Blood–brain barrier disruption in dementia: Nano‐solutions as new treatment options

Candidate drugs targeting the central nervous system (CNS) demonstrate extremely low clinical success rates, with more than 98% of potential treatments being discontinued due to poor blood–brain barrier (BBB) permeability. Neurological conditions were shown to be the second leading cause of death globally in 2016, with the number of people currently affected by neurological disorders increasing rapidly. This increasing trend, along with an inability to develop BBB permeating drugs, is presenting a major hurdle in the treatment of CNS‐related disorders, like dementia. To overcome this, it is necessary to understand the structure and function of the BBB, including the transport of molecules across its interface in both healthy and pathological conditions. The use of CNS drug carriers is rapidly gaining popularity in CNS research due to their ability to target BBB transport systems. Further research and development of drug delivery vehicles could provide essential information that can be used to develop novel treatments for neurological conditions. This review discusses the BBB and its transport systems and evaluates the potential of using nanoparticle‐based delivery systems as drug carriers for CNS disease with a focus on dementia.


K E Y W O R D S
Alzheimer's disease, blood-brain barrier, dementia, drug delivery, nanogels, nanoparticles

| INTRODUCTION
Dementia is a neurodegenerative disease made up of multiple neuropathological processes that lead to a deficit in cognitive function.All dementias share common aetiologies and progression including hypoxia, neuroinflammation, neurodegeneration, oxidative stress mitochondrial bioenergetics and blood-brain barrier (BBB) permeability (Raz et al., 2016).Age is the most common risk factor for dementia development with its prevalence being higher in individuals aged over 60.Despite this generally limited age range, dementia affects 47 million people globally with this number expecting to rise to 131 million people by 2050 (Arvanitakis et al., 2019).Alzheimer's disease (AD) accounts for 60-80% of all dementia cases (Alzheimer's & Dementia, 2021) and was listed as the leading cause of death in August and September 2021, in England, accounting for 11% of deaths, and yet, its complex processes remain largely not understood.Economically, the current global cost is $305 billion, which is expected to reach over $1 trillion as the world's population rises, with these figures likely to have been underestimated (Wong, 2020).Despite this, there are very limited treatment options available to prevent or cure the disease.It is imperative that further research into understanding the disease is prioritised along with developing new therapeutic interventions.One of the major hurdles in treating neurological diseases is penetration of the BBB, which functions as a highly selective physiological barrier between the blood and the brain (Abbott et al., 2010).This review will discuss the role of the BBB and its complexity and dysfunction in relation to different dementia types and the implication this has for the treatment of these life debilitating diseases.It will assess the use of nanoparticles (NPs) as a potential method for BBB mitigation and dementia treatment.A variety of NPs will be discussed in relation to crossing the BBB for dementia targeting including exosomes, liposomes and nanogels (NGs), before analysing the potential future direction for these nanomedical interventions in order to progress them clinically.

| THE BBB
The BBB is the term used to describe the complex microvasculature of the central nervous system (CNS), formed by non-fenestrated capillaries that create a highly selective, semipermeable membrane between the lumen of the brain capillaries and the brain parenchyma (Daneman & Prat, 2015).It is critical in maintaining the chemical composition of the neuronal 'milieu', required for proper neuronal function, synaptic transmission and remodelling and neuro/angiogenesis (Abbott et al., 2010;Zlokovic, 2008).Moreover, the BBB plays a crucial protective role, preventing blood-borne agents from entering the CNS (Patabendige & Janigro, 2023).The BBB is also part of the neurovascular unit (NVU) predominantly consisting of endothelial cells (ECs), forming the vessel wall, and mural cells, which reside on the EC abluminal surface.These work in concert with pericytes, astrocytes and microglia to develop and maintain the BBB structure and unique properties (Figure 1).
Whilst the BBB conjures images of a physical wall preventing the movement of molecules between brain and blood, the reality is a series of physiological properties.An alteration to one of these properties can have significant impact on the neural environment (Profaci et al., 2020).For example, dysfunction of L-type amino acid transporter (LAT1) has been linked to autism spectrum disorder, whereas changes in glucose transporter 1 (GLUT1) has links to seizure (Seidner et al., 1998;T arlungeanu et al., 2016).Disruption in the BBB is generally linked to brain insult or ageing; however, certain monogenic neurological diseases exist with genetic association.These genetic mutations impact cells of the BBB resulting in developmental and maintenance defects (Zhao et al., 2015).Current treatments for many neurological conditions are unable to penetrate the BBB due to its highly selective nature.Furthermore, current techniques use a single modality to detect BBB dysfunction, whether quantifying markers within the blood or cerebrospinal fluid (CSF) or examining post-mortem tissue.In order to advance treatment options for neurological conditions, it is imperative to understand the complex physiology of BBB changes in each disease, where overlaps exist, and to use this information to design therapeutic strategies with a range of applications.
BBB dysregulation plays a key role in cellular damage in neurological diseases, including AD, ischaemia, multiple sclerosis and Parkinson's disease (Table 1).During pathological events, a cascade of molecular events occurs.These events include activation of matrix metalloproteinases (MMPs) that attack TJ proteins such as occludin and claudin, resulting in increased BBB permeability (Yang et al., 2007).The production of hypoxia-inducible factor-1α (HIF-1α) under low oxygen environments, associated with most pathologies, further activates MMPs (Yuan et al., 2008).BBB disruption also allows for an influx of immune cells into the brain, resulting in cytokine production and inflammation.
Neuroinflammation along with increased production of cyclooxygenases (COXs) results in further damage to the BBB and injury amplification (Rosenberg, 2012).Free radicals of nitrogen and oxygen can also enter the brain tissue, causing further damage to TJ proteins and ECs, ultimately resulting in a leaky BBB and brain tissue death (Kim et al., 2003).
The number of people currently affected by CNS disorders is rapidly increasing, including neurodegenerative diseases, such as dementia, which is now recognised as the second leading cause of death globally (Feigin et al., 2019).The urgent need for novel therapeutics and the significance of the exploration of new avenues for CNS drug delivery have become clearer than ever.The BBB whilst essential for proper neural function is equally limiting for therapeutic intervention.BBB dysregulation plays a crucial role in dementia, and its effects can be seen as both an initiator and an accelerator.Crossing or bypassing the BBB is essential for CNS disease treatment, yet currently, there has been little to no success in doing this without detrimental effect on the patient.A new approach to CNS treatment must be considered, and the use of NPs for the encapsulation and delivery of drugs is showing great potential as a safe and efficient method for CNS targeted treatments.
Therefore, this review will focus on the effects of dementia on the BBB and the different nanomedicine approaches that are currently available to make a difference in the hunt towards understanding and conquering the BBB and in the treatment of CNS disorders.

