Fentanyl receptor assay. I. Development of a radioreceptor assay for analysis of fentanyl and fentanyl analogs in urine.

Radioreceptor binding assays may be useful methods for determining the plasma and urine concentration of drugs. We have evaluated the possibility of employing CNS receptors in an assay to measure fentanyl and fentanyl-like drugs. This in vitro assay is based on the competition of these drugs with [3H]fentanyl for opioid receptors in membrane preparations of rat forebrain. The binding is stereospecific, reversible, and saturable. Scatchard plots of saturation suggest the presence of high and low affinity binding sites. Naloxonazine, which selectively binds to mu 1-opioid site, competed with [3H]fentanyl for its high affinity binding site. Morphine and hydromorphone competed with [3H]fentanyl for the opioid receptor, but other morphine-like compounds were relatively weak displacers of [3H]fentanyl. Many other commonly abused drugs did not displace [3H]fentanyl from the opiod receptors. Urine samples from animals injected with fentanyl were evaluated by other analytical techniques, including radioimmunoassay (RIA) and gas chromatography/mass spectrometry (GC/MS), and results were compared to those from the radioreceptor assay. Urinary analysis of fentanyl showed a good correlation between all three methods.