Increased specificity of the 3 H-acetic anhydride coupling method for plasma analysis of drugs containing secondary amino groups.

Abstract

: For the plasma analysis of drugs containing primary or secondary amino groups the method of HAMMER & BRODIE (1967) has been widely used. By this technique amines are isolated by extraction and coupled with 3H-acetic anhydride. After removal of excess labelling agent, the amount of labelled coupling product is assayed by liquid scintillation counting. The possibility of masking primary amines by adding salicylic aldehyde to the hexane phase in the first extraction step of this method has been investigated. The presence of 0.1 M salicylic aldehyde was found to reduce considerably the amount of primary amine measured as 3H-amide, while secondary amines were only slightly influenced. Thus increased specifity for secondary amines was achieved. The practical importance of the modified method was illustrated by experiments on plasma from rats given nortriptyline and human subjects given Lu 5-003, a bicyclic thymoleptic. In the former case the modified method is of less importance, since no primary amine metabolite is present - in the latter case, however, it would be advantageous, because the corresponding primary amine is present as a metabolite in plasma.