Interaction Between Ropivacaine and a Self-Assembling Peptide: A Nanoformulation for Long-Acting Analgesia.

Introduction: Ropivacaine as a conventional local anesthetic has been used more and more frequently in the treatment of postoperative pain, but its analgesic effect can only last for several hours. In order to fulfill the clinic requirement for long-term analgesia, a long-acting ropivacaine nanocrystal formulation was fabricated through the interaction between ropivacaine and a self-assembling peptide. Methods: Transmission electron microscopy, dynamic light scattering, circular dichroism and fluorescence spectrometry were used to examine the structural changes caused by the interaction between ropivacaine and the peptide. Scanning electron microscopy, dynamic light scattering, Fourier transform infrared spectrometry, X-ray diffraction and optical microscopy were used to characterize the ropivacaine-peptide nanocrystal. In vitro drug release and pharmacokinetics study were conducted to evaluate the slow-release profile of the nanocrystal formulation. A rodent cutaneous trunci muscle reflex model was used to evaluate the nociceptive blockade effects, and histological analysis was used to evaluate the local toxicity. A rodent plantar incisional pain model was used to evaluate the analgesic effect. Results: Soluble ropivacaine monomers interacted with the Q11 peptide through π-π stacking and remolded its self-assembling structure, leading to the formation of drug/peptide nanoparticles which could be mineralized to form drug/peptide nanocrystals by adjusting the pH. Under physiological condition, the nanocrystals could release free ropivacaine slowly. As evaluated in rodent models, the anesthetic and analgesic effects of this formulation were significantly extended without causing toxicity. Conclusion: Based on the interaction between ropivacaine and Q11, a controllable biomineralization process could be induced to obtain homogeneous nanocrystals, which could be used as an injectable long-acting analgesic formulation. This crystallization strategy utilizing the peptide-drug interaction also provided a promising pathway to fabricate long-acting formulations for many other small molecular drugs.