Nasal delivery of nanotherapeutics for CNS diseases: challenges and opportunities

CNS disorders & delivery challenges

CNS disorders represent neurological disorders that affect either the structure or function of the brain or spinal cord. CNS diseases include Alzheimer's disease, Parkinson's disease (PD), dementia, epilepsy, stroke, multiple sclerosis, encephalopathy and other cerebrovascular diseases [1]. Despite enormous research in disease pathogenesis and treatment strategies, there are generally significantly lower levels of new drug approvals for CNS disorders compared with other non-CNS indications. Recently, the US FDA accelerated the approval of β-amyloid-targeted aducanumab-avwa (Aduhelm^™, Biogen Inc., MA, USA), which is the first and only treatment for Alzheimer's disease. However, it is under continued approval contingent upon confirmatory trials [2]. In the current clinical setting, it appears that targeting the CNS is an unreached holy grail. The current situation portrays the unmet need to identify new targets, improve existing pharmacological treatments, repurpose old drugs for new indications and improve the form of drugs or delivery, thus reducing the time taken to develop a CNS drug and resources.

The biggest impediment to creating biological disease-modifying therapies for CNS diseases is the challenge drugs face in crossing the nearly insurmountable blood–brain barrier (BBB) and blood–cerebrospinal fluid barrier, especially with regard to large molecules [3]. With the exception of those endogenous materials needed for regulation of brain homeostasis (e.g., glucose, electrolytes, amino acids), the brain capillary endothelial cells, with extensive tight junctions, impede the transport of foreign and toxic substances into the brain from the blood supply. The unilateral transport pathways across the polarized and regulated BBB are controlled by tight junctions and active efflux systems or ATP-binding cassette transporters such as multidrug resistance-associated proteins and transmembrane P-glycoproteins.

Various delivery approaches have been investigated to achieve effective drug concentrations in the CNS. Invasive techniques harness intrathecal, intracerebral, intraventricular and interstitial administration. However, invasive procedures may not be viable and are complex. Alternatively, the BBB can also be temporarily disrupted using ultrasound radiation or hyperosmotic solutions. Pharmacological approaches for noninvasive delivery exploring the natural properties of the BBB and those confirming Lipinski's ‘rule of five’ often reveal challenges for large molecular weight and hydrophilic proteins. Nevertheless, the active efflux system extrudes the drug from the brain back into the external environment [4].

Intranasal delivery

Over the past decade, researchers have investigated the intranasal route as a noninvasive and better method for delivering vaccines, peptides and proteins. The uniqueness of this route is that it can circumvent the BBB, potentially allowing treatment of CNS disorders. It is the most accessible delivery route, with well-reported passive and active targeting approaches noted in the literature. Other substantial advantages include reduced systemic exposure, avoidance of sterile techniques, improved bioavailability, faster absorption and onset of action, circumvention of the hepatic first-pass effect and gastrointestinal degradation and increased patient compliance. Drugs administered via the nasal route are absorbed directly through either the olfactory sensory neuron pathway or the trigeminal nerve pathway and the extracellular convection/diffusion pathway and effectively reach the brain [5].

Low molecular weight and hydrophobic drugs are generally taken up intracellularly through absorption via olfactory sensory neurons that converge to form fila olfactoria in the olfactory bulb. Large hydrophilic molecules like antibodies are taken up extracellularly through paracellular diffusion from the respiratory and olfactory epithelia, creating a continuous channel to the olfactory bulb and further diffusing in the perineural space, reaching the CNS. The olfactory and trigeminal nerve systems terminate at the respiratory or olfactory epithelium of the nasal cavity. Nevertheless, the olfactory neuron or epithelium pathway is the major, faster route to accessing the CNS by either passive diffusion or receptor-mediated or fluid-phase endocytosis. This is evidenced by the large accumulation of drugs in the olfactory epithelium, olfactory bulb and cortex, followed by further distribution to other parts of the CNS within 25–30 min [6]. The trigeminal nerve enters the brainstem and spinal cord via the ophthalmic, maxillary and mandibular divisions, which further unify to form the trigeminal ganglion. Indirect transport to the CNS is also possible via the nasal vascular plexus, which infiltrates the carotid artery and lymph nodes. Formulation, lipophilicity, degree of ionization and molecular weight/size and nasal epithelial permeability enhancers influence the transportation pathways into the brain [7]. Uptake via unique nasal, anatomical and functional pathways is possible for most macromolecules, with the exception of specific ligand or receptor-mediated delivery.

