. Brain delivery of therapeutic proteins using an Fc fragment blood-brain barrier transport vehicle in mice and monkeys. Sci Transl Med. 2020 May 27;12(545) PubMed.


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  1. This is an elegant study by Ullman and colleagues of the Denali group where the proof of concept of a therapeutic approach is provided to treat the neurological symptoms in Hunter disease (iduronate-2-sulfatase (IDS) deficiency). The study employs a novel type of fusion protein to overcome the blood-brain barrier, which can hardly be passed by “classical“ enzyme replacement therapies (ERT), although exceptions for the successful CNS corrections by ERT have been described (e.g. Stroobants et al., 2017; Damme et al., 2015; Damme et al., 2011). This is also the case when IDS is intravenously applied to an IDS knock-in mouse model. The enzyme does not reach the brain where it is urgently needed.

    To overcome this problem the authors used a newly developed ETV:IDS protein, which represents a fusion of the lysosomal IDS to an Fc domain modulated to interact with the transferrin receptor. This fusion protein turned out to be extremely successful in reaching the brain and all affected cells, including microglia. Most importantly the treatment with rather high doses (40mg/kg) led to a correcting of lysosomal storage and improved the neurological phenotype, i.e., neuroinflammation, neuroaxonal injury, and neurodegeneration. This is certainly a major step forward in the development of an effective therapy of Hunter disease (MPSII) and maybe other lysosomal-storage disorders. It may well be suited for other lysosome-related neurological diseases.

    However, a word of caution may be justified. The fusion protein, which requires regular dosing, is unknown to the body and may well cause unwanted immunological reactions and the generation of antibodies, which could affect the long-term efficacy of such a treatment protocol. Also, blocking the transferrin receptor with the therapeutic fusion enzyme may cause unexpected long-term problems. It will be interesting to learn more about the intracellular delivery routes and fate of ETV:IDS proteins. It should also be mentioned that in 2018 a similar study of a blood-brain-barrier-penetrating fusion protein consisting of an anti-human transferrin receptor (hTfR) antibody and intact hIDS was shown to be successful in mice and monkeys (Sonoda et al., 2018). 

    Despite the above-mentioned questions, I am convinced that this, or a modified treatment regime, may be exploited for the preclinical and later clinical treatment of a variety of neurological disorders.


    . Long-term enzyme replacement therapy improves neurocognitive functioning and hippocampal synaptic plasticity in immune-tolerant alpha-mannosidosis mice. Neurobiol Dis. 2017 Oct;106:255-268. Epub 2017 Jul 15 PubMed.

    . Chronic enzyme replacement therapy ameliorates neuropathology in alpha-mannosidosis mice. Ann Clin Transl Neurol. 2015 Nov;2(11):987-1001. Epub 2015 Sep 19 PubMed.

    . Cerebellar alterations and gait defects as therapeutic outcome measures for enzyme replacement therapy in α-mannosidosis. J Neuropathol Exp Neurol. 2011 Jan;70(1):83-94. PubMed.

    . A Blood-Brain-Barrier-Penetrating Anti-human Transferrin Receptor Antibody Fusion Protein for Neuronopathic Mucopolysaccharidosis II. Mol Ther. 2018 May 2;26(5):1366-1374. Epub 2018 Mar 6 PubMed.

    View all comments by Paul Saftig
  2. This is an elegant and comprehensive study that describes development of an engineered Fc fragment (called a transport vehicle, TV) that binds to the apical domain of the human transferrin receptor (TfR) at the blood-brain barrier (BBB) at a TfR site that is distinct from its binding sites for transferrin, a natural ligand for TfR and FcRn. Using anti-BACE1 antibodies bound to their TV increased substantially brain uptake of the antibody and effectively reduced endogenous Aβ levels in mice and nonhuman primates.

    Brain capillary TfR was proposed years ago by Bill Pardridge as an excellent candidate target for delivery across the BBBs of mice and nonhuman primates of neuroactive proteins and peptide biologics, for example growth factors, when they are linked to anti-TfR antibodies. The new platform described in the current STM paper advances importantly this approach by allowing for numerous additional configurations, including bispecific antibodies and protein fusions, that could be delivered to high concentrations in the CNS using the novel, highly modular CNS delivery platform.

    Some questions in the field, however, persist, such as how the BBB function is compromised in many human neurological diseases, particularly in the regions of therapeutic interest, for example the medial temporal lobe in Alzheimer’s. Whether TfR at the BBB remains functional or not at these affected sites remains to be seen. Thus, site-specific delivery of therapeutics, including proteins, to affected CNS regions could be important for many neurological disorders. Interestingly, some of these issues seem to be at least partially solved by CSF delivery thereby completely circumventing the BBB, as shown for example with designer DNA drug therapy for neurodegenerative disorders.

