Crossing the Blood-Brain Barrier: RMT Technology Breaks Through the Bottleneck of CNS Disease Treatment

The treatment of central nervous system (CNS) diseases, such as Parkinson's and Alzheimer's, has long been limited by the blood-brain barrier (BBB). While the BBB maintains brain homeostasis, it blocks the entry of most large-molecule therapeutic drugs. Therapeutic antibodies and other biologics have extremely low efficiency in crossing the BBB due to the restrictive tight junctions of endothelial cells, limited vesicular transport capacity, and lysosomal degradation of drugs, among other factors. Furthermore, drugs that do enter the brain often rely on diffusion, which limits their distribution to the brain surface and perivascular spaces rather than deep lesion sites. Currently, common strategies to cross the BBB include Receptor-Mediated Transcytosis (RMT), Adsorptive-Mediated Transcytosis (AMT), and Carrier-Mediated Transport (CMT). Of these strategies, RMT is one of the most widely researched methods.[1]

Transcytosis across the BBB[2]

 

Receptor-Mediated Transcytosis (RMT)

RMT utilizes specific receptors highly expressed on brain endothelial cells as "entry points". By leveraging the brain's extensive vascular network, drugs can be delivered throughout the brain parenchyma, significantly increasing the drug’s exposure and target binding rates in the brain. Proteomic studies show that the Transferrin Receptor (TfR) and CD98 heavy chain (CD98hc) are highly expressed on brain endothelial cells, making them appealing targets.

Transferrin Receptor (TfR)

TfR exists in two forms: TfR1 and TfR2. TfR1 is widely expressed and has a higher affinity for its ligand Transferrin (Tf), whereas TfR2 is mainly expressed in hepatocytes.

TfR1 is a type II transmembrane glycoprotein that is expressed on the cell surface as a disulfide-linked dimer. Each monomer consists of a large extracellular C-terminal domain, a transmembrane domain, and an intracellular N-terminal domain. The extracellular C-terminal domain is composed of three parts: the apical domain (A), the helical domain (H), and the protease-like domain (P).

TfR plays an essential role in cellular iron uptake and shuttling across the BBB endothelium. The process begins with iron-loaded transferrin (holo-Tf) binding to TfR1, forming a complex that is constitutively internalized through clathrin-mediated endocytosis. Within the endosome, an acidic environment triggers the release of iron from transferrin. The iron is then transported out of the endosome, while the iron-free complex (apo-Tf/TfR1) is recycled back to the cell surface. Here, the apo-Tf is released to rebind with circulating iron for the next round of transport.

Structure of TfR1[3]

The high expression of TfR1 on brain endothelial cells and its cyclical mechanisms of moving through the endothelial layer have led to its development as a key target for BBB-crossing drug delivery in CNS therapy[4].

TfR-based BBB-crossing Drugs 

Currently, several pharmaceutical companies, including JCR Pharma, Roche, Alector, and Denali Therapeutics, have established differentiated TfR technology platforms. Each platform employs distinct structural design strategies and has propelled multiple drugs to achieve breakthrough progress.

Established TfR-based BBB-crossing platforms[5]

Among them, the most advanced is Pabinafusp alfa (JR-141 or IZCARGO®). Developed by JCR using the J-Brain Cargo® platform, it is already approved in the Japanese market. It is an antibody/enzyme fusion protein indicated for Mucopolysaccharidosis Type II (MPS II, also known as Hunter Syndrome), a genetic disease where sugars accumulate in the body (especially the CNS) due to a deficiency of iduronate-2-sulfatase. The IDS enzyme is responsible for degrading glycosaminoglycans (GAGs). The drug is formed by fusing IDS with an anti-TfR antibody, which binds to the apical domain of TfR1. The binding epitope does not overlap with the Tf binding site, thereby avoiding interference with TfR’s iron transport function while triggering TfR-mediated transcytosis[6]. The IDS portion carries polysaccharide chains containing mannose-6-phosphate (M6P), which can recognize the mannose-6-phosphate receptor (M6PR). After binding to TfR, Pabinafusp alfa undergoes endocytosis along with TfR, is subsequently transported across the cell and enters the brain parenchyma via exocytosis. It is then taken up by neurons, astrocytes, and pericytes through M6PR recognition, delivering IDS into the brain to produce therapeutic effects.

