Improved radioreceptor assay of opiate narcotics in human serum: application to fentanyl and morphine metabolism.

A recently developed radioreceptor assay (RRA) (1) that employs 3H-naloxone and rat brain membrane homogenates was improved two ways. First, the brain membranes were preincubated in the presence of sodium ions, and second, manganase-II ions were added to the sample incubations. These changes enhanced the assay sensitivity and reproducibility with stored membrane preparations and reduced the effects of serum constituents (Na+) on ligand-receptor binding. Patient sera were assayed by radioimmunoassay (RIA) and RRA after fentanyl administration and by high-performance liquid chromatography (HPLC) and RRA after morphine administration. The results with both fentanyl assays were comparable, and no fentanyl metabolites were detectable by RRA after HPLC of serum extracts. In contrast, preliminary results with the HPLC-RRA procedure suggest the presence of an active morphine metabolite of unknown structure in sera obtained from patients on morphine therapy.