Page - (000323) - in Biomedical Chemistry: Current Trends and Developments

except 37, were classified as having high permeability, with the highest permeability observed for the α-methylated analogs 34 and 36, 18 × 10-6 cm s-1 and 20 × 10-6 cm s-1, respectively. However, the methylated compounds were not actively transported, as can be seen from the PepT1/K1 ratio. Furthermore, the peptides also displayed efflux (the ba/ab ratio ranging from 3 to 14), where compounds 35 and 36 showed the highest tendency towards efflux and were equal to 26. Compared to the methylated analogs, the rigidified and the C- terminal-phenylalanine-modified analogs possessed much lower permeability (ranging from 0.02 × 10-6 cm s-1 to 4.4 × 10-6 cm s-1). Satisfyingly, the high affinity analog 40 turned out to have high permeability (3.6 × 10-6 cm s-1). In this series, the efflux was much higher; 8−95 times, in the b–a direction than in the a–b direction. The stereochemistry of the compounds also seemed to influence the predisposition for efflux. Thus, the compounds 38, 40 and 42 all showed lower efflux ratios than their diastereomeric counterparts 39, 41 and 43. The replacement of the phenylalanine in the N- or C-terminal by the 3-phenylproline moiety seemed advantageous in improving the active uptake of these compounds, since a slight increase in the PepT1/K1 ratio was observed (cf. 26 vs. 38, 39, 40 and 41). 3.3.3 Conclusion The case study presented herein is an illustrative example of rational design of drug-like molecules mimicking the actions of a bioactive peptide. It highlights the importance working in an iterative manner, where structural modifications of compounds are evaluated both from a pharmacodynamic and pharmacokinetic perspective. The optimization process, starting with the bioactive heptapeptide SP1-7 and the tetrapeptide EM-2, resulted in the remarkable discovery of the dipeptide H-Phe-Phe-NH2, which was equipotent to endogenous SP1-7 and had a higher binding affinity than EM-2. However, in the ADME assessment, the dipeptide showed poor metabolic stability and permeability, and suffered from high efflux rates. By introducing local constraints in the C-terminal of H-Phe-Phe-NH2, via the introduction of