The acute or chronic drug treatments for different neurodegenerative and psychiatric disorders are challenging from several aspects. The low bioavailability and limited brain exposure of oral drugs, the rapid metabolism, elimination, the unwanted side effects and also the high dose to be added mean both inconvenience for the patients and high costs for the patients, their family and the society. The reason of low brain penetration of the compounds is that they have to overcome the blood-brain barrier which protects the brain against xenobiotics. Intranasal drug administration is one of the promising options to bypass blood-brain barrier, to reduce the systemic adverse effects of the drugs and to lower the doses to be administered. Furthermore, the drugs administered using nasal route have usually higher bioavailability, less side effects and result in higher brain exposure at similar dosage than the oral drugs. In this review the focus is on giving an overview on the anatomical and cellular structure of nasal cavity and absorption surface. It presents some possibilities to enhance the drug penetration through the nasal barrier and summarizes some in vitro, ex vivo and in vivo technologies to test the drug delivery across the nasal epithelium into the brain. Finally, the authors give a critical evaluation of the nasal route of administration showing its main advantages and limitations of this delivery route for CNS drug targeting.

The aim of this review is to provide an understanding of the anatomical and histological structure of the nasal cavity, which is important for nasal drug and vaccine delivery as well as the development of new devices. The surface area of the nasal cavity is about 160 cm2, or 96 m2 if the microvilli are included. The olfactory region, however, is only about 5 cm2 (0.3 m2 including the microvilli). There are 6 arterial branches that serve the nasal cavity, making this region a very attractive route for drug administration. The blood flow into the nasal region is slightly more than reabsorbed back into the nasal veins, but the excess will drain into the lymph vessels, making this region a very attractive route for vaccine delivery. Many of the side effects seen following intranasal administration are caused by some of the 6 nerves that serve the nasal cavity. The 5th cranial nerve (trigeminus nerve) is responsible for sensing pain and irritation following nasal administration but the 7th cranial nerve (facial nerve) will respond to such irritation by stimulating glands and cause facial expressions in the subject. The first cranial nerve (olfactory nerve), however, is the target when direct absorption into the brain is the goal, since this is the only site in our body where the central nervous system is directly expressed on the mucosal surface. The nasal mucosa contains 7 cell types and 4 types of glands. Four types of cells and 2 types of glands are located in the respiratory region but 6 cell types and 2 types of glands are found in the olfactory region.

Nasal mucociliary clearance is one of the most important factors affecting nasal delivery of drugs and vaccines. This is also the most important physiological defense mechanism inside the nasal cavity. It removes inhaled (and delivered) particles, microbes and substances trapped in the mucus. Almost all inhaled particles are trapped in the mucus carpet and transported with a rate of 8-10 mm/h toward the pharynx. This transport is conducted by the ciliated cells, which contain about 100-250 motile cellular appendages called cilia, 0.3 µm wide and 5 µm in length that beat about 1000 times every minute or 12-15 Hz. For efficient mucociliary clearance, the interaction between the cilia and the nasal mucus needs to be well structured, where the mucus layer is a tri-layer: an upper gel layer that floats on the lower, more aqueous solution, called the periciliary liquid layer and a third layer of surfactants between these two main layers. Pharmacokinetic calculations of the mucociliary clearance show that this mechanism may account for a substantial difference in bioavailability following nasal delivery. If the formulation irritates the nasal mucosa, this mechanism will cause the irritant to be rapidly diluted, followed by increased clearance, and swallowed. The result is a much shorter duration inside the nasal cavity and therefore less nasal bioavailability.

Galectins are glycan-binding proteins that regulate innate and adaptive immune responses, and some confer maternal-fetal immune tolerance in eutherian mammals. A chromosome 19 cluster of galectins has emerged in anthropoid primates, species with deep placentation and long gestation. Three of the five human cluster galectins are solely expressed in the placenta, where they may confer additional immunoregulatory functions to enable deep placentation. One of these is galectin-13, also known as Placental Protein 13 (PP13). It has a “jelly-roll” fold, carbohydrate-recognition domain and sugar-binding preference resembling other mammalian galectins. PP13 is predominantly expressed by the syncytiotrophoblast and released from the placenta into the maternal circulation. Its ability to induce apoptosis of activated T cells in vitro, and to divert and kill T cells as well as macrophages in the maternal decidua in situ, suggests important immune functions. Indeed, mutations in the promoter and an exon of LGALS13 presumably leading to altered or non-functional protein expression are associated with a higher frequency of preeclampsia and other obstetrical syndromes, which involve immune dysregulation. Moreover, decreased placental expression of PP13 and its low concentrations in first trimester maternal sera are associated with elevated risk of preeclampsia. Indeed, PP13 turned to be a good early biomarker to assess maternal risk for the subsequent development of pregnancy complications caused by impaired placentation. Due to the ischemic placental stress in preterm preeclampsia, there is increased trophoblastic shedding of PP13 immunopositive microvesicles starting in the second trimester, which leads to high maternal blood PP13 concentrations. Our meta-analysis suggests that this phenomenon may enable the potential use of PP13 in directing patient management near to or at the time of delivery. Recent findings on the beneficial effects of PP13 on decreasing blood pressure due to vasodilatation in pregnant animals suggest its therapeutic potential in preeclampsia.

Therapeutic vaccination offers great promise as an intervention for a diversity of infectious and non‐infectious conditions. Given that most chronic health conditions are thought to have an immune component, vaccination can at least in principle be proposed as a therapeutic strategy. Understanding the nature of protective immunity is of vital importance, and the progress made in recent years in defining the nature of pathological and protective immunity for a range of diseases has provided an impetus to devise strategies to promote such responses in a targeted manner. However, in many cases, limited progress has been made in clinical adoption of such approaches. This in part results from a lack of safe and effective vaccine adjuvants that can be used to promote protective immunity and/or reduce deleterious immune responses. Although somewhat simplistic, it is possible to divide therapeutic vaccine approaches into those targeting conditions where antibody responses can mediate protection and those where the principal focus is the promotion of effector and memory cellular immunity or the reduction of damaging cellular immune responses as in the case of autoimmune diseases. Clearly, in all cases of antigen‐specific immunotherapy, the identification of protective antigens is a vital first step. There are many challenges to developing therapeutic vaccines beyond those associated with prophylactic diseases including the ongoing immune responses in patients, patient heterogeneity, and diversity in the type and stage of disease. If reproducible biomarkers can be defined, these could allow earlier diagnosis and intervention and likely increase therapeutic vaccine efficacy. Current immunomodulatory approaches related to adoptive cell transfers or passive antibody therapy are showing great promise, but these are outside the scope of this review which will focus on the potential for adjuvanted therapeutic active vaccination strategies.