Utilization of Combined Chemical Modifications to Enhance the Blood-Brain Barrier Permeability and Pharmacological Activity of Endomorphin-1

Abstract

The endogenous -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, Cterminal 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. Guanidinoaddition and chloro-halogenation attenuated the -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 guanidinoaddition, 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 DPro-Gly-containing pentapeptide elicited significant and prolonged central-mediated analgesia upon subcutaneous administration, indicating that more peptides reached the CNS, eliciting greater analgesic effect.

Brainpeps: the blood–brain barrier peptide database

“…The lipophilicity (Liu et al 2006a) is expressed as log P, log D or delta log P. The partition coefficient (P) (Ecker and Noe 2004;Kumar et al 2007;Liu et al 2006a) for test compounds is defined as the concentration ratio of test compound found in the organic phase such as octanol (C org [nl -3 ]) and that in the aqueous phase (C wat [nl -3 ]), quantified as log P: BW body weight, GFR glomerular filtration rate (Lin 1998) log P ¼ log…”

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Endomorphins: potential roles and therapeutic indications in the development of opioid peptide analgesic drugs

“…132 Inspired by the ancient Chinese ''Yin'' and ''Yang'' balance theory, a series of EM-1 analogues 140-149 were synthesized to enhance stability and penetration to the CNS. We combined the useful methods of structural modification like cationization of the amino acid N-terminus, halogenation of the amino acid C-terminus, which can achieve a balance between ''anions'' and ''cations,'' and D-Ala, Sar, D-Pro-Gly substitution in the second position.…”

Development of PACAP38 Analogue with Improved Stability: Physicochemical and In Vitro/In Vivo Pharmacological Characterization

“…Metabolic stability study was carried out according to previously reported method with some modifications (Liu et al 2006). Male Wistar rats (ca.…”