Serum levels of butorphanol by radioimmunoassay

A radioimmunassay was developed for the analysis of butorphanol in human serum. The procedure involves extraction of serum with hexane containing 2% isoamyl alcohol, followed by evaporative solvent removal and radioimmunoassay of the reconstituted residue. The antibody significantly cross-reacts with unidentified butorphanol metabolites but not with two known metabolites, hydroxybutorphanol and norbutorphanol. Extraction eliminated interference from the butorphanol metabolites normally present in serum. The antibody also cross-reacts with pentazocine and cyclazocine but not with morphine, hydromorphone, oxymorphine, codeine, methadone, and meperidine. Butorphanol tartrate was administered intravenously (2 mg) to normal male volunteers. Serum butorphanol levels declined biexponentially with an average terminal half-life of 2.7 hr. Enzymatic serum hydrolysis prior to extraction yielded additional butorphanol, indicating the presence of butorphanol conjugates. The specificity of the assay for butorphanol was confirmed by GLC--mass fragmentography.