Abstract 2730: Anti-Her2/neu immunotoxins: Impact of monovalent and bivalent designs on in vitro and in vivo antitumor efficacy

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Immunotoxins and designed fusion constructs have significant potential for improved cancer therapy and reduced toxic side effects. We compared the efficacy of anti-Her2/neu immunotoxins containing recombinant gelonin (rGel) in either monovalent or bivalent formats. The bivalent conjugate was Herceptin/rGel chemical conjugate and the monovalent constructs were Herceptin-derived scFv (4D5) fused to rGel in two orientations: 4D5/rGel and rGel/4D5. All three immunotoxins retained the affinity and specificity of the original antibody as well as the biological activity of rGel toxin. Cytotoxicity studies against a panel of Her2/neu expressing tumor cell lines revealed significant differences among these constructs. Both Herceptin/rGel conjugate and rGel/4D5 displayed the best cytotoxicity and specificity whereas 4D5/rGel was the least potent. IC50 values ranged from 0.1-1nM for Herceptin/rGel conjugate and rGel/4D5, but were 10-1000 fold higher for 4D5/rGel. BT474 M1 HR cells with acquired resistance to Herceptin, either by constitutive Akt activation or external factor stimulation, remained sensitive to these constructs compared to the parental cells. The cytotoxicity of all constructs was generally restricted to cells expressing high levels of Her2/neu indicating that Herceptin-based immunotoxins may not be toxic to normal tissues with endogenous (low or medium) levels of Her2/neu. Treatment of nude mice bearing well-developed SKOV3 tumor xenografts with either Herceptin/rGel or rGel/4D5 resulted in impressive tumor inhibition compared to either Herceptin or 4D5/rGel. These studies demonstrate that engineered fusion constructs are competitively effective compared with chemically-generated immunotoxins with longer plasma circulation times. Further characterization of the constructs on Her2/neu overexpressing tumor cells with MDR/MRP resistance is currently ongoing. In conclusion, our studies suggested that smaller, engineered monovalent fusion constructs have excellent, comparable activity compared to intact bivalent antibody conjugates. In addition, the orientation of rGel/scFv vs scFv/rGel can significantly impact the in vitro and in vivo therapeutic activity and should be a consideration in design optimization. Research conducted, in part, by the Clayton Foundation for Research. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2730. doi:1538-7445.AM2012-2730