Advances in Antibody-Based Therapeutics for Cerebral Ischemia

Abstract

Cerebral ischemia is an acute disorder characterized by an abrupt reduction in blood flow that results in immediate deprivation of both glucose and oxygen. The main types of cerebral ischemia are ischemic and hemorrhagic stroke. When a stroke occurs, several signaling pathways are activated, comprising necrosis, apoptosis, and autophagy as well as glial activation and white matter injury, which leads to neuronal cell death. Current treatments for strokes include challenging mechanical thrombectomy or tissue plasminogen activator, which increase the danger of cerebral bleeding, brain edema, and cerebral damage, limiting their usage in clinical settings. Monoclonal antibody therapy has proven to be effective and safe in the treatment of a variety of neurological disorders. In contrast, the evidence for stroke therapy is minimal. Recently, Clone MTS510 antibody targeting toll-like receptor-4 (TLR4) protein, ASC06-IgG1 antibody targeting acid sensing ion channel-1a (ASIC1a) protein, Anti-GluN1 antibodies targeting N-methyl-D-aspartate (NMDA) receptor associated calcium influx, GSK249320 antibody targeting myelin-associated glycoprotein (MAG), anti-High Mobility Group Box-1 antibody targeting high mobility group box-1 (HMGB1) are currently under clinical trials for cerebral ischemia treatment. In this article, we review the current antibody-based pharmaceuticals for neurological diseases, the use of antibody drugs in stroke, strategies to improve the efficacy of antibody therapeutics in cerebral ischemia, and the recent advancement of antibody drugs in clinical practice. Overall, we highlight the need of enhancing blood-brain barrier (BBB) penetration for the improvement of antibody-based therapeutics in the brain, which could greatly enhance the antibody medications for cerebral ischemia in clinical practice.

Keywords: antibody; blood brain barrier; cerebral ischemia; hemorrhagic; ischemic.

Figure 1

Schematic representative diagram demonstrating intravenous or subcutaneous injection of different forms of antibody-based medications to a stroke patient. The development of stroke triggers several mechanisms, including activation of glutamate receptors, which leads to the release of glutamate and influx of calcium ions that activate nitric oxide, caspases, and proteases. This activation leads to inflammation, producing free radicals, and protein damage, resulting in neuronal cell death. Other important aspects of stroke include blood–brain barrier (BBB) damage, oxidative stress, cytokine-mediated toxicity, excitotoxicity, and loss of neuronal function [14,15].

Figure 2

Schematic representative of recently developed antibody-based drug in different neurological diseases. Antibody-based drugs exert their therapeutic effects through a variety of mechanisms. Some antibodies block the function of the membrane receptors. Others target specific complement proteins for cell depletion. Finally, antibody-based drugs can target specific effector molecules from interacting with their ligands. Immunogens include IL-6L/R, interleukin 6 ligand/receptor; CGRPL/R: calcitonin gene-related peptide ligand/receptor; CD19/20: cluster of differentiation 19/20; C5: complement factor 5 protein.

Figure 3

Schematic illustration of function of HMGB1 or TLR4 in the pathology of stroke. The high mobility group box 1 (HMGB1) protein is a typical damage-associated molecular pattern (DAMP) protein that can bind to the toll-like receptor 4 (TLR4) and initiates and activates the NF-κB signaling axis. The activation of the NF-κB pathway significantly increases the production of pro-inflammatory cytokines, including TNF-α, IL-6, and IL-1β, which causes exaggerated neuroinflammation, BBB disruption, and neuronal degeneration after cerebral ischemia. Inhibition of TLR4 or HMGB1 with antibody-based drugs can ameliorate the neuroinflammatory responses and neuronal damage after stroke.

Figure 4

Schematic illustration for the function of ASIC1a or NMDAR in the development of stroke. Acidosis caused by anaerobic glycolysis is a common side effect of stroke. When pH levels drop after stroke, ASIC1a is activated, allowing calcium to flood the cell, creating an improper ionic balance and osmotic pressure. On the other hand, glutamate is the chief excitatory neurotransmitter in the central nervous system and displays critical roles in synaptic communications and flexible synaptic plasticity. However, the extensive and rapid accumulation of glutamate at the synaptic cleft after ischemic stroke can lead to over-excitation of the NMDARs (N-methyl-D-aspartate receptors), which can eventually be lethal to neurons. Both of the membrane-originated pathological events lead to edema and depolarization, which in neurons causes massive excitotoxicity from increased intracellular calcium ions and eventually, inflammation, hence ischemia injury. Inhibition of the ASIC1a or GluN1 with antibody-based drugs can reverse the neuronal death and protect against cerebral ischemia.

Figure 5

Schematic illustration of MAG or Nogo-A signaling cascades in the pathology of cerebral ischemia. The overproduction of Myelin-associated glycoprotein (MAG) Neurite outgrowth inhibition protein-A (Nogo-A) after cerebral ischemia activates the signaling cascades linked to the RhoA/ROCK pathway, which collapses neuronal growth cones and inhibits axonal growth. Such inhibitory events significantly suppress the functional recovery after the suffering of cerebral ischemia. Inhibition of the MAG or Nogo-A with highly specific antibody-based therapeutics can enhance the axonal regeneration and neurite outgrowth dramatically, which leads to better functional recovery after cerebral ischemia.

Figure 6

Schematic illustration of ATP and P2X7 signaling cascades in the pathology of cerebral ischemia. Under cerebral ischemia, energy deprivation causes anoxic and irreversible depolarization, which leads to excessive release of glutamate and ATP. ATP could act as a damage-associated molecular pattern (DAMP) that activates P2X7 receptors on both neurons and glia cells. Neuronal-expressed P2X7 receptors initiate calcium overload and extensive excitotoxic neurological injury. On the other hand, microglia-expressed P2X7 receptors regulate its aberrant activation and over-production of reactive oxygen species and pro-inflammatory cytokines. These ATP-P2X7 signaling cascades modulate immune responses, ROS production, and eventually progressive neurological damage.

Figure 7

Sketch illustration of the brain’s processes for antibody-based drug transport via the blood–brain barrier. In a receptor-mediated target (RMT), the ligand and antibody in the blood specifically bind to RMT receptors in the apical cell membrane; (2) the receptors and ligands are internalized in an energy-dependent manner to form an intracellular vesicle; and (3) these vesicles are shuttled through the cell cytoplasm, and the antibodies and ligands are released into the brain via the fusion of vesicles with the basolateral cell membrane. In an adsorptive-mediated target (AMT), the positively charged antibodies interact electrostatically with the negatively charged apical cell membrane, forming intracellular vesicles containing antibodies through energy-independent endocytosis. Finally, after traveling through the cell cytoplasm and fusing with the basolateral cell membrane, these vesicles release their contents into the brain.

Figure 8

Strategies to improve the efficacy of antibody-based drugs for cerebral ischemia. The over-activation of immunological responses by the antibody drugs could cause serious biological and clinical consequences. The recent development of fully human or humanized antibodies has greatly reduced the risk for inducing exaggerated immune responses compared to the mouse or chimeric antibodies. Meanwhile, bivalent conjugation of an antibody of BBB enriched receptors, such as TfR, LDLRP, or NACR, with the target antibody drug could significantly increase the efficacy of receptor-mediated transport of antibodies into the brain. Nanoparticles or liposomes, which prevent the degradation of antibody drugs and show better efficacy in BBB penetration, can be used as vehicles to transport the antibody into the brain after cerebral ischemia.