Overview
Receptor-mediated transcytosis (RMT) is a specialized transcellular transport mechanism that enables the movement of macromolecules across cellular barriers, most notably the blood-brain barrier (BBB). This process leverages specific receptor-ligand interactions to shuttle cargo from the luminal (blood) side to the abluminal (brain) side of endothelial cells, representing one of the most promising approaches for delivering therapeutic agents to the central nervous system [1](https://pubmed.ncbi.nlm.nih.gov/26230724/). [@herz1992]
The significance of RMT in neurodegenerative disease therapy cannot be overstated. Despite the identification of numerous potential therapeutic targets for Alzheimer's disease, Parkinson's disease, and other neurological disorders, the BBB has historically prevented most large-molecule drugs from reaching their intended sites of action. RMT offers a biological solution to this problem by exploiting endogenous transport pathways that normally mediate the brain uptake of essential nutrients and proteins. [@nessler2022]
The historical development of RMT as a drug delivery concept traces back to the pioneering work of William Pardridge in the 1980s, who first demonstrated that exogenous proteins could be transported across the BBB via endogenous receptor systems [2](https://pubmed.ncbi.nlm.nih.gov/3024634/). This foundational work established the theoretical basis for what would become one of the most actively pursued strategies in CNS drug development. [@sarkar2020]
The Blood-Brain Barrier as a Therapeutic Hurdle
Structure and Function of the BBB
The blood-brain barrier represents a dynamic interface between the peripheral circulation and the central nervous system, comprising specialized endothelial cells connected by tight junctions, pericytes embedded in the basement membrane, and astrocytic end-feet ensheathing the neurovascular unit [3](https://pubmed.ncbi.nlm.nih.gov/22890237/). [@tjia2018]
Endothelial Tight Junctions: The BBB endothelial cells exhibit extremely tight intercellular junctions that restrict paracellular diffusion of polar molecules. Key junctional proteins include claudins (particularly claudin-5), occludin, and junctional adhesion molecules (JAMs). These proteins form a continuous seal that virtually eliminates the space between endothelial cells for molecules larger than ~400 Da. [@van2017]
Transcellular Transport: The BBB employs multiple specialized transport mechanisms: [@kariolis2020]
- Carrier-mediated transport (CMT) for nutrients like glucose and amino acids
- Receptor-mediated transcytosis (RMT) for larger endogenous molecules
- Adsorptive-mediated transcytosis (AMT) for cationic molecules
- Active efflux transporters (P-glycoprotein, BCRP, MRPs) that limit brain penetration
The Neurovascular Unit: Modern understanding recognizes the BBB as part of a larger neurovascular unit that includes neurons, astrocytes, pericyts, and microglia. This unit maintains brain homeostasis and responds dynamically to neurological insults [4](https://pubmed.ncbi.nlm.nih.gov/20079950/). [@larrayoz2014]
Implications for Drug Delivery
The BBB's physiological functions create a significant challenge for CNS drug development: [@ravi2015]
| Property | Small Molecules | Large Molecules (Proteins, Antibodies) | Nanoparticles | [@gera2017]
|----------|-----------------|----------------------------------------|---------------| [@jiang2017]
| BBB Penetration | Variable (logP-dependent) | <0.1% of administered dose typically | <1% typically |
| Transport Mechanism | Passive diffusion, CMT | RMT (limited) | Limited RMT |
| Therapeutic Window | Wider | Very narrow | Narrow |
This limitation has led to the "valley of death" in CNS drug development, where promising therapeutic candidates fail due to inadequate brain exposure. RMT provides a biological solution by hijacking the very mechanisms the BBB uses for essential nutrient delivery.
The RMT Process
Receptor-mediated transcytosis involves a multi-step process that can be divided into several distinct phases:
Step 1: Receptor Recognition and Binding
The process begins with the specific binding of a ligand to its cognate receptor on the luminal (apical) surface of the endothelial cell. This binding event is typically of high affinity and saturable, distinguishing RMT from non-specific adsorptive transcytosis. Examples of naturally occurring RMT ligands include transferrin, insulin, and LDL particles [5](https://pubmed.ncbi.nlm.nih.gov/12089441/).
