Abstract
$[Lys^7$]dermorphin, abbreviated K7DA, which has structural features similar to a metabolically stable $\mu$-opioid peptide agonist $[D-Arg^2, Lys^4$]dermorphin analogue (DALDA), but is intrinsically more potent with respect to binding to the $\mu$-opioid peptide receptor. The present studies report on attempts to enhance brain uptake of systemically administered K7DA by conjugation to a complex of streptavidin (SA) and the OX26 murine monoclonal antibody to the rat transferrin receptor, which undergoes receptor-mediated transcytosis through the blood-brain barrier (BBB). SA-OX26 conjugate mediates BBB transport of biotinylated therapeutics. The K7DA is monobiotinylated at the $\varepsilon$-amino group of the $[Lys^7$] residue with cleavable linker using NHS-SS-biotin. The brain uptake of $^{125}I$ labeled biotinylated K7DA ($^{125}I$-bio-SSa-K7DA) was very small and rapidly metabolized after intravenous injection. The brain uptake, expressed as percent of injected dose delivered per gram of brain, of the $^{125}I$-bio-55-K7DA bound to the SA-OX26 conjugate $^{125}I$-bio-SS-K7DA/SA-OX26) was 0.14$\pm$0.01, a level that is 2-fold greater than the brain uptake of morphine. The cleavability of the disulfide linker in vivo in rat plasma and brain was assessed with gel filtration HPLC and intravenous injection of labeled opioid chimeric peptides. The disulfide linker is stable in plasma in vivo but is cleaved in rat brain in vivo. In conclusion, these studies show that delivery of these potential opioid peptides to the brain may be improved by coupling them to vector-mediated BBB drug delivery system.
