Improved bioavailability to the brain of glycosylated Met-enkephalin analogs

The blood–brain barrier prevents the entry of many potentially therapeutic peptide drugs to the brain. Glycosylation has shown potential as a methodology for improving delivery to the CNS. Previous studies have shown improved bioavailability and improved centrally mediated analgesia of glycosylated opioids. In this study we investigate the effect of glycosylation on the cyclic opioid peptide [ d-Cys 2,5,Ser 6,Gly 7] enkephalin. The peptide was glycosylated on the Ser 6 via an O-linkage with various sugar moieties and alignments. The peptides were then investigated for receptor binding, physiochemical attributes, in situ brain uptake in female Sprague–Dawley rats and antinociception in male ICR mice. Glycosylation resulted in a slight decrease in affinity to the δ-opioid receptor, and mixed effect on binding to the μ-opioid receptor. There was a significant decrease in lipophilicity resulting from glycosylation and a slight reduction in binding to bovine serum albumin. In situ perfusion showed that brain uptake was improved by up to 98% for several of the glycosylated peptides, and the nociceptive profiles of the peptides, in general, followed the rank order of peptide entry to the brain with up to a 39-fold increase in A.U.C.