Bispecific antibodies are the antibodies capable of binding to two different antigens simultaneously.
Over hundred different formats of the bispecific antibodies (bsAbs) are in the pipeline making bsAbs one of the fastest growing class of investigational drugs. However, only two of the bsAbs namely, Blinatumomab and Catumaxomab are approved till date.
By simultaneously binding two different antigens or two epitopes on the same antigen, bsAbs can master functions that cannot be performed by conventional monospecific antibodies.
With the ability to recognize and bind two different target antigens, bsAbs can act as mediators to redirect immune cells to tumor cells enhancing their destruction. In addition, by targeting two different receptors on the same cell, bsAbs can cause modification of cell signalling, such as inactivating the process of division of cancer cells or inactivating other inflammatory pathways.
BsAbs are created through use of recombinant technology or by somatically fusing hybridoma cells or through chemical means.
Based on the presence or absence of Fc domain, bsAbs are divided into two types IgG-like and non IgG-like bispecific antibodies.
IgG-like bsAbs have conserved Ig domain and bear Fc region which contributes to their improved solubility and stability. Also these bsAbs cab exhibit Fc mediated effector functions such as antibody-dependent cellular toxicity (ADCC) and complement fixation (CDC). This is an add-on to the therapeutic efficacies of these antibodies.
The smaller, non-IgG bsAbs lack constant domain and rely completely on their antigen-binding capacity to exhibit their therapeutic action.
Mechanism of Action of Bispecific Antibodies
1. Redirecting cellular immunity toward tumor cells
The bispecific antibodies showing this function activates the immune cells and recruits them for destruction of tumor cells bearing the target antigens. One of the two antigen binding sites recognizes and binds to the target antigen on the tumor cell and the other site binds to the suitable leukocyte.
Bispecific T-Cell engagers (BiTEs) represent a very efficient format for this function. Blinatumomab is one such BiTE that achieved an accelerated approval by FDA in December 2014 for the treatment of children and adults with Philadelphia chromosome – negative relapsed or refractory B-cell precursor acute lymphoblastic leukemia. In July 2017, its indication was expanded by FDA to patients with Philadelphia chromosome-positive ALL giving it full approval.
2. Delivering cytotoxic substances to the malignant cells
BsAbs that bind cell surface antigens as well as haptens are used for targeted and pretargeted therapies. Payloads such as fluorophores or chelated radioisotopes, nanoparticles, peptides etc. are haptenylated for example with digoxigenin to make them able to bind to bsAbs. Haptens are small, seperable part of antigen that reacts specifically with an antibody but is inicapable of stimulating antibody production except in combination with a carrier protein molecule.
Bispecific antibodies carrying unmodified hapten form non-covalent complexes with the cell surface antigen and the payload. Once this complex reaches inside the cell bsAbs can be separated and payloads can be released.
Pre-targeted delivery is another method of payload delivery through hapten binding. In this, the targeting vehicles are administered first which gets distributed and bound to target sites and the non-bound targeting vehicles are cleared from circulation. Then, the haptenylated payloads are administered which becomes captured at the desired target sites by hapten binding bispecific antibodies.
3. Overcoming drug resistance
The development of resistant towards anticancer drugs is likely due to the inhibitory checkpoint molecules as well as crosstalk between various signalling pathways. Bispecific antibody can engage the inhibitory molecule with one of the sites while binding the target molecules with the other.
4. Inhibiting multiple signalling/ligands
Multiple signalling pathways are involved in leading the diseases like cancer.
Receptor Tyrosine Kinase (RTKs) are a superfamily of cell surface receptors that are involved in mediating intracellular signalling by phosphorylating substrate proteins involved in division, differentiation and migration of cancer cells. Although many monospecific antibodies are already in therpapeutic practice to target these receptors, the cancer cells are able to escape the blocking of one signalling pathways by taking up other signalling pathways.
BsAbs that are able to block two pathways simultaneous are effective in reducing the possibility of tumor escape through such mechanisms.
5. Inhibition of tumor angigiogenesis
Angiogenesis, the formation of new blood vessels is regulated by the release of specific growth factors from cancer cells, endothelial cells or macrophages associated with cancer cells. These growth factors include vascular endothelial growth factor (VEGF), basic fibroblast growth factor and likes. Angiopoietin 2 secreted by endothelial cells on the other hand increase vascular permeability and cause endothelial cells proliferation. It is known to be upregulated in wide range of cancers.
Simultaneously blocking two or more of such angiogenic factors can reduce tumor growth and increase therapeutic efficacy.
Mechanism of Action of Bispecific Antibodies for Treating Diseases Other Than Cancer
1. Two factors dimerization
Emicizumab is a bsAb that can bind to both the coagulation factor, namely factor IX and factor X, facilitating the cascade reaction by mediating the activation of factor X. Factor X is normally activated by coagulation factor VIII which is deficient in haemophilia A patients.
2. Targeted apoptosis
RG7386, a bsAb binds Fibroblast activation protein (FAP) on cancer associated fibroblast and Death receptor-5 on cancer cells subsequently inducing apoptosis (death) of cancer cells. The bsAb showed positive result in preclinical trial.
3. Hormone mimetic action
RG-7992, a bispecific antibody that completed phase 1 of clinical trials mimics the hormone FHF1. It does so by targeting Klotho beta protein and fibroblast growth factor receptor-q (FGFR-1).
4. bsAb against bacteria
Medi-3902 is a bsAb that attacks Pseudomonas aeroginosa bacterium and neutralizes its defences. The bacterium has two antigens PcrV and Psl. Psl plays a role in phagocytosis whereas PcrV neutralizes the phagocytosis factors released by the bacteria. Patients infected with this were known to lack pre-existing immunity and thus could not generate immune response against these antigens.
The passage of large molecules across blood brain barrier is restricted due to the presence of tight junctions between the endothelial cells in brain capillaries. Specific receptors such as TfR allow the transport across bbb. A bispecific antibody binding TfR and BACE1 simultaneously was developed.
BACE1 (Beta amyloid precursor protein cleavage enzyme) is an enzyme that cleaves beta amyloid precursor protein and release soluble amyloid beta into brain interstitium. By binding BACE1, bsAb ensures its inhibition which leads to reduction in soluble amyloid beta in the brain preventing amyloid plaque formation.
Broadly neutralizing antibodies against the HIV1 envelope glycoprotein have been shown to suppress viremia in animal models of HIV1 and humans.
Bispecific antibodies have shown great therapeutic potential in treatment of both cancer and non-cancer diseases. Moreover, these great little inventions have shown outstanding usefulness in diagnosis and other healthcare fields. Immunotherapy has become one of the most researchable fields with great potential in therapeutics most importantly in cancer treatment. A better understanding of mechanisms of these agents can help in both, developing new drugs and exploring the abilities of already established drugs.
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© 2018 Sherry Haynes