Anatomy and Physiology of the nose: Key points relating to nasal drug delivery
Table of Contents
Nasal Cavity function: Olfaction, filtration, heating and humidification:
The nose has two primary functions. The first is olfaction – the sense of smell. However, the second function is of primary interest to this discussion – filtration, heating and humidification of the inhaled air. To accomplish the second task, the nasal cavity contains a convoluted set of passageways called the turbinates on the lateral wall of each nasal cavity (see cross-sectional CT scan of turbinates to right). These turbinates interrupt the flow of air into the nasal passage, forcing it through narrow passages that are covered with moist nasal respiratory mucosa. The total surface area available in the nasal mucosa is estimated to be about 180 cm2, of which 10 cm2 is olfactory mucosa and 170 cm2 is the richly vascularized respiratory mucosa. During the passage across this broad mucosal surface, the air is warmed and humidified by a rich vascular capillary bed that is directly beneath the surface see diagram to left). These capillaries are specifically designed for rapid passage of fluids through the vascular wall and out into the dry air.[1-3] The amount of blood flow to this area is considerable – it is higher per unit of tissue than the blood flow to the brain, liver or muscle.[2-4]
This large mucosal surface covered with a rich vascular bed of highly permeable capillaries creates an opportunity for intranasal medication delivery. Not only will fluids cross the capillary bed into the air stream to humidify the air, but fluids delivered in the air stream onto the mucosa will also cross the capillary bed into the blood stream.[1-3] For this reason, when medications of proper concentration and molecular character are delivered onto the nasal mucosa, they are rapidly transported into the capillary bed and delivered to the patient’s circulation. (See diagram comparing nasal morphine plasma concentrations to those of intramuscular and oral morphine).
There are some patient factors that would theoretically limit nasal absorption. A significant amount of mucous or blood on the nasal mucosa may prevent drug absorption or result in rapid washing away of the medication. Prior use of vasoconstrictors such as oxymetazoline, phenylephrine or cocaine might also limit absorption. Investigators have confirmed that vasoconstrictors will reduce (but not eliminate) drug absorption while a mild URI will have little or no effect (see diagram).
Nose-brain pathway – nasal mucosal absorption of medications directly into the cerebral spinal fluid and brain:
If the nasally administered medication contacts the olfactory mucosa, there is good evidence that suggests molecule transport can occur directly across this tissue and into the cerebral spinal fluid.[5-8] The olfactory mucosa is located in the upper nasal cavity, just below the cribriform plate of the skull. It contains olfactory cells which traverse the cribriform plate and extend up into the cranial cavity. When medication molecules come in contact with this specialized mucosa they are rapidly transported directly into the brain, skipping the blood-brain barrier, and achieving very rapid cerebrospinal fluid levels (often faster than if the drug is given intravenously). This concept of transfer of molecules from the nose to the brain is referred to as the nose-brain pathway and has implications when centrally acting medications such as sedatives, anti-seizure drugs and opiates are delivered nasally. Multiple authors demonstrate that the nose-brain pathway leads to nearly immediate delivery of some nasal medications to the cerebral spinal fluid, by-passing the blood brain barrier.[5-9,11]
Literature related to Nose brain pathway:
In summary, the nasal mucosa consists of a highly vascularized surface that easily absorbs many medications directly into the venous circulation. This medication is then transported to the heart and pumped out to the body where it can have its therapeutic effect. Because the absorptive surface is not the intestinal mucosa, the drug never enters the portal circulation and is not subjected to hepatic metabolism – thereby leading to far higher drug levels than oral or rectal medications. In addition, the nose brain pathway across the olfactory mucosal transports some of the nasally delivered medication directly into the CSF and brain – leading to early effects of centrally acting medications.
- Hanson, IN drugs for neurodegenerative disorders, Policy Holder Advocate.pdf
- Pires, Intranasal drug delivery - How why what for -extensive review, J Pharm Pharmaceut Sci 2009
- Talegaonkar, Intranasal delivery - An approach to bypass the blood brain barrier, I J Pharm 2004.pdf
- Ying, Nose-brain path and neurologic diseases, Future medicine.pdf
Veldhorst-Janssen, N. M. (2013). Intranasal delivery of rapid acting drugs: Studies of pharmacokinetics, effect, tolerability and satisfaction, Universitaire Pers Maastricht. PhD Thesis: 141
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9. Cros, C. D., I. Toth, et al. (2014). "Delivery of a lactose derivative of endomorphin 1 to the brain via the olfactory epithelial pathway." Bioorg Med Chem Lett 24(5): 1373-1375.
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