The Blood-Brain Barrier: A Double-Edged Sword in Neurology
The brain, our most intricate and vital organ, is safeguarded by a unique structure known as the blood-brain barrier (BBB). Acting as a fortress, the BBB keeps harmful substances out while preserving the delicate environment of the brain. This protective mechanism, however, also poses significant challenges in developing drugs for the central nervous system (CNS). With fewer than 5% of evaluated drugs successfully crossing the BBB, this field of pharmaceutical research faces notably high costs and lengthy development cycles (Haumann et al., 2020).
The BBB as a Bottleneck in CNS Drug Development
More than one in three people worldwide are affected by neurological disorders, leading to profound suffering for individuals and their families (Steinmetz et al., 2024). These conditions have emerged as the leading causes of disability and ill health globally, with the overall impact of disability, illness, and premature death rising by 18% since 1990 (World Health Organization, 2024). Despite significant research investment, the BBB remains a major bottleneck in developing effective treatments. To overcome this hurdle, considerable work has also gone into devising effective methods for drug delivery across the BBB. Some key areas of ongoing research include:
- Receptor-Mediated Transport (RMT): This strategy leverages the BBB’s natural transport mechanisms. Drugs are attached to ligands that bind to specific receptors, such as transferrin or insulin receptors, on the BBB. This interaction triggers receptor-mediated endocytosis, allowing drugs to be transported into the brain within vesicles.
- Nanoparticle-Based Delivery: Nanoparticles are engineered to encapsulate drugs, which are then coated to evade immune detection. These coatings may include ligands that enhance uptake through RMT, improving the delivery process.
- Focused Ultrasound: This method employs ultrasound waves in conjunction with microbubbles to create a temporary disruption in the BBB, allowing drugs to pass through more readily.
- Molecular Trojan Horses: By linking drugs to molecules that naturally cross the BBB, this technique supports RMT by using peptides or other carriers to ferry drugs across the barrier.
Animal Models for BBB Research at Biocytogen
To advance the development of more effective drug delivery methods across the BBB, Biocytogen has developed a series of humanized animal models. In these models, key mouse targets involved in natural BBB transport, which also hold potential for delivering therapeutic agents, have been replaced with their human counterparts. This enables in vivo testing of drug delivery mechanisms using humanized candidates, allowing for more accurate evaluations of therapies aimed at crossing the BBB in preclinical studies.
Humanized mice of TFR1 (B-hTFR1 mice)
(Source: Huang et al., 2024)
TFR1, predominantly expressed on brain endothelial cells, acts as a crucial gateway for transferrin-bound iron to enter the CNS. It is part of the RMT system and the most widely studied target for CNS drug delivery. Biocytogen has developed humanized TFR1 mice by replacing exons 4-19 of the mouse Tfr1 gene, which encode the extracellular region, with the corresponding human TFR1 exons. A recent study published in Science using our B-hTFR1 mouse model revealed that their optimized AAV, which binds to human TFR1, can not only get into the CNS but also deliver a therapeutically relevant protein that is lacking in Gaucher disease (Huang et al., 2024). Our humanized TFR1 mice enable researchers to more accurately study drug delivery pathways in a human gene context in preclinical studies, accelerating the development of CNS-targeted therapies.
Immunofluorescence staining of brain sections from WT and B-hTFR1 mice. Nuclei are stained blue, the mouse endothelial cell marker mCD31 is red, and human TFR1 (hTFR1) is green. hTFR1, absent in WT (left), was observed in B-hTFR1 mice (right) and expressed in brain endothelial cells, suggesting a method for drugs to cross the BBB via hTFR1-mediated transendocytosis.
B-hTFR1 mice received intravenous injections of control IgG or anti-human TFR1 BsAbs. Brain and serum samples collected at various time points showed quantified antibody concentrations, revealing higher brain exposure and serum clearance for the BsAbs. This suggests the BsAbs can cross the blood-brain barrier in B-hTFR1 mice.
Humanized mice of CD98HC (B-hCD98HC mice)
CD98HC plays a pivotal role in maintaining the integrity of the BBB and in nutrient transport, including amino acid transport, integrin signaling, and cellular stress responses. Given its critical functions, CD98HC is being explored as a potential target for drug delivery to the brain, opening new avenues for treating CNS disorders. Biocytogen has developed humanized mice of CD98HC to support related research.
Strain-specific CD98 expression in mouse brains was analyzed using immunofluorescence staining. Brain sections from WT (left) and homozygous B-hCD98HC (right) mice were stained with anti-human CD98HC (green) and anti-mouse CD31 antibodies (red). Human CD98 was exclusive to the microvascular endothelium of B-hCD98HC mice, absent in WT mice.
Customizable Models and Pharmacological Services
In addition to our standard models, Biocytogen offers customizable options to develop mouse models tailored to meet your specific research needs. We also provide comprehensive pharmacological services, which include in vitro and in vivo evaluations of drug efficacy, toxicity, and precise pharmacokinetics/pharmacodynamics (PK/PD) profiling. Contact us to learn how our products and services can enhance your research involving the BBB!
Animal Models for BBB Research at Biocytogen
Upcoming Webinar on Neuroscience
Join Dr. Dong Han, Sr. Scientist and Neurological Disease Team Lead at Biocytogen, for a webinar on Oct 2 at 1 PM EST. Explore the fundamentals of neurological disorders and the crucial role of animal models in revealing disease mechanisms and evaluating therapies. We’ll also highlight Biocytogen’s specialized animal models and innovations for neurological conditions and BBB transport. Don’t miss this opportunity to learn how these advanced tools are transforming studies of neurological diseases, providing new insights, and advancing therapeutic approaches!
References
Haumann, Rianne, et al. “Overview of current drug delivery methods across the blood–brain barrier for the treatment of primary brain tumors.” CNS drugs 34.11 (2020): 1121-1131.
Steinmetz, Jaimie D., et al. “Global, regional, and national burden of disorders affecting the nervous system, 1990–2021: a systematic analysis for the Global Burden of Disease Study 2021.” The Lancet Neurology 23.4 (2024): 344-381.
World Health Organization. “Over 1 in 3 people affected by neurological conditions, the leading cause of illness and disability worldwide.” (2024)
Huang, Qin, et al. “An AAV capsid reprogrammed to bind human Transferrin Receptor mediates brain-wide gene delivery.” Science 384.6701 (2024): 1220-1227.
