Different generations of therapeutic antibodies
Murine Antibodies (suffix – “momab”)The murine antibodies were the first antibodies to be developed at lab scale using the hybridoma technology in rodents. But because of the differences between the human and rodent immune system, usage of murine antibodies resulted in cytotoxicity. Thus their continuous administration often resulted in allergic reactions and anaphylactic shock, termed as human anti-mouse or anti-murine antibody (HAMA) response. Anti-CD3 mAb of murine origin (OKT-3), the first therapeutic mAb, approved for treatment of transplantation rejection was discontinued primarily due to severe HAMA response in patients. Nevertheless, murine antibodies serve as frameworks for antibody development. Also due to the improved and optimized protocols for antibody generation from hybridoma mouse cell lines, this technology is considered popular and economical to produce new antibodies.
Chimeric MAbs (suffix – “ximab”)In order to minimize the HAMA response, chimeric antibodies were manufactured. These antibodies consist of 65% human genetic component by recombination of human constant regions and mouse variable regions in a suitable expression system. There are some chimeric antibodies approved by the FDA for use in human therapy and research, for eg., Infliximab, Rituximab, Abciximab. Presently even though there is a declining interest in developing chimeric antibodies for clinical applications, chimeric molecules can be widely used in the initial stage of antibody humanization strategies and also serve as controls and calibrators for specific research needs or in diagnostics applications.
Humanised MAbs (suffix – “zumab”)The urgency to overcome the obstacles presented by low complementarity of the mouse line developed antibodies to the patient’s immune system, gave way to the creation of humanized antibodies. The humanized antibodies are close to 95% in human origin, with only the complementarity determining regions (CDRs) of the variable regions having mouse-sequence origin. However, these Abs sometimes present lower affinity than the parent murine mAbs with respect to binding with antigens. So as to increase the antibody-antigen binding affinity, techniques such as chain-shuffling randomization can be employed to introduce some transformations into the CDR. Daclizumab is the first FDA-approved humanized antibody, used in the treatment of multiple sclerosis.
Fully human MAbs (suffix – “mumab”)Fully human sequence derived antibodies have no murine sequence, and are largely produced via two sources: phage display technologies and transgenic mice. The first fully human sequence-derived antibody to be approved for therapeutic use was adalimumab (Humira), a fully human IgG1 antibody specific for TNFalpha that was selected via phage display of human VH and VL sequences.
- Phage display technology
- Transgenic mice
Different generations of Antibody therapeutics
Novel therapeutic antibody structures
Bispecific antibodies (BsAb)
Antibody-drug conjugates (ADC)
Mechanisms of action of therapeutic antibodies
Signaling blockade by therapeutic antibodiesThe antibody can bind to the receptor on the target cell in an antagonistic fashion. The mAb can inhibit an undesired signaling pathway by blocking ligand-binding or interfering with the di/trimerization process to bring about the preferred physiological effect. For example, the mAb-ligand binding can give rise to inhibition of proliferative signals, blockade of immune checkpoints, induction of pro-apoptotic programs or re-sensitization of cell to a cytotoxic agent. Bispecific antibodies can target two ligands simultaneously so as to block redundant pathways.
Fc-domain mediated activation by therapeutic antibodiesFc mediated response occurs when the target cell is opsonized with the several mAb drug molecules. The Fc region can bind to various molecules eliciting a cytotoxic response which can be via (i) Antibody-dependent cell-mediated cytotoxicity (ADCC) – The Fc domain can bind to FcγRIIIA on NK cells triggering cell destruction via lytic factors secreted by the NK cells. (ii) Complement‐dependent cytotoxicity (CDC) – The C1q subunit of the C1 compliment factor binds to the Fc domain initiating a signalling cascade that finally leads to the formation of a membrane attack complex (MAC), a pore that causes cell lysis. (iii) Antibody-dependent cellular phagocytosis (ADCP or ADPh) – FcγRI expressed on macrophages, neutrophils, and eosinophils can bind to the Fc domain resulting in phagocytosis.
Agonist therapeutic antibodiesHere, the mAb-receptor binding activates a desired cellular pathway which is, in most cases, the activation of immune cells. Agonism of co-stimulatory ligands on T-cells and dendritic cells are currently being explored as an anti-cancer therapeutic strategy.
Receptor-mediated therapeutic antibody internalizationIt is the main moA utilized by ADCs. Binding of the ADC to the target cell leads to endocytosis of the mAb complex, enabling intracellular delivery of the drug resulting in cell death. Certain payloads have the ability to diffuse and kill surrounding cells, a phenomenon termed as “bystander killing” which is useful in the case of solid tumors.
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