By Stephanie Baringer

Since the FDA’s founding in 1906, the agency has only approved approximately 140 drugs to treat diseases of the central nervous system¹, compared to over 450 drugs to treat various cancers². While there are a variety of reasons for this dichotomy, one is the difficulty of getting drugs into the brain due to the blood-brain barrier (BBB). The BBB is the barrier that separates the brain tissue from its nutrient-supplying blood vessels. The barrier is firstly comprised of endothelial cells that encircle the blood vessel and form tight junctions to prevent the crossing of molecules (Fig. 1). The only things that can cross the BBB are water, certain gases such as oxygen, lipid-soluble molecules, or substances transported through a transport protein³. Some small molecule drugs can cross the BBB by being lipid-soluble, but this is not always possible for a therapeutic target. Macromolecules, such as monoclonal antibodies, must be transported. Common transporters at the BBB include, but are not limited to, glucose transporters, P-glycoprotein, and the transferrin receptor (TfR)³. TfR is increasingly becoming the subject of drug delivery studies.

Though often studied for drug delivery purposes, TfR’s primary role is to delivery iron to the brain, which is necessary for proper functioning (Fig. 1). Transferrin (Tf), with iron in tow, binds to the TfR and is then endocytosed. From there, the receptor complex has two known paths. The complex can be disassociated, leaving the iron to be freely exported and the remaining Tf and receptor to be recycled back to the blood membrane. Alternatively, the receptor complex can be transcytosed directly into the brain. The latter is a golden ticket for a drug to enter the brain. However, use of this pathway is more challenging than attaching your favorite drug to Tf and waiting for TfR to take it up.

Uptake of Tf into the brain is subject to many forms of regulation:

1) The iron status of brain-side Tf can control how much iron is taken up and released by endothelial cells of the BBB^4–6, resulting in localized regulation of iron uptake.

2) Many iron-related genes, such as TfR, contain iron response elements on their mRNA. These elements can control the translation of iron-related proteins depending on the how much iron in is the cell- resulting in a potential for variable Tf uptake.

3) It is unclear how much iron is delivered to the brain via TfR transcytosis, export of free iron, or release of extracellular vesicles containing iron. Additionally, exactly how much of the TfR-Tf complex is fully transcytosed to the brain versus how much is recycled back to the blood-side membrane remains unknown, though evidence suggests that most is recycled^6,7.

Many companies have tried to utilize the Tf uptake pathway without fully considering the above complexities. However, there is one company who is devoted to these studies and is making more progress in the optimized use of this pathway for drug delivery in 9 years than others have made in decades.

Denali Therapeutics is a small biopharmaceutical company founded in 2013 with the focus of delivering much-needed therapeutics to the neurodegenerative disease space. To address the brain drug delivery issues, Denali developed their patented transport vehicle (TV)^8–10. On one end, the TV binds to the TfR, away from the Tf binding site, as to not disrupt normal iron uptake. On the other end, the TV binds the therapeutic molecule to deliver to the brain. To date, Denali has developed drug candidates that use the TV transportation scheme to deliver antibodies, enzymes, oligonucleotides, and proteins¹¹. A weak binding affinity of the TV to the TfR allows for easy detachment once endocytosed and allows the TV freedom to diffuse throughout the brain once across the BBB⁸. While a large amount of TV is recycled from the endothelial cells back to the blood, just as Tf is (Fig 1), scientists at Denali see brain penetrance of some TV-delivered therapeutics⁸. Even with the low ratio of TV delivered to the brain compared to TV recycled back to the blood, a study of an enzyme-TV in mice showed significant improvement of disease biomarkers and behaviors¹⁰.

Denali has over a dozen TV-related molecules in clinical testing. DNL310, an enzymatic TV designed to replenish iduronate-2-sulfatase (IDS)¹², is their furthest along. IDS is mutated in Hunter’s syndrome, a rare inherited disorder primarily found in young boys. The mutation of IDS in Hunter’s syndrome results in the body’s inability to break down sugar molecules, leading to a build-up of sugar in tissues, and can cause lysosomal dysfunction and subsequent neurodegeneration, along with muscle complication and physical deformities. Currently, the standard of care for patients is infusion enzyme replacement therapy; however, these treatments have not been shown to penetrate the brain to resolve cognitive deficits¹³. Denali initiated a phase 1/2 clinical trial with DNL310 in July 2020, and while these data are not yet publicly available, Denali presented positive preliminary data from this trial at the WORLD Symposium in February 2022¹⁴. They demonstrated that their biomarker measurements of drug efficacy improved when compared to standard of care and that a number of patients had improved cognitive and physical abilities after only 6-months of treatment¹⁵. In May 2022, Denali initiated a phase 2/3 trial with DNL310, underscoring the likelihood of the drug to deliver an unmet need to patients¹⁵.

