Abstract

Alzheimer's disease (AD) is a neurodegenerative condition affecting over 50 million people globally, marked by cognitive decline and memory loss. Its pathological presentations include the accumulation of amyloid beta protein, neurofibrillary tangles, and neuroinflammation. The challenges in developing effective treatments stem from the inconclusive causes of AD and the blood-brain barrier, which restricts drug delivery to the brain. Research efforts are thus focused on three main areas: bypassing the blood-brain barrier, creating effective therapeutic drugs, and managing all the possible causes. This review highlights advancements in these areas, such as the use of intranasal and targeted nanocarriers for transporting drugs to combat amyloid beta and tau tangles, and how the use of insulin-inspired neuronal targeting can be used in delivering larger therapeutic agents, such as proteins. It also describes some of the challenges still faced amidst the progress achieved so far.

Introduction

Alzheimer's disease is one of the most common forms of dementia, the different forms of neurodegenerative condition that disrupts neural functions and cognition[1]. While it was initially believed to affect those over 65, Alzheimer's is now characterized as an early-onset progressive disease that can begin at the age of 25[2]. Concurrently, with the current improvement in healthcare access, there is an increase in the older populations necessitating an effective treatment to alleviate the burden of Alzheimer’s disease(AD) financially and psychosocially.

Pathophysiological explanations for AD include Amyloid Beta(Aβ), Tau protein, Glutamate Cytotoxicity, and Cholinergic hypotheses. In the Aβ hypothesis, the breakdown of Amyloid Precursor Protein(APP) by β- and γ-secretases (rather than α-secretases) leads to the formation of insoluble Aβ, which aggregates to form neurotoxic plaques[1, 3]. In the Tau Protein Aggregation hypothesis, the normally soluble tau proteins are hyperphosphorylated, destabilizing neuronal microtubules and leading to the formation of neurofibrillary tangles, and hence, cytoskeletal dysfunction and neurodegeneration[4]. The Cholinergic hypothesis links Alzheimer’s to insufficient acetylcholine (ACh) levels in the brain, while the Glutamate Cytotoxicity Hypothesis associates the disease with excessive glutamate neurotransmitter, which causes increased calcium (Ca²⁺) influx and neuronal death[1, 4].The Aβ and Tau aggregation pathways have also been shown to be correlated, hence the need for multitargeting therapeutics[3].

This non-specificity in causes and onset variability of ADis a challenge indeveloping therapeutic solutions, in addition to developing treatments that can effectively bypass the blood-brain barrier (BBB). The BBB is a protective barrier in the brain that consists of pericytes, neurovascular endothelial cells, and astrocyte end feet, forming a physical barrier with tight junctions that limits the paracellular transport of molecules, including therapeutic drugs larger than 400 Da[1]. Moreover, the ATP-dependent efflux pump P-glycoprotein (P-gp) actively pumps drugs out of the central nervous system(CNS), reducing their effectiveness[5]. Some FDA-approved drugs for treating Alzheimer's disease include Donepezil and Rivastigmine, both acetylcholinesterase inhibitors, administered orally[2, 6]. Rivastigmine also acts as a butyrylcholinesterase inhibitor to reduce amyloid-beta production[2, 6]. However, these drugs often have poor stability and BBB permeability, requiring modification or target vehicles to achieve a higher permeability and retention[2]. Although many ways have been developed to traverse the BBB, most FDA-approved drugs are monoclonal antibody-based, used to reduce the accumulation of the Aβ plaques and tau proteins[2, 3]. The following sections describe the progress in AD treatments, including the various targeting mechanisms that may be useful in developingmore effective therapeutics.

Bypassing the blood-brain barrier

Mechanisms for bypassing the BBB can be divided into disruptive and non-disruptive methods[1]. Disruptive methods temporarily compromise the integrity of the BBB and involve using ultrasound and hyperosmotic solutions (e.g., polysorbate 80, dimethyl sulfoxide)to allow the temporary passage of molecules. While effective for delivering drugs, these methods may also allow harmful neurotoxins to enter, potentially causing neuronal damage or inflammation[1, 7].

Figure 1. Pathological progression of Alzheimer’s disease illustrating key mechanisms: A) Cholinergic hypothesis, B) Glutamate excitotoxicity, C) Amyloid cascade, and D) Tau protein aggregation. ChAT (Acetylcholine transferase), ACh (Acetylcholine), AChE (Acetylcholinesterase), Aβ (Beta-amyloid), and APP (Amyloid precursor protein). Adapted from Jang et al.., Journal of Nanobiotechnology, 2025[4].

