Monoclonal antibody therapy

Initial therapeutic antibodies were murine analogues (suffix -omab). These antibodies have: a short half-life in vivo (due to immune complex formation), limited penetration into tumour sites and inadequately recruit host effector functions.[12] Chimeric and humanized antibodies have generally replaced them in therapeutic antibody applications.[13] Understanding of proteomics has proven essential in identifying novel tumour targets.

Initially, murine antibodies were obtained by hybridoma technology, for which Jerne, Köhler and Milstein received a Nobel prize. However the dissimilarity between murine and human immune systems led to the clinical failure of these antibodies, except in some specific circumstances. Major problems associated with murine antibodies included reduced stimulation of cytotoxicity and the formation complexes after repeated administration, which resulted in mild allergic reactions and sometimes anaphylactic shock.[12] Hybridoma technology has been replaced by recombinant DNA technology, transgenic mice and phage display.[13]

To reduce murine antibody immunogenicity (attacks by the immune system against the antibody), murine molecules were engineered to remove immunogenic content and to increase immunologic efficiency.[12] This was initially achieved by the production of chimeric (suffix -ximab) and humanized antibodies (suffix -zumab). Chimeric antibodies are composed of murine variable regions fused onto human constant regions. Taking human gene sequences from the kappa light chain and the IgG1 heavy chain results in antibodies that are approximately 65% human. This reduces immunogenicity, and thus increases serum half-life.

Humanised antibodies are produced by grafting murine hypervariable regions on amino acid domains into human antibodies. This results in a molecule of approximately 95% human origin. Humanised antibodies bind antigen much more weakly than the parent murine monoclonal antibody, with reported decreases in affinity of up to several hundredfold.[14][15] Increases in antibody-antigen binding strength have been achieved by introducing mutations into the complementarity determining regions (CDR),[16] using techniques such as chain-shuffling, randomization of complementarity-determining regions and antibodies with mutations within the variable regions induced by error-prone PCR, E. coli mutator strains and site-specific mutagenesis.[1]

Human monoclonal antibodies (suffix -umab) are produced using transgenic mice or phage display libraries by transferring human immunoglobulin genes into the murine genome and vaccinating the transgenic mouse against the desired antigen, leading to the production of appropriate monoclonal antibodies.[13] Murine antibodies in vitro are thereby transformed into fully human antibodies.[5]

The heavy and light chains of human IgG proteins are expressed in structural polymorphic (allotypic) forms. Human IgG allotype is one of the many factors that can contribute to immunogenicity.[17][18]

Anti-cancer monoclonal antibodies can be targeted against malignant cells by several mechanisms. Ramucirumab is a recombinant human monoclonal antibody and is used in the treatment of advanced malignancies.[19] In childhood lymphoma, phase I and II studies have found a positive effect of using antibody therapy.[20]

Monoclonal antibodies used for autoimmune diseases include infliximab and adalimumab, which are effective in rheumatoid arthritis, Crohn's disease and ulcerative colitis by their ability to bind to and inhibit TNF-α.[21] Basiliximab and daclizumab inhibit IL-2 on activated T cells and thereby help preventing acute rejection of kidney transplants.[21] Omalizumab inhibits human immunoglobulin E (IgE) and is useful in moderate-to-severe allergic asthma.

Alzheimer's disease (AD) is a multi-faceted, age-dependent, progressive neurodegenerative disorder, and is a major cause of dementia.[22] According to the Amyloid hypothesis, the accumulation of extracellular amyloid betapeptides (Aβ) into plaques via oligomerization leads to hallmark symptomatic conditions of AD through synaptic dysfunction and neurodegeneration.[23] Immunotherapy via exogenous monoclonal antibody (mAb) administration has been known to treat various central nervous disorders, such as AD, by inhibiting Aβ-oligomerization thereby preventing neurotoxicity. However, mAbs are large for passive protein channels and are therefore inefficient due to the blood-brain barrier preventing mAb passage into the brain. However, the Peripheral Sink hypothesis proposes a mechanism where mAbs may not need to cross the blood-brain barrier.[24] Therefore, many research studies are being conducted from failed attempts to treat AD in the past.[23]

However, anti-Aβ vaccines can promote antibody-mediated clearance of Aβ plaques in transgenic mice models with amyloid precursor proteins (APP), and can reduce cognitive impairments.[22] Vaccines can stimulate the immune system to produce its own antibodies,[25] in this case by introducing Aβ into transgenic animal models, known as active immunization. They can also introduce antibodies into animal models, known as passive immunization. In mice expressing APP, both active and passive immunization of anti-Aβ antibodies has been shown to be effective in clearing plaques, and can improve cognitive function.[23] Currently, there are no approved monoclonal antibody therapies for Alzheimer's disease, but several clinical trials using passive and active immunization approaches by development of certain drugs approved by the FDA are currently underway, and are expected to yield results in a couple of years.[23] The implementation of these drugs is during the onset of AD. Other research and drug development for early intervention and AD prevention is ongoing. Various drugs that are under research to treat AD include Bapineuzumab, Solanezumab, Gautenerumab, and BAN2401.

