## 1 Introduction  
In recent decades, solid lipid nanoparticles (SLNs) have been widely investigated as carriers for drug delivery by different administration routes [1; 2; 3; 4; 5]. The great deal of interest focused on SLNs has been due to their many advantages compared to other colloidal systems, including increased drug bioavailability and stability, adjustable technological properties, ability to control and/or to target drug delivery, low production cost, and ease scale-up [6; 7]. In addition, as SLNs are made up of a solid lipid core stabilized by surfactants in aqueous media, lipophilic drugs could be entrapped in the SLNs matrix,resulting in an improvement of drug water solubility. Therefore, such nanocarriers provide a useful tool to design aqueous formulations for poorly soluble drugs [8; 9; 10]. Recently, a dual-target mu opioid peptide receptor (MOPr)/delta opioid receptor (DOPr) anti-nociceptive ligand, named LP2 [11], containing a benzororphan nucleus (Figure 1), has been synthesized and investigated for its in vivo biological activities in animal models of acute and persistent pain [12]. The development of multitarget ligands has emerged as a promising strategy to enhance therapeutic efficacy while reducing adverse effects [13; 14; 15]. In particular, compounds with multitarget opioid activity are effective anti-nociceptive agents for pain management with a limited incidence of adverse effects, usually associated with clinically used opioid analgesics acting at a single target [16; 17; 18]. The improved pharmacological fingerprint of dual-target MOPr/DOPr agonists, which are able to simultaneously target MOPr and DOPr, is related to the co-expression of MOPr and DOPr in key sites for pain modulation, allowing their inter-modulatory interactions, both physically and functionally. LP2 was able to bind to and simultaneously activate MOPr (Ki = 1.08 nM, IC\({}_{50}\) = 21.5 nM) and DOPr (Ki = 6.6 nM, IC\({}_{50}\) = 4.4 nM) [11]. In the tail-flick test, LP2 produced a significant long-lasting anti-nociceptive effect, naloxone-reversed, with an ED\({}_{50}\) of 0.9 mg/kg i.p. in mice. Tests in models of persistent pain showed that LP2 elicited a significant anti-inflammatory effect (ED\({}_{50}\) = 0.88 and 0.79 mg/kg i.p) in both phases I and II of the formalin test in mice, without motor impairments in the Rotarod test [19]. In addition, in unilateral sciatic nerve chronic constriction injury (CCI) neuropathy in rats, LP2 ameliorated mechanical allodynia signs from the early phase of treatment up to 21 days post-ligatures [20]. Unfortunately, the lipophilic nature of LP2 is a major concern for its further development as a new therapeutic anti-nociceptive agent. As reported in the literature [21], many (more than 40%) drug candidates are poorly soluble in water [21]; therefore, several strategies have been investigated to overcome this drawback, including chemical modifications of drugs and/or incorporation into suitable delivery systems [8; 9; 10]. To address the issue of LP2's low water solubility allowing the design of formulations for parenteral administration, in the present work, this dual-target MOPr/DOPr anti-nociceptive ligand was incorporated into SLNs, and the resulting colloidal dispersions were assessed to determine their technological properties (mean particle sizes, polydispersity index, zeta potential, morphology, and stability) and in vitro LP2 release. As LP2 is a diastereoisomeric mixture due to the presence of a stereocenter at the (\(R\)/S)-2-methoxy-2-phenylethyll group as an N-substituent of the 6,7-benzororphan scaffold, a further aim of this work was the development of an effective analytical method to identify and quantify each LP2 diastereoisomer. The availability of a suitable and reliable analytical method was fundamental to assess the release rate of each diastereoisomer after loading LP2 as a racemic mixture into SLNs. Figure 1: Chemical structure of (\(2R\)/S)-N-2-methoxy-2-phenylethyll-6,7-benzororphan (LP2).