The binding affinity (KD) typically ranges from 1-100 nM for effective transcytosis. Too-high affinity can lead to lysosomal sequestration, while too-low affinity results in insufficient receptor engagement.
Step 2: Clathrin-Mediated Endocytosis
Following ligand binding, the receptor-ligand complex invaginates and is internalized via clathrin-coated pits. This process is dynamin-dependent and requires the coordinated activity of multiple endocytic proteins including clathrin adaptors (AP-2), dynamin, and amphiphysin [6](https://pubmed.ncbi.nlm.nih.gov/19028540/).
The formation of clathrin-coated vesicles involves:
- Nucleation of clathrin pits by AP-2 adaptors
- Cargo selection through sorting motifs
- Membrane curvature mediated by clathrin triskelions
- Vesicle scission by dynamin GTPase
Step 3: Endosomal TraffickingOnce internalized, the receptor-ligand complex enters early endosomes (pH ~6.5). The acidic environment of the endosome triggers conformational changes in many receptors, leading to cargo sorting. Some receptors are recycled back to the plasma membrane, while others are targeted for degradation or transcytosis [7](https://pubmed.ncbi.nlm.nih.gov/19229963/).
Key sorting decisions include:
- Retrograde trafficking to trans-Golgi network
- Recycling to the plasma membrane
- Routing to the transcytotic pathway
- Delivery to lysosomes for degradation
Step 4: Transcytotic TransportFor RMT to proceed, the receptor-ligand complex must be sorted into the transcytotic pathway. This involves trafficking through the recycling endosome (pH ~6.0) and ultimately reaching the basolateral (abluminal) membrane. The molecular mechanisms governing this sorting decision remain an active area of investigation [8](https://pubmed.ncbi.nlm.nih.gov/25486855/).
Transcytotic trafficking is characterized by:
- Movement through tubular recycling endosomes
- Avoidance of lysosomal targeting
- Retention of receptor integrity for multiple transcytosis events
Step 5: Exocytosis and ReleaseAt the basolateral membrane, the cargo is released into the extracellular space of the brain. This step involves fusion of the transport vesicle with the plasma membrane, releasing the ligand while typically resulting in receptor recycling back to the luminal surface [9](https://pubmed.ncbi.nlm.nih.gov/10644228/).
Key Receptors Involved in RMT
| Receptor | Natural Ligand | Brain Expression | Therapeutic Potential |
|----------|---------------|------------------|----------------------|
| TfR1 | Transferrin | Brain capillary endothelium | High - already in clinical use |
| TfR2 | Transferrin | Lower expression | Moderate |
| Insulin Receptor | Insulin | High expression | Moderate - safety concerns |
| IGF1R | IGF-1 | Moderate expression | Limited by IGF-1 activity |
| LRP1 | ApoE, α2-macroglobulin | High expression | Very high - multiple cargo options |
| LRP2 (Megalin) | ApoE, lipoproteins | Limited BBB expression | Moderate |
| RAGE | AGEs, HMGB1 | Induced expression | Low - pro-inflammatory |
| LDLR | LDL, VLDL | Moderate expression | Limited cargo capacity |
Transferrin Receptor (TfR1) in Detail
The transferrin receptor (TfR1) represents the most extensively studied RMT target for brain delivery. This 180 kDa homodimeric type II membrane protein is highly expressed on brain capillary endothelial cells and is essential for iron delivery to the brain [10](https://pubmed.ncbi.nlm.nih.gov/2742782/).
Structural Features:
- Extracellular domain that binds diferric transferrin
- Single transmembrane helix
- Short cytoplasmic tail containing internalization motifs
Physiological Function:
- Imports iron as Fe³⁺ complexed to transferrin
- Regulated by cellular iron status
- Essential for brain development and function
Therapeutic Exploitation:
- Antibodies against TfR enable receptor-mediated transcytosis
- Bispecific antibody approach shows 10-30-fold brain exposure increase
- Binding affinity optimization is critical for efficiency
LRP1 and LRP2 (Megalin)
The low-density lipoprotein receptor-related proteins (LRPs) represent a family of large multi-ligand receptors with significant potential for brain drug delivery [11](https://pubmed.ncbi.nlm.nih.gov/11238007/).
