A nanomedicine approach for the treatment of long-lasting pain.

This study explores the potential of a nanomedicine approach, using Leu-enkephalin-squalene nanoparticles (LENK-SQ NPs) for managing long-lasting pain. It was observed that the nanomedicine significantly improved the pharmacological efficacy of the Leu-enkephalin, a fast metabolized neuropeptide, in a rat model of acute inflammatory pain, providing local analgesic effect, while minimizing potential systemic side effects by circumventing central nervous system. The LENK-SQ NPs were tested in a rat model of postoperative pain (Brennan's rodent plantar incision model) using continuous infusion via Alzet® pump, with an additional bolus injection. The analgesic activity was assessed through stimulus-evoked methods, such as the von Frey and Hargreaves tests. Both mechanical and thermal hyperalgesia were significantly reduced at days 2 and 3 post-incision. An additional pharmacokinetic study was conducted, showing that LENK-SQ NPs allowed a sustained circulation of the neuropeptide under its prodrug form. On the other hand, the biodistribution of fluorescently labelled LENK-SQ NPs revealed their selective accumulation in the incised paw within the first hour post administration, followed by a disassembly of the NPs, starting 24 h later. The study proposes the following multi-step mechanism for the anti-nociceptive pharmacological activity of LENK-SQ NPs: (i) protection of the neuropeptide from metabolization into the bloodstream, (ii) targeted accumulation of the nanoparticles within the incised painful tissue and (iii) gradual release of LENK at the onset of the inflammatory process, leading to the observed analgesic activity.