Nanobodies to Cross the Blood-Brain Barrier

Effective delivery of antibodies across the blood-brain barrier (BBB) is a serious limiting factor in the further development of antibody based therapies for AD but also for other diseases of the central nervous system (CNS). The BBB is composed of a tightly sealed monolayer of brain endothelial cells (BECs), which normally precludes free exchanges of solutes between blood and brain (and vice versa), including biologics, but yet enabling the supply of nutrients (e.g. amino acids, glucose, insulin, etc.). Therefore, the potential treatment of many CNS-related diseases is severely hampered because of the fact that numerous potential drug candidates are not able to cross the BBB. To overcome this problem, much research efforts are currently focused on the development and application of safe and efficient delivery methods to cross the BBB. These efforts include the delivery by means receptor mediated transcytosis (RMT). One of the more successful targeting strategies makes use of monoclonal antibodies (mAbs) or antibody fragments targeting a BBB nutrient transporter. For example, mAbs have been generated against human insulin receptor (InsR) and transferrin receptor (TfR). For both antibodies an increased transport over the BBB was observed. When coupled to an appropriate drug candidate pharmacological effects could, for example, be observed in rats (anti-InsR coupled to glial-derived neurotrophic factor) and non-human primates (bispecific antibody, anti-BACE1/anti-TfR). However the efficiency of BBB transport is still rather low as shown by Pardridge et al. (2009) for their anti-InsR mAb and Yu et al. (2014) for their anti-TfR mAb (respectively 0.55 % and 1-2 % of the injected dose reached the brain in their respective monkey studies). In the current project an innovative method is followed to identify new potential RMT targets to cross the BBB. Simultaneously, because of the intelligent design of the proposal, not only new RMT targets might be discovered but also Nanobodies (heavy-chain only antibody fragments originating from camelids) capable of shuttling the BBB. Next to this unbiased approach, a specific potential RMT target expressed at the BBB is selected to generate Nanobodies against. Initially, based on its expression both on human and mice BECs by Uchida et al. (2011), the heavy chain of the Na+ independent neutral amino acid transporter (CD98hc) is selected as potential RMT target. Recently (December 2015), indeed, CD98hc was confirmed as a robust RMT pathway for antibody delivery to the brain. In vivo proof of concept in mice will be obtained by coupling a BBB-shuttling Nanobody to a BACE1 inhibiting Nanobody, previously discovered by the host lab (patent EP 2281005 B1). If a therapeutically relevant dose of the construct crosses the BBB successfully, a reduction in brain Aβ levels should be observed. The host lab has an ELISA and other assays available which makes an easy read out of therapeutic efficiency possible. This will allow us to provide proof of concept that the brain is targeted in a therapeutic relevant way. Discovery of BBB-shuttling Nanobodies would be a major breakthrough in the treatment of CNS related diseases as they can be used as generic entities which can be coupled to drug candidates with poor BBB permeability.

Publication year:2021

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