Morphine and etorphine: XIV. Detection by ELISA in equine urine

We have raised antibodies to morphine and etorphine and developed one-step enzyme-linked immunosorbent assays (ELISA) for these drugs as part of a panel of post race tests for drugs in racing horses. These tests are simple, can be completed in 2 h, and can be read by visual inspection. The morphine ELISA has an I₅₀ for morphine of about 1.5 ng/mL, while the etorphine ELISA has an I₅₀ for etorphine of 250 pg/mL. Cross-reactivity studies show that the antimorphine antibody cross-reacts well with levorphanol, hydromorphone, and oxycodone, while the anti-etorphine antibody showed no cross-reactivity with buprenorphine, diprenorphine, oxymorphone, morphine, or thebaine. The morphine test readily detected parent morphine or its metabolites in equine urine for at least 8 h after administration of 50 mg/horse, while a 0.1 μg/kg dose of etorphine was detectable for up to 48 h post dosing. For each test the background activity in post-race urines was equal to or less than the I₅₀ for the standard curves, making them useful equine forensic tests. Each of these tests has detected 'positives' in post race urine samples and as such these tests are capable of substantially improving the speed and efficacy of both pre-race and post-race testing for morphine, etorphine, and their congeners in racing horses.