Solid lipid nanoparticles as carriers for the synthetic opioid lp2: Characterization and in vitro release

A synthetic dual-target mu opioid peptide receptor/delta opioid peptide receptor anti-nociceptive ligand, named LP2, has emerged as a promising candidate for the management of acute and/or persistent pain, but its lipophilicity limits further developments as a therapeutic agent. In this work, to allow designing aqueous formulations of LP2 for parenteral administration, solid lipid nanoparticles (SLNs) were investigated as LP2 nanocarriers. LP2-loaded SLNs were prepared by the phase-inversion temperature method, showing good technological properties (small mean particle, size, low polydispersity index, good stability). As LP2 was a diastereoisomeric mixture of 2R/2S-LP2, an HPLC method was developed to identify and quantify each diastereoisomer, and this method was used to assess LP2 in vitro release from SLNs. The developed method, based on reverse-phase chromatography using an isocratic mobile phase consisting of 50% methanol and 50% triethanolamine at 0.3% (pH = 3 with trifluoroacetic acid), allowed efficient separation of 2R-and 2S-LP2 peaks and reliable quantification with intra-and inter-day precision and accuracy within the acceptability limit, expressed as relative standard deviation set at ≤15%. The results of this study suggest that the incorporation of LP2 into SLNs could be a promising strategy to design suitable formulations for further pharmacological studies involving LP2.