Given the promise of piggybacking off of the TfR for brain drug delivery, Denali is not the only company to have it in their pipeline. However, compared to their competitors, Denali has found rapid and diverse preliminary success and is making waves in the space. In 2021, Takeda announced,  in collaboration with JCR Pharmaceuticals, their plan to commercialize a TfR-delivered treatment, termed J-Brain Cargo, outside of the United States, but domestic approval awaits a Phase 3 trial estimated to be completed in 2026. Roche also developed their own TfR delivery platform, termed Brain Shuttle, in 2014, but there has been no update of its potential uses since. No TfR-delivered therapeutics have fully made it to market yet, but as we understand more about Tf and TfR dynamics, we can better utilize the pathway for successful drug delivery to the brain.  

TL:DR

• The blood-brain barrier is a significant roadblock for drug delivery to the brain, but Denali Therapeutics is optimizing use of the transferrin receptor pathway for such purpose and has an expansive portfolio on the horizon.

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References

1.         Nervous System Drugs | Nerve System Treatment. https://patient.info/medicine/nervous-system-drugs-1265.

2.         Cancer drugs A to Z list | Treatment for cancer | Cancer Research UK. https://www.cancerresearchuk.org/about-cancer/cancer-in-general/treatment/cancer-drugs/drugs.

3.         Banks, W. A. Characteristics of compounds that cross the blood-brain barrier. BMC Neurology 9, S3 (2009).

4.         Baringer, S. L., Neely, E. B., Palsa, K., Simpson, I. A. & Connor, J. R. Regulation of brain iron uptake by apo- and holo-transferrin is dependent on sex and delivery protein. Fluids Barriers CNS 19, 49 (2022).

5.         Chiou, B. et al. Endothelial cells are critical regulators of iron transport in a model of the human blood-brain barrier. J. Cereb. Blood Flow Metab. 39, 2117–2131 (2019).

6.         Simpson, I. A. et al. A novel model for brain iron uptake: introducing the concept of regulation. J Cereb Blood Flow Metab 35, 48–57 (2015).

7.         Gkouvatsos, K., Papanikolaou, G. & Pantopoulos, K. Regulation of iron transport and the role of transferrin. Biochimica et Biophysica Acta (BBA) – General Subjects 1820, 188–202 (2012).

8.         Arguello, A. et al. Molecular architecture determines brain delivery of a transferrin receptor-targeted lysosomal enzyme. J Exp Med 219, e20211057 (2022).

9.         Kariolis, M. S. et al. Brain delivery of therapeutic proteins using an Fc fragment blood-brain barrier transport vehicle in mice and monkeys. Science Translational Medicine 12, (2020).

10.       Ullman, J. C. et al. Brain delivery and activity of a lysosomal enzyme using a blood-brain barrier transport vehicle in mice. Science Translational Medicine 12, (2020).

11.       Engineering Brain Delivery. Denali https://www.denalitherapeutics.com/science/engineering.

12.       Arguello, A. et al. Iduronate-2-sulfatase transport vehicle rescues behavioral and skeletal phenotypes in a mouse model of Hunter syndrome. JCI Insight 6, e145445.

13.       Sato, Y. & Okuyama, T. Novel Enzyme Replacement Therapies for Neuropathic Mucopolysaccharidoses. Int J Mol Sci 21, 400 (2020).

14.       Denali Therapeutics Announces Presentations on DNL310 (ETV:IDS) Development Program in MPS II (Hunter Syndrome) at the Upcoming WORLDSymposium^TM. Denali https://www.denalitherapeutics.com/investors/press-release.

15.       Denali Therapeutics Announces Continued Progress in DNL310 (ETV:IDS) Program for MPS II (Hunter Syndrome) Supporting Planned Initiation of Phase 2/3 Clinical Trial. Denali https://www.denalitherapeutics.com/investors/press-release.