Non-disruptive methods use alternative administration routes, such as intranasal or intrathecal delivery, or targeting molecules that exploit BBB receptors. For instance, intranasal delivery, in which drugs are directly injected into neural tissues without absorbance into the systemic circulation, as seen with chitosan-coated solid lipid nanoparticles [1]. Targeted vesicles offer a non-invasive approach that utilizes BBB carriers and receptors, such as insulin receptors (IR) and transferrin receptors (TfR) [2]. For instance, mannose can target glucose transporter-1 and be combined with penetratin, a substrate for nicotinic acetylcholine receptors, and Rabies Virus Glycoprotein (RVG) for liposomal transport [3].

The uptake of these vesicles occurs through adsorptive-mediated transcytosis (AMT), involving interactions between positively charged compounds and negatively charged BBB membranes[4]. Carrier-mediated transcytosis (CMT) utilizes specific transporters in the BBB responsible for the uptake of essential nutrients, while receptor-mediated transcytosis (RMT) involves target molecules binding to specific receptors, forming complexes that are internalized through Clathrin-coated pits, enabling efficient delivery across the BBB[1, 5].

Use of Lipid-Based Nanocarriers

Exosomes

Exosomes are naturally occurring extracellular vesicles. They are membrane-bound nano-sized vesicles secreted for intercellular communication and can cross the blood-brain barrier due to their small size(30 -100 nm)[4, 8]. For example, RVG-modified mesenchymal stem cell exosomes (MSC-RVG) have been used to reduce amyloid plaque deposition in the hippocampus compared to other MSCs[3, 8]. Exosomes provide potential vehicles for delivering protein-based molecules or siRNA that target tau aggregates or Aβ plaques[4]. The structure of an exosome is shown in Figure 3.

**Figure 2.** Structure of the Blood–Brain Barrier Neurovascular Unit. ABC = ATP-binding cassette transporter; JAM = junction adhesion molecule; LAT1 = L-type amino acid transporter 1. Adapted from Pedder et al. (2025), The Lancet Neurology[7].

Liposomes

These are artificially synthesized, spherical, phospholipid bilayer nanovesicles that can be used to encapsulate hydrophilic and hydrophobic drugs or can be functionalized with targeting ligands like transferrin to enhance neuronal intake[4]. For example, liposomescan deliver microRNAs, such as microRNA-195, to lower the expression of APPand BACE1, reducing Aβ production[4]. Liposomes can also be dual-targeted, e.g., in the delivery of doxorubicin liposomes in which liposomes are conjugated with folate and transferrin. The structure of a liposome is shown in Figure 3.

Solid lipid nanoparticles (SLNs)

Solid lipid nanoparticles have a hydrophobic lipid core enclosed by phospholipids(glycerides and fatty acids)and are stabilized using surfactants[1, 4]. They can also be used to encapsulate both hydrophobic and hydrophilic drugs; for example, the use of chitosan-coated SLNs, as seen in the FA-chitosan-SLN formulation, which delivers dopamine while offering a slow release for sustained effects, albeit with a potential lag time[4]. The structure of an SLN is shown in Figure 3.

**Figure** 3: Different types of lipid-based nanoparticles: Exosome, liposome, solid lipid nanoparticle, and nanostructured lipid carrier. Adapted from Jang et al.., Journal of Nanobiotechnology, 2025 [4].

Use of Prodrugs

Prodrugs are chemically modified drugs designed to improve pharmacokinetic properties such as solubility, stability, and bioavailability, allowing for enhanced delivery and efficacy of therapeutic agents[9]. In AD, they are intended to enhance the delivery of drugs across the BBB, improve metabolic stability, and enable the multitargeting of the various pathways involved in AD[10]. The different formulations include mutual prodrugs, in which two active molecules are linked together to simultaneously target multiple therapeutic pathways, and pleiotropic prodrugs, which are designed to release multiple active agents in response to specific biological conditions within the AD pathology[10].

For example, the ibuprofen-tacrine prodrug, which is a mutual prodrug linked by a Phenyl Boronate ester, has improved the pharmacokinetic properties over its parent compounds. This pro-drug has enhances solubility and bioavailability, allowing for more effective CNS delivery[10]. It is activated by esterase enzymes present in the brain, releasing ibuprofen, which provides anti-inflammatory effects, and tacrine, a cholinesterase inhibitor that enhances cholinergic function. Ladostigil is a pleiotropic prodrug, with both cholinesterase inhibition and monoamine oxidase B inhibition, for oxidative stress, neuroinflammation, and improved cognitive function[9, 10].

