Passive Immunotherapies for Central Nervous System Disorders: Current Delivery Challenges and New Approaches

Abstract

Passive immunotherapy, i.e., the administration of exogenous antibodies that recognize a specific target antigen, has gained significant momentum as a potential treatment strategy for several central nervous system (CNS) disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and brain cancer, among others. Advances in antibody engineering to create therapeutic antibody fragments or antibody conjugates have introduced new strategies that may also be applied to treat CNS disorders. However, drug delivery to the CNS for antibodies and other macromolecules has thus far proven challenging, due in large part to the blood-brain barrier and blood-cerebrospinal fluid barriers that greatly restrict transport of peripherally administered molecules from the systemic circulation into the CNS. Here, we summarize the various passive immunotherapy approaches under study for the treatment of CNS disorders, with a primary focus on disease-specific and target site-specific challenges to drug delivery and new, cutting edge methods.

Figure 1.

Summary of IgG, Fab, and sdAb structure and sizes. (A) Full length IgG is a Y shaped molecule made up of four polypeptide chains – two heavy chains (red) and two light chains (grey) that are linked by disulfide bonds. Each polypeptide chain has constant domains (C) and variable domains (V). There are two Fab arms, each containing an antigen-binding site made up of the variable domains of the heavy and light chains, which can recognize antigens with high specificity. The crystallizable fragment or Fc arm can interact with Fc receptors. (B) Camelids, sharks and other cartilaginous fish (Chondrichthyes) produce a unique IgG molecule consisting of heavy chains alone. A camelid IgG molecule is depicted here. A single heavy chain variable domain is also referred to as a single domain antibody or nanobody. Unlike the antibody variable domains in other species, camelid and cartilaginous fish variable domains do not aggregate when isolated and retain their antigen binding capacity; this has generated interest in their use as therapeutics when a smaller size and no Fc interactions are desired .

Figure 2.. Passive immunotherapy strategies for brain cancer using anti-angiogenic antibodies.

VEGFA binding to VEGFR2 triggers an increase in paracellular permeability, downregulation of tight junctional proteins, and the promotion of angiogenesis. Anti-VEGFA antibodies can bind to VEGFA and prevent angiogenesis thus inhibiting tumor growth and survival. Adapted from: . Abbreviations: VEGF – vascular endothelial growth factor; VEGFR – VEGF receptor.

Figure 3.. T cell immune response and immune checkpoints in brain cancer.

T cells may recognize tumor antigen peptides presented via MHC class I/II molecules on tumor cells or antigen presenting cells (APCs) via the TCR, resulting in a weak immune stimulatory signal. Interaction between the TCR and tumor antigen peptide/MHC complex can only activate the T cell in the presence of other co-stimulatory immune signaling. However, tumor cells and APCs in the tumor microenvironment express high levels of programmed cell death-ligand-1 (PD-L1), a ligand for the programmed cell death - (PD-1) receptor expressed by T cells, which inhibits T cell activation. APCs presenting the tumor antigen peptide/MHC complex may migrate to the cervical lymph nodes where T cells recognizing the tumor antigen may be activated and directed to the tumor. In addition to the TCR-tumor antigen/MHC interaction, the T cell must receive co-stimulatory signals in order to be activated. This co-stimulatory signal is typically received when the classification determinant 28 (CD28) receptor on T cells interacts with the B7 ligand expressed by APCs. However, regulatory T cells (Treg cells) express high amounts of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) – a receptor that mimics CD28 and has an even higher affinity for the B7 ligand. Thus, CTLA-4-B7 interaction can compete with the CD28-B7 interaction, resulting in the lack of appropriate co-stimulatory signaling to activate tumor antigen recognizing T cells. Adapted from: ^–. Abbreviations: PD-1 – programmed cell death protein-1; PD-L1 – programmed cell death protein ligand-1; CTLA-4 – cytotoxic T-lymphocyte-associated protein 4; Treg – regulatory T cells; CD28 – classification determinant 28.

Figure 4.. Passive immunotherapy strategies for brain cancer using immune checkpoint inhibitory antibodies.