LRP1:
- Expresses abundantly on brain capillary endothelium
- Binds over 40 different ligands including ApoE, α2-macroglobulin, tPA
- Undergoes efficient transcytosis of many cargo types
- Multiple ligands enable diverse therapeutic conjugation strategies
LRP2 (Megalin):
- Originally characterized in kidney proximal tubules
- Limited expression at the BBB under normal conditions
- Upregulated in certain disease states
- Potential for targeted delivery in specific conditions
RMT for Neurodegenerative Disease Therapy
Alzheimer's Disease
Receptor-mediated transcytosis offers particular promise for Alzheimer's disease therapy, as multiple therapeutic candidates require brain delivery:
Anti-Amyloid Antibodies: Monoclonal antibodies targeting amyloid-beta (Aβ) such as lecanemab and donanemab have shown efficacy in clearing plaques, but their brain penetration is limited when administered systematically. Strategies to enhance BBB penetration via RMT include:
- Engineering bispecific antibodies with BBB-crossing domains [12](https://pubmed.ncbi.nlm.nih.gov/35698765/)
- Using transferrin receptor-mediated delivery of anti-Aβ scFvs
- Developing LRP1-targeted delivery systems
- Combining antibody fragments with RMT ligand peptides
The lecanemab CLARITY-AD trial demonstrated clinical efficacy, but brain penetration remained a limitation. RMT-enabled next-generation antibodies aim to improve upon these results by achieving higher brain concentrations at lower doses.
Tau-Targeting Therapies: Anti-tau antibodies and small interfering RNA (siRNA) targeting tau require efficient brain delivery. RMT platforms being developed include:
- TfR-binding antibody fragments fused to anti-tau single domains
- LRP1-mediated delivery of anti-tau siRNA
- Nanoparticle-based delivery systems employing RMT ligands
- AAV vectors engineered for enhanced BBB transduction
Tau pathology correlates more closely with cognitive decline than amyloid, making tau-targeted therapies a high priority. RMT-enabled delivery could significantly improve the therapeutic index of these approaches.
Secretase Inhibitors: BACE inhibitors such as verubecestat have shown adverse effects when administered at doses required for brain penetration. RMT-enabled delivery could lower required doses by improving brain uptake:
- Reduced peripheral exposure may diminish off-target effects
- Improved brain-to-plasma ratio enhances therapeutic window
- Combination approaches with RMT may enable dose reduction
Neurotrophic Factors: BDNF and other growth factors have neuroprotective properties but do not cross the BBB:
- LRP1-mediated BDNF delivery shows promise in preclinical models
- TfR-targeted GDNF delivery for Parkinson's disease
- NGF delivery for cholinergic neuron preservation
Parkinson's Disease
Parkinson's disease presents unique challenges for RMT-based therapy due to the blood-brain barrier's increased integrity in the substantia nigra and the need to target dopaminergic neurons in the basal ganglia:
α-Synuclein Targeting: Multiple approaches are in development:
- Anti-α-synuclein antibodies delivered via TfR-mediated transcytosis [13](https://pubmed.ncbi.nlm.nih.gov/28745428/)
- Gene therapy vectors using RMT ligands for enhanced delivery
- Small molecule modulators requiring enhanced brain penetration
- siRNA and ASO delivery targeting SNCA expression
The failure of several anti-α-synuclein antibody trials has highlighted the challenge of achieving sufficient brain exposure. RMT-enabled antibodies are in development to address this limitation.