| AD
AD accounts for 60-70% of all dementia cases and is generally characterised by an accumulation of amyloid-β in the form of plaques and Tau protein in neurofibrillary tangles (Breijyeh & Karaman, 2020).Morphological changes have been observed in post-mortem brains of AD patients, such as basal membrane thickening, decreased microvessel density, capillary leakages and accumulation of fibrinogen, albumin, prothrombin and haemoglobinderived peptide levels (Bowman et al., 2007;Nelson et al., 2016;Zipser et al., 2007), indicative of BBB damage (Thomsen et al., 2017).
BBB disruption is proposed to contribute to both the onset and progression of AD (Montagne et al., 2017), as neuroimaging has shown that BBB disruption is observed in the hippocampus of AD-affected brains predating brain atrophy and dementia (Montagne et al., 2015).Additionally, the presence of peripheral macrophages and neutrophils in the AD-affected brain further suggests that BBB breakdown occurs, leading to an influx of circulating leukocytes into the brain (Hultman et al., 2013;Zenaro et al., 2015).Damage to the brain's vasculature  Proescholdt et al. (2002), Wolburg et al. (2003).

Vascular dementia
Reduced EC and pericyte integrity.Astrocyte end-feet swelling and retraction from vessel wall.Upregulation of oxidative stress.Increased production of nitric oxide (NO) ROS and free radicals.Infiltration of inflammatory cytokines.Montagne et al. (2018), Ma et al. (2013).
Note: CNS pathologies and the mechanisms associated with BBB disruption as noted within literature.Abbreviations: BBB, blood-brain barrier; CNS, central nervous system; ECs, endothelial cells; NVU, neurovascular unit; TJ, tight junction.
system can lead to pronounced BBB dysfunction, which manifests as cerebral microbleeds, commonly associated with AD (Goos et al., 2009;Poliakova et al., 2016).Cerebral amyloid angiopathy is seen as a causative factor for microhaemorrhages within lobar regions (Greenberg et al., 2009).AD-associated microbleeds, whilst present across the brain, are more frequently found in the occipital lobe, and their frequency is seen to be positively correlated to amyloid deposition (Kantarci et al., 2013;Viswanathan & Greenberg, 2011).BBB impairment is also associated with diminished glucose uptake into the brain via GLUT1, preceding neurodegeneration (Simpson et al., 1994).As BBB breakdown appears to precede clinical neurodegeneration, vascular deficit and associated BBB dysregulation should be considered in staging preclinical AD as well as in potential early treatment options.

| Vascular dementia (VaD)
VaD is the second most common dementia type accounting for 10-20% of all dementia cases (Wolters & Ikram, 2019).VaD is caused by reduced blood flow to the brain, most commonly as a result of hypoperfusion and thrombosis, which subsequently lead to oxidative stress, hypoxia and neuroinflammation.VaD is also closely linked to cerebral small vessel disease, a condition that affects brain arterioles and capillaries, eventually leading to reduced brain perfusion, BBB damage, lacunar infarcts and ultimately dementia (Østergaard et al., 2016).Cerebral small vessel disease can be caused by thromboembolic strokes and atherosclerosis of the small calibre vessels in white matter, such as in Binswanger syndrome (Loeb, 2000), or can be inherited.Cerebrovascular disruption associated with both cerebral small vessel disease and VaD includes reduced microvascular density, loss of angiogenic capacity and microvascular plasticity (Srinivasan et al., 2016).Often, cerebral microvessels also contain micro-atheroma or lipid emboli along with focal subclinical inflammation, increased permeability and perivascular oedema (Østergaard et al., 2016).BBB dysfunction is seen as a contributing factor to VaD as well as being shown to be one of VaD's critical signs, along with hypoperfusion and hypoxia.Hypoxia is seen as the initiating factor for BBB dysfunction during VaD, resulting in reduced EC and pericyte integrity, swelling of astrocyte endfeet and retraction from the vessel wall, ultimately leading to BBB disruption (Patabendige et al., 2021;Wardlaw et al., 2003).Hypoxia has been shown to upregulate oxidative stress resulting in the production of nitric oxide (NO), reactive oxygen species (ROS) and free radicals (Ma et al., 2013).It also disrupts antioxidant levels, ultimately causing damage to ECs, glial cells and neuronal cells, increasing BBB permeability and reducing cerebral blood flow (Liu & Zhang, 2012).Reduced cerebral blood flow has been shown to further induce ROS production and exacerbate damage to myelin, axons and oligodendrocytes (Montagne et al., 2018).An increase in ROS causes mitochondrial dysfunction, which results in hypoxia and further induces oxidative stress.Hypoxia has also been linked with infiltration of inflammatory cytokines such as IL-6 and MMPs into the brain.This results in neuroinflammation causing demyelination and damage to axons and oligodendrocytes (Chen et al., 2011), eventually resulting in enhanced BBB permeability.Additionally, this oligodendrocyte damage represses remyelination, disrupting the transmission of neuron signals which manifests as cognitive impairment (Hussain et al., 2021).Other factors have been associated with BBB disruption during VaD, including perivascular collagen accumulation in the hippocampus and white matter regions of the brain, as a result of hypertension (Verhaaren et al., 2013).BBB leakage is also associated with an accumulation of albumin, fibrogen and IgG, due to TJ protein loss and decreased pericyte concentration (Goodall et al., 2018).Furthermore, there is an association between an upregulation of intercellular adhesion molecule 1 (ICAM-1) and vascular adhesion molecule 1 (VCAM-1) and BBB damage in VaD; however, further research is needed to understand this fully.

