On the Importance of Metabolic Stability to Achieve High Oral Exposures for Cyclic Peptides

Following up on a previous publication in which we reported a high liver first pass effect in rats for the cyclic peptide (1) [Ala-Leu-NMe-D-Leu-NMe-Leu-Leu-D-Pro], we decided to investigate the type of metabolites formed and to suggest solutions to this problem. As a result of a bile duct cannulation study in rats and subsequent derivatization of this peptide by an isolated Cyp-enzyme, several hydroxylated variants were identified. Cyclopropyl-Ala (Cpa) residues as surrogates for Leu alleviated metabolism at these particular side chains. Significant progress was achieved, when in addition the D-Pro residue was exchanged by 4,4 difluoro-D-Pro (DiF-D-Pro). Albeit the Ala was kept constant in this process, in the corresponding in-vivo studies in rats, peptide (6) [Ala-Cpa-NMe-D-Cpa-NMe-Cpa-Cpa-4,4 difluro-D-Pro] exhibited mM exposures at 3mg/kg and an absolute oral bioavailability of > 90%. Thus, we emphasize the importance of controlling metabolism to achieve significant systemic exposure upon oral administration and suggest the Cpa- and DiF-D-Pro residues as metabolically stable isosteres for Leu, and D-Pro respectively.