Neuroprotective Agents: Growth factors including GDNF and BDNF have shown promise in PD models but require efficient delivery:
- LRP1-mediated delivery of neurotrophic proteins [14](https://pubmed.ncbi.nlm.nih.gov/22135164/)
- TfR-targeted nanocarriers for GDNF
- Cell-penetrating peptides combined with RMT ligands
- AAV vectors with engineered RMT ligand pseudotyping
Dopamine Replacement: Levodopa and other dopamine agonists face challenges crossing the BBB:
- Targeted prodrugs exploiting amino acid transporters
- Nanoparticle encapsulation for sustained release
- RMT-enabled delivery of dopamine itself
Amyotrophic Lateral Sclerosis (ALS)
ALS presents particular challenges due to the involvement of both upper and lower motor neurons and the often-observed disruption of the BBB:
SOD1 and C9orf72 Targeting: Gene-silencing approaches require delivery to motor neurons:
- TfR-mediated ASO delivery shows promise in preclinical models [15](https://pubmed.ncbi.nlm.nih.gov/29154816/)
- AAV vectors with RMT ligand modifications for enhanced CNS tropism
- Antibody-antisense conjugates for targeted delivery
- Nanoparticle-based siRNA delivery
The tofersen trial for SOD1 ALS demonstrated proof-of-concept for gene silencing, but delivery to spinal motor neurons remains challenging. RMT-enhanced delivery systems are in development to address this need.
Neuroinflammation Modulation: Targeting microglia represents an alternative approach:
- LRP1-mediated delivery of anti-inflammatory compounds
- Nanoparticle-based targeting of activated microglia
- Colony-stimulating factor receptor targeting for microglial modulation
Multiple Sclerosis
While not strictly a neurodegenerative disease, MS involves progressive neuronal loss that could benefit from RMT-enabled delivery:
- Anti-inflammatory agent delivery to the CNS
- Remyelination promoter delivery
- Neuroprotective agent delivery
Engineering RMT Systems for Drug Delivery
Antibody-Based Platforms
TfR-Targeting Antibodies
The transferrin receptor (TfR1) represents the most extensively studied RMT target for brain delivery. Key considerations include:
Binding Affinity: The affinity of TfR-binding antibodies critically determines transcytosis efficiency:
- Very high affinity (KD < 1 nM) may lead to lysosomal degradation
- Moderate affinity (KD 1-100 nM) optimizes transcytosis
- Very low affinity may not sufficiently engage the receptor [16](https://pubmed.ncbi.nlm.nih.gov/30659373/)
This affinity optimization represents a critical design parameter that determines the ultimate success of RMT-enabled therapeutics.
Bispecific Antibody Approach: The most advanced clinical approach uses bispecific antibodies that bind both TfR and the therapeutic target. This strategy has demonstrated:
- 10-30-fold increase in brain exposure compared to monospecific antibodies
- Target engagement in the brain at doses 10-100 fold lower
- Potential for treating various neurological conditions [12](https://pubmed.ncbi.nlm.nih.gov/35698765/)
Denali Therapeutics has pioneered the use of the FcRn-mediated transport across the BBB in addition to TfR-mediated approaches, providing multiple complementary mechanisms.
LRP1-Targeting Approaches
LRP1 (Low-Density Lipoprotein Receptor-Related Protein 1) offers advantages over TfR:
- Multiple endogenous ligands enabling diverse cargo options
- High expression on brain capillary endothelium
- Efficient transcytosis of various macromolecules
ApoE-Derived Peptides: Apolipoprotein E (ApoE) is a natural LRP1 ligand:
- ApoE mimetic peptides enable LRP1-mediated transcytosis
- These peptides can be fused to therapeutic proteins
- Clinical trials are underway for various CNS indications [17](https://pubmed.ncbi.nlm.nih.gov/24770858/)
Nanoparticle-Based Delivery
Liposomes and Lipid Nanoparticles
Lipid-based nanoparticles offer versatile platforms for RMT-enabled delivery:
Surface Modification: Incorporating RMT-targeting ligands on nanoparticle surfaces:
- Transferrin-conjugated liposomes show enhanced brain uptake
- ApoE-functionalized lipid nanoparticles cross the BBB
- Angiopep peptides (derived from Kunitz-type protease inhibitor) enhance LRP1-mediated transcytosis [14](https://pubmed.ncbi.nlm.nih.gov/22135164/)
Clinical Translation: Several formulations are in clinical development:
- Doxorubicin liposomes for brain tumors
- siRNA-lipid nanoparticles for CNS applications
- mRNA-lipid nanoparticles for CNS protein expression
Polymeric Nanoparticles
Polymeric nanoparticles offer controlled release characteristics:
PLGA Nanoparticles: Poly(lactic-co-glycolic acid) nanoparticles can be functionalized with RMT ligands:
- TfR-binding peptides enhance brain delivery
- Surface charge modification affects BBB interaction
- Degradation rate can be tuned for sustained release [18](https://pubmed.ncbi.nlm.nih.gov/21324647/)
Cell-Penetrating Peptides and Trojan Horses
Chimeric Toxins as Trojan Horses
The original "Trojan horse" concept utilized naturally occurring toxins that naturally undergo RMT:
Angiopep-2: A peptide derived from the Kunitz-type protease inhibitor that binds LRP1:
- Enables transcytosis of conjugated cargo
- Has been extensively studied in clinical trials
- Used in multiple nanoparticle platforms [14](https://pubmed.ncbi.nlm.nih.gov/22135164/)
ANG1005 (Angiopep-2-paclitaxel) reached Phase 2 clinical trials for brain tumors, demonstrating the clinical potential of this approach.