| NeuroAIDS dementia
Prior to anti-human immunodeficiency virus (HIV) treatments, 30% of adults with acquired immune deficiency syndrome (AIDS), and 50% of paediatric AIDS cases were affected by HIV-induced dementia (HID).HID is often referred to as HIV-associated neurocognitive disorder.HID occurs when HIV replicates within macrophages and microglia in the brain early after primary infection due to reduced integrity of the EC monolayer (Sansing et al., 2012).It is worth noting that HIV is unable to productively replicate within neurons, oligodendrocytes or brain ECs (Kaalund et al., 2020;Koenig et al., 1986;Yadav & Collman, 2009).However, studies have shown low levels of astrocyte infection resulting in increased dickkopf-1 protein (DKK 1 ) expression leading to an increase in BBB permeability, abnormal endothelium interactions and neuronal damage (Eugenin et al., 2011;Orellana et al., 2014).Along with brain cell infection, HID results in multinucleated giant cell formation, monocyte infiltration of the CNS, astrogliosis and myelin pallor, which clinically manifest as cognitive and motor dysfunctions associated with dementia (Pereira & Nottet, 2000).Neuropathological studies have established that BBB disruption is a consistent feature of HID with both its presence and severity considered key factors of the disease (Berger et al., 2000;Berger & Avison, 2004).
Initially, HIV gains entry to the brain via blood-derived macrophages (Nottet & Gendelman, 1995).During the later stages of brain HIV infection, levels of proinflammatory cytokines such as tumour necrosis factor-a (TNF-a) and IL-1b, that are known to increase BBB permeability, are elevated (Strazza et al., 2011), resulting in BBB damage.This has been attributed to many factors including HIV-viral protein R (Vpr) production and associated glyceraldehyde 3-phosphate dehydrogenase (GAPDH) activity (Ferrucci et al., 2013).In addition, these cytokines induce expression of endothelial adhesion molecules such as E-selectin and VCAM-1 (Floris et al., 2002).This is mediated by the HIV-1 tat molecule, which is secreted by HIV-infected macrophages and microglia (HIV-I M/M), furthering BBB damage.The production of tat, gp120 and the presence of HIV-1 also causes EC to secrete neuroimmune-active substances exacerbating injury (Banks et al., 2006).Tat has also been shown to decrease the expression of occludin and ZO-1 in TJs, reducing BBB integrity (Pu et al., 2007).HIV-I M/M also produces higher levels of NO, which is linked to BBB EC damage and increased BBB permeability (Dhawan et al., 1992).
HIV-1 encodes Gag, Pol and Env polyproteins, which are subsequently proteolysed into individual proteins including gp-120, a potent neurotoxin detected in HIVinfected patient serum (Ellaurie et al., 1990).When exposed to HIV-gp120, EC monolayer permeability and monocyte migration are increased (Kanmogne et al., 2007) along with down-regulation of TJ proteins, leading to disruption of BBB integrity (Kanmogne et al., 2005).Gp-120 along with HIV-1 negative regulatory factor (Nef) has also been shown to produce gelatinase B, which degrades extracellular matrix proteins that surround ECs, further increasing BBB permeability (Dhawan et al., 1995).HIV-induced gelatinase B is also shown to antagonise MMPs, which attack TJ proteins and increase BBB permeability allowing more immune and inflammatory cells to influx to the brain.This, in turn, increases expression of MMP-2 and MMP-9.The first MMP related to brain dysfunction during early HIV infection was shown to be MMP-2 (Li et al., 2013).Along with MMP-9, MMP-2 has also demonstrated influence over BBB permeability as well as displaying a neurotoxic effect (Zhang et al., 2003).This increase in BBB permeability further exacerbates HIV-related dementia, speeding up its progression.
BBB disruption has also been linked to HIV-Nef production, which facilitates down-regulation of CD4, rupturing the BBB and depleting T cells.Whilst detrimental, Nef exposed microglia have been the target of potential HID treatment options.Nef myristoylation sites containing Nef peptides have been shown to reduce microglial release of Nef exosomes and prevent BBB disruption (Raymond et al., 2016).Whilst HID treatment is currently limited, potential avenues for reducing its progression such as through the introduction of Nef peptides should be explored.

| Strategies for treating CNS disorders
In many cases, CNS disorders could be treated with drugs, enzymes or genes that have already been discovered; however, they are unable to cross the BBB by any of these methods of transport.In particular, CNS acting therapeutics have the lowest clinical success rate (Kola & Landis, 2004) with over 98% of treatments being revoked from clinical trials due to their inability to permeate the BBB and exert their therapeutic effects.In these cases, an alternative 'Trojan Horse' approach could be used (Santiago-Tirado et al., 2017).This would conceal the molecule and allow it to pass through the BBB to reach its target area.
Current research in using this 'Trojan Horse' method to pass through the BBB transport system typically requires the use of an NP and is providing promising results in research.NPs can be administered using different routes of delivery including intravascular, intraventricular, intranasal and via direct intraparacellular injection.Once at the BBB, NPs enter the brain using multiple strategies including passive diffusion, carrier transport and adsorptive-and receptor-mediated transcytosis (Hersh et al., 2022).Modern advancements in technology have led to the development of a range of NPs with more than 50 NPs approved by the FDA as drug delivery systems between 1995 and 2016 (Bobo et al., 2016;Mitchell et al., 2021) (Table 2).NPs are believed to be the beginning of the future for medical advancement and can be categorised based on physical and/or chemical parameters.
Despite providing a new potential treatment option for CNS disease treatment, NPs can experience the same limitations as other xeno-compounds to cross the BBB and access the brain; therefore, composition must be tailored to application.Several strategies have been approached to enable BBB crossing by NPs, taking advantage of different physiological mechanisms (Table 3).T A B L E 3 Current methods commonly employed to increase nanoparticle dissipation across the BBB.

Provoked transient BBB disruption
Ultrasound, microbubbles, osmotic pressure and magnetic force can all be used to disrupt TJ between ECs to temporarily increase BBB permeability.

Absorptive transcytosis
NP binding to luminal plasma membrane of ECs or NPs decorated with compounds that are taken up by nerve terminals.

Receptor-mediated transcytosis
NP decorated with specific ligands bind to corresponding receptors promoting endocytosis.

Clathrin-mediated endocytosis
In clathrin-enriched areas of cell membrane endocytotic vesicles form.These clathrin coated vesicles then fuse together forming early endosomes, progressing to late endosomes where they will facilitate deposition of cargo before fusing with lysosomes.

Caveolin-mediated endocytosis
Plasma membrane invaginations are created in lipid rafts.Caveolin vesicles fuse with other caveolin vesicles generating cavesomes.
The most common NPs used in biomedicine include metal NPs, liposomes, polymeric NPs and NGs.NPs offer a new level of pharmaceutical control with solubility, diffusivity, half-life and biodistribution all being modifiable and controllable parameters (Carissimi et al., 2021).More recently, NPs are being used that combine therapy and diagnostics in a single application known as 'theranostics' (Kenny et al., 2012;Rafique et al., 2019).The main advantages of NPs as drug delivery systems are (i) enhanced penetration and retention of the NP (Kalyane et al., 2019); (ii) potential to transport insoluble drugs through the blood to otherwise impermeable areas, such as the brain (Grabrucker et al., 2016); and (iii) a controlled release of the drug at its target site (Tagalakis et al., 2018).
NPs can also be coated with different modalities to increase their biocompatibility and enable higher NP concentrations to permeate the BBB.One NP coating option that has shown positive results is poly(ethylene glycol) (PEG) (Kafetzis et al., 2023).PEG has a dense near neutral composition enabling particles to 'experience' the brain ECs as a fluid rather than an impermeable solid.Nance et al. (2012) demonstrated PEG-coated NPs as large as 114 nm were able to penetrate ex vivo human brain tissue and mouse brain in vivo significantly better than similar sized COOH-coated particles providing beneficial results for the potential use of PEG coating on CNS drug delivery molecules.Additionally, PEG-g-chitosan containing NGs, with a suitable pore size, were able to release T lymphocytes and retain their anti-glioblastoma activity.They also showed greater efficacy in killing glioblastoma cells when compared with the non-targeted control (Tsao et al., 2014), further supporting the use of PEG in BBB penetrating NPs.Another surfactant coating option is polysorbate 80 (PS80).The first BBB permeable NP system used PS80-coated poly(n-butyl cyanoacrylate) (PBCA) NPs to successfully deliver dalargin, an antinociceptive peptide unable to penetrate the BBB on its own (Kreuter et al., 1995).Since then, polylactide-co-glycolide (PLGA) NPs loaded with either loperamide or doxorubicin and coated with PS80 or P188 have demonstrated an ability to cross the BBB and deliver their cargo to the brain (Gelperina et al., 2010).PS80 coating was also successful for PLA (polylactic acid)-b-PEG NPs (Ren et al., 2009) but was unsuccessful for PLA NPs (Raudszus et al., 2017).This suggests that effectiveness of the coating is influenced by the composition of the NP core and the components used in its development (Kreuter, 2004).