Interactions between T cells, antigen presenting cells (APCs), and tumor cells that inhibit appropriate activation of T cell cytotoxic immune responses may be modulated via passive immunotherapy. For example, anti-PD-1 antibodies can bind to the PD-1 receptor that is expressed by T cells and disrupt PD-1’s interaction with its ligand PD-L1, which is highly expressed on tumor cells and APCs in the tumor microenvironment. Alternatively, anti-PD-L1 antibodies can neutralize the PD-L1 ligand’s ability to bind to PD-1. Anti-CTLA-4 antibodies may be used to block the interaction between the CTLA-4 receptor on Treg cells and the B7 ligand on tumor cells and APCs; this would subsequently allow B7 interaction with the CD28 receptor on T cells, which provides a stimulatory signal for T cell activation. Adapted from: ^–. Abbreviations: PD-1 – programmed cell death protein-1; PD-L1 – programmed cell death protein ligand-1; CTLA-4 – cytotoxic T-lymphocyte-associated protein 4; Treg – regulatory T cells; CD28 – classification determinant 28.

Figure 5.. Passive immunotherapy strategies for antibodies recognizing lymphocyte antigens.

Antibodies that recognize molecules expressed by malignant infiltrating lymphocytes may be used to treat certain CNS lymphomas. Adapted from: ^,.

Figure 6.. Passive immunotherapy strategies for brain cancer using antibody drug conjugates (ADCs).

ADCs combine the ability of antibodies to recognize specific antigens overexpressed by tumor cells (e.g., EGFR, IL-13R, or IL-4R) and the ability to deliver a cytotoxic payload that can lead to tumor cell death or arrest tumor growth. Typically an ADC has 3 main components – an antibody that can recognize a tumor antigen, a linker, and a cytotoxic payload. The cytotoxic payloads may be radioisotopes (e.g., I) that can cause DNA damage within the tumor cell, bacterial immunotoxins (e.g., diphtheria toxin) that may interfere with microtubule assembly or protein translation, or anti-tumor chemotherapeutic drugs (e.g., MMAF). Since ADCs can deliver a cytotoxic payload to the tumor target with high specificity they minimize off-target effects. Adapted from: . Abbreviations: EGFR – epidermal growth factor receptor; IL – interleukin; I – iodine radioisotope; MMAF – monomethyl auristatin F

Figure 7.. Passive immunotherapy strategy to treat breast cancer brain metastases using anti-HER2 antibodies.

HER2 overexpressing breast cancer brain metastases may be treated with anti-HER2 antibodies. Anti-HER2 passive immunotherapy may have several effects. First, HER2 homo or hetero dimerization that drives downstream signaling that promotes tumor cell survival may be disrupted using anti-HER2 antibodies. Second, the extracellular domain of HER2 is typically shed in tumor cells, leaving behind a phosphorylated P95 that is membrane bound and can drive downstream signaling promoting tumor cell growth and survival; anti-HER2 antibodies can bind to the HER2 extracellular domain and prevent its cleavage. Third, anti-HER2 antibodies may bind to HER2 expressed on tumor cell surfaces and initiate an Fc-mediated immune effector function that targets tumor cells. Fourth and finally, anti-HER2 antibodies may bind to HER2 and cause its internalization by endocytosis, resulting in HER2 degradation. Adapted from: . Abbreviations: HER2 – human epidermal growth factor receptor 2.

Figure 8.

