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

Abstract

In an attempt to improve the membrane permeabilities of opioid peptides, we have synthesized cyclic prodrugs of [Leu5]-enkephalin and DADLE using a coumarinic acid or a phenylpropionic acid linker. The synthesis of the coumarinic acid- and phenylpropionic acid-based cyclic prodrugs followed similar strategies. Key intermediates were the compounds with the C-terminal amino acids of opioid peptides (L-Leu, [Leu5]-enkephalin; D-Leu, DADLE) attached to the phenol hydroxyl group and the remaining amino acids of the peptide linked via the N-terminal amino acid (L-Tyr) attached to the carboxylic acid groups of the prodrug moieties (coumarinic acid or propionic acid). Cyclization of these linear precursors gave the cyclic prodrugs in 30-50% yields. These cyclic prodrugs exhibited excellent transcellular permeation characteristics across Caco-2 cell monolayers, an in vitro model of the intestinal mucosa. To correlate the cellular permeabilities of these cyclic prodrugs with their physicochemical properties, we calculated their Stokes-Einstein molecular radii from their diffusion coefficients which were determined by NMR and we determined their membrane interaction potentials using immobilized artificial membrane (IAM) column chromatography. The cyclic prodrugs exhibited molecular radii similar to those of the parent compounds, [Leu5]-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 prodrugs is apparently due to the alteration of their lipophilicity and hydrogen bonding potential, but not their molecular sizes.

Evaluation of the Permeation Characteristics of a Model Opioid Peptide, H-Tyr-d-Ala-Gly-Phe-d-Leu-OH (DADLE), and Its Cyclic Prodrugs across the Blood-Brain Barrier Using an In Situ Perfused Rat Brain Model

“…The P app values were determined using single time point analysis, as described under 2002a,b), we were not surprised to observe the low P app values of the three cyclic prodrugs across the BBB, which were comparable with the P app values of DADLE and sucrose (a nonpermeable marker) ( Table 1). The poor BBB permeation of these prodrugs did not correlate with their physicochemical properties (e.g., no charge, high hydrophobicity, and low hydrogen-bonding potential) (Bak et al, 1999b;Wang et al, 1999;Ouyang et al, 2002a), but they were very consistent with their poor cell permeation characteristics observed in cell culture models (Bak et al, 1999a;Ouyang et al, 2002b;Tang and Borchardt, 2002a,b).…”

“…DADLE and [Leu 5 ]-enkephalin were purchased from SigmaAldrich (St. Louis, MO). Prodrugs of DADLE and [Leu 5 ]-enkephalin were synthesized in our laboratory following procedures described previously (Bak et al, 1999b;Wang et al, 1999;Ouyang et al, 2002a). All other chemicals were high grade and were purchased from Aldrich Chemical Co. (Milwaukee, WI) or Acros Organics distributed by Fisher Scientific (Houston, TX).…”

“…1). Unlike DADLE, which is hydrophilic and charged, AOA-DADLE (Bak et al, 1999b), CA-DADLE (Wang et al, 1999), and OMCA-DADLE (Ouyang et al, 2002a) are lipophilic and uncharged. The physicochemical properties of these cyclic prodrugs of DADLE are indicative of solutes that have good cell membrane permeation characteristics (Pauletti et al, 1997).…”

“…In contrast, GF-120918 had no effect on the P app value of DADLE. In addition, the observed bioconversions of the prodrugs to DADLE in the rat brains after 240-s perfusion were very low (5.1% from AOA-DADLE, 0.6% from CA-DADLE, and 0.2% from OMCA-DADLE), which was consistent with the in vitro bioconversion rates determined previously in rat brain homogenates.In an attempt to improve the blood-brain barrier (BBB) permeation of H-Tyr-D-Ala-Gly-Phe-D-Leu-OH (DADLE), an opioid peptide (Hill and Pepper, 1978;Iyengar et al, 1987;Prokai-Tatrai et al, 1996), our laboratory has synthesized cyclic prodrugs of this peptide using an acyloxyalkoxy (AOA) linker (Bak et al, 1999b), a coumarinic acid (CA) linker (Wang et al, 1999), and an oxymethyl-modified coumarinic acid (OMCA) linker (Ouyang et al, 2002a) (Fig. 1).…”

In Vitro Stability and In Vivo Pharmacokinetic Studies of a Model Opioid Peptide, H-Tyr-d-Ala-Gly-Phe-d-Leu-OH (DADLE), and Its Cyclic Prodrugs

“…Having these criteria in mind, our laboratory synthesized cyclic prodrugs of the opioid peptide H-Tyr-D-Ala-Gly-Phe-DLeu-OH (DADLE) using an acyloxyalkoxy (AOA) linker (Bak et al, 1999b), a coumarinic acid (CA) linker (Wang et al, 1999), and an oxymethyl-modified coumarinic acid (OMCA) linker (Ouyang et al, 2002b) (Fig. 1).…”