endomorphin-1 and 2--6--dimethyltyrosine

Three dimethyl-L-tyrosine (Dmt) based peptide analogues were identified in a previous study as excellent agonists for the mu-opioid receptor showing very low K(i) values and good in vivo antinociceptive activity upon intracerebroventricular administration to mice. This activity decreased markedly when the compounds were delivered subcutaneously or orally. To establish the cause of this decrease of activity the apparent permeability across Caco-2 cell monolayers of each compound and their relative stability to the digestive enzymes present in the cell line has been determined and compared to that of the native peptide endomorphin 2. The compounds' permeabilities clearly correlate with their increasing lipophilicity suggesting that the analogues cross the monolayer via passive diffusion and the results show that the compound with high K(i) value for the mu-receptor (K(i)mu=0.114 nM) exhibited the highest permeability suggesting that this may be the better lead compound despite the lower binding affinity than that of compound 2 or 3.

Topics: Analgesics, Opioid; Blood-Brain Barrier; Caco-2 Cells; Chemistry, Pharmaceutical; Drug Design; Humans; Inhibitory Concentration 50; Injections, Intraventricular; Kinetics; Molecular Conformation; Oligopeptides; Peptides; Permeability; Receptors, Opioid; Tyrosine

[N-allyl-Dmt1]-endomorphin-1 and -2 ([N-allyl-Dmt1]-EM-1 and -2) are new selective micro-opioid receptor antagonists obtained by N-alkylation with an allyl group on the amino terminus of 2',6'-dimethyl-L-tyrosine (Dmt) derivatives. To further characterize properties of these compounds, their intrinsic activities were assessed by functional guanosine 5'-O-(3-[35S]thiotriphosphate) binding assays and forskolin-stimulated cyclic AMP accumulation in cell membranes obtained from vehicle, morphine, and ethanol-treated SK-N-SH cells and brain membranes isolated from naive and morphine-dependent mice; their mode of action was compared with naloxone or naltrexone, which both are standard nonspecific opioid-receptor antagonists. [N-allyl-Dmt1]-EM-1 and -2 were neutral antagonists under all of the experimental conditions examined, in contrast to naloxone and naltrexone, which behave as neutral antagonists only in membranes from vehicle-treated cells and mice but act as inverse agonists in membranes from morphine- and ethanol-treated cells as well as morphine-treated mice. Both endomorphin analogs inhibited the naloxone- and naltrexone-elicited withdrawal syndromes from acute morphine dependence in mice. This suggests their potential therapeutic application in the treatment of drug addiction and alcohol abuse without the adverse effects observed with inverse agonist alkaloid-derived compounds that produce severe withdrawal symptoms.

Topics: Analgesics, Opioid; Animals; Brain; Cell Line; Cell Membrane; Cyclic AMP; Guanosine 5'-O-(3-Thiotriphosphate); Male; Mice; Oligopeptides; Protein Binding; Radioligand Assay; Receptors, Opioid, mu; Substance Withdrawal Syndrome; Tyrosine