New opioid affinity labels containing maleoyl moiety

Affinity labeling mu opioid receptors with novel radioligands

1. A series of novel opiate ligands based upon 6alpha-naloxamine have been examined in opioid receptor binding assays. 2. Coupling an ethylamine spacer alone to 6-alpha-naloxamine gave a compound with relatively poor affinity for mu opioid receptors compared to naloxone, although it retained high affinity for kappa1 opioid receptors. Coupling a benzoyl group significantly increased the affinity. The presence at the 4-position of the benzoyl moiety of an amino-(NalAmiBen) or an azido-substituent (NalAziBen) did not significantly effect the affinity at mu receptors. However, iodinating the benzoyl moiety at the 3-position increased the affinity of the derivatives. 3. Two compounds were radiolabeled and evaluated in receptor binding assays. Both radioligands labeled sites in CHO cells stably transfected with the mouse MOR-1 clone. The amino coupound [125I]NalAmiBen and the azido derivative [125I]NalAziBen reversibly bound to membranes from CHO cells transfected with MOR-1 with high affinity in the dark. Exposure of [125I]NalAmiBen to UV did not alter the reversibility of binding, but exposure of [125I]NalAziBen to UV light led to the covalent coupling of the radioligand to the receptor. When run on SDS-PAGE, [125I]NalAziBen binding showed a band at approximately 70-80 kDa. A control corresponding to nonspecific binding failed to reveal any labeling. No bands were observed from membranes labeled with [125I]NalAmiBen.

An affinity label for δ-opioid receptors derived from [d-Ala2]deltorphin I*

A series of potential affinity label derivatives of the amphibian opioid peptide [D-Ala2]deltorphin I were prepared by incorporation at the para position of Phe3 (in the 'message' sequence) or Phe5 (in the 'address' sequence) of an electrophilic group (i.e. isothiocyanate or bromoacetamide). The introduction of the electrophile was accomplished by incorporating Fmoc-Phe(p-NHAlloc) into the peptide, followed later in the synthesis by selective deprotection of the Alloc group and modification of the resulting amine. While para substitution decreased the delta-opioid receptor affinity, selected analogs retained nanomolar affinity for delta receptors. [D-Ala2,Phe(p-NCS)3]deltorphin I exhibited moderate affinity (IC50=83 nM) and high selectivity for delta receptors, while the corresponding amine and bromoacetamide derivatives showed pronounced decreases in delta-receptor affinity (80- and >1200-fold, respectively, compared with [D-Ala2]deltorphin I). In the 'address' sequence, the Phe(p-NH2)5 derivative showed the highest delta-receptor affinity (IC50=32 nM), while the Phe(p-NHCOCH2Br)5 and Phe(p-NCS)5 peptides displayed four- and tenfold lower delta-receptor affinities, respectively. [D-Ala2,Phe(p-NCS)3]deltorphin I exhibited wash-resistant inhibition of [3H][D-Pen2,D-Pen5]enkephalin (DPDPE) binding to delta receptors at a concentration of 80 nM. [D-Ala2, Phe(p-NCS)3]deltorphin I represents the first affinity label derivative of one of the potent and selective amphibian opioid peptides, and the first electrophilic affinity label derivative of an agonist containing the reactive functionality in the 'message' sequence of the peptide.

Josef Rudinger Memorial Lecture 2002. The chemistry of the opioid receptor binding sites

Since the discovery of the opioid receptors and their endogenous ligands an immense research work has been devoted to the exploration of their specificity, the mechanism of ligand binding and ligand-receptor interactions. One of the main goals has been the location and characterization of the binding sites. The present review compiles the results achieved in this field in the last quarter of a century, and puts some questions concerning the success of these efforts.