Radioimmunoassay for Atropine and l‐Hyoscyamine

Abstract

A simple, specific and sensitive radioimmunoassay is described for atropin (dl-hyoscyamine) and l-hyoscyamine. Antiserum was obtained from rabbits immunized with an immunogen prepared by coupling l-hyoscyamine to human serum albumin. By using 3H-atropine as tracer, the assay can detect atropine and l-hyoscyamine concentrations down to 9 nmol/l (2.5 ng/ml) in a 0.1 ml serum or plasma sample. The recovery of atropine was near 100% when the drug was added at different concentrations to normal, pooled human plasma. Atropine and l-hyoscyamine are recognized equally well by the antibodies, but some other structurally related drugs (homatropine, scopolamine) and atropine hydrolysis products (tropine, tropic acid) do not interfere. The usefulness of the method in pharmacokinetic studies was shown by assaying atropine concentrations in serial serum samples from 2 patients, who were given 1.3 mg atropine in connection with anesthesia. A biexponential serum decay curve was demonstrated in both cases with a very rapid distribution phase (t1/2, half-life = 0.63 and 1.38 min) and much slower elimination phase (t1/2 1.86 and 2.09 h).