**Acknowledgements**

PD-L1 antibodies was attempted to recapitulate these results. Despite peripheral immune activation, in all instances neither reductions in Aβ pathology nor infiltration of peripheral monocytes were detected [109]. Further studies are needed to elucidate the potential of check-

**4.2. Increasing blood-brain barrier (BBB) penetrance for passive immunotherapeutics**

A significant barrier in the development of passive immunotherapeutics for AD is the low percentage of circulating antibody that crosses the BBB. Animal studies have indicated that ~0.1–0.5% of IgG enters the CSF from the periphery [110, 111], which is borne out by preclinical [112] and clinical [113, 114] data obtained with antibodies tested for use in AD. This has led to trials with increasing amounts of antibody administered to patients ([82]; clinicaltrials.gov, Identifier NCT03318523) with the hope of delivering sufficient amounts of antibody to the CNS to achieve a clinical effect. There are, however, indications that concentrations of antibodies are higher in brain parenchyma than what is present in CSF. The chimeric form of aducanumab reported brain:plasma AUC ratios when tested in a transgenic APP model of 1.3% [38]. This is in agreement with the finding that the concentration of protein analyte present in the interstitial fluid is approximately 10-fold higher than in the ISF [62, 115]. This could be due to the rapid turnover of CSF volume [116] compared to ISF, longer elimination times of antibodies in brain parenchyma compared with CSF, or increased residence time due to target-mediated binding. Nevertheless, methods and technologies to increase BBB penetrance of biomolecules urgently need to be applied to

One of the more promising approaches to increase penetrance of protein therapeutics into the brain utilize endogenous receptors that transcytose between the brain and periphery, such as transferrin receptor (TfR) [117], insulin receptor [118], and LDL receptor-related protein 1 (LRP1) [119]. Protein engineering approaches feature fusion of the therapeutic molecule to proteins, ligands, or peptides that bind these receptors and facilitate transcytosis across the BBB (reviewed in [120]). One of the best understood receptor-mediated delivery systems is the use of TfR, though a similar path has been taken in the development of technologies that utilize insulin receptor. Increased brain uptake of transferrin/antibody fusion proteins were detected in rats [121], though the relatively large size (~80 kDa) of full-length transferrin make this impractical for biotherapeutic use. The detection of increased transcytosis of anti-TfR antibodies and antibody fragments [122, 123], and later advances in antibody generation technologies, enabled bispecific antibodies that bind TfR as well as target [124]. As understanding of the transcytotic properties of TfR binding moieties have increased, so has the understanding of how best to incorporate properties to ensure delivery to the brain. For example, reducing TfR affinity improves delivery, as a low affinity anti-TfR moiety will release from the receptor faster than a high affinity moiety [124]. As receptor-binding fusions enter the clinic, further questions regarding safety and distribution changes brought about by higher CNS concentrations will need to be continually addressed [125, 126]. Work continues to identify receptors that may be useful for increasing BBB concentrations of antibodies to allow engage-

point modulation.

140 Alzheimer's Disease - The 21st Century Challenge

antibodies and other proteins.

ment with wider range of drug targets [127, 128].

There is a wealth of excellent studies in the areas of study covered by this chapter—unfortunately, we were unable to list them all. We thank the many researchers not recognized here who have contributed greatly to the field. We would also like to thank the thoughtful review and comments provided by Enchi Liu and Ellen Rose.
