Introduction of a cis-prolyl mimic in position 7 of the peptide hormone oxytocin does not result in antagonistic activity

Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology, EPFL-BCH, CH-1015 Lausanne, Switzerland.
Journal of Medicinal Chemistry (Impact Factor: 5.48). 11/2005; 48(21):6553-62. DOI: 10.1021/jm049205z
Source: PubMed

ABSTRACT New insights into the structure-activity relationship of the peptide hormone oxytocin are presented. Incorporation of the novel cis-prolyl mimic 2,2-dimethyl-1,3-thiazolidine-4-carboxylic acid (pseudoproline, PsiPro) at position 7 of the hormone yielded the analogue [Cys(Psi(Me,Me)pro)]7oxytocin (1) that showed a 92-95% induction of the cis peptide bond conformation between Cys6 and PsiPro7, as determined by one- and two-dimensional NMR spectra in water and in DMSO-d6. The impact of the dimethyl moiety regarding conformation and bioactivity was investigated by the synthesis of the corresponding dihydro compound, [Cys(Psi(H,H)pro)]7oxytocin (2). Biological tests of the uterotonic activity, the pressor activity, and the binding affinity to the rat and human oxytocin receptors were carried out. As a most significant result, no antagonistic activities were found for both the cis-constrained analogue 1 and analogue 2, suggesting that a cis conformation between residues 6 and 7 of the molecule does not result in antagonistic activity. However, the about 10-fold reduction in agonistic activity of 1 as compared to oxytocin is consistent with the reduction of the trans conformation from 90% for oxytocin to 5-8% for compound 1. Compound 1 retained a high binding affinity for the oxytocin receptor, with K(i) values of 8.0 and 1.9 nM for the rat and the human receptor, respectively. The correlation between the biological activities and the cis contents obtained from NMR analysis for compounds 1, 2, and oxytocin leads to the hypothesis that a cis/trans conformational change plays an important role in oxytocin receptor binding and activation.

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    • "OT and VP hormones are intensively investigated using different methods and methodologies. E.g., the structure-activity relationship of the peptide hormone oxytocin is constantly investigated using NMR experiments [20], or can be a target of numerous patents as in the case of vasopressin [21]. Both studied peptides are nonapeptides. "
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    • "The results lead however to the hypothesis that the cis/trans conformational change is playing a role in OT receptor binding and activation. There was one interesting finding and thus that in the absence and presence of magnesium ions, a considerable change was observed in the pattern of the HR protons of residues 3 and 1, indicating conformational changes that influence activity (Wittelsberger et al. 2005). Also in the case of cyclic analogues, the structural analysis revealed b-turns around residues Tyr 2 and Ile 3 , which differed from the previously discussed b-turn geometry around residues 3 and 4 that was ascribed to the conformation of OT agonists (Hruby and Lebl 1987; Oldziej et al. 1995). "
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