8-A842A Halperin7e_CH29_CROPPED2

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S E C T I O N I I  Techniques, Modalities, and Modifiers in Radiation Oncology

FIGURE 29.1.  Antibody configurations for radioimmunotherapy. A: Typical structure of a humanized IgG antibody. B: Antibody fragments and size. C: Attachment of a nuclear localization signal. D: Humanization: green represents murine portion of IgG and red represents human portion. CDR, complementarity determining regions; CH, constant domain heavy chain; CL, constant domain light chain; Fab, antigen-binding fragment; F(ab’) antigen-binding fragment (retaining a portion of the hinge region following enzymatic digestion); Fc, crystallizable fragment; Fv, variable fragment; PKKKRKV, amino acid sequence of nuclear localization signal; scFv, single-chain variable fragment.

cytotoxicity (CDC). 5 Concerning ADCC, interaction of the Fc region of the antibody with Fc receptors (located on immune effector cells) results in the subsequent phagocytosis or lysis of the antibody-bound cancer cell. CDC is initiated by the inter- action of soluble blood proteins and the Fc region. Epitope- dependent (Fc-independent) functions of the mAb may result in the inhibition of ligand binding, inhibition of ligand-induced dimerization, and inhibition of receptor shedding. These epi- tope-dependent functions are characteristic of modern-day biologics that target growth factor receptors, such as cetux- imab and trastuzumab. The original technology used to produce mAbs was first published by Kohler and Milstein in 1975 and is referred to as the hybridoma technique. The technique has propagated the use of murine mAbs for research and for therapy in the clinic. In fact, the two U.S. Food and Drug Administration (FDA) RIT agents used to treat non-Hodgkin lymphoma (NHL; ibritumomab tiuxetan and tositumomab) are murine mAbs. Although these agents are delivered as single instillations in patients typically with decreased immune recognition capa- bilities, there is a concern that human antiglobulin antibodies (HAGAs) will develop. If this phenomenon occurs in response to murine antibodies, then the resulting HAGAs will be called human antimouse antibodies (HAMAs). The formation of HAMAs will expedite blood clearance of the antibody and decrease targeting capabilities as well as potentially cause various adverse symptoms. Two main strategies, through the use of genetic engineering, have emerged 12 that reduce the immunogenicity of mAbs: (a) the production of antibody chimeras derived from both murine and human DNA and

light (C L

), one variable heavy (V H

), and one variable light (V L )

domain. The ABS consists of a V L region. Within each variable domain, there are three hypervariable regions (about 10 amino acid residues per hypervariable region) that form a three-dimensional surface that is “complementary” to the shape of the antigen surface; they are called complementarity determining regions (CDRs). A total of six CDRs come together to form the ABS. There are two ABS for each IgG mAb; hence, each IgG mAb is considered bivalent. Affinity refers to the strength of the bond between the ABS and the antigen.The strength of this bond is represented by the dissociation constant (K d ).Avidity refers to the overall strength of the ABS–antigen interaction, depending on both the affinity and the valency of the interaction. It should be noted that a high-affinity interaction can improve specific delivery of the RIT agent and reduce overall dosing requirements. Increasing the affinity indefinitely, however, may decrease tumor pen- etration. It has been demonstrated that an affinity of 10 − 7 to 10 − 8 M is needed for tumor retention, whereas affinities in the range of ≥10 − 10 to 10 − 11 M will result in retention in normal tissue and asymmetric binding in tumor tissue, termed the binding site barrier . 13 The binding site barrier phenomenon may be at least partially overcome by increasing the antibody mass, or the overall delivered quantity of antibody. Unconjugated antibodies—those not attached to a radio- nuclide or cytotoxic agent—will also mediate biologic activi- ties. These activities may be mediated by the Fc region of the mAb or may be Fc independent. Fc-mediated interactions are termed effector functions and consist of antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent and a V H