Acid-catalyzed isomerization in the peptide part of ergot alkaloids

Conformation of ergopeptam and ergopeptine alkaloids (ergocristam and ergocristine)

The crystal structures of two ergot alkaloids ergocristam (I) and ergocristine acetone solvate (II), have been determined by X-ray diffraction and compared with their conformation in solution obtained from NMR data. The presence of D-proline in I essentially influences the conformation of the alkaloid tripeptide moiety which seems likely to explain why I does not undergo further enzymatical hydroxylation and is found as a terminal step of ergot alkaloid biosynthesis.

Rate and equilibrium constants for the epimerization of the endothelin receptor antagonist J-104,132 in aqueous solution

The degradation of [5S-[5alpha,6beta,7alpha(R*)]]-2-butyl-5-(1,3-benzodioxol-5-yl)-7-[(2-carboxypropyl)-4-methoxyphenyl]-6-dihydro-5H-cyclopenta[b]pyridine-6-carboxylic acid (J-104,132) was studied in aqueous solution as a function of temperature and pH. The degradation reaction does not proceed to completion; rather, a stable equilibrium is attained in which approximately 2% of the degradate is produced. Kinetic data for the formation of the degradate are analyzed using an integrated form of the rate law for a reversible first-order reaction, and the forward and reverse rate constants and overall equilibrium constants are presented. Isolation and spectroscopic structural determination indicate that the degradate is the C7 beta-epimer of the drug. A mechanism for the epimerization reaction involving a novel enamine-like intermediate is proposed and shown to be consistent with the kinetic data. The rate and equilibrium constants are used to predict the room temperature stability of an injectable formulation of J-104,132, and these predictions are compared to actual data from long-term stability studies. It is concluded that the preformulation kinetic studies provide essential data needed for optimum drug product development.

Selective Reduction of Peptidic Ergot Alkaloids

Five 6'-deoxoergopeptines were prepared in 51-68% yield by selective reduction of parent alkaloids with lithium aluminium hydride in tetrahydrofuran at low temperature. New compounds were characterized by mass spectrometry and NMR spectroscopy. The conformation of the peptide part in starting compounds and reduced derivatives is discussed on the basis of crystal structure determination of 6'-deoxo-9,10-dihydroergotamine dihydrate butan-2-one solvate as a representative member of the series.

Evidence for an irreversible interaction of bromocryptine with central dopamine receptors

The effects of the dopamine agonist bromocryptine on several measures dopaminergic function were assessed in the rat. Following inhibition of impulse flow with gamma-butyrolactone, and after dopa decarboxylase inhibition, dopa accumulation and its reversal by dopamine agonists is easily studied. In this model, bromocryptine (10 mg/kg, i.p.) caused a significant decrease in dopa accumulation in both the striatum and olfactory tubercle which was prevented, but not reversed, by the dopamine antagonist (+)-butaclamol (4 mg/mg, i.p.). The inactive isomer, (-)-butaclamol was without effect. Analysis of an vitro 3H-spiperone binding 2h after bromocryptine (10 mg/kg, i.p.) revealed a 30% decrease in the number of striatal dopamine receptors labelled (Bmax), with no change in receptor affinity for 3H-spiperone. No changes in binding were seen when animals were sacrificed 30 min or 48 h after bromocryptine. In extracellular single unit recording experiments, bromocryptine-induced depression of nigrostriatal dopamine cell firing was found to be largely reversible by the dopamine antagonist haloperidol when injected within 5 min of intravenous bromocryptine. However, when haloperidol was injected more than 20 min after bromocryptine, no reversal of bromocryptine-induced depression of cell firing was obtained. These results strongly suggest that bromocryptine interacts in an irreversible fashion with central dopaminergic receptors.