Structural and pharmacological studies of synthetic and endogenous opioid receptor ligands

The interaction of a diverse set of opioid alkaloids and peptides with various opioid receptors has been examined using biochemical and pharmacological techniques. Structural information on the compounds was obtained from single crystal X-ray diffraction and nuclear magnetic resonance studies, and modelled by computational methods. The introduction of a dithiocarbazate moiety into the 7a-position of a bridged thebaine was shown to afford a degree of μ selectivity in this class of nonselective compounds. X-ray diffraction analysis of this compound and comparison with the structure of [Met5]enkephalin showed the importance of the sulphydryl moiety. The conformation of [Leu5]enkephalin, in which the amino acid methionine is replaced by leucine, at the same receptor is unlikely to be similar. A series of morphinan derivatives which had been developed as μ-antagonists were evaluated. Substitution patterns of the morphinan ring nucleus and their effect upon activity were examined. X-ray analysis of several key compounds was performed. Unexpectedly a 3-hydroxymorphinan-6-one analogue showed an ability to differentiate apparently similar opioid Kreceptors. The implications in terms of K-receptor subtypes are discussed. The opioid receptor binding characteristics of structurally diverse K-receptor ligands were examined in two different buffer systems. Electrostatic modelling of the K-ligands, based upon crystal structure coordinates, was performed. From electrostatic potential maps a requirement for ligands acting at Kreceptors is postulated. Solution conformations of the endogenous K-ligand, dynorphin A(1-8), were determined by nuclear magnetic resonance studies and compared with the wo preferring [Leu5]enkephalin. Models were proposed based upon dihedral angles determined from HCtl-NH coupling constants, amide proton-deuteron exchange and amide proton temperature coefficient data. Candidate conformations were shown to be stable under dynamic simulation conditions. Electrostatic modelling of a chosen dynorphin An-8) conformation gave results comparable with the observed electrostatic model of the K-ligands. The proposed model is discussed in terms of its suitability as a retro-model for the active site ofthe K-opioid receptor.