Lois Hinman

Chapter 16. Chemical Modification of Antibodies for Cancer Chemotherapy

Publisher Summary This chapter discusses the chemistry behind conjugation of agents to MoAbs. Recent reviews dealing with other aspects of MoAb-based therapy have appeared, covering subjects, such as the generation of hybridomas, the various tumor-associated antigens, and the problems inherent in MoAb-based approaches to tumor therapy, and clinical experiences with MoAbs and their conjugates. Modifications at more specific sites have involved cysteine residues or carbohydrate chains. Several recent reviews have dealt with the restrictions placed on useful conjugation reactions because of the nature of MoAbs. Briefly, all methods of activating agents for conjugation must yield intermediates that are stable to, but reactive, under aqueous conditions. Although maximum loadings achieved by a particular method are cited, such loadings differ from one MoAb to the next, and often reflect a point, at which immunoreactivity was lost upon further loading. Because direct conjugation of MTX to some MoAbs leads to the precipitation of the product after as little as two or three molecules of MTX have been conjugated, human serum albumin (HSA) has been used extensively as an intermediate carrier for MTX-MoAb conjugates. HSA contains numerous amino groups for amide formation and solubilization is relatively small (-65,000 M W) and contains a cysteine at residue 34 that makes the complex amenable to later attachment to a MoAb. Cobra venom factor (CVF), a 140,000 MW functional analog of complement C 3 /C 5 convertase, has been attached to several MoAbs in an effort to achieve cytotoxicity by complement fixation. A number of hetero-bifunctional cross-linking reagents were evaluated for their potential as spacers between CVF and MoAbs, including 4-(N-maleimidomethyl)cyclohexanecarboxyl N-hydroxysuccinimide, γ-(N-maleimidobutyryl) N-hydroxysuccinimide, MBS, SPDP, and iodoacetyl N-hydroxysuccinimide.

Chapter 25. New Approaches to Non-Immunogenic Monoclonal Antibody Cancer Therapies

Publisher Summary Despite a great deal of research, only a small number of MoAb-based products have been approved for clinical use, including OKT3, a murine IgG for the treatment of renal transplant rejection and myoscint, an in-labeled murine antimyosin MoAb, for the imaging of myocardial damage but no products for cancer therapy. The significance of most of the fundamental technical problems varies among therapeutic approaches and tumor targets; however, immunogenicity is a general concern that limits the therapeutic possibilities for murine MoAbs or MoAb-based products in any protocol requiring multiple or repeat injections. Immune responses in the form of human anti-mouse antibodies (HAMA) often result from even a single injection of murine-based MoAbs in all but the most immunosuppressed patients. As repeated doses are expected to be required for most treatment regimens in anti-cancer therapy, a great deal of effort has recently been directed toward the engineering of less immunogenic anti-body constructs. Many trials have been made that can undoubtedly provide further information on the properties of humanized antibodies and their likely efficacy in cancer therapy. Higher mammals have five classes of immunoglobulin, termed IgG, IgM, IgA, IgE, and IgD. The structure and function of these are conserved across species. Nearly all those used for cancer therapy have been of the IgG class. In the field of drug targeting, many investigators are looking for drugs with potencies similar to the phytotoxins for the next generation of drug-MoAb conjugates and are investigating the targeting of highly potent low molecular weight xenobiotic drugs. While too toxic as stand-alone therapies, these drugs theoretically combine the best features of drug and toxin conjugates, having potencies up to 1000-fold higher than the clinically approved chemotherapeutic agents without the inherent immunogenicity of the phytotoxins. Antibody directed prodrug therapy (ADEPT) may also address the problems of tumor penetration and antigenic heterogeneity.