DETERMINATION OF SERUM MORPHINE BY THE SPIN-LABEL ANTIBODY TECHNIQUE

¹ Clinical Pharmacology Section, Minneapolis Veterans Administration Hospital, and Department of Pharmacology, University of Minnesota, School of Medicine

The spin-label antibody technique has been adapted to the determination of serum morphine concentrations. Our results indicate that with a 0.30-ml solution cell, the ultimate sensitivity of the assay is 35 nM (10 ng of morphine per ml) which is very close to the instrumental detection limit for spin-labeled morphine of 20 nM (5.7 ng/ml). Comparison of t'S for disappearance of [N-methyl/-¹⁴C]morphine in a rabbit yielded 1.9 hr by ¹⁴C counting and 1.6 hr by the spin-label technique. Serum binding of morphine can be easily and rapidly determined utilizing the spin-labeled drug; the percentage of free (unbound) morphine in human serum was found to be 69.5 ± 0.9% by the spin-label technique and 64.6 ± 1.4% by ultrafiltration. Our results would suggest that this method holds promise for the assay of serum concentrations and binding not only of morphine but of other drugs as well.