| NPs in CNS drug delivery
In 1995, the first liposome NP was approved by the FDA for the delivery of doxorubicin under the name Doxil (Youn & Bae, 2018).Since then, several NP formulations have reached human clinical trials with vast treatment implications (Vieira & Gamarra, 2016;Anselmo & Mitragotri, 2021; Table 2).NPs designed for CNS drug delivery are an emerging field with great potential.Brain delivery can be actively targeted by surface modification with molecules recognised by the receptors/transporters abundantly expressed in the BBB and surrounding brain tissue.NPs would then be able to enter the brain via adsorptive-mediated transcytosis, transporter-mediated transcytosis and receptor-mediated transcytosis, where they would then pass to their target site.Whilst no NP has received approval for clinical use in CNS disease treatment or prevention as of yet, there are several clinical trials underway and much research into nano approaches to CNS disease treatment.A current phase-1 trial (NCT03603379) is assessing levels of doxorubicin in the CSF and the peripheral blood following intravenous administration of anti-EGFR-immunoliposomes containing doxorubicin (Wicki et al., 2021).Additionally, a phase-II trial NCT03806478 is due to start in June 2023 for evaluating APH-1105 for the treatment of mildmoderate AD (ClinicalTrails.gov, 2019).
These trials, along with many others, bring great promise for CNS disease treatment.However, it is essential that research into the development of different NP compositions suitable for CNS delivery is needed to truly push the field of CNS nanomedicine to ensure effective treatments can be developed.In this review, we will explore several NP options and their progress within the field and their future potential as CNS therapeutics with a focus on dementia.

| NPs for dementia treatment
NPs are fast becoming a viable option for dementia treatment.Their adaptability and targeted approach have shown great success both in vitro and in vivo for delivering therapeutic substances across the BBB.Here, this review presents some key advancements for the development of NPs to most effectively treat dementia.Theranostics have, in recent years, seen much more interest and funding in the biomedical field.This approach will benefit the patients as they would be able to be both diagnosed and treated in a personalised and less invasive approach.Theranostics is an area of intense research within oncology and has already seen clinical success (Table 4).Theranostics are reliant on target-specific NPs capable of carrying dual cargo.This approach could be effectively used within dementia treatment and would allow for diagnosis, treatment and regular monitoring of effects, which currently remain unavailable (Li et al., 2018).
Additionally, advances in NP research could allow for genetic intervention within the genome to prevent and cure currently untreatable diseases such as dementia.CRISPR/Cas9 is a revolutionary gene editing tool that identifies a targeted gene sequence, creates a double strand break and allows for gene inactivation or correction (Bhardwaj et al., 2022).With the genetic evidence for dementia becoming more evident, tools such as this should be thoroughly explored for the prevention and treatment of the disease.Current limitations with CRISPR/Cas9 include an inability to effectively reach target genes without the use of a viral vector.Use of a viral vector within the brain could potentially introduce fatal side effects and further brain tissue damage (Hanafy et al., 2020).Therefore, an alternative carrier must be found.NPs make an ideal carrier, having already shown that they are able to pass through the BBB and remain biocompatible within brain tissue.Further research should look to explore the use of NPs for delivery of CRISP/Cas9 systems for dementia treatment.

| Exosomes
Produced in the eukaryotic endosomal compartment, membrane bound extracellular vesicles known as exosomes act as biological NP carriers, uptaking and releasing cell constituents (Kalluri & LeBleu, 2020) in different parts of the body.Exosomes are constantly secreted by the cells of the body and are consistently present in many bodily fluids.They have been linked to intracellular communication observed in various pathological pathways including that in neurodegeneration, with the majority of proteins responsible for neurodegeneration being transported via exosomes (Guo et al., 2021;Howitt & Hill, 2016).Additionally, exosomes have been found to express MHC class II molecules, linked to inflammatory response promotion (Zitvogel et al., 1998) as well as being shown to play a part in angiogenesis, programmed cell death, inflammation, morphogen transportation, tau and Aβ transportation and carrying misfolded or mutant copper-zinc superoxide dismutase one (mSOD1) (Colombo et al., 2014;Prada et al., 2013;Wang et al., 2016).Exosomes have, however, also demonstrated a scavenging role, with microglia having been shown to uptake neuronal exosomes carrying both intact and hypophosphorylated tau or Aβ (Prada et al., 2013;Wang et al., 2016) highlighting a key avenue for potential treatment options.This has been demonstrated by several researchers for the successful delivery of doxorubicin in mouse models that resulted in reduced tumour mass (Jang et al., 2013;Tian et al., 2014), as well as successful delivery of catalase for potential Parkinson's Disease treatment (Haney et al., 2015), and the ability to carry and deliver mRNAs to target areas (Del Pozo-Acebo et al., 2021;Kim & Rhee, 2020).
There has also been evidence to support the movement of exosomes through the BBB through conjugation with target molecules (Choi et al., 2022).Kim et al. have shown that T7-peptide decorated exosomes are efficient carriers of antisense miRNA oligonucleotides against miR-21 with higher delivery efficiency to C6 glioblastoma cells than unmodified exosomes (Kim et al., 2020).Additionally, in mouse models, Alvarez-Erviti et al. demonstrated RVG-targeted exosomes were able to deliver GAPDH siRNA specifically to neurons, microglia and oligodendrocytes with significant knockdown of BACE1, a target for AD (Alvarez-Erviti et al., 2011).There has also been recent success in the use of mesenchymal stem cellderived exosomes and their ability to ameliorate pathology and improve cognitive function in VAD and AD (Joo et al., 2023;Wang et al., 2023).This potential use of exosomes to deliver therapeutics across the BBB could open new CNS treatment options, if properly explored.

