Improving the Biopharmaceutical Properties of Cyclic Opioid Peptide Prodrugs

In an attempt to improve the "drugability" of opioid peptides, cyclic prodrugs of the opioid peptide DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH) were developed using a coumarinic acid linker (CA). CA-DADLE was metabolically unstable in the present of liver microsomes and human recombinant cytochrome P450-3A4 (hCYP3A4). The sites of metabolite formation were located on the Tyr1 and Phe4 amino acids. New cyclic prodrug analogs of CA-DADLE, CA-[Cha4,D-Leu5]-Enk and CA-[Cha4,D-Ala5]-Enk were designed, synthesized, and characterized. They showed increased lipophilicity, decreased hydrogen-bonding potential, and a unique solution conformation compared to DADLE. The cyclic prodrugs were metabolically unstable in liver microsomes, but the new cyclic analogs were found to be stable in the presence of hCYP3A4. The metabolite identification data and the microsomal stability data correlated well, showing metabolite formation only in the liver microsomal assays. The new cyclic prodrug analogs showed strong substrate specificity for P-glycoprotein, which limited their permeation in vitro and in vivo