/ml) at 2 h after oral administration (1 mg/kg) to dogs.