Urine drug testing of chronic pain patients. III. normetabolites as biomarkers of synthetic opioid use

Opioids are important therapeutic agents available to patients with moderate to severe pain. The synthetic opioids, buprenorphine, fentanyl, meperidine, methadone, and propoxyphene have been utilized for decades as analgesics. One of the major biotransformation pathways of these drugs occurs through N-demethylation leading to the formation and excretion of normetabolites. Normetabolites generally exhibit longer half-lives than the parent drug leading to accumulation with prolonged use. As part of continuing research efforts to improve monitoring programs of chronic pain patients undergoing opioid treatment, we evaluated the prevalence and relative abundance of normetabolites of buprenorphine, fentanyl, meperidine, methadone, and propoxyphene in patients' urine specimens. Selected sets of specimens were analyzed without prior immunoassay screening by liquid chromatography- tandem mass spectrometry for buprenorphine, fentanyl, meperidine, methadone, propoxyphene, and their respective normetabolites. Limits of quantitation (LOQ) were as follows: buprenorphine, 1 ng/mL; fentanyl, 0.5 ng/mL; meperidine, 50 ng/mL; methadone, 50 ng/mL; and propoxyphene, 50 ng/mL. LOQs for normetabolites were equal to the parent drug with the exception of norbuprenorphine (2.5 ng/mL). The percentage of positive specimens that contained normetabolite (only) ranged from 8.0% for EDDP (2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine) to 53.1% for norpropoxyphene. Inclusion of the five normetabolites in the test panel produced an increase in detection rates for parent drug use as follows: buprenorphine, 10.0%; fentanyl, 42.1%; meperidine, 98.7%; methadone, 8.7%; and propoxyphene, 113.2%. The authors conclude that testing for synthetic opioid normetabolites enhances the effectiveness of monitoring programs for pain patients.