Passive immunotherapy strategies to treat Alzheimer’s disease (AD). Some of the major hallmarks of pathology in AD include: (i) excess production of amyloid β-peptide (Aβ) fragments catalyzed by the beta-site APP-cleaving enzyme 1 (BACE1) and γ-secretase enzyme complex cleavage of the amyloid precursor protein (APP); (ii) accumulation and aggregation of hyperphosphorylated tau within neurons leading to cell death and cell-to-cell transmission of extracellular tau; and (iii) accumulation and aggregation of Aβ within the brain parenchyma (Aβ42) and the perivascular compartments of cerebral arteries (Aβ40). Passive immunotherapy may be used to target these different features of AD pathology. (A) Anti-BACE1 antibodies can be used to block the BACE1 cleavage of APP and thus minimize abnormal and excess production of Aβ fragments. (B) Anti-tau antibodies that target hyperphosphorylated tau can be used to block intracellular tau aggregation (likely using intrabodies ) and prevent the extracellular cell-to-cell transmission of pathologic tau (conventional antibodies ^,). (C) Anti-Aβ42 antibodies can be used to target Aβ42 in the brain parenchyma and halt or reverse disease pathology by aiding microglia mediated Aβ42 clearance via Fc interactions, binding to monomers and oligomers and preventing their aggregation, and resolving plaques via serine protease activity. Anti-Aβ40 antibodies may be used to target Aβ40 accumulation in the perivascular compartment of cerebral arteries (also referred to as cerebral amyloid angiopathy or CAA) in a similar manner. Adapted from: ^,–.

Figure 9.. Passive immunotherapy strategies to treat Parkinson’s disease.

Disease pathology in Parkinson’s disease typically entails the accumulation and aggregation of abnormal alpha synuclein protein, subsequently leading to neuronal cell death and cognitive decline. Anti-alpha synuclein antibodies may be used to block the intracellular aggregation of abnormal alpha synuclein which typically leads to the formation of intracellular Lewy bodies (thus the most likely strategy would be to use intrabodies) or prevent the cell-to-cell transmission of extracellular abnormal alpha synuclein (using conventional antibodies). Extracellular anti-alpha synuclein antibodies may prevent abnormal alpha synuclein monomers and oligomers from aggregating further and may recruit microglia to phagocytose abnormal protein via Fc mediated interactions. Lymphocyte activation gene 3 (LAG3) protein was recently implicated in the internalization of pathologic alpha synuclein during cell-to-cell transmission so an anti-LAG3 antibody strategy may therefore be promising to prevent the spread of alpha synuclein pathology. Abbreviations: ECS – extracellular space. Adapted from: ^,.

Figure 10.. Passive immunotherapy strategies to treat Huntington’s disease.

Huntington’s disease (HD) pathology is characterized by the intracellular accumulation and aggregation of the mutant huntingtin protein (mHTT), which results in subsequent cell death, and the spread of pathology due to cell-to-cell transmission of extracellular mHTT. Other hallmarks of HD pathology include pro-inflammatory signals (e.g., SEM4D/plexinB1 signaling pathway), and down-regulation of cell survival/neurotrophic signals (e.g., BDNF/TrkB signaling pathway). HD progression may potentially be blocked by passive immunotherapy strategies that target one or more aspects of this pathology. For example, anti-HTT antibodies (e.g., intrabodies) may be used to target intracellular mHTT. It is important to note that anti-HTT intrabodies typically bind to both normal HTT and mHTT. The ratio of mHTT to normal HTT is indicative of HD pathology and mHTT mRNA transcripts were found to exceed normal HTT in the cortex and striatum of nearly 75% patients in an HD clinical study . The increase in mHTT compared to normal HTT may be attributed to increased transcription of the mHTT allele, or decreased clearance of mHTT, or both . Therefore, engineering antibodies that recognize and bind with higher affinity to mHTT than normal HTT may be important since an equimolar inhibition of mHTT and normal HTT may increase the mHTT to normal HTT ratio . Additionally, normal HTT is thought to play a role in promoting cell survival and depleting it may further exacerbate disease pathology and clinical outcome .

Figure 11.. Immunoglobulin G (IgG) access to the perivascular space (PVS) surrounding cerebral blood vessels following intrathecal and intranasal delivery.

(A, B) Examples of blood vessels at the rat the cortical surface and in the striatum respectively showing intrathecally administered rat IgG accessing the PVS. (C, D) Examples of blood vessels in the rat olfactory bulb and at the cortical surface respectively showing intranasally administered rat IgG accessing the PVS. Abbreviations: IT – intrathecal; IN – intranasal; RECA-1 – rat endothelial cell antigen-1 (endothelial cell marker); GFAP – glial fibrillary acidic protein (astrocyte marker); DAPI – 4’, 6-diamidino-2-phenylindole (cell nucleus marker).