Neurodegenerative diseases impact the lives of patients and their families. As the brain slowly deteriorates, patients lose control of their bodies, thoughts, and ultimately succumb to the disease. An aging population generates a higher incidence of Alzheimer’s disease, Parkinson’s disease, and other neurological disorders. Concurrently, the rise of Autism Spectrum disorders has become an interesting area of potential research. As of now, neurological medicines only treat the symptoms and rarely address the root cause – dying or misfiring neurons. Getting drugs into the brain has proven a difficult task because of the impermeable blood-brain barrier (BBB). Newly developed tools circumvent the BBB by going through the nose. Intranasal-to-brain (INB) delivery provides a hopeful avenue to slow, treat, and prevent the progression of the diseases of the central nervous system (CNS) with greater efficacy, less invasion and reduced toxicity. Medical professionals should recognize the difficulty of delivering drug to the brain and the need for easier, direct route such as nasal administration.
The Blood Brain Barrier
As the controlling organ of the body, the brain is locked behind two major defense mechanisms. The cranium provides a hard case that limits access to only highly invasive techniques that inject drugs through the skull directly into the parenchyma or cerebrospinal fluid. The BBB internally protects the CNS from potentially damaging foreign chemicals travelling through the bloodstream. Toxins and potential medications are blocked from neurons by several mechanisms. Despite the rich network of blood vessels reaching all areas of the brain, the endothelial lining of the capillaries are fused together with extremely tight junctions preventing the passage of many molecules.1 Only the smallest hydrophilic molecules can enter the brain through the paracellular route around the tight junctions. Some small, lipophilic molecules can cross through the epithelial cells via the transcellular route.2 Prodrugs can increase lipophilicity and nanoparticles can hide polar regions inside a PLGA liposome. Larger endogenous molecules are selectively allowed across the barrier through transport proteins, like insulin and transferrin, which we can use to transport drugs across.1 While we can make drugs lipophilic or hijack transporter proteins to cross the membrane, P-gp efflux pumps work to actively remove foreign compounds. Some drugs can be given with P-gp inhibitors to increase the chances of remaining inside the brain.3 All of these defense mechanisms make getting drugs into the brain incredibly difficult. Indeed, as little as 1% of an intravenous dose may reach and remain within the CNS. The larger doses required leads to toxic side effects elsewhere in the body.4 To treat the growing problem of neurodegenerative disorder, we need a less invasive, patient directed route of administration that provides rapid onset of action, higher brain concentrations, and lower systemic distribution and side effects. Going through the nose to the olfactory bulb provides each of our requirements.
The Nasal Anatomy
The nasal cavity performs three functions for the human body: warms air, filters large particles, samples odors. In direct contact with the external environment, the cavity is lined with mucosal epithelium. Mucus, secreted from surrounding glands, traps foreign bodies, while the cilia continuously push the mucus to the esophagus for elimination. Any nasal medication must be absorbed quickly or be washed into the stomach. When we breathe, air enters through the nares, past the vestibule and into a large nasal cavity. The empty space contains specialized folds, called conchae, which increase the surface area and create turbulence to help warm the air on the way to the lungs. Most nasal sprays for the treatment of rhinitis target the inferior and middle conchae. The rich vasculature in the area provides access to systemic circulation, but effects are limited due to nasal mucus. To target the CNS however, specialized INB sprays target the superior concha to directly access the olfactory bulb. Comprised of millions of nerve endings, the olfactory bulb sends out those nerve endings through the holes in the cribriform plate to test the air for smells and provides a pathway from the nasal cavity to the brain. As such, the nasal cavity is a rare location with open access to the nervous system. INB drugs can bypass the difficult BBB treat CNS disorders.5
Reaching the Brain through the Nose
To reach our goal, INB drugs do not target the vasculature, but rather the top of the nasal cavity – olfactory bulb and cribriform. If absorbed systematically, the drug would still have to pass through the BBB reducing the neuronal availability and potentially cause toxicity. INB drugs avoid systemic circulation by entering the brain through the more permeable epithelial membrane above the superior concha. Any INB drug reaches the CNS through two main pathways. The epithelial pathway uses paracellular transport around the olfactory epithelium, across the cribriform plate and into the subarchnoid space. The CSF carries the drug particle throughout the brain or clears into the systemic circulation.6 Using the epithelial pathway requires a small, hydrophilic molecule. An olfactory pathway uses the nerve endings to internalize the drug and ferry it to the olfactory bulb and into the olfactory region of the brain. These molecules must trigger endocytosis to promote transport.
