| Description |
Detection and quantitation of fentanyl and fentanyl class compounds present a significant analytical problem. Due to their extreme potency, their concentrations in biological fluids are well below the limits of detection of most routine screening techniques. The purpose of the present investigation is to develop a radioreceptor binding assay that may be a useful screening method for fentanyl and its analogs. The assay is based on the competition of these drugs with [3H] fentanyl for opioid receptors in membrane preparations of rat forebrain in vitro. 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 µ1-opioid sites, completes with [3H] fentanyl for its high affinity binding site, suggesting that the majority of the central nervous system fentanyl binding occurs with µ1-opioid receptors. Morphine and hydromorphone complete with [3H] fentanyl for the opioid receptor, but other morphine-like compounds were relatively weak displacers of [3H] fentanyl. Many other commonly abused drugs do not compete with [3H] fentanyl for the opioid receptors. Urine samples from animals injected with fentanyl, ±-cis-3-methylfentanyl, alpha-methylfentanyl, butyrylfentanyl and benzylfentanyl were analyzed by radioreceptor assay, radioimmunoassay and gas chromatography/mass spectrometry. Urinary analysis of fentanyl showed a good correlation with these three methods; however, discrepancies were observed in the analysis of fentanyl analogs. Urinary concentrations of ±-cis-3-methylfentanyl measured by radioreceptor assay were 5 to 10 times higher than values obtained by gas chromatography/mass spectrometry, and undetectable by radioimmunoassay. Concentrations of alpha-methylfentanyl obtained by radioreceptor assay and gas chromatography were similar; however, these samples were negative by radioimmunoassay. Urinary concentrations of benzylfentanyl were not detected by either radioreceptor assay or by radioimmunoassay; however, concentrations of benzylfentanyl were measurable by gas chromatography/mass spectrometry. Urine samples from animals injected with butyrylfentanyl showed high cross-reactivity with fentanyl antibody but low affinity for the opioid receptors, giving values measured by radioimmunoassay about 10 to 20 times higher than those obtained by the other methods. This radioreceptor assay is well-suited as an initial assay for the detection of active analogs of fentanyl in urine with good correlation with other techniques in the analysis of fentanyl; however, there is substantial disagreement between techniques in the quantitation of fentanyl analogs. The implications of these discrepancies are discussed. |