    View all comments by Berislav Zlokovic
  3. Monoclonal antibodies are very interesting as therapeutics, also for Alzheimer’s disease. They can be made highly specific for their target, which is an obvious advantage when focusing on aggregating proteins where not all forms are toxic. However, one problem is the low penetrance of antibodies into the brain. It is usually claimed that about 0.1 percent of the given drug eventually crosses the blood-brain barrier (BBB) and passes into the brain. This is also the case for other biologics like enzymes, which prohibits the use of standard administration of enzyme replacement therapy for neurological disorders.

    The papers from the Denali team describe an excellent step in evolving a technology platform using the transferrin receptor (TfR) as a way of crossing the BBB. It is difficult to envision a smaller adjustment to an antibody structure to get it to function as a vehicle to pass the BBB. By randomizing a specific region involving nine amino acids of the Fragment crystallizable (Fc) region, in the CH3 domain, they manage to identify binders to the TfR. This technology was originally developed by the company F-star for making bispecific antibodies (Fcab).

    The modified Fc, which Denali named transport vehicle (TV), can ferry different types of biologics over the BBB. The TV binds to the TfR at the apical domain without interfering with the endogenous ligand transferrin. However, recently it was described that another endogenous ligand to TfR, ferritin, also binds to a large epitope on the apical domain (Montemiglio et al., 2019). If the TV overlaps with this epitope on TfR, an obvious question is if this will have consequences, as ferritin serves to store iron in a nontoxic form, and to transport it to areas where it is required. Ultimately only the coming clinical data will inform us about this.

    One interesting aspect of the TV’s design is using a monovalent binding mode to TfR by using the knob-into-hole (KiH) technology. It would be interesting understand why this is necessary, as the group has previously only focused on relatively low affinity to TfR to successfully getting across the BBB (Yu et al., 2011). 

    A group at Roche has developed a similar strategy to deliver biologics to the brain, the Brain Shuttle technology. They argue that the monovalent binding mode is absolutely crucial for an efficient and safe transport by the TfR in order not to interfere with the intracellular trafficking of TfR (Niewoehner et al., 2014). This Brain Shuttle approach is currently in Phase 1 trials for Alzheimer’s disease (RG6102). The gain in brain exposure by using lower affinity binding to TfR is likely due to the longer plasma half-life, as the TV is not rapidly sequestered by cells in the periphery that also express TfR.

    A standard bivalent antibody with high affinity to TfR, where the therapeutic is linked in the back of the Fc, could potentially negatively affect the homeostasis of TfR, especially when using frequent dosing. This approach is currently being investigated in Phase 2/3 in the clinic (see trial) for the treatment of Mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome, by the company JCR Pharmaceuticals. Denali will investigate its TV platform in the clinic for the same indication.

    Recently it was shown that the Brain Shuttle platform developed by Roche had reduced engagement with Fc gamma receptor (FcγR) when it was bound to the TfR. This could be of importance when using antibody subtypes with effector function, as it attenuates immune responses involving cells expressing TfR in the periphery. Therefore, it would be of importance to investigate if the platform developed by Denali interacts with other endogenous Fc ligands, e.g. FcγR, in its normal mode when it is bound to the TfR.

    One obvious question is whether the amino-acid changes made in the Fc part of the antibody will induce immune responses. Immunogenicity can limit the use of biopharmaceuticals, particularly for the treatment of chronic diseases.

    Overall, the paper from the Denali group is of high quality and further enhances our knowledge on how to deliver biologics to the brain in a noninvasive manner. It is a beautiful combination of antibody engineering and biological understanding of the BBB that drives this brain delivery platform development.

    In the future, with effective platforms for BBB penetration being used for biological drugs, we expect not only less cost of goods, but also better efficacy as these molecules also seem to penetrate better into different compartments of the brain.


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    . Boosting brain uptake of a therapeutic antibody by reducing its affinity for a transcytosis target. Sci Transl Med. 2011 May 25;3(84):84ra44. PubMed.

    . Increased brain penetration and potency of a therapeutic antibody using a monovalent molecular shuttle. Neuron. 2014 Jan 8;81(1):49-60. PubMed.

    . Brain Shuttle Antibody for Alzheimer's Disease with Attenuated Peripheral Effector Function due to an Inverted Binding Mode. Cell Rep. 2018 Jan 2;22(1):149-162. PubMed.

    View all comments by Per-Ola Freskgård

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  1. Molecular Transport Vehicle Shuttles Therapies into Brain