Synthesis and evaluation of the physicochemical properties of esterase‐sensitive cyclic prodrugs of opioid peptides using an (acyloxy)alkoxy linker

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Abstract
The objective of this work was to synthesize the cyclic prodrugs 1 and 2 of [Leu5]‐enkephalin (Tyr‐Gly‐Gly‐Phe‐Leu‐OH) and DADLE (Tyr‐D‐Ala‐Gly‐Phe‐D‐Leu‐OH), respectively, using an (acyloxy)alkoxy linker. The cyclic prodrugs 1 and 2 were synthesized via a convergent method using the (acyloxy)alkoxy promoiety that connected the C‐ and N‐terminus of the peptides. The key intermediates were compounds 6a and 9a for cyclic prodrug 1 and compounds 6b and 9b for cyclic prodrug 2. The key intermediates 6a and 9a (or 6b and 9b) were coupled to give compound 10a (or 10b). The N‐ and C‐terminus protecting groups were removed from 10a and 10b to give compounds 11a and 11b, respectively, which were then treated with HBTU to give 1 and 2 in 40% and 53% yields, respectively. The cyclic prodrugs 1 and 2 exhibited Stokes–Einstein molecular radii similar to those of [Leu5]‐enkephalin and DADLE; however, the cyclic prodrugs were shown to be significantly more lipophilic than the corresponding opioid peptides, as determined by partitioning experiments using immobilized artificial membrane (IAM) column chromatography. In addition, the cyclic prodrugs exhibit stable solution conformations, which reduce their hydrogen bonding potentials. Based on these physicochemical characteristics, the cyclic prodrugs 1 and 2 should have exhibited better transcellular flux across the Caco‐2 cell monolayer than [Leu5]‐enkephalin and DADLE, respectively. However, the cyclic prodrugs 1 and 2 were shown in separate studies to be substrates for P‐glycoprotein, which significantly reduced their ability to permeate across Caco‐2 cell monolayers. When P‐glycoprotein was inhibited, the permeability characteristics of prodrugs 1 and 2 were consistent with their physicochemical properties.