Stereospecific radioimmunoassays for d-pseudoephedrine in human plasma and their application to bioequivalency studies

Antiserum to d-pseudoephedrine was raised in New Zealand White rabbits in response to immunization with a conjugate of bovine serum albumin and d-pseudoephedrine-N-3-propionic acid. The hapten was prepared by reaction of methyl acrylate with d-pseudoephedrine, followed by ester hydrolysis. Sodium boro[3H]hydride reduction of dl-ephedrine gave [alpha-3H]-dl-ephedrine, and a Welsh rearrangement with acetic anhydride followed by deacetylation gave [alpha-3H]-dl-pseudoephedrine, which was used as a radioligand in radioimmunoassay procedures for direct plasma analyses. Three sensitive radioimmunoassay procedures were developed, two using [3H]pseudoephedrine as the radioligand and either adsorption on coated charcoal or polyethylene glycol precipitation for separation of antibody-bound from free radioligand. The third method used an [125I]tyrosine methyl ester analog of pseudoephedrine and charcoal separation, preceded by extraction and derivatization of pseudoephedrine with methyl acrylate. All three assays could detect less than or equal to 2.5 ng of pseudoephedrine/ml. The antiserum was stereospecific, showing low cross-reactivities with l-pseudoephedrine and d- and l-ephedrines. d-Norpseudoephedrine and some other related compounds also had low cross-reactivity in these radioimmunoassay procedures. Excellent agreement was found between pseudoephedrine concentrations in human plasma determined by radioimmunoassay and by a standard GLC method. The utility of radioimmunoassay was illustrated by application of one of these procedures to an assessment of the bioequivalence of immediate- and sustained-release pseudoephedrine formulations in normal volunteers. A sustained-release preparation containing 120 mg of pseudoephedrine hydrochloride given every 12 hr was shown by AUC comparisons to be bioequivalent to an immediate-release tablet (containing 60 mg of pseudoephedrine hydrochloride) given every 6 hr.