Synthesis and evaluation of the physicochemical properties of esterase-sensitive cyclic prodrugs of opioid peptides using coumarinic acid and phenylpropionic acid linkers

In an attempt to improve the membrane permeabilities of opioid peptides, we have synthesized cyclic prodrugs of [Leu(5)]-enkephalin and DADLE using a coumarinic acid or a phenylpropionic acid linker. The synthesis of the coum arinic acid- and phenylpropionic acid-based cyclic prodrugs followed simila r strategies. Key intermediates were the compounds with the C-terminal amin o acids of opioid peptides (L-Leu, [Leu(5)]-enkephalin; D-Leu, DADLE) attac hed to the phenol hydroxyl group and the remaining amino acids of the pepti de linked via the N-terminal amino acid (L-Tyr) attached to the carboxylic acid groups of the prodrug moieties (coumarinic acid or propionic acid). Cy clization of these linear precursors gave the cyclic prodrugs in 30-50% yie lds. These cyclic prodrugs exhibited excellent transcellular permeation cha racteristics across Caco-2 cell monolayers, an in vitro model of the intest inal mucosa. To correlate the cellular permeabilities of these cyclic prodr ugs with their physicochemical properties, we calculated their Stokes-Einst ein molecular radii from their diffusion coefficients which were determined by NMR and we determined their membrane interaction potentials using immob ilized artificial membrane (IAM) column chromatography. The cyclic prodrugs exhibited molecular radii similar to those of the parent compounds, [Leu(5 )]-enkephalin and DADLE. However, these cyclic prodrugs were shown to have much higher membrane interaction potentials than their corresponding opioid peptides. Therefore, the enhanced cellular permeation of the cyclic prodru gs is apparently due to the alteration of their lipophilicity and hydrogen bonding potential, but not their molecular sizes.