Structure of Pabinafusp alfa and its cellular uptake processes in the brain[7]

Trontinemab is a bispecific 2+1 structure Aβ antibody developed by Roche based on its proprietary Brainshuttle™ technology platform. This technology fuses an anti-Aβ monoclonal antibody with a TfR1 shuttle module containing a single TfR1 binding site. Due to its distinct structural design, Trontinemab can achieve high-efficiency BBB crossing, targeting aggregated forms of Aβ and clearing amyloid plaques from the brain. It can achieve CNS exposure at low doses, and can rapidly reduce Aβ in Alzheimer's patients[8]. At the 2025 CTAD conference, Roche disclosed interim clinical data for Trontinemab: for patients receiving a 3.6 mg/kg dose, 91% saw their amyloid PET centiloids drop below the positive threshold at 28 weeks. The incidence of ARIA-E was less than 5% across cohorts, demonstrating good overall safety and tolerability.

Mechanism for Trontinemab function[9]

The Transport Vehicle™ (TV) platform developed by Denali is another leading example. This platform optimizes affinity and effectively drives drug distribution in the brain by engineering the loop region of the human IgG1 Fc domain to embed TfR binding sites. As a modular platform, this design requires no additional sequences. While retaining the natural IgG1 structure and FcRn binding capability, it can be flexibly coupled with antibodies, enzymes, or oligonucleotides to construct Antibody Transport Vehicles (ATV), Enzyme Transport Vehicles (ETV), and Oligonucleotide Transport Vehicles (OTV), achieving efficient brain delivery of various biological macromolecules[4].

Denali Transport Vehicle (TV) platform[10]

CD98hc

CD98hc is a type II transmembrane protein that can form covalent heterodimers with various large amino acid transporters such as LAT1, participating in the transport of amino acids in the brain. Studies have confirmed that CD98hc is highly expressed on brain endothelial cells and has been validated as a BBB-crossing target in both mice and cynomolgus monkeys. 

Unlike Denali's TfR-based TV platform, which modifies the Fc loop region, the CD98hc TV platform is engineered on the β-sheet surface of the CH3 domain. The modified Fc is then fused with a Fab to generate an Antibody Transport Vehicle (ATV). Research has confirmed significant differences between ATVCD98hc and ATVTfR in terms of clearance rate, uptake kinetics, and in vivo distribution characteristics. ATVCD98hc has a slower uptake speed and more persistent brain exposure[4].

CD98hc TV structure[5]

Different RMT technology platforms possess different in vivo distribution characteristics, pharmacokinetic behaviors, and brain exposure profiles. Delivery platforms targeting TfR can achieve rapid drug uptake in the brain, with the core advantage being fast internalization, while delivery platforms targeting CD98hc can provide relatively longer exposure times for the brain delivery of biological therapies. Therefore, for different types of CNS diseases, it is necessary to find the most appropriate delivery platform to better guarantee therapeutic efficacy.

KACTUS Supplies CNS Drug-Related Recombinant Proteins

By harnessing the endogenous shuttling mechanisms of proteins across the BBB, CNS drug development is overcoming previous accessibility challenges, leading to a steady expansion of related treatment pipelines. KACTUS has developed a series of BBB-crossing delivery-related proteins, such as Tf, TfR, and CD98, as well as CNS disease-related target proteins, such as tau and alpha-synuclein. All proteins have passed strict quality testing to assist in the development of CNS drugs.

Product data

Human Transferrin, His Tag immobilized on CM5 Chip can bind Human Transferrin R, His Tag with an affinity constant of 11.26 nM as determined in SPR assay (Biacore T200).

Immobilized Human Transferrin R, His Tag at 2μg/ml (100μl/well) on the plate. Dose response curve for Biotinylated Human Transferrin, His Avi Tag with the EC50 of 14.3ng/ml determined by ELISA. (QC Test)

Loaded Anti-Transferrin R Antibody, hFc Tag on ProA-Biosensor can bind Human Transferrin R, His Tag with an affinity constant of 0.65 nM as determined in BLI assay (Gator® Prime).

