For decades scientists have sought a way to sneak large drugs across the blood-brain barrier. These efforts received a boost in 2011, when researchers led by Ryan Watts at Genentech, South San Francisco, California, reported using bispecific transferrin receptor (TfR)/β-secretase (BACE) antibodies to suppress amyloid-β in the mouse brain. Now, in the May 1 Science Translational Medicine, the group extends their findings, showing that anti-TfR antibodies can have serious consequences for immature red blood cells. Side effects can be eliminated by engineering the antibody to avoid interacting with macrophages, the authors report. The principles may be broadly applicable to other strategies to penetrate the blood-brain barrier (BBB), Watts suggested. Several companies are working on similar drug delivery systems, which may eventually help clinicians administer medications for the treatment of Alzheimer’s disease or other neurodegenerative disorders. “It’s essential for the field to find a way to deliver large molecules to the brain,” Watts said.
Commentators agreed the study is informative. “This is an elegant study,” said Einar Sigurdsson at NYU Langone Medical Center, New York City. “The search for brain drug delivery vectors has not yielded much success,” noted Frédéric Calon at Université Laval, Québec, Canada, in an e-mail to Alzforum. “Since the safety profile of a drug is key to its clinical success, such data early in the drug development process is always welcome,” he wrote.
Transferrin receptors decorate endothelial cell membranes and transport iron-carrying transferrin from the blood into the brain via endocytosis. Many researchers have speculated that TfRs could serve as an entry point to the brain for drugs. In practice, this has yet to pan out, as antibodies that bind TfR remain stuck to the endothelial cells inside the brain vasculature. In previous work, Watts and colleagues found that lowering the antibody’s affinity for the receptor allowed it to diffuse inside the brain. To deliver a drug, the researchers generated a bispecific antibody in which one arm recognized TfR, while the other bound to and blocked activity of BACE1. The antibody reached 10-fold higher concentrations in mouse brain compared to monospecific anti-BACE1 and lowered Aβ40, showing that the method could deliver an effective drug dose (Yu et al., 2011).
The next question was how safe and feasible such a therapy might be in humans. “TfR has been considered as a [drug] transport mechanism since the late 1980s, yet there is no robust description of pharmacokinetics and safety in the literature,” Watts pointed out. “[These data] are essential for the foundation of these types of platform and for understanding how realistic they are.”
Despite the presence of TfR in endothelial cells, the authors found no damage to these cells or the BBB. First author Jessica Couch then turned her attention to immature red blood cells called reticulocytes, because they express even higher levels of the receptor. These cells need TfR to load iron into hemoglobulin, but lose the receptor as they mature. The authors saw two types of negative effect in mouse reticulocytes, one immediate and one longer-lasting. First, they found that a monospecific anti-TfR antibody ruptured the cells and caused acute toxicity and lethargy in the mice. Surprisingly, the animals tolerated bispecific antibodies quite well. The researchers traced the difference to the Fc domain, which forms the stem of the Y-shaped antibody. Fc activates the Fcγ receptor found on macrophages and monocytes, signaling them to attack and lyse the immature red blood cells. Because the bispecific antibodies were made in Escherichia coli and lacked glycosylated Fc domains, they ignored the Fcγ receptor, sparing the reticulocytes. Antibodies made in eukaryotes may be less innocuous, said Watts, suggesting this could be a major drawback that needs to be appreciated when developing these types of antibodies. The authors engineered an anti-TfR antibody that lacked glycosylation sites and was safe for reticulocytes, demonstrating that the problem can be solved.
However, bispecific antibodies still caused a long-term loss of circulating reticulocytes. The effect depended on antibody affinity and dose. At the highest dose (50 mg/kg), reticulocyte levels crashed by as much as 90 percent 24 hours after administration, recovering in about a week. The culprit was the complement cascade that drives the innate immune system and stimulates macrophages, the authors report. Mice lacking a key complement protein resisted the effect. While this safety issue could potentially cripple the approach, there are signs that this second problem may be confined to rodents. In primates, including humans, reticulocytes mature primarily in the bone marrow, which lacks complement components, and by the time the cells enter the bloodstream, they are loaded with hemoglobin and have little need for TfR. In fact, using flow cytometry, the authors confirmed that almost no reticulocytes in monkey or human blood express the transferrin receptor. “[These data] lay the foundation for the future and give great hope to the platform,” Watts said.
Watts said Genentech is generating several antibodies to primate TfR and will select one of those for eventual clinical trials. As they did in rodents, the authors need to find an antibody that allows uptake of transferrin into the brain. They also want to improve the potency of their anti-BACE1 antibody, which will allow them to use a lower dose. If successful, the technique could likely be adapted to ferry other large molecules into the brain by conjugating them to anti-TfR antibodies, Watts noted.
Other companies are also working on ways to penetrate the BBB, using the transferrin receptor or other molecular shuttles. Hoffmann-La Roche, based in Basel, Switzerland, which owns Genentech, has its own independent “brain shuttle” program (see Jan 2014 news).
Couch JA, Yu YJ, Zhang Y, Tarrant JM, Fuji RN, Meilandt WJ, Solanoy H, Tong RK, Hoyte K, Luk W, Lu Y, Gadkar K, Prabhu S, Ordonia BA, Nguyen Q, Lin Y, Lin Z, Balazs M, Scearce-Levie K, Ernst JA, Dennis MS, Watts RJ. Addressing Safety Liabilities of TfR Bispecific Antibodies That Cross the Blood-Brain Barrier. Sci Transl Med. 2013 May 1;5(183):183ra57. PubMed.
Yu YJ, Zhang Y, Kenrick M, Hoyte K, Luk W, Lu Y, Atwal J, Elliott JM, Prabhu S, Watts RJ, Dennis MS. 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.
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