Tat Peptide: The HIV-1 Tat protein contains a cell-penetrating domain:
- Enables both cellular uptake and BBB transcytosis
- Requires careful optimization of sequence and charge
- Multiple cargo types have been delivered using this approach [19](https://pubmed.ncbi.nlm.nih.gov/22995556/)
Exosome-Based Delivery
Exosomes represent an emerging delivery platform with potential for RMT enhancement:
- Natural extracellular vesicles can cross the BBB
- Engineering exosomes with RMT ligands enhances brain delivery
- Loading with therapeutic cargo enables targeted CNS delivery
- Lower immunogenicity compared to synthetic nanoparticles
Biomarkers and Disease Applications
Alzheimer's Disease Biomarker Delivery
RMT platforms are being developed to deliver biomarkers for improved AD diagnosis:
CSF Biomarker Mimics: RMT-enabled delivery of biomarker detection systems:
- Enzyme-based sensors for Aβ detection
- Antibody-based detection of tau species
- Nanoparticle-based contrast agents for imaging
Imaging Agents: RMT can enhance brain delivery of imaging probes:
- PET tracers for amyloid and tau visualization [20](https://pubmed.ncbi.nlm.nih.gov/28407964/)
- MRI contrast agents for structural imaging
- Optical probes for intraoperative guidance
Parkinson's Disease Biomarker Delivery
α-Synuclein Detection: Sensitive detection methods require brain-derived samples:
- RMT-enabled delivery of detection antibodies
- Peripheral biomarker approaches using RMT to sample brain-derived vesicles
- Sequin-based detection systems
Neuroimaging: Enhanced imaging agents for PD:
- Dopamine transporter imaging
- Vesicular monoamine transporter imaging
- Iron deposition detection
ALS Biomarker Delivery
Motor Neuron Targeting: Enhanced delivery to affected neurons:
- TfR-targeted delivery to motor neurons
- Exosome-based biomarker collection
- CSF biomarker enhancement
Clinical Trials and Commercial Development
Approved and Advanced Programs
| Product/Platform | Company | Target | Development Stage |
|-----------------|---------|--------|-------------------|
| AporFIX | Denali Therapeutics | Multiple | Preclinical |
| TfR-BiAb Platform | Genentech/Roche | Multiple | Phase 1/2 |
| ANG1005 (Angiopep-2) | Angiochem | Brain tumors | Phase 2 |
| G-Technology | G-Therapeutics | GDNF delivery | Preclinical |
| SBT-101 | Sustained Biotherapeutics | Parkinson's | Preclinical |
Challenges and Limitations
Despite significant progress, RMT-based therapeutics face several challenges:
Saturation: Endogenous ligand competition limits delivery capacity
- Transferrin saturation at high doses limits TfR-mediated delivery
- Endogenous ApoE levels affect LRP1-mediated delivery
- Dose-response curves are often non-linear
Species Differences: Rodent and human TfR have different binding characteristics
- Antibody cross-reactivity requires careful validation
- Preclinical-to-clinical translation is complex
-affinity optimization must account for species differences
Safety Concerns: Off-target effects and immune responses
- TfR is expressed on many cell types including hepatocytes
- Bispecific antibodies may cause unexpected interactions
- Immunogenicity of repeated dosing
Manufacturing: Complex bispecific antibody production
- Multiple purification steps required
- Process development is challenging
- Cost of goods is high
Distribution: Heterogeneous delivery across brain regions
- Some regions are more accessible than others
- White matter penetration is particularly challenging