Intranasal delivery of nanotherapeutics

Nanotherapeutics offer improved nose-to-brain delivery owing to their small size and ability to safeguard payloads from biological/chemical degradation and active efflux systems, control their release kinetics and harness the potential for specific targeting [3]. The physicochemical characteristics of nanoparticles, such as composition, surface hydrophobicity/hydrophilicity, size and surface charge, play a critical role in determining the fate of nanoparticles and their ability to interact with the biological environment. Traditional nanotherapeutics use excipients like hydrophilic polymers and gums to overcome nasal mucociliary clearance and increase mucoadhesion. Nanotherapeutics with high surface area to volume and hydrophilic polymers provide a greater interface for entangling and forming prolonged interactions with highly glycosylated mucins of the mucosa compared with larger structures. For example, Godfrey et al. reported enhanced nose-to-brain delivery of the metabolically labile endogenous peptide Leu5-enkephalin hydrochloride by encapsulating it in chitosan-based nanoparticles [8]. The synergism of the mucoadhesion exhibited by chitosan with the transient opening of tight junctions present on the mucosal epithelium resulted in perivascular pathway delivery of the nanoparticles to the olfactory bulb and cortex. The capillary pore size of the nasal cavity (13–17 nm) and olfactory bulb (<10 nm) may have restricted the peripheral systemic distribution of nanoparticles with a size of 20–300 nm.

Moreover, positively charged nanoparticles have been demonstrated to be more efficiently absorbed across olfactory cell membranes because of electrostatic attractions [9]. Lipid carriers are also reported to exhibit mucoadhesion attributable to higher electrostatic interactions, especially cationic lipids. Intranasal administration of cationic lipid-based nanoparticles loaded with ovalbumin in rats enables higher concentration and distribution of peptide in the brain at 1 h than aqueous ovalbumin solution [10]. Cationic lipids interact electrostatically with sialic acid residues of mucosal proteins, leading to greater cellular binding and adsorptive endocytosis and increased protein residence time in the nasal epithelium [11].

The semipermeable mucosal barrier also exhibits size and interaction filtering. Mistry et al. investigated the effect of polystyrene nanoparticle size on transit across the olfactory bulb after intranasal administration in mice and observed that the optimal size for olfactory bulb axonal transport was <100 nm [12]. However, both 100- and 200-nm-sized particles reached the olfactory epithelial cells, with smaller particles being able to penetrate the mucosal layer faster. Thus, a narrow particle size range with a low polydispersity index is crucial for better transport of drugs. Surface engineering of nanotherapeutics with mucus-penetrating polymers (e.g., polyethylene glycol and polyvinyl alcohol) and penetration enhancers (poloxamer 188 and polysorbate 80) enhances the transport of nanoparticles laden with peptides, proteins and small molecules [13].

Active targeting directly to the nasal epithelium is possible with lectin-engineered (wheat germ agglutinin) nanotherapeutics or cell-penetrating peptides (TAT, pentratin and oligoarginine) or other ligands like lactoferrin and Arg-Gly-Asp. However, unlike different receptor-mediated delivery approaches where the aim is accumulation in specific cells or receptors, active targeting in intranasal delivery aims at interactions with epithelial cells and favors translocation to the brain [13]. Intranasal delivery of anticancer drugs like paclitaxel loaded in RGD-modified PLGA nanoparticles has been found in a rat model to target regions of glioblastoma and inhibit growth [14]. In a mouse model, concurrent administration of temozolomide and chitosan–tripolyphosphate siRNA nanoparticles intranasally silences the galectin-1 gene (gene driving chemoresistance/immunotherapy resistance) and sensitizes the tumor microenvironment to chemotherapy, thereby increasing survival [15]. Small molecules, siRNA and genes have thus successfully demonstrated nose-to-brain delivery in preclinical models.

Challenges in clinical translation of intranasal delivery

The nasal route has emerged as the preferred route for CNS delivery, but challenges remain with regard to anatomical, physiological and nasal aerodynamics. The low volume of the nasal cavity restricts the volume that can be administered, thereby impeding high drug potency via nasal administration. Rapid mucociliary clearance resulting in cyclic dynamic physiological changes rarely permits prolonged exposure time with the nasal epithelium and may influence the absorption of drugs, predominantly hydrophilic peptides, proteins and nucleotides. Within this limited residence time, enzymes like proteases, endopeptidases and carboxypeptidases are reported to degrade proteins and peptides [16]. Finally, these drugs need to diffuse through the olfactory epithelium to access the nerve sheaths. Nevertheless, these properties no longer constrain the use of intranasal delivery, as nanotechnology modulates the physicochemical properties of the delivery carrier instead of the drug.

The main remaining limitation of nose-to-brain delivery is that the olfactory and respiratory epithelia used to assess the olfactory and trigeminal pathways are present in the posterior and top region of the nasal cavity. The posterior region is at times difficult to access even with normal inspiratory airflow. The respiratory epithelial region of the nasal cavity exhibits no direct absorption to the CNS and can be cleared via the nares; thus, there may be 0 to minimal chances for systemic absorption [7]. Nasal health can also impair absorption. For instance, rhinorrhea or a blocked nose may lead to expulsion of the medication from the nasal cavity. A short nasal cavity hold time is also possible. Thus, as a result of various physiological and anatomical considerations, accuracy and consistent drug administration may be limited.