| Liposomes
Composed of one or more concentric membranes of lipid bilayers, liposomes have the ability to encapsulate both lipophilic and hydrophilic molecules within their membranes and cavities (Irving et al., 2020;Yadav et al., 2017).Liposomes used in medical applications range between 50 and 450 nm in size (Bozzuto & Molinari, 2015) and can be made up of natural or synthetic lipids and surfactants giving modifiable physicochemical properties (Cattel et al., 2004).Their molecular makeup directly influences their interactions with cells, their half-life and their penetrative ability, making them highly flexible.
Liposomes can reach their target region using numerous methods.Direct methods involve bypassing the BBB and include intrathecal or intraventricular routes of delivery to the CSF, intranasal delivery and interstitial delivery using either biodegradable wafers or convection enhanced delivery (CED) (Hersh et al., 2016).CED is an interstitial method that generates a pressure gradient at the tip of an infusion catheter to allow for therapeutic delivery through the interstitial spaces of the CNS (Kenny et al., 2013;Mehta et al., 2017).CED has been successful for anionic peptide-lipid nanocomplexes (Tagalakis et al., 2014) and PEGylated, anionic nanocomplexes (Tagalakis et al., 2014).Interstitial wafers are the alternative route for this method and are composed of biodegradable polymers with macromolecules encapsulated within.The most studied interstitial wafer is Gliadel ® composed of p (CPP:SA) (poly(1,3-bis-(p-carboxyphenoxy propane)-co-(sebacic anhydride) polymer loaded with 3.8% biodegradable 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU; also known as Carmustine).These wafers line the surgical cavity post maximal resection of malignant glioma (Brem et al., 1991) and are still used clinically, marking an important step in interstitial delivery techniques.Intrathecal routes deliver therapeutics directly into the lumbar subarachnoid space whereas intraventricular routes deliver them directly into the ventricular system.Whilst research has shown these to be promising routes for CNS drug delivery, particularly for leptomeningeal drug treatments of various meninges, they are limited by rapid CSF turnover (Beauchesne, 2010;Fleischhack et al., 2005).Intranasal delivered therapeutics reach the brain via crossing the nasal olfactory epithelium or nasal mucosa (Graff & Pollack, 2005).Chemotherapeutic drugs, proteins, small molecules and NPs have all shown success when delivered via intranasal routes, and this is a method that is rapidly being favoured due to its non-invasive technique (Costantino et al., 2007;Keller et al., 2021;Xu et al., 2020).
Furthermore, research is showing great potential in the ability of modified liposomes to target specific regions, including crossing the BBB for dementia targeted treatment.Evidence for BBB penetration and dementia targeting using liposomes was shown by Chen et al. who demonstrated that liposomes, modified with transferrin, promoted penetration of α-mangostin, a potential AD treatment, through the BBB whilst maintaining it intact.With the use of a rat model, they also showed effective distribution of α-mangostin in the brain, indicative of improved delivery across the BBB (Chen et al., 2016).Additionally, Rompicherla et al. (2021) showed nasal delivery of rivastigmine loaded liposomes (composed of soya lecithin and cholesterol) induced rapid drug absorption to target tissue, enhanced systemic bioavailability and half-life as well as reversing memory deficit in both acute and chronic AD models (Rompicherla et al., 2021).Similarly, Vasileva et al. (2023) found that cholesterol-based liposomes modified with tetradecyltriphenylphosphonium bromide were efficient in dual loading of α-tocopherol and donepezil hydrochloride for intranasal delivery.Upon delivery within transgenic AD mouse models, they observed a reduced number of Aβ plaques in the entorhinal cortex, dentate gyrus and CA1 region of the hippocampus, supporting their use as a AD therapeutics carrier.Whilst ideally suited to the encapsulation of hydrophilic drugs/antigens (Bozzuto & Molinari, 2015), liposomes are also able to carry nucleic acids (Ewert et al., 2021;Grant-Serroukh et al., 2022;Lechanteur et al., 2018;Liang et al., 2019;Tagalakis et al., 2017), proteins (Colletier et al., 2002), peptides (Martins et al., 2007;Yang et al., 2021) and pathogen-associated molecular patterns that target pattern recognition receptors and lead to immunostimulation (Perrie et al., 2016;Schwendener, 2014).Despite these advancements, liposomes have presented various problems over time.One important issue is the rapid capture of liposomes following intravenous administration by the reticuloendothelial system and a tendency to aggregate in the liver and spleen due to an abundance of blood flow and phagocytic cells (Chrai et al., 2002;Zamboni, 2008).This could potentially cause liposomal drugs to accumulate, increasing toxicity and interfering with host defence functions (Daemen et al., 1995;Juliano & Stamp, 1978;Sercombe et al., 2015).In order to prevent toxicity and rapid capture, the liposome surface can be coated with biocompatible hydrophilic polymer conjugates, such as PEG and chitosan, to reduce immunogenicity and macrophage uptake (Nisini et al., 2018).The PEG coating has been shown to inhibit both electrostatic and hydrophobic reactions with plasma proteins and/or cells minimising liposomal uptake by the reticuloendothelial system (RES) (ishida et al., 2001;Sawant & Torchilin, 2012;Tagalakis et al., 2021).However, there has been some caution as to the use of PEG due to unexpected immune responses to PEGylated nanocarriers.These included accelerated blood clearance due to the production of anti-PEG antibodies at the injection site, along with hypersensitivity, often referred as complement activation-related pseudo allergy (CARPA) (Tenchov et al., 2023).This was most recently seen with two mRNA COVID-19 vaccines, mRNA-1273 andBNT162b2, where 11 (mRNA-1273) and 2.5 (BNT162b2) cases of anaphylaxis per million doses were observed, as of Jan 2021 (CDC and FDA, 2021).