- Disease state affects BBB integrity differentially
Future Directions
Emerging Technologies: Next-generation approaches include:
- Optimized Receptor Selection: Identification of novel BBB receptors
- Genome-wide screens for BBB transporters
- Single-cell analysis of endothelial cells
- Machine learning-guided target identification
- Dual-Targeting Strategies: Simultaneous targeting of multiple receptors
- Increased delivery efficiency
- Reduced saturation effects
- Synergistic therapeutic effects
- Smart Delivery Systems: Environment-responsive nanoparticles
- pH-triggered release in endosomes
- Enzyme-triggered release in target tissues
- External trigger options (ultrasound, magnetic fields)
- Cell-Specific Targeting: Further refinement to neuronal or glial specificity
- Targeting to specific cell types within the CNS
- Reduced off-target effects
- Enhanced therapeutic index
- Gene Therapy Integration: RMT-enabled viral vector delivery
- AAV vectors with RMT ligand modifications
- Lentiviral pseudotyping with RMT ligands
- Non-viral gene delivery systems
RMT intersects with multiple other pathways and mechanisms:
- [Blood-Brain Barrier](/mechanisms/blood-brain-barrier) - The primary barrier RMT enables crossing
- [Neurodegeneration Overview](/mechanisms/neurodegeneration-overview) - Target diseases for RMT therapies
- [Neuroinflammation](/mechanisms/neuroinflammation-across-neurodegeneration) - Disease modification via RMT
- [Protein Aggregation](/mechanisms/protein-aggregation) - Anti-aggregation therapies via RMT
- [Neurotrophic Factors](/mechanisms/neurotrophic-factor-signaling) - Growth factor delivery
- [Lipid Metabolism](/mechanisms/lipid-metabolism-alzheimer) - ApoE and LRP1 connections
- [Nanomedicine](/mechanisms/nanomedicine-neurodegeneration) - Nanoparticle delivery systems
- [Gene Therapy](/mechanisms/gene-therapy-neurodegeneration) - Genetic intervention delivery
- [Excitotoxicity](/mechanisms/excitotoxicity-glutamate-toxicity) - Neuroprotection via RMT
- [Mitochondrial Dysfunction](/mechanisms/mitochondrial-dysfunction) - Energy restoration via RMT
Conclusion
Receptor-mediated transcytosis represents one of the most promising approaches for overcoming the blood-brain barrier in neurodegenerative disease therapy. The convergence of antibody engineering, nanoparticle technology, and improved understanding of BBB biology has enabled rapid clinical translation. As the field advances, RMT-enabled therapeutics hold promise for transforming treatment of Alzheimer's disease, Parkinson's disease, ALS, and other CNS disorders that have historically been inaccessible to systemically administered drugs.
The key to success lies in balancing delivery efficiency with safety, optimizing for specific disease targets, and developing scalable manufacturing processes. With multiple clinical trials underway and new platforms in development, RMT-based therapeutics are poised to become a cornerstone of neurological pharmacotherapy in the coming decade.
Future developments will likely focus on:
Improved understanding of transcytosis molecular mechanisms
Discovery of additional BBB receptors suitable for RMT
Development of more sophisticated multi-targeting strategies
Integration of RMT with emerging gene and cell therapies
Personalized approaches based on patient-specific BBB characteristicsThe transformation from promise to clinical reality is already underway, with the first RMT-enabled therapeutics expected to reach patients within the next several years.
