The development of a radioimmunoassay for cannabinoids in blood and urine

Abstract

Antibodies, for use in radioimmunoassay, have been raised in sheep by immunization with a conjugate of Δ⁹‐tetrahydrocannabinol hemisuccinate and bovine serum albumin. Antiserum titre and avidity were increased by successive booster doses of conjugate. The high degree of non‐specific binding encountered in the radioimmunoassay of cannabinoids was reduced by the use of the solubilizing detergent Triton X‐405 and by restricting protein concentration in the assay medium. Plasma samples were deproteinized with ethanol before assay, but urine was directly assayed. High avidity antibodies and high specific activity [³H]‐ Δ⁹‐tetrahydrocannabinol permitted the detection of 50 pg of cross‐reacting cannabinoids–‐a sensitivity of 7.5 ng ml⁻¹ of plasma and 1.0 ng ml⁻¹ of urine. Whilst apparently specific for the three‐ringed cannabinoid nucleus, the assay antiserum cross‐reacted with several cannabinoids, both natural compounds and metabolites. Partial identification of cross‐reacting cannabinoids was achieved by the use of pure compounds and by the assay of plasma and urine samples collected from rabbits given pure cannabinoids intravenously.