| Lipid NPs
Lipid NPs and liposomes are similar by design but differ slightly in composition.Lipid NPs can be further categorised dependent on their composition; if composed of solid lipids, they form solid lipid NPs (SLNPs) or if composed of a combination of solid and liquid lipids, they form nanostructured lipid carriers (Estanqueiro et al., 2016).Lipid NPs have shown success in a range of fields including cutaneous (Beloqui Garcia et al., 2016;Garcês et al., 2018), pulmonary (Gaspar et al., 2016), cosmetic (Wissing & Müller, 2003) and genetic (del Pozo-Rodríguez et al., 2010).
Lipid NPs are of particular interest as mRNA carriers.The mRNA is three to four times too large to readily diffuse into cells, and its negative charge electrostatically repulses anionic cell membranes, preventing its uptake (Hajj & Whitehead, 2017).Additionally, Lipid NPs are positively charged at low pH allowing for RNA complexation (Kowalski et al., 2019) and neutral at physiological pH reducing toxic effects (Sanghani et al., 2021).Lipid NPs carrying mRNA have been tested for protein replacement therapies (Ryals et al., 2020) and antiviral therapies (Kim et al., 2022;Lu et al., 2020).Importantly, multiple lipid NPs are already approved clinically as drug carriers with BioNTech/Pfizer's and Moderna's mRNA COVID-19 vaccines all utilising lipid NPs in order to carry mRNA into the cell as well as protecting the nucleic acid from degradation (Schoenmaker et al., 2021).
SLNPs have been shown to be able to cross the reticuloendothelial system of the BBB due to their solid lipid composition that acts as a protective barrier to active drug compounds (Müller et al., 2000).Additionally, SLNPs have been shown to uptake and deliver donepezil, an acutely toxic hydrophilic drug unable to cross the BBB in high amounts.SLNPs were able to carry donepezil to its target site within the brain more effectively than the free drug (Yasir et al., 2018) suggesting an ability to utilise BBB transport systems.More specifically, studies have demonstrated the ability of lipid NPs in transporting potential dementia treatments.Prathipati et al. (2021) showed that curcumin loaded SLNPs were able to successfully ameliorate the oxidative stress in VaD in vivo via the activation of the Nrf2/HO-1 pathway.Furthermore, Loureiro et al. (2017) highlighted the use of SLNPs for targeted delivery of grape extracts and resveratrol, for Aβ aggregate inhibition in VaD.Additionally, quercetin loaded SLNPs decorated with transferrin were shown to successfully facilitate passage across the BBB along with showing a capacity to inhibit fibril formation with the potential for their use in AD treatment to inhibit Aβ aggregate formation (Pinheiro et al., 2020).
Permeability of lipid NPs across the BBB for CNS and specifically dementia treatment can be improved by surface modification with various molecules.For example, PEG and polysorbate 80 (Manjunath & Venkateswarlu, 2006) have improved delivery of antipsychotic drugs, clozapine and nitrendipine when encapsulated within coated lipid NPs than administered on their own.Modification of SLNPs with apolipoprotein E (ApoE), a ligand of BBB receptors, can also be used to increase BBB permeability.This resulted in increased uptake of ApoE-decorated SLNPs across a co-culture BBB model when compared with SLPS alone, allowing for increased delivery of the AD drug, donepezil (Topal et al., 2020).More recent findings suggest lipid NP diffusion across the BBB and into the parenchyma is sizedependent with NPs that have 30-nm diameter able to reach neurons whilst their 80 nm counterparts remained in the vessel lumen (Khalin et al., 2022).This provides crucial information on the specific requirements on lipid NP carriers and aids their potential catapult into clinical use.

| Polymeric NPs and peptide nanocomplexes
Polymeric NPs are colloidal solid particles that can be composed of either synthetic polymers or natural polymers with either hydrophilic, hydrophobic or amphiphilic properties dependent on composition.Common polymer types include alginic acid (Li et al., 2008), gelatin (Xu et al., 2012), polylactic acid (Casalini et al., 2019), chitosan (Chattopadhyay et al., 2017), PLGA (Dinarvand et al., 2011) and polycaprolactone (Karthik et al., 2017).Polymeric NPs can be loaded with active compounds either entrapped within or surface-absorbed on to the polymeric core (Zieli nska et al., 2020) and are either structured as nanospheres or nanocapsules dependent on preparation (Jawahar & Meyyanathan, 2012).The use of polymerics NPs is of interest in the medical field as they have been shown to increase the stability of volatile pharmaceuticals and deliver higher concentrations of pharmaceutical agents to the desired location with modifiability dependent on target disease (Nagavarma et al., 2012).
Additionally, peptide nanocomplexes can be formed that utilise a multi-material approach to their design for a potential personalised treatment (Kwok et al., 2016;Ma & Cao, 2022;Peng et al., 2020).RALA, a peptide with repeat units of arginine-alanine-leucine-alanine (R-A-L-A) developed by McCarthy et al. (2014), is an example of a peptide nanocomplex.RALA involves the separation of the cationic hydrophilic arginine and hydrophobic leucine residues during peptide folding and alpha helix formation.This allows the complex to have a combined positive charge and amphipathicity that allows for NP formation through electrostatic interaction between anionic nucleic acids and small molecules.Furthermore, the cationic nature of the complexes allows embedding into the phospholipid bilayer of the BBB and endocytic uptake of the NPs.Other materials have also shown success as BBB peptide nanocomplexes such as the fluorescent amphiphile-peptide conjugate, termed FAM-CGY.These, when complexed with β-secretase 1 (BACE1) siRNA, were able to target cerebral ECs through the transferrin receptor, cross the BBB and reach neurons and microglial cells for effective BACE1 down-regulation in the brain without toxicity and inflammation (Wu et al., 2019), highlighting their potential as AD therapeutics.
Alginate is a polymer that is pH-sensitive, which allows it to control drug release from NPs. Agili and Aly demonstrated that sodium alginate/pectin/tannic acidsilver NPs were pH-sensitive leading to enhanced drug release (Agili & Aly, 2019).Chitosan has the ability to combine with DNA to produce a polyelectrolyte complex, increasing DNA resistance to nuclease degradation (Li et al., 2000).Additionally, the chitosan backbone contains many hydroxyl and amino groups, making it ideal for chemical modification and increased targeting (Zhao et al., 2010).This targeting ability also applies within the CNS where a chitosan containing NP increases interaction with the negatively charged cell membrane in the BBB and increases passage into the brain (Cortés et al., 2020).PLGA is also used in vaccine treatments alongside chitosan due to its ability to be internalised in cell by fluid phase pinocytosis and/or clathrinmediated endocytosis (Danhier et al., 2012;Vasir & Labhasetwar, 2007).PLGA has also been shown to increase BBB penetration and allowing NPs to entrap small molecule drugs, both of which are key considerations for dementia-targeted therapeutic devices (Zhi et al., 2021).Recently, Zhou et al. have developed a glycosylated polymeric siRNA nanomedicine that utilises glycemia-controlled GLUT1 recycling to facilitate BBB penetration.This siRNA-carrying NP resulted in the decrease of the expression of β-site APP cleavage enzyme, an enzyme that controls the cleavage of Aβ in AD.Administration of the nanomedicine showed restored cognitive capacity in AD mice and supports the use of RNA interference therapy via nanocarriers in the treatment of neurodegenerative diseases (Zhou et al., 2020).Additionally, withaferin-A loaded PLGA NPs were developed as a natural alternative to AD treatment and prevention.These demonstrated efficient drug loading and protection of the cargo as well as sufficient antioxidant activity retention post release from the NP (Madhu et al., 2021), further highlighting the need for in depth research into polymeric NPs and their capabilities within dementia treatment.