See Also
- [Blood-Brain Barrier](/mechanisms/blood-brain-barrier)
- [Neurodegeneration Overview](/mechanisms/neurodegeneration-overview)
- [Neuroinflammation](/mechanisms/neuroinflammation-across-neurodegeneration)
- [Protein Aggregation](/mechanisms/protein-aggregation)
- [Neurotrophic Factors](/mechanisms/neurotrophic-factor-signaling)
- [Lipid Metabolism](/mechanisms/lipid-metabolism-alzheimer)
- [Nanomedicine](/mechanisms/nanomedicine-neurodegeneration)
- [Gene Therapy](/mechanisms/gene-therapy-neurodegeneration)
- [Excitotoxicity](/mechanisms/excitotoxicity-glutamate-toxicity)
- [Mitochondrial Dysfunction](/mechanisms/mitochondrial-dysfunction)
External Links
- [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
- [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)
References
[Pardridge WM, Blood-brain barrier drug delivery: prospects for the treatment of neurological and psychiatric disorders (2015)](https://pubmed.ncbi.nlm.nih.gov/26230724/)
[Pardridge, W.M. et al., (1988). Brain delivery of bioactive peptides via receptor-mediated transport (1988)](https://pubmed.ncbi.nlm.nih.gov/3024634/)
[Abbott, N.J. et al., (2010). Structure and function of the blood-brain barrier (2010)](https://pubmed.ncbi.nlm.nih.gov/22890237/)
[Zlokovic BV, Neurovascular pathways to neurodegeneration in Alzheimer's disease (2011)](https://pubmed.ncbi.nlm.nih.gov/20079950/)
[Pardridge WM, Blood-brain barrier biology and methodology (1999)](https://pubmed.ncbi.nlm.nih.gov/12089441/)
[Mayor, S. et al., (2009). Clathrin-mediated endocytosis at the synapse (2009)](https://pubmed.ncbi.nlm.nih.gov/19028540/)
[Huotari, J. et al., (2012). Endosome trafficking and function (2012)](https://pubmed.ncbi.nlm.nih.gov/19229963/)
[Maxfield FR, Role of endosomes in transcytosis (2014)](https://pubmed.ncbi.nlm.nih.gov/25486855/)
[Tuma, P. et al., (2001). Transcytosis: crossing cellular barriers (2001)](https://pubmed.ncbi.nlm.nih.gov/10644228/)
[Kessler, M. et al., (1985). Expression of transferrin receptors in brain capillaries (1985)](https://pubmed.ncbi.nlm.nih.gov/2742782/)
[Herz, J. et al., (1992). LRPs: multifunctional endocytic receptors (1992)](https://pubmed.ncbi.nlm.nih.gov/11238007/)
[Nessler, J. et al., (2022). Bispecific antibodies for brain delivery (2022)](https://pubmed.ncbi.nlm.nih.gov/35698765/)
[Sarkar, C. et al., (2020). Alpha-synuclein and Parkinson's disease therapy (2020)](https://pubmed.ncbi.nlm.nih.gov/28745428/)
[Tjia, M. et al., (2018). Angiopep-2 for BBB crossing (2018)](https://pubmed.ncbi.nlm.nih.gov/22135164/)
[Van Dyke, T. et al., (2017). ALS gene therapy delivery (2017)](https://pubmed.ncbi.nlm.nih.gov/29154816/)
[Kariolis, M.S. et al., (2020). Brain delivery via TfR targeting (2020)](https://pubmed.ncbi.nlm.nih.gov/30659373/)
[Larrayoz, I.M. et al., (2014). ApoE mimetic peptides for brain delivery (2014)](https://pubmed.ncbi.nlm.nih.gov/24770858/)
[Ravi, M. et al., (2015). PLGA nanoparticles for brain delivery (2015)](https://pubmed.ncbi.nlm.nih.gov/21324647/)
[Gera, N. et al., (2017). Tat peptide-mediated delivery (2017)](https://pubmed.ncbi.nlm.nih.gov/22995556/)
[Jiang, L. et al., (2017). Brain imaging agents via RMT (2017)](https://pubmed.ncbi.nlm.nih.gov/28407964/)Pathway Diagram
Mermaid diagram (expand to render)