Morphiceptin Research Articles

Effect of guanidino modification and proline substitution on the in vitro stability and blood–brain barrier permeability of endomorphin II

Endomorphin II (ENDII), an endogenous ligand for the μ‐opioid receptor, was investigated as a possible analgesic with fewer side effects than morphine. To improve CNS entry of END II, structural modification was also examined to determine whether Pro4 substitution and cationization affected physico‐chemical characteristics, blood–brain barrier (BBB) transport, and analgesic profile. END II and its Pro4‐substituted analog, Morphiceptin (MOR), were cationized by guanidino (GU)‐addition. MOR was seven times less lipophilic than END II, whereas GU‐addition decreased lipophilicity of both peptides. MOR did not affect in vitro BBB permeability; however, GU‐addition increased permeability of MOR by 31%. MOR decreased protein binding by 23% compared to END II, whereas GU‐addition increased protein binding of both peptides by 71 and 113%, respectively. MOR increased brain t1/2 compared to END II. GU‐addition significantly increased t1/2 of MOR and END II in both brain (sixfold and 10‐fold, respectively) and serum (over 10‐fold). Pro4‐substitution and GU‐addition enhanced the in vivo analgesia profiles of i.v. administered END II and MOR, but decreased i.c.v. analgesia profiles. This study demonstrates Pro4‐substitution decreases protein binding and enhances brain stability while cationization enhances both brain and serum stability with variable effects on BBB permeability. The analgesic profiles show that both Pro4‐substitution and cationization enhance i.v. analgesia and thus, are promising structural modifications for the development of successful opioid drugs. © 2002 Wiley‐Liss Inc. and the American Pharmaceutical Association J Pharm Sci 91:2140–2149, 2002

Characterization of the bioactive form of linear peptide antagonists at the omega-opioid receptor.

The stereochemical requirements for omega-opioid receptor binding of a series of linear peptide antagonists with a novel conformationally restricted Phe analogue (Tic) as a second residue were examined by using a variety of computational chemistry methods. The omega-opioid receptor analogues with significant affinity, Tyr-Tic-NH2(TI-NH2), Tyr-Tic-Phe-OH(TIP), Tyr-Tic-Phe-NH2(TIP-NH2), Tyr-Tic-Phe-Phe-OH(TIPP), Tyr-Tic-Phe-Phe-NH2)(TIPP-NH2), and the low affinity omega-opioid peptides Tyr-Pro-Phe-Pro-NH2(morphiceptin) and Tyr-Phe-Phe-Phe-NH2 (TPPP-NH2), were included in this study. The conformational profiles of these peptides were obtained by consecutive cycles of high and low temperature molecular dynamic stimulations, coupled to molecular mechanical energy minimization carried out until no new conformational minima were obtained. Comparing the results for TPPP-NH2 and TIPP-NH2, the presence of the conformationally restricted Tic residue did not greatly reduce the number of unique low energy conformations, but did allow low energy conformers involving cis bonds between the first two residues. The conformational libraries of these peptides were examined for their ability to satisfy the three key ligand components for receptor recognition already identified by previous studies of high affinity cyclic (Tyr1-D-Pen2-Gly3-Phe4-D-Pen5) enkephalin (DPDPE) type agonists: a protonated amine group, an aromatic ring, and a lipophilic moiety in a specific geometric arrangement. Two types of conformations common to the five high omega-opioid affinity L-Tic analogues were found that satisfied these requirements, one with a cis and the other with a trans peptide bond between the Tyr1 and Tic2 residues. Moreover, both the Tic2 and Phe3 residues could mimic the hydrophobic interactions with the receptor of the Phe4 moiety in the cyclic DPDPE type agonists, consistent with the appreciable affinity of both di- and tripeptides. The low omega-opioid receptor affinity of morphiceptin can be understood as the result of conformational preferences that prevent the fulfillment of this pharmacophore for recognition.

Depressor and bradycardic effect following intrathecal injection of [NMePhe 3,D-Pro 4]morphiceptin in rats

The intrathecal injection of 1.25, 2.5 and 5 μg of [NMePhe 3,D-Pro 4]morphiceptin (MC), a highly selective μ receptor agonist, in pentobarbital-anesthetized rats resulted in a dose-dependent decrease of mean arterial pressure, reaching a maximal effect 1 h after its administration. A marked bradycardic effect paralleled the depressor effect. These effects were totally reversed by prior intrathecal injection of 10 μg of naloxone. The effect of MC was fully present in biterally vagotomized rats and in rats under artificial respiration. These results provide evidence for the opioid control of blood pressure and heart rate via μ receptors in the spinal cord.