Radioimmunoassay of indomethacin in biological fluids

A radioimmunoassay was developed for the determination of indomethacin in biological fluids at concentrations as low as 50 ng/ml. Antibodies were produced in rabbits immunized with a conjugate of bovine serum albumin and indomethacin. This conjugate was prepared by an N-hydroxysuccinimide active ester procedure. Antiserums exhibited minimal cross-reactivity with the O-desmethyl and deschlorobenzoyl metabolites. However, the glucuronide conjugate was about three times as reactive as indomethacin, thus invalidating direct determinations of indomethacin in urine. This difficulty was circumvented by analyzing urine aliquots before and after conjugate hydrolysis. Concentrations of free and conjugated indomethacin were calculated by difference.