Currently, the FDA provides guidelines that are limited to nasal devices such as pressurized metered-dose inhalers and mechanical liquid spray pumps for local use. It fails to offer any guidance for nasal delivery intended for systemic absorption. Moreover, mucosal inflammation, nasal polyps and septal deviations can limit deposition of sprays or inhalations [7].

Further, clinical translation of nanomedicines requires in-depth fundamental studies, including with regard to aggregation and clearance due to nanoparticle size, final fate in terms of toxicity, reticuloendothelial system elimination and systemic toxicity, and detailed investigations of the mechanism of action. The potential toxicity of nanoparticles to the nasal mucosa and CNS must also be taken into consideration. The actual contribution of each physicochemical property is not yet clear and has interrelated mechanisms. Careful selection of excipients and assessment of their potential toxicities are warranted while developing innovative delivery systems. Other issues such as the aggregation of nanoparticles and premature drug release have also hindered the clinical translation of nanocarriers.

Minimally invasive nasal depot approach

Creative delivery strategies have been explored to facilitate nasal delivery to the brain, harnessing the capacity of the nasal anatomical pathways coupled with the latest surgical advances. In a study by Bleier et al., endoscopic skull base surgery was used to remove the arachnoid membrane associated with the BBB and replace it with nasal mucosa, which is >1000 times more permeable than the native barrier [17]. This permitted direct communication between the nasal cavity and subarachnoid brain space. The researchers investigated a preclinical heterotopic mucosal engrafting technique within the nasal cavity. The intervening nasal mucosa, bone, dura and arachnoid membrane were removed, which created a direct window on top of the head of rats, exposing the brain. The cranial window was then repaired using a nasal mucosal graft from a donor rat in the same species, and a reservoir was placed over the implanted graft to deliver therapeutics to the brain across the permeable nasal graft. In an exploratory pilot study, GDNF (35 kDa) was given to a murine PD model and evaluated for therapeutic efficacy [18]. A thermosensitive liposome-in-gel (LiG) delivery system was also formulated to protect the encapsulated ovalbumin from degradation and achieve sustained release from the administration site to the brain through the implanted nasal graft. In the surgical model, the implanted nasal mucosal graft combined with the LiG delivery system resulted in efficient and sustained drug delivery to the brain. The delivery platform was further evaluated for the efficacy of BDNF AntagoNAT (AT) encapsulated as LiG [19]. BDNF AT-LiG was applied to the brain in both healthy rats and a diseased rat model of PD using the heterotopic mucosal engrafting technique. Results showed that BDNF AT was successfully delivered to the substantia nigra and cerebellar regions of the brain. Upregulated protein expression of BDNF and tyrosine hydroxylase in the two PD-relevant brain regions confirmed the neuroprotective effect of BDNF AT. This study proved that the endoscopic skull base approach of mucosal grafting could be adopted to overcome the inability of macromolecules to cross the BBB and also demonstrated that nanoscale liposomal formulations could further enhance both drug distribution and efficacy within the brain.

A minimally invasive nasal depot (MIND) technique was later developed as a safer alternative to the invasive mucosal engrafting approach, which requires surgical intervention [20]. In the MIND technique, the drug carrier is directly implanted or injected within the submucosal space of the olfactory epithelium, allowing the entire administered dose to be implanted within the tissue surrounding the olfactory neurons by a noninvasive endoscopic process with minimal discomfort. This technique overcomes the limitations associated with dose uniformity differences and ensures transepithelial diffusion, distribution and retention. BDNF AT-LiG was directly injected into this submucosal compartment using the MIND technique. The pharmacokinetic–pharmacodynamic modeling data indicated that the implantation of a depot containing BDNF AT enabled sustained drug delivery to the brain and significant upregulation of BDNF expression, with efficiency of intracerebroventricular administration approaching 40%. Considering the ease and safety of MIND, this technique holds tremendous potential for clinical translation in CNS drug delivery. The MIND technique provides a promising outlook and may support various therapeutic interventions in the treatment of CNS disease.

Conclusion

Demand for newer and advanced CNS therapies will be ongoing until the need to deliver or bypass the BBB with a safe, effective and cost-effective treatment is met. Even as the tremendous challenge of formulating medications and ensuring effective drug delivery to the desired brain regions for CNS indications is overcome, reliable biomarkers to track disease progression will still be necessary. Moreover, disease pathology may influence the cerebral perivascular space and deposition of specific proteins, such as β-amyloid or hyperphosphorylated tau, and increased perivascular space in ischemic stroke and traumatic brain injuries requires a thorough investigation. Nonetheless, intranasal delivery is paving a new path for the delivery of miRNA, dsDNA, siRNA, etc. Getting large, full-length protein antibodies for immunotherapies, antibody fragments such as Fab or single-domain antibodies through the CNS barrier to engage with their pathological target will necessitate a combination of techniques, including smart nanodelivery systems and advanced approaches like MIND.