Strategies to target kyotorphin analogues to the brain

The design, synthesis, and pharmacological evaluation of brain-targeted chemical delivery systems (CDS) for a kyotorphin analogue (Tyr-Lys) are described. The brain-targeted compound contains the active peptide in a packaged, disguised form, flanked between the lipophilic cholesteryl ester on the C-terminus and the 1,4-dihydrotrigonellyl redox targetor, attached to the N-terminus through strategically selected L-amino acid(s) spacer. It was found that for successful brain targeting, the ε-amine of Lys needs to be also converted to a lipophilic function. Through sequential enzymatic bioactivation, the Tyr-Lys dipeptide is released in a sustained manner, producing significant and prolonged analgesic activity as demonstrated by the rat tail latency test. An alternate strategy was also employed. Lys was replaced by a redox amino acid pair, Nys⁺ mutually implies Nys, the nicotinamide mutually implies 1,4-dihydronicotinamide analogues of Lys (Nys⁺ is 2-amino-6-(3-carbamoyl-1-pyridiniumyl)hexanoic acid). The Nys form is lipophilic and facilitates delivery in addition to the C- and N-terminal lipophilic functions. Enzymatic oxidation to Nys⁺ provides the lock-in, followed by removal of the lipophilic groups, releasing Tyr-Nys⁺ from the brain-targeted analogue (BTRA). Nys⁺ was shown to be an effective substitution for Arg or Lys. The activities of the CDS and BTRA, respectively, were antagonized by naloxone, supporting the designed brain- targeted processes. The most potent compound is the two-proline spacer containing CDS (CDS-PP), followed by the BTRA.