Radioreceptor assay of narcotic analgesics in serum

A sensitive radioreceptor assay (RRA) to determine the serum concentrations of fentanyl, pentazocine and morphine was developed on the basis of the drug's competition with a labeled tracer ((3)H-naloxone) for the membrane bound opioid receptor in rat brain homogenates. The binding data were computer-fitted to a standard curve by means of nonlinear least square regression. Sensitivity of the assay applied directly to serum samples without extraction was limited to approximately 3, 5 and 25 ng/ml for fentanyl, morphine and pentazocine, respectively, because of endogenous plasma constituents that interfere with the opioid receptor binding. With the use of petrol-ether extraction the sensitivity was improved to 0.3 ng/ml fentanyl and 3 ng/ml pentazocine (0.3 ml serum samples). No RRA-active metabolites were detectable after HPLC separation of serum from a patient treated with fentanyl. The plasma concentration time course of fentanyl in a patient, measured by RRA, was similar to that obtained by a radioimmunoassay (RIA). The RRA represents a general procedure for the detection of clinically used opioid analgesics and their active metabolites.