| NGs
NGs are NPs that have received a lot of interest in recent years due to their modifiability and potential to pass through the BBB (Kaushik et al., 2016;Kaushik et al., 2018).NGs are 3-dimensional, porous, polymeric molecules with a particle size in the nanometre range, between 20 and 200 nm (Kim et al., 2019).They are composed of either natural or synthetic polymers that are crosslinked either physically or chemically and are mainly spherical in shape.Many crosslinking options are available for NG preparation as shown in Figure 2. NGs are composed of either a core-shell or coreshell-corona structure retaining at least one crosslinked layer for structural integrity and are mostly hydrophilic (Soni et al., 2016).This not only makes them biocompatible but also gives them an ability to swell without dissolving into solvent due to the presence of hydrophilic functional groups such as -OH and CONH 2 along the macromolecular chains (Neamtu et al., 2017).Additionally, their porous structure allows NGs to have a high loading capacity for a range of cargoes along with an ability to respond to stimuli via phase transitions instigated by external cues such as pH, temperature or magnetic field (Raju et al., 2018), resulting in the release of encapsulated molecules.These factors, along with their size and softness, make them of high interest in the biotechnology sector as the manipulation of their volumetric phase change could result in a controllable drug delivery system (Kinoshita et al., 2020;Vicario-de-la-Torre & Forcada, 2017).
Whilst still in its infancy, research into clinical potential of NGs has already shown great promise in developing a drug delivery molecule that is able to cross the BBB.A carboxyl-functionalised poly(N-vinyl pyrrolidone) NG produced via ionising radiation was used for covalent attachment of insulin.This insulin functionalised NG was then able to cross the BBB using the insulin receptor and reach target sites whilst retaining neuroprotection for the encapsulated insulin against dysregulation caused by Aβ (Picone et al., 2016).Additionally, there has been great promise in the use of NGs as personalised therapeutics due to an ability for dual loading.Jiang et al. (2018) have developed a dual inhibitor-modified hyaluronic acid NG loaded with both epigallocatechin-3-gallate (EGCG) and curcumin for AD treatment.This NG demonstrated increased Aβ inhibitory effect of >50% and an increased cell viability when used in combination compared with each compound on its own.This demonstrates the need for dual loading therapeutic mechanisms not currently available clinically to reach the full potential for dementia treatment.
Not only have NGs proved themselves as potential therapeutic carriers, there is ongoing research into their ability to act as a theranostic tool allowing for a less invasive one stop shop for both diagnosis and treatment of dementia.Whilst this research remains in early stages, Kimura et al. have demonstrated that intravenously administered ultra-small gadolinium-conjugated gelatine NGs were able to pass through the BBB into the parenchyma to enable gadolinium to reach its target site (Kimura et al., 2021).This demonstrates an ability to carry diagnostic tools and can be used alongside the previously discussed ability for NGs to carry therapeutics.This theory has been recently demonstrated by Chen et al. (2022) who developed a hyaluronic acid NG loaded with iron oxide NPs.This showed noticeable superparamagnetic properties with an MRI favourable magnetic saturation value along with an ability to bring about relative Aβ fibril disaggregation and prevent Aβ aggregation.This is a huge development for NGs, and it opens up new potentials for their use within clinical settings to improve the health care for dementia patients.
Whilst research into NGs is still developing, hydrogels have shown equal potential as drug delivery systems and scaffolds.NGs, in essence, are a nanosized hydrogel with similar functions and applications.Hydrogels are also classed as three-dimensional, crosslinked, hydrophilic networks of polymers that are able to swell whilst maintaining their structure, due to their crosslinking, and are also stimuli responsive, due to volume phase transitions.Unlike NGs, hydrogels are currently used clinically as (i) contact lenses, making softer more comfortable lenses with equal durability to stiffer versions; (ii) hygiene products; and (iii) wound dressing, where hydrogels maintain a moist environment local to the wound site accelerating wound healing whilst containing fluid spread (Granugel ® , Intrasite Gel ® , Aquaflo™) (Jones et al., 2006).Additionally, several hydrogel-based products have been FDA approved for use as drug delivery and disease treatments such as Cervidil ® , a hydrogel polymer containing dinoprostone for labour induction and post-delivery bleeding.
Both NGs and hydrogels are of particular interest in the medical industry due to their characteristic properties such as swelling, stimuli-responsive behaviour and softness.These protect the therapeutic molecule from degradation and elimination and increase the concentration of drug able to reach its target tissue (Oh et al., 2008).NGs also have the capacity to encapsulate more than one bioactive substance in the same carrier with different physical properties, an ability that is crucial for several diseases displaying multidrug resistance, narrow therapeutic windows and undesirable side effects of current treatments (Gurunathan et al., 2018;Napier & DeSimone, 2007).Additionally, they are mostly hydrophilic in nature; therefore, they are often more biocompatible than free drugs or other NPs such as copper and zinc containing NPs (Lanone et al., 2009).Compared with other drug delivery systems, NGs and hydrogels have higher potential for site targeting and controllable release of bioactive substances with little to no side effects.They have also been shown to be fully/semi biodegradable; thus, no removal technique is needed and are also easy to scale up for mass production, a common issue with many NPs (Neamtu et al., 2017).
Whilst a lot of promising work has been done on NGs as drug delivery agents to the brain, there are still some large hurdles to overcome before they reach clinical standard.One main issue with NGs is their potential to cause cytotoxicity.Some long-term studies have shown cytotoxicity as a result of NP introduction, highlighting the need for further NG cytotoxicity research (Ansari et al., 2017).One potential way to overcome this is to use multifunctional NGs.The use of multi-purpose NGs able to distribute a variety of molecules to target areas must be explored (Khan et al., 2021), not only to limit cytotoxicity but also to offer more successful treatment options for a variety of disorders.Another potential limitation of NGs as drug delivery systems is their tendency for early release of drug where they release their drug in an unwanted or undesirable location (Anooj et al., 2021).
However, the composition of NGs can be altered to allow for temperature, pH and magnetic field release making this aspect a lot more controllable.