While easier to reach than through the blood, the nasal cavity still possesses a number of factors that can affect absorption. As stated before, small molecular weight drugs are ideal as absorption is limited at 1000 daltons. The secretion and removal of mucus can affect absorption clearing drug from the site of absorption. Few enzymes exist in the area, but degradation remains a concern for drug stability, especially for proteins. Finally, rhinitis can alter absorption.5 Still, the nasal cavity remains a valid alternative to administer drugs to the CNS, especially if the drug is orally ineffective, blocked by the BBB, or requires a rapid onset of action. Several modifications can protect from these protections and improve INB delivery.
To improve uptake through the olfactory bulb, either the drug or the formulation can be modified. Prodrugs take advantage of local enzymes and increase lipophilicity. Excipients transiently open up mucosal pores to increase absorptions. Chitosan is a natural polysaccharide that binds to the mucosal membrane and loosens the tight junctions between epithelial cells allowing more drug to enter.5 PLGA nanoparticles can increase absorption by attaching a lectin ligand, triggering receptor-mediated endocytosis.7 Currently, several drugs are being designed and studied to use these methods to treat a variety of CNS disorders. Mouse models show an increase in brain distribution through INB relative to normal intravenous administration, opening the door to potential treatments for Alzheimer’s disease and autism.
Potential Treatments through INB
Alzheimer’s patients face a dark diagnosis with little positive light. Chi Zhang and his group in Shanghi hope to make the outlook a little brighter. Basic fibroblast growth factor (bFGF) simulates the growth of neurons, but remains blocked by the BBB. Intravascular delivery of the peptide shows that only 1% reaches the brain. INB offers an alternative route but must overcome mucusal clearance and protein degradation. Incorporating bFGF into PEG-PLGA nanoparticles coated with Solanum tuberosum lectin, Zhang’s group showed increased residence time, CNS concentration of bFGF in Alzheimer-model rats and corresponding improvement in memory tests. Direct bFGF administration to the brain through the nose decreases plasma concentrations resulting in fewer toxicities.7
Each day, more children receive diagnoses of autism spectrum disorders, but few treatments exist. Evdokia Anagnostou and her Toronto-based lab want to administer intranasal oxytocin as a potential cure. Children with ASD have poor social skills, tend to perform repetitive behaviors, and have reduced levels of oxytocin in the blood. Medical oxytocin could hopefully ameliorate many autism symptoms. Because oxytocin degrades in the intestines by chymotrypsin, it cannot be given orally. Intravascular injection of oxytocin shows positive effects, but is too invasive for children. INB produced better CNS distribution with easier administration. While early in the process, children treated with INB oxytocin showed promising improvements in autism-related behaviors.8
Getting medication into the brain remains a difficult task as many molecules fail to reach the desired destination. The BBB protects the neurons but makes treatment more difficult. As the population grows and ages, more patients will be diagnosed with neurodegenerative diseases, like Alzheimer’s. A targeted nasal spray to the olfactory bulb could potentially deliver therapeutic compounds for a variety of CNS disorders. INB is a rapid acting route that avoids the restrictive BBB and potentially toxic systemic side effects. More studies must determine the safety of this route on a larger scale and show more examples of efficacy. We should learn more about local side effects of penetration enhancers on an important protective membrane or sensitivity in the nasal cavity leading to rhinitis. Overall, INB offers a safer, less invasive route of administration directly to the brain and can potentially help the patients with the frightening diseases of the CNS.
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