Immunotherapeutics

Monoclonal antibodies (mAbs) are manufactured in vitro to recognize specific targeted antigens (Ags); they are used to treat solid and hematopoietic tumors, inflammatory disorders, and infections. Most mAbs in clinical use target a single Ag, but a few are engineered to be bispecific. The monoclonal antibodies that are currently in clinical use include

• Murine

• Chimeric

• Humanized

• Fully human

Murine monoclonal antibodies are produced by injecting a mouse with an antigen, harvesting its spleen to obtain B cells that are producing antibody specific to that antigen, fusing those cells with immortal mouse myeloma cells, growing these hybridoma cells (eg, in cell culture), and harvesting the antibody. Although mouse antibodies are similar to human antibodies, clinical use of murine monoclonal antibodies is limited because they induce human anti-mouse antibody production, can cause immune complex serum sickness (a type III hypersensitivity reaction), and are rapidly cleared.

To minimize the problems due to use of pure mouse antibody, researchers have used recombinant DNA techniques to create monoclonal antibodies that are part human and part mouse. Depending on the proportion of the antibody molecule that is human, the resultant product is termed one of the following:

• Chimeric

• Humanized

In both chimeric and humanized monoclonal antibodies, the process usually begins as above with production of mouse hybridoma cells that make antibody to the desired antigen. Then the DNA for some or all of the variable portion of the mouse antibody is merged with DNA for human immunoglobulin. The resultant DNA is placed in a mammalian cell culture, which then expresses the resultant gene, producing the desired antibody. If the mouse gene for the whole variable region is spliced next to the human constant region, the product is termed "chimeric." If the mouse gene for only the antigen-binding hypervariable regions of the variable region is used, the product is termed "humanized."

Chimeric monoclonal antibodies activate antigen-presenting cells (APCs) and T cells more effectively than murine monoclonal antibodies but can still induce production of human anti-chimeric antibodies.

Humanized monoclonal antibodies against various antigens are available for the treatment of colorectal cancer, breast cancer, leukemia, allergy, autoimmune disease, transplant rejection, and respiratory syncytial virus infection.

Fully human monoclonal antibodies are produced using transgenic mice that contain human immunoglobulin genes or using phage display (ie, a bacteriophage-based cloning method) of immunoglobulin genes isolated from human B cells. Fully human monoclonal antibodies have decreased immunogenicity and therefore may have fewer adverse effects.

Monoclonal antibodies that target checkpoint molecules on either T cells or tumor cells (termed checkpoint inhibitors—see table Some Immunotherapeutic Agents in Clinical Use) are used to prevent downregulation of antitumor responses and effectively treat some heretofore resistant cancers. However, because checkpoint molecules are also involved in other types of immune response, checkpoint inhibitors can cause severe immune-related inflammatory and autoimmune reactions (both systemic and organ specific).

These hybrid proteins are created by linking together the gene sequences encoding all or part of 2 different proteins to generate a chimeric polypeptide that incorporates desirable attributes from the parent molecules (eg, a cell-targeting component combined with a cell toxin). The circulating half-life of therapeutic proteins can also often be improved by fusing them to another protein that naturally has a longer serum half-life (eg, the Fc region of IgG).

Soluble versions of cytokine receptors can block the action of cytokines by binding with them before they attach to their normal cell surface receptor.

rheumatoid arthritis, polyarticular juvenile idiopathic arthritis, ankylosing spondylitis, psoriatic arthritis, and plaque psoriasis.

Colony-stimulating factors (CSF), such as erythropoietin, granulocyte CSF (G-CSF), and granulocyte-macrophage CSF (GM-CSF), are used in patients undergoing chemotherapy or transplantation for hematologic disorders and cancers and in patients with severe chronic neutropenia (see table Some Immunotherapeutic Agents in Clinical Use). Interferon-alpha (IFN-alpha) and IFN-gamma are used to treat cancer, immunodeficiency disorders, and viral infections. IFN-beta is used to treat relapsing multiple sclerosis.

rheumatoid arthritis, is a recombinant, slightly modified form of the naturally occurring IL-1R antagonist; this agent attaches to the IL-1 receptor and thus prevents binding of IL-1, but unlike IL-1, it does not activate the receptor.

Cells expressing cytokine receptors can be targeted by modified versions of the relevant cytokine (eg, denileukin diftitox, which is a fusion protein containing sequences from IL-2 and from diphtheria toxin). Denileukin is used in cutaneous T-cell lymphoma to target the toxin to cells expressing the CD25 component of the IL-2 receptor.

Small linear peptides, cyclicized peptides, and small organic molecules are being developed as agonists or antagonists for various applications. Screening libraries of peptides and organic compounds can identify potential mimetics (eg, agonists for receptors for erythropoietin, thrombopoietin, and G-CSF).

Immune system cells are harvested (eg, by leukapheresis) and activated in vitro before they are returned to the patient. The aim is to amplify the normally inadequate natural immune response to cancer. Methods of activating immune cells include using cytokines to stimulate and increase numbers of antitumor cytotoxic T cells and using pulsed exposure to antigen-presenting cells such as dendritic cells with tumor antigens. Before being returned to the patient, T cells can be genetically engineered to express chimeric antigen receptors (CAR) or T cell receptors (TCR) capable of recognizing tumor antigens, an approach that has shown efficacy in patients with leukemia and lymphoma.