Immobilized Human CD98, His Tag at 0.5μg/ml (100μl/well) on the plate. Dose response curve for Anti-CD98 Antibody, hFc Tag with the EC50 of 14.2ng/ml determined by ELISA. (QC Test)

Product list

References

[1] Johnsen KB, Burkhart A, Thomsen LB, Andresen TL, Moos T. Targeting the transferrin receptor for brain drug delivery. Prog Neurobiol. 2019 Oct;181:101665. doi: 10.1016/j.pneurobio.2019.101665. Epub 2019 Jul 31. PMID: 31376426.

[2] Zhao P, Zhang N, An Z. Engineering antibody and protein therapeutics to cross the blood-brain barrier. Antib Ther. 2022 Nov 9;5(4):311-331. doi: 10.1093/abt/tbac028. PMID: 36540309; PMCID: PMC9759110.

[3] Candelaria PV, Leoh LS, Penichet ML, Daniels-Wells TR. Antibodies Targeting the Transferrin Receptor 1 (TfR1) as Direct Anti-cancer Agents. Front Immunol. 2021 Mar 17;12:607692. doi: 10.3389/fimmu.2021.607692. PMID: 33815364; PMCID: PMC8010148.

[4]Chew KS, Wells RC, Moshkforoush A, Chan D, Lechtenberg KJ, Tran HL, Chow J, Kim DJ, Robles-Colmenares Y, Srivastava DB, Tong RK, Tong M, Xa K, Yang A, Zhou Y, Akkapeddi P, Annamalai L, Bajc K, Blanchette M, Cherf GM, Earr TK, Gill A, Huynh D, Joy D, Knight KN, Lac D, Leung AW, Lexa KW, Liau NPD, Becerra I, Malfavon M, McInnes J, Nguyen HN, Lozano EI, Pizzo ME, Roche E, Sacayon P, Calvert MEK, Daneman R, Dennis MS, Duque J, Gadkar K, Lewcock JW, Mahon CS, Meisner R, Solanoy H, Thorne RG, Watts RJ, Zuchero YJY, Kariolis MS. CD98hc is a target for brain delivery of biotherapeutics. Nat Commun. 2023 Aug 19;14(1):5053. doi: 10.1038/s41467-023-40681-4. Erratum in: Nat Commun. 2023 Sep 7;14(1):5516. doi: 10.1038/s41467-023-41355-x. PMID: 37598178; PMCID: PMC10439950.

[5] https://investors.denalitherapeutics.com

[6] Yamamoto R, Yoden E, Tanaka N, Kinoshita M, Imakiire A, Hirato T, Minami K. Nonclinical safety evaluation of pabinafusp alfa, an anti-human transferrin receptor antibody and iduronate-2-sulfatase fusion protein, for the treatment of neuronopathic mucopolysaccharidosis type II. Mol Genet Metab Rep. 2021 Apr 18;27:100758. doi: 10.1016/j.ymgmr.2021.100758. PMID: 33981582; PMCID: PMC8081988.

[7]Fukatsu T, Morio H, Furihata T, Sonoda H. Transferrin receptor-targeting property of pabinafusp alfa facilitates its uptake by various types of human brain-derived cells in vitro. Front Drug Deliv. 2023 Jul 3;3:1082672. doi: 10.3389/fddev.2023.1082672. PMID: 40838062; PMCID: PMC12363330.

[8] Roche presents novel therapeutic and diagnostic advancements in Alzheimer’s at AD/PD 2025

[9]Brainshuttle™ AD: New interim results of a randomized, placebo-controlled Phase Ib/IIa proof-of-concept study with trontinemab, a novel anti-amyloid monoclonal bispecific antibody for the treatment of Alzheimer’s disease - CTAD-2025-presentation-kulic-brainshuttle-tm-ad-new-interim-results.pdf

[10]Denali Therapeutics_DNLI CORPORATE PRESENTATION - SEPTEMBER 2025