Bapineuzumab, a humanized anti-Aβ mAb, is directed against the N-terminus of Aβ. Phase II clinical trials of Bapineuzumab in mild to moderate AD patients resulted in reduced Aβ concentration in the brain. However, in patients with increased apolipoprotein (APOE) e4 carriers, Bapineuzumab treatment is also accompanied by vasogenic edema,[26] a cytotoxic condition where the blood brain barrier has been disrupted thereby affecting white matter from excess accumulation of fluid from capillaries in intracellular and extracellular spaces of the brain.[27] In Phase III clinical trials, Bapineuzumab treatment is associated with reduced rate of accumulation of Aβ in the brain in APOE e4 patients, and no significant reduction of Aβ concentration in APOE e4 patients and non-APOE e4 patients. Therefore, Aβ plaque concentration was not reduced, and there is no significant clinical benefits in cognitive functioning. Bapineuzumab was discontinued after failing in Phase III clinical trial[27]

Solanezumab, an anti-Aβ mAb, targets the N-terminus of Aβ. In Phase I and Phase II of clinical trials, Solanezumab treatment resulted in cerebrospinal fluid elevation of Aβ, thereby showing a reduced concentration of Aβ plaques. Additionally, there are no associated adverse side effects. Phase III clinical trials of Solanezumab brought about significant reduction in cognitive impairment in patients with mild AD, but not in patients with severe AD. However, Aβ concentration did not significantly change, along with other AD biomarkers, including phospho-tau expression, and hippocampal volume. Phase III clinical trials are currently ongoing.[24]

BAN2401, is a humanized mAb that selectively targets toxic soluble Aβ protofibrils,[28] and the therapy is currently undergoing a phase 3 clinical trial which is expected to be completed in 2022.[29]

Failure of several drugs in Phase III clinical trials has led to AD prevention and early intervention for onset AD treatment endeavours. Passive anti-Aβ mAb treatment can be used for preventive attempts to modify AD progression before it causes extensive brain damage and symptoms. Trials using mAb treatment for patients positive for genetic risk factors, and elderly patients positive for indicators of AD are underway. This includes anti-AB treatment in Asymptomatic Alzheimer's Disease (A4), the Alzheimer's Prevention Initiative (API), and DIAN-TU.[24] The A4 study on older individuals who are positive for indicators of AD but are negative for genetic risk factors will test Solanezumab in Phase III Clinical Trials, as a follow up of previous Solanezumab studies.[24] DIAN-TU, launched in December 2012, focuses on young patients positive for genetic mutations that are risks for AD. This study uses Solanezumab and Gautenerumab. Gautenerumab, the first fully human MAB that preferentially interacts with oligomerized Aβ plaques in the brain, caused significant reduction in Aβ concentration in Phase I clinical trials, preventing plaque formation and concentration without altering plasma concentration of the brain. Phase II and III clinical trials are currently being conducted.[24]

Radioimmunotherapy (RIT) involves the use of radioactively-conjugated murine antibodies against cellular antigens. Most research involves their application to lymphomas, as these are highly radio-sensitive malignancies. To limit radiation exposure, murine antibodies were chosen, as their high immunogenicity promotes rapid tumor clearance. Tositumomab is an example used for non-Hodgkin's lymphoma.

Antibody-directed enzyme prodrug therapy (ADEPT) involves the application of cancer-associated monoclonal antibodies that are linked to a drug-activating enzyme. Systemic administration of a non-toxic agent results in the antibody's conversion to a toxic drug, resulting in a cytotoxic effect that can be targeted at malignant cells. The clinical success of ADEPT treatments is limited.[30]

Antibody-drug conjugates (ADCs) are antibodies linked to one or more drug molecules. Typically when the ADC meets the target cell (e.g. a cancerous cell) the drug is released to kill it. Many ADCs are in clinical development. As of 2016 a few have been approved.

Immunoliposomes are antibody-conjugated liposomes. Liposomes can carry drugs or therapeutic nucleotides and when conjugated with monoclonal antibodies, may be directed against malignant cells. Immunoliposomes have been successfully used in vivo to convey tumour-suppressing genes into tumours, using an antibody fragment against the human transferrin receptor. Tissue-specific gene delivery using immunoliposomes has been achieved in brain and breast cancer tissue.[31]

Checkpoint therapy uses antibodies and other techniques to circumvent the defenses that tumors use to suppress the immune system. Each defense is known as a checkpoint. Compound therapies combine antibodies to suppress multiple defensive layers. Known checkpoints include CTLA-4 targeted by ipilimumab, PD-1 targeted by nivolumab and pembrolizumab and the tumor microenvironment.[2]

The tumor microenvironment (TME) features prevents the recruitment of T cells to the tumor. Ways include chemokine CCL² nitration, which traps T cells in the stroma. Tumor vasculature helps tumors preferentially recruit other immune cells over T cells, in part through endothelial cell (EC)–specific expression of FasL, ET_BR, and B7H3. Myelomonocytic and tumor cells can up-regulate expression of PD-L1, partly driven by hypoxic conditions and cytokine production, such as IFNβ. Aberrant metabolite production in the TME, such as the pathway regulation by IDO, can affect T cell functions directly and indirectly via cells such as T_reg cells. CD8 cells can be suppressed by B cells regulation of TAM phenotypes. Cancer-associated fibroblasts (CAFs) have multiple TME functions, in part through extracellular matrix (ECM)–mediated T cell trapping and CXCL12-regulated T cell exclusion.[32]