Design of a Potent Combined Pseudopeptide Endothelin-A/Endothelin-B Receptor Antagonist, Ac-dBhg¹⁶-Leu-Asp-Ile-[NMe]Ile-Trp²¹(PD 156252):  Examination of Its Pharmacokinetic and Spectral Properties

Abstract

The endothelins (ETs) are a family of bicyclic 21-amino acid peptides that are potent and prolonged vasoconstrictors. It has been shown that highly potent combined ETA/ETB receptor antagonists can be developed from the C-terminal hexapeptide of ET (His16-Leu17-Asp18-Ile19-Ile20-Trp21), such as Ac-(D)Dip16-Leu-Asp-Ile-Ile-Trp21 (PD 142893) and Ac-DBhg16-Leu-Asp-Ile-Ile-Trp21 (PD 145065). However, these compounds are relatively unstable to enzymatic proteolysis as determined in an in vitro rat intestinal perfusate assay. This instability is thought to be due to carboxypeptidase activity. In fact, incubation of PD 145065 with carboxypeptidase inhibitors greatly increased its half-life in rat intestinal perfusate. By performing a reduced amide bond and N-methyl amino acid scan, it was discovered that N-methylation of Ile-20 resulted in a compound (Ac-DBhg16-Leu-Asp-Ile-[NMe]Ile-Trp21, PD 156252) that retained full receptor affinity at both endothelin receptor subtypes along with enhanced proteolytic stability and cellular permeability. Interestingly, N-methylation of this bond allows the cis configuration to be readily accessible which greatly alters the preferred structure of the entire molecule and may be responsible for the observed enhanced metabolic stability.

Improving Oral Bioavailability of Peptides by Multiple N‐Methylation: Somatostatin Analogues

“…One of the techniques suggested to improve the enzymatic stability of peptides is N-methylation. [8,9] We recently developed a simplified method which allows fast and efficient multiple N-methylation of peptides on solid support. [10] This simplified synthetic capability led us to study the influence of multiple N-methylation of the peptide backbone on its conformation and bioactivity.…”

Synthesis and structure–activity relationship studies of peptidomimetic PKB/Akt inhibitors: The significance of backbone interactions

“…1) is a powerful tool for structure-activity relationship studies and is often used to examine the effects of local backbone modifications on the potency of a known peptide sequence. N-Methylation has been shown to improve important pharmacological parameters such as lipophilicity, 18 bioavailability, 18,19 proteolytic stability, 19,20 conformational rigidity, 21 and duration of action. 19 N-Methylation may also result in enhanced potency, [22][23][24] new receptor subtype selectivity 25,26 and the conversion of an agonist into an antagonist.…”

Recent Advances in the Synthesis and Applications of Reduced Amide Pseudopeptides

“…Other research groups have also described the preparation of reduced amide analogues of different endogenous peptides, such as, corticotropin-releasing factor [41], endothelin-A and B [42], and osteogenic growth peptide [43].…”