The earliest clinical trial with a radiolabeled antibody was carried out in solid tumors with polyclonal antibodies (8). Development of the hybridoma technique (9) allowed monoclonal antibody production with reproducible characteristics. Initial production of these antibodies was in murine systems, and the inherent immunogenicity of xenogeneic proteins became apparent in the initial studies. Most humans who receive murine antibodies develop human antimouse antibodies (HAMA) that preclude effective multiple dosing regimens (10). The search for nonimmunogenic molecules led to the use of chimeric antibodies (11), with the murine Fv, grafted genetically to a human constant region, and to humanized antibodies where only the complementarity-determining region (CDR) remains of murine origin (12). Development of suitable nonimmunogenic antibodies would be essential to regimens that require multiple administrations. However, genetically engineered proteins have sometimes been immunogenic as well.
An obvious initial strategy to increase antibody tumor delivery was to decrease the size of the labeled protein. The resulting more rapid clearance would decrease the levels of circulating species, and therefore decrease the absorbed dose to marrow. Deletion of whole antibody constant regions, either by digestion or by genetic modification, resulted in bivalent F(ab)'2 fragments (no CH2 and CH3 domains) and monovalent Fab fragments. However, the resulting faster blood clearance has not improved relative tumor to bone marrow dose significantly (13). There is also increased renal retention of these smaller molecules (Fab > F(ab)'2). Single-chain antigen binding proteins (sFv) are linear constructs of light and heavy Fv fragments that clear rapidly from the blood and may have lower renal retention compared with the Fab' fragments (the lack of CH1/CL domains results in a molecule of approximately 26 kDa) (14,15). Other constructs with rapid clearance are minibodies, which consist of two sFv fragments linked by a component of the heavy-chain region (e.g., CH3), and diabodies, which comprise two sFv fragments joined chemically by disulfide bonds or by genetic engineering of the sequences (16).
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