Lipid Nanoparticles as Tools for the Administration of Active Natural Products Aimed to the Treatment of Nervous System Disorders

The nervous system is the most complex system of our body. It is divided into peripheral (PNS) and central nervous system (CNS), the latter being composed of the brain and spinal cord that constitute the control center of all functions body. The brain microenvironment, essential for its normal function, is ensured by the blood–brain barrier (BBB), a physical physiological barrier that represents the major obstacle to reach CNS. Due to this complex anatomy, most of the current therapeutic strategies remain ineffective for the CNS treatment. Nowadays, the incidence of CNS disorders has greatly increased and they represent a serious problem, not only concerning the high costs of treatment, but in addition, the quality of life of the patient. Among these, Alzheimer’s disease (AD) is a neurodegenerative disorder of which the main cause is a marked oxidative stress at the level of the encephalic cells. Recent studies indicate that the normalization of antioxidant capacity could represent a very promising therapeutic for the treatment of AD. Moreover, research has made a significant effort to develop innovative therapies based on natural compounds, as old synthetic drugs cause numerous side effects. Curcumin (CUR) and astaxanthin (AST) are two natural antioxidants that possess therapeutic properties for the treatment of AD, although they show poor biovailability due to their high lipophilicity. In order to overcome these limits that compromise their therapeutic use, the researchers' attention is focused on the development of innovative and efficient stealth carriers (sSLN) loaded with these antioxidants, capable of avoiding the defense line represented by the macrophages and improving the drug stability, in order to achieve good bioavailability in the brain. The best strategies to realize sSLN, suitable for parenteral administration, were to coat the CUR-SLN surface with hydrophilic polymer PEG (CUR-pSLN), while the AST-SLN with surfactant polysorbate 80 (AST-p80SLN). CUR-pSLNs were prepared by the solvent evaporation method obtaining spherical nanoparticles having a particle size suitable for parenteral administration (<200 nm). Nanoparticles showed a good stability for six months at room temperature, as confirmed by Turbiscan Technology. Furthermore, the obtained data showed that the freeze-drying of CUR-pSLNs under optimized conditions lead to a lyophilized sample with good reconstitution properties, thus protecting the drug. Finally, CUR-pSLNs showed greater antioxidant activity over time than free CUR, confirming the key role of encapsulation in preserving and therefore increasing the antioxidant activity of powerful active compounds. Regarding AST, AST-p80SLNs were prepared by solvent-diffusion technique showing a good mean particle size suitable for parenteral administration (<200 nm), as confirmed by TEM. In term of stability, AST-p80SLN showed an acceptable stability during six months of storage. Moreover, the formulation did not produce toxic effects on the cell cultures. In addition, AST-p80SLNs showed greater antioxidant activity over time than free AST. The protective effect of SLN was further demonstrated by UV stability assay confirming that the lipid shell protected AST from photodegradation. In conclusion, the pharmacological activity of the formulations have been evaluated by in vivo assay using transgenic mice TgCRND8. The obtained results showed that the cognitive deficit was completely recovered. Therefore, CUR-pSLNs and AST-p80SLNs could be regarded as a promising carriers for the treatment of CNS disorders, through systemic administration. An injury to the nerves of the PNS or the structures of the CNS causes a chronic painful sensation called neuropathic pain. The perception of pain involves nociceptors, of which TRPV1 receptor is the most studied. Capsaicin (CPS) is a natural agonist highly selective of TRPV1 that allows used for the treatment of thermal hyperalgesia in chronic pain conditions. Since prolonged exposure to CPS induces TRPV1 defunctionalization, CPS loaded lipid nanocarriers (CPS-SLNs) were formulated in order to optimize CPS release, thus preventing TRPV1 internalization and degradation. CPS-SLNs were prepared by using the solvent injection method showing a good homogeneity and a high encapsulation efficiency. AFM study revealed a regular shape of the SLN. In vivo study pointed out that CPS inclusion into SLN induced a lower pain response compared to drug dissolved in a standard vehicle (CPS-STD). Therefore, CPS-SLNs were able to provide a long term activation of TRPV1 receptors without the unwanted reduction of TRPV1 receptor expression. In conclusion, SLNs could be useful as a valid alternative to conventional vehicles when a controlled release of CPS is required to prevent TRPV1 defunctionalization after a prolonged activation of the receptor. Retina is defined the “window to the brain” as it considered integral parts of the CNS. It is the site of many ocular diseases. The conventional therapies (eye drop, ocular injection or ocular implant) show many disadvantages such as low bioavailability, high costs and low patient compliance. Recently, the development of lipid-based nanocarriers, in particular the NLC, has made it possible to overcome these problems and improve the ocular bioavailability of natural ophthalmic drugs. N-palmitoylethanolamide (PEA) is a molecule that has numerous therapeutic properties, in fact it can be considered a good candidate for the treatment of retinal pathologies, such as diabetic retinopathy. It has never been formulated as eye drops due to its high lipophilicity. This obstacle has been overcome by encapsulating this compound in the NLCs, as they are able to deliver PEA in the posterior eye segment and to improve the ocular bioavailability of drug. NLCs have been formulated using two well-known techniques, such as the high shear homogenization technique (HSH) and the method based on a combination of HSH technique and ultrasound (HSH / US), to obtain NLCs suitable for ocular administration. NLC1 formulated following the HSH/US method provided better results in terms of mean particle size (Z-Ave), population homogeneity (PDI), and ZP with respect to NLC2 formulated by HSH technique. This result outlines the important role of US in controlling the final dimension of a lipid nanoparticle dispersion. Stability studies by Turbiscan demonstrated that NLC1 have a greater physical stability compared to NLC2 due to the higher energy applied in the emulsification process used in the HSH/US method. As demonstrated by the Draize test, PEA-NLC formulation has high tolerability and stability. In vivo studies showed that the drug reaches the target site, the retinal tissue, through a transscleral route. This is demonstrable as no significant drug concentrations were found at the level of the inner segments of the eye. Furthermore, a progressive increase of the PEA concentration at the retinal level has been observed; the maximal concentration was about 4 hours after instillation. Therefore, the results demonstrated that PEA-NLC-based formulation for ophthalmic application, has a unique pharmacological profile both in terms of retinal distribution and inhibition of inflammation. Mangiferin (MGN) is another potential natural compound for the potential treatment of eye diseases. Although it has a wide range of pharmacological properties, including anti-inflammatory and antioxidant activities, its use in ophthalmology is compromised due to its high lipophilicity. This obstacle has been overcome by encapsulating this compound in the NLC. MGN-NLCs, formulated by HSH/US method, showed an appropriate particle size, good stability, high ophthalmic tolerability and adequate corneal permeability. Furthermore, the antioxidant activity of MGN-NLC was higher than free compound. This demonstrated that the carrier preserved the drug. In conclusion, these findings suggest that the NLC system is a potential strategy to improve ocular bioavailability of lipophilic drugs.