| Future direction for nanosolutions
In the last decade, NP development has come a long way with a more profound comprehension for their composition procedures along with advancement in their ability to express cargo molecules (Anooj et al., 2021;Poudel & Park, 2022).
Despite this, the range of NPs investigated clinically is currently limited.Whilst many NPs have shown promise in crossing the BBB and delivery of drugs/nucleic acids for neurological conditions, further work needs to be carried out at the single cell level.Studies into the molecular and cellular mechanisms of uptake across the BBB and neurons and glial cells are required to demonstrate different compositions of NPs and the effect this has towards cytosolic, endosomal and nuclear destinations.This information would be crucial to allow a larger range of NPs to stand as drug delivery systems for targeting at the subcellular level (Mukhopadhyay & Trivedi, 2021) and would give rise to the potential for development of drug delivery systems for neurological conditions currently seen as untreatable.
There is also the option to use different NPs in conjugation with other NP types to combat undesirable features such as the potential of NGs to leak or release their cargo prematurely.Liposomes have been shown to not only stabilise hydrogel release pattern but also increase their viscoelastic properties and gel strength (Ruel-Gariépy et al., 2002).Furthermore, De Smedt and co-workers demonstrated that hydroxyethyl methacrylated dextran (dex-HEMA) NGs coated with SOPC/ DOTAP liposomes were able to impart cellular uptake in VERO-1 cells as well as showing better stability in phosphate-buffered saline (PBS) at 37 C (De Geest et al., 2006;Van Thienen et al., 2005).Additionally, pH-responsive NGs containing 2-vinylpyridine and divinylbenzene have been shown as an adequate cargo system for simultaneous loading and controlled pH-mediated release of magnetic NPs containing oligonucleotide sequences, which would otherwise be trapped (Deka et al., 2010).More recently, gold NP (Au NP)-incorporated molecularly imprinted polymer NGs (MIP-NGs) (Au MIP-NGs) have been developed for use as stealth radiosensitisers for cancer treatment.These were shown to have high selectivity towards human serum albumin and formed an albumin-rich protein corona as well as having a passive targeting property towards refractory pancreatic cancer, based on the enhanced permeability and retention (EPR) effect.Ultimately, results showed tumour growth was suppressed more effectively in Au MIP-NG-injected mice compared with PBS-injected models and that radiation therapy was successful even at a low X-ray irradiation dose (Kitayama et al., 2022).Other studies have however showed minimal positive effect when combining hydrogels and liposomes (Billard et al., 2015) showing further work is required to understand the potential for this kind of combinational therapy.
Exosomes have also shown promising results when combined with liposomes.It was found that cellular uptake and interactions with the exosome-liposome hybrid could be modified by altering the lipid composition and properties of the liposomal component.This suggests that the membrane engineering approach of hybrid exosome systems could advance their use in drug delivery (Sato et al., 2016).Similarly, Liu et al. (2022) have recently supported this with the development of a delivery system known as membrane fusion-based hybrid exosomes (MFHEs).This combination of exosomes with membrane bound liposomes leads to a high drug loading rate and targeted cellular uptake whilst maintaining high cell compatibility and low immunogenicity.
Despite being a potential new avenue for drug development, combining different types of NPs requires further monitoring and testing on the interacting charge, pH and chemical components of both NPs; the effect this has on the drug; and ultimately biocompatibility and degradability.It has been shown that clinical progression of NGs on their own or in combination has been hindered by the time-consuming reality of testing each dosing scenario experimentally.However, recent advances have shown that artificial intelligence in the form of machine learning may be able to create predictive models from a multidimensional dataset and would be invaluable for its use in analysing complex interactions of NPs with biological systems allowing for outcomes to be anticipated (Adekoya et al., 2022;Camacho et al., 2018;Li et al., 2012).It is hypothesised that machine learning would be able to effectively anticipate optimal drug dosing, drug stability and the stability of drug delivery systems with toxicity predictions (Bernick, 2015), and whilst still in its infancy, the use of machine learning has already shown positive results within the lab.Li et al. were able to use machine learning to explore the relationship between hydrogel characteristics and the molecular skeleton allowing them to further develop hydrogels (Li et al., 2019).With this technology, the future of NP development could be accelerated, with their clinical use closer than anticipated.
Additionally, as with a lot of research, there is a growing concern as to the potential for gender bias.Many in vivo studies focusing on the use of NPs within the brain are limited to male animal models.Despite the 1993 NIH mandate for women in human clinical trials, no such mandate is extended to animal models (Beery & Zucker, 2011).There are vast differences between male and female anatomy, which could affect drug interactions at cellular and molecular levels.However, almost all studies that focus on developing treatments at the preliminary stages are catered to male anatomy.It is possible that these treatments, when at clinical trial level, may have differing effects on female participants that were not taken into consideration, rendering the treatment unsuccessful or give unexpected side effects.This is unnecessarily holding back potential treatments due to an avoidable gender bias in the developmental stages of therapeutics.Therefore, it is imperative that this is monitored and addressed in future drug discovery and development pipelines to support the development of NPs suitable for all genders.

| CONCLUSION
Our understanding of the role of the BBB has advanced considerably in recent years, with confirmation of its role as a protective, intrinsic interface essential for brain homeostasis rather than a restrictive impermeable barrier.This review has discussed the regulatory processes of the BBB, the molecular transport routes across its interface and ways to use and manipulate these routes for CNS drug development with a particular focus on dementia.It explored current research using nanomedicine approaches within the dementia field and found great potential for their use clinically, as well as highlighting key areas for development in order to further advance the field.Ultimately, the goal of future CNS drug development needs to expand beyond current drug design following Lipinski's rule of 5 and explore the use of specialised drug carriers able to exploit BBB transport routes.NPs offer an exciting research prospect for CNS research due to their modifiability, biocompatibility and high drug loading potential and have already shown promising results in research.Building upon this knowledge may give the best potential for developing CNS treatments that are so desperately needed to improve the lives of millions of people living with CNS diseases worldwide.

AUTHOR CONTRIBUTIONS
Conceptualisation: AT, AP; draft: CC; review-editing: AT, AP, CC, KK. and the Health Research Institute and the Research Investment Fund, Edge Hill University.AP is supported by a Springboard Award by the Academy of Medical Sciences, the Wellcome Trust, the Government Department of Business, Energy and Industrial Strategy and the British Heart Foundation and Diabetes UK (SBF008\1135).

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I G U R E 1 Composition of the blood-brain barrier (BBB) comprising endothelial cells, astrocytes, basement membrane, pericytes, neurons and tight junctions and showing the interaction of microglia.Transport pathways are also shown: (a) paracellular aqueous pathway: controlled by BBB tight junctions via size and charge selectivity, which severely restrict diffusion of ions and polar solutes.(b) Lipidmediated passive diffusion of lipid soluble, non-polar molecules.(c) Solute carrier (SLC) transport proteins, which can be unidirectional or bidirectional.(d) ATP binding cassette (ABC) transporter efflux pumps.(e) Receptor-mediated transcytosis.(f) Absorptive-mediated transcytosis.

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I G U R E 2 Nanogel polymerisation and crosslinking strategies.(a) Direct polymerisation of monomers.(b) Polymer modified with hydrophobic moieties for self-assembly.(c) Positively and negatively charged polymer self-assembled through electrostatic interactions.
Mechanisms of BBB disruption in neurological conditions.
T A B L E 1 Theranostic approaches used in clinical practice within oncology.
Note: Diagnostic and therapeutic agents used in current clinical theranostics within oncology.Table adapted from Gomes Marin et al. (2020) and Okamoto et al. (2021).