Preclinical Development Single-Chain Antibody-Based Immunotoxins Targeting Her2/neu: Design Optimization and Impact of Affinity on Antitumor Efficacy and Off-Target Toxicity

Recombinant immunotoxins, consisting of single-chain variable fragments (scFv) genetically fused to polypeptide toxins, represent potentially effective candidates for cancer therapeutics. We evaluated the affinity of various anti-Her2/neu scFv fused to recombinant gelonin (rGel) and its effect on antitumor efficacy and off-target toxicity. A series of rGel-based immunotoxinswere created from the human anti-Her2/neu scFv C6.5 and various affinity mutants (designatedML3-9, MH3-B1, and B1D3) with affinities ranging from 10 8 to 10 mol/L. Against Her2/neu-overexpressing tumor cells, immunotoxinswith increasing affinity displayed improved internalization and enhanced autophagic cytotoxicity. Targeting indiceswere highest for the highest affinity B1D3/rGel construct. However, the addition of free Her2/neu extracellular domain (ECD) significantly reduced the cytotoxicity of B1D3/rGel because of immune complex formation. In contrast, ECDaddition had little impact on the lower affinity constructs in vitro. In vivo studies against establishedBT474M1xenografts showed growth suppression by all immunotoxins. Surprisingly, therapy with the B1D3-rGel induced significant liver toxicity because of immune complex formation with shed Her2/neu antigen in circulation. The MH3-B1/rGel construct with intermediate affinity showed effective tumor growth inhibition without inducing hepatotoxicity or complex formation. These findings show that while high-affinity constructs can be potent antitumor agents, they may also be associated with mistargeting through the facile formation of complexeswith soluble antigen leading to significant off-target toxicity. Constructs composed of intermediateaffinity antibodies are also potent agents that are more resistant to immune complex formation. Therefore, affinity is an exceptionally important consideration when evaluating the design and efficacy of targeted therapeutics. Mol Cancer Ther; 11(1); 143–53. 2011 AACR.