Radioreceptor assay of anticholinergic drugs

The radioreceptor assay (RRA) of anticholinergic drugs in plasma and other biological fluids presents methodological difficulties. The specificity is not comparable with some pure chemical methods because drug metabolites which are active in binding to muscarinic receptors participate in the assay. There are also problems with serum dilution, with the lipophilicity of the radioligands and drugs, and with the protein binding of the drugs. However, the RRA of anticholinergic drugs is rapid and sensitive. In racemic drugs, only active stereoisomers show affinity for the muscarinic receptors. Results with RRA have been shown to correlate with anticholinergic effects, both desired and side effects. A careful familiarization with the method used for each individual drug analysis allows its useful clinical application.