Specific radioimmunoassay of ergot peptide alkaloids in plasma
Schran, H.F.; Schwarz, H.J.; Talbot, K.C.; Loeffler, L.J.
Clinical Chemistry 25(11): 1928-1933
1979
ISSN/ISBN: 0009-9147 PMID: 498503 Document Number: 152757
Radioimmunoassay [RIA] methods for the ergot peptide alkaloid (ergopeptine) drugs ergotamine, dihydroergotamine and bromocriptine are described. Antisera were elicited against conjugates consisting of human serum albumin and ergopeptine linked via the indole N of the alkaloid. This site is most distal to the metabolically labile peptide protion of the ergopeptine. The resulting high specificity of the antisera toward the peptide portion prevents assay interference by peptide metabolites. The antisera showed less specificity towards the lysergic acid moiety of the ergopeptines. It is possible to use dihydroergotamine antisera in the analysis for ergotamine, dihydroergocryptine antisera in the analysis for ergocryptine and bromocriptine, and to use their 9, 10-3H derivatives as radiolabeled standard. The intact ergopeptine structure is a prerequisite for cross reactivity. Lysergic acid and peptide aminocyclol derivatives formed by metabolic scission of the C-8 amido bond of the ergopeptines do not cross react. The high specificity and sensitivity of the assays enable direct determination of therapeutic concentrations of ergopeptines in plasma after their oral administration. Currently available RIA methods for ergopeptines based on lysergic acid antisera require an extraction procedure to achieve the necessary sensitivity and also lack specificity, as shown by the extensive cross reactivity of ergopeptine metabolites with the lysergic acid and analogous dihydrolysergic acid antisera. Significantly lower values for plasma concentrations and more rapid excretion rates for s.c. administered dihydroergotamine were obtained with the dihydroergotamine than the dihydrolysergic acid antisera. This suggests the presence in vivo of slowly excreted metabolite species cross reacting with the latter antisera and demonstrates the preferential applicability of the specific ergopeptine antisera for studies of concentrations in human plasma.