Page - (000319) - in Biomedical Chemistry: Current Trends and Developments

a Ki value determined on the same occasion as for 33–46. b Previously determined Ki value c The metabolic stability data are expressed as mean ± SD. d Clint = in vitro intrinsic clearance. e t1/2 = in vitro half-life. In the methylated series, only compound 35 with internal N- methylation, retained binding affinity comparable to that of H-Phe-Phe- NH2 (26); unfortunately, no significant improvement in the metabolic stability was achieved. Methylation of the N-terminal resulted in 22-fold lower affinity, but was found to have a pronounced impact on the stability, which increased the half-life 50 times (cf. 26 and 33). C- terminal methylation (37) was accompanied by a roughly 20-fold decrease in binding affinity, but without improvement of the stability. A potential reason for the reduced binding affinity of 37 is that the secondary amide loses a hydrogen bond donor feature compared to 26, which may be important for the affinity. Incorporation of an α-methyl amino acid (34 and 36) also reduced the binding affinity, but less than for the N-methyl amino acids in the terminal parts of H-Phe-Phe-NH2. A 2- to 3-fold increase in the half-life