Use of Fusion proteins

Although small-molecule drugs and antibodies have successfully been delivered across the BBB, delivering large molecular therapeutics, such as proteins, remains a challenge. Kamei et al. proposed a potential solution by engineering hippocampal neuron-targeting (Ht) proteins that leverage the existing transport mechanisms of insulin to enhance the delivery of therapeutic proteins specifically to hippocampal neurons[11]. This is somehow analogous to the use of prodrugs.

The study aimed to improve the accumulation and internalization of therapeutic proteins in the hippocampus, a primary brain region affected by AD. This system employs insulin-fusion proteins designed to bind specifically to IR on hippocampal neurons. Insulin is an excellent candidate because it naturally crosses the BBB through receptor-mediated endocytosis and is selectively taken up by neuronal cells, hence allowing better delivery of larger therapeutic molecules, as shown in Figure 4(bottom right )[11].The results indicated that Ht proteins significantly increased the internalization of fused therapeutic agents in neuronal cells by inducing macropinocytosis via IR, enhancing delivery efficiency to hippocampal neurons in both in vitro and in vivo models[11]. Additionally, strong results were observed with individual insulin subunits, such as comparable binding to the IR and cellular uptake. This suggests that even truncated forms of the insulin molecule can effectively target and deliver cargo into neurons, hence the feasibility of using these fusion proteins for therapeutic applications[11].Wang et al. also developed a transthyretin-Penetratin fusion protein with enhanced permeability and increased inhibition of Aβ aggregation[12].

Payloads

Drugs for Alzheimer’s target specific pathways related to its pathophysiology. Acetylcholinesterase inhibitors, such as Donepezil, Rivastigmine, and Galantamine, increase ACh levels to improve neuron communication. Memantine, an FDA-approved NMDA receptor antagonist, reduces the excitotoxic effects of excess glutamate. Therapeutics targeting the Aβ pathway mainly use immunogenic approaches. Aducanumab is an antibody that targets aggregated Aβ plaques, while Lecanemab targets soluble protofibrils, reducing plaques(Figure 4, top right)[13]. UBITh amyloid-beta is a non-FDA-approved peptide vaccine that promotes antibody production against Aβ. Lecanemab and Donanemab are FDA-approved monoclonal antibodies that target tau aggregates for their clearance(Figure 4, left)[14]. Although approved, like the AchE inhibitor drugs, their effectiveness is limited to improving the prognosis since they only target specific pathways, which are now thought ofpossiblyas only downstream causes rather than the actual causes.

Figure 4:(Left) Mechanisms of action for tau antibodies in targeting tau pathology. The antibodies target the tau aggregates both intracellularly(uptake by receptor-mediated endocytosis or mass endocytosis)and extracellularly to promote their disassembly., Congdon et al. (2023), Nature Reviews Neurology[14]; (Top right) Aβ aggregation pathway and action of aducanumab by binding the Aβ to increase their clearance, Gunawardena et al. (2021), Brain Sciences.[13]; (Bottom right) Illustration of an insulin fusion protein, Kristensen, M. (2024), Proceedings of the National Academy of Sciences[15].

Limitations

Despite the progress in Alzheimer’s treatment, there hasn’t been a promise of complete recovery. Some drugs show increased effectiveness in single-drug systems but not in multidrug systems required to address the multifactorial causes of AD, including the BBB. Lipid-based nanocarriers may be subject to degradation, physical instability, and low production[4]. Moreover, they often have low drug loading capacity and poor controlled release, leading to unpredictable biodistribution. Nanocarriers like exosomes may also trigger immune responses. On the other hand, some therapies demonstrate suboptimal efficacy with minimal impact on the progression of AD; for example, Solanezumab, an antibody designed to target soluble amyloid-beta aggregates, failed to show significant cognitive benefits in patients with mild to moderate AD despite initial hopes for efficacy in early-stage patients[1]. Nanoparticles like exosomes and even prodrugs are generally costly to produce in therapeutic amounts.

Conclusion

This review has highlighted the potential of targeted neuronal drug delivery strategies through disruptive and non-disruptive methods that enhance the bioavailability of therapeutics for AD. Exosomes serve as a natural vehicle for delivering protein-based drugs and siRNA. At the same time, liposomes can encapsulate both hydrophilic and hydrophobic drugs, enhancing neuronal uptake through targeted ligands. SLNs provide stable delivery with sustained effects. Additionally, prodrugs are chemically modified drugs to improve pharmacokinetics and target multiple pathways involved in AD. Novel engineering of insulin-fusion proteins has also shown promise by utilizing IR on hippocampal neurons, facilitating the delivery of larger therapeutic molecules. While no therapy has successfully managed all aspects of AD, the progress for multifactorial therapeutic is promising. The success of the field will alleviate a significant burden on humanity, both economically by reducing care-related costs and scientifically by opening new avenues for addressing other neurodegenerative diseases.

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