A new painkiller nanomedicine to bypass the blood-brain barrier and the use of morphine

The clinical use of endogenous neuropeptides has historically been limited due to pharmacokinetic issues, including plasma stability and blood-brain barrier permeability. In this study, we show that the rapidly metabolized Leu-enkephalin (LENK) neuropeptide may become pharmacologically efficient owing to a simple conjugation with the lipid squalene (SQ). The corresponding LENK-SQ bioconjugates were synthesized using different chemical linkers in order to modulate the LENK release after their formulation into nanoparticles. This new SQ-based nanoformulation prevented rapid plasma degradation of LENK and conferred on the released neuropeptide a notable antihyperalgesic effect that lasted longer than after treatment with morphine in a rat model of inflammation (Hargreaves test). The biodistribution study as well as the use of brain-permeant and -impermeant opioid receptor antagonists indicated that LENK-SQ NPs act through peripherally located opioid receptors. This study represents a novel nanomedicine approach, allowing the specific delivery of LENK neuropeptide into inflamed tissues for pain control.