title: Utilization of combined chemical modifications to enhance the blood-brain barrier permeability and pharmacological activity of endomorphin-1.
abstract: The endogenous mu-opioid receptor agonist, endomorphin (EM)-1, cannot be delivered into the central nervous system (CNS) in sufficient quantity to elicit analgesia when given systemically because it is severely restricted by the blood-brain barrier (BBB). To improve the physicochemical characteristics of EM-1 and subsequently achieve greater BBB permeation, we synthesized a series of EM-1 analogs by combining successful chemical modifications, including N-terminal cationization, C-terminal chloro-halogenation, and unnatural amino acid (d-Ala, Sar, and d-Pro-Gly) substitutions in position 2. Presently, their binding and bioassay activity, lipophilicity, stability, and antinociceptive activity were determined and compared. Guanidino-addition and chloro-halogenation attenuated the mu-receptor affinity to some extent, but they demonstrated differences in the influence on stability. It appeared that guanidino-addition contributed to brain stability enhancement for the greater part, whereas chloro-halogenation together with amino acid substitutions in position 2 was of more importance for the stability enhancement in serum than in brain. Determination of the octanol/buffer coefficient revealed that chloro-halogenation did compromise the decreased lipophilicity caused by guanidino-addition, and introduction of d-Ala as well as d-Pro-Gly, but not Sar, in place of l-Pro(2), also increased the overall lipophilicity to some extent. Among the peptides tested, intracerebroventricular injection of guanidino-[d-Ala(2), p-Cl-Phe(4)]EM-1 showed the strongest analgesia, being 3 times more potent than the parent peptide. We also found that in comparison with EM-1, the four d-Ala-containing tetrapeptides and the chloro-halogenated d-Pro-Gly-containing pentapeptide elicited significant and prolonged central-mediated analgesia upon subcutaneous administration, indicating that more peptides reached the CNS, eliciting greater analgesic effect.
authors.affiliation: Department of Biochemistry and Molecular Biology, School of Life Sciences, Lanzhou University, 222 Tianshui South Road, Lanzhou 730000, People's Republic of China.
authors.forename: Hong-Mei
authors.initials: HM
authors.lastname: Liu
authors.forename: Xue-Feng
authors.initials: XF
authors.lastname: Liu
authors.forename: Jin-Long
authors.initials: JL
authors.lastname: Yao
authors.forename: Chang-Lin
authors.initials: CL
authors.lastname: Wang
authors.forename: Ye
authors.initials: Y
authors.lastname: Yu
authors.forename: Rui
authors.initials: R
authors.lastname: Wang
mesh_terms: D000701:Analgesics, Opioid / Q000493:pharmacokinetics*; D000818:Animals; D001812:Blood-Brain Barrier; D004355:Drug Stability; D005260:Female; D006168:Guinea Pigs; D008297:Male; D051379:Mice; D009842:Oligopeptides / Q000737:chemistry / Q000493:pharmacokinetics* / Q000494:pharmacology*; D010539:Permeability; D011869:Radioligand Assay; D051381:Rats; D012995:Solubility; D013329:Structure-Activity Relationship
chemical_list: D000701:Analgesics, Opioid; D009842:Oligopeptides; C105293:endomorphin 1