Antibody Targeting Program

Antibody Engineering

The characteristics of an antibody that may prevent its use, or reduce its efficacy as an immunotherapy, can be altered through genetic engineering. Thus the immunogenicity of an antibody (its propensity to be destroyed by the immune system) can be reduced or eliminated, and the effector function (ability to cause cell death), size, structure, and affinity (strength of binding to the antigen) of the antibody can be altered.

Introduction

Antibody engineering may be required to refine or optimize one or more of the four key characteristics relating to the efficacy of a therapeutic antibody. First, the antibodies that are used in research and as cancer therapies are monoclonal antibodies (mAbs), which are identical antibodies derived originally from mouse cells. These mAbs can generate an anti-mouse antibody response in patients, rapidly clearing the injected antibody from the blood and rendering it useless. Then, depending upon the manner in which it will be used, a therapeutic antibody may need a stronger or weaker effector function, increased or decreased antibody affinity, and/or reduction in size, for maximum efficacy.

Immunogenicity

Most mAbs are originally produced in a mouse cell system, because it produces the largest amount of identical antibodies. Thus the human immune system may recognize the mAbs as ‘foreign’, even though mouse and human mAbs are structurally identical. Should a mAb be found to be immunogenic during clinical characterization, LICR scientists use genetic engineering techniques to substitute mouse gene sequences with the corresponding human gene sequence. By altering the protein sequence of immunogenic mouse-derived mAbs, the risk that the patient’s own immune system will ‘reject’ the therapy can be reduced or eliminated. Alternatively, LICR scientists may generate a fully human antibody that targets the same antigen as the mouse mAb.

Effector Function

LICR investigators use a variety of genetic engineering methods to enhance or reduce the effector function of an antibody in order to suit the strategy for its clinical application. A strong effector function is required for a ‘naked’ antibody therapy, in which the antibody is designed to be cytotoxic (cause cell death). The antibody can be engineered to enhance the effector function so that the cytotoxicity is via complement-dependent cytotoxicity (CDC), which is part of the humoral (CD4+/CD8+ T cell) response, or via antibody-dependent cellular cytotoxicity (ADCC), which is part of the cellular (antibody, B cell) immune response. However, LICR may choose to engineer the mAb sequence such that it has a weak effector function, which is required for an antibody that is not, by itself, cytotoxic, but is designed to deliver an attached toxin or radioisotope to the cancer cells.

Antibody Size and Structure

The size of an antibody has a direct impact on its ability to penetrate the tumor and other tissues, as well as its pharmacokinetics (clearance from the body). If necessary, as determined from the Antibody Characterization, LICR scientists reduce the size of the antibody, by removing non-essential components through genetic engineering, to increase its tumor penetration and improve its pharmacokinetics.

Therapies that utilize the delivery of a toxin or radioisotope have traditionally been comprised of an antibody with a conjugated (chemically attached) toxin or radioisotope. However LICR scientists now fuse the toxin or radioisotope directly to the antibody structure; minimizing the potential for loss of the toxin from the antibody, and thus maximizing the delivery of antibody-toxin conjugate to the tumor.

Affinity

The affinity of an antibody for its antigen can also be increased or decreased through genetic engineering, by altering the amino acids within the antibody’s binding site. Existing data from LICR now show that the optimization of an antibody’s affinity may improve its clinical efficacy.

Key Publications

Centers Involved in this Research