Abstract
The endogenous μ-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 μ-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-Pro2, also increased the overall lipophilicity to some extent. Among the peptides tested, intracerebroventricular injection of guanidino-[d-Ala2, p-Cl-Phe4]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.
Footnotes
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This work was supported by the National Natural Science Foundation of China (Grants 20525206, 20372028, and 20472026), by the Specialized Research Fund for the Doctoral Program in Higher Education Institutions, and by the Chang Jiang Program of the Ministry of Education of China.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.106.106484.
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ABBREVIATIONS: CNS, central nervous system; BBB, blood-brain barrier; EM, endomorphin; TLC, thin-layer chromatography; MS, mass spectrometry; GPI, guinea pig ileum; MVD, mouse vas deferens; RP-HPLC, reversed-phase high-performance liquid chromatography; CL, control latency; %MPE, percentage of maximal possible effect; s.c., subcutaneous; i.c.v., intracerebroventricular
- Received April 18, 2006.
- Accepted June 26, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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