Radioimmunoassay for atropine and l-hyoscyamine.

A simple, specific and sensitive radioimmunoassay is described for atropine (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 concentratins 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 two patients, who were given 1.3 mg atropine on connection with anaesthesia. A biexponential serum decay curve was demonstrated in both cases with a very rapid distribution phase (t 1/2 0.63 and 1.38 min.) and a much slower elimination phase (t 1/2 1.86 and 2.09 hrs.).