Abstract There is increasing evidence that T cells are able to control tumor growth and increase survival of cancer patients. However, tumor-specific T cell responses are difficult to mount and sustain in cancer patients, and are limited by numerous immune escape mechanisms of tumor cells. A promising approach in the immunotherapy of cancer is to engage T cells to target tumor cells using bispecific therapeutics targeting a tumor-associated surface antigen and CD3e on T cells. Such therapeutics elicit T cell mediated lysis of tumor cells independent of T cell specificity. Fynomers are small 7 kDa globular proteins derived from the SH3 domain of the human Fyn kinase (Fyn SH3) that can be engineered to bind with antibody-like affinity and specificity to virtually any target of choice. Fynomers can be fused to N-terminal and/or C-terminal ends of antibodies to generate multispecific therapeutics (FynomAbs) with tailored architectures. FynomAbs can be produced using standard antibody technology (GMP production yield of 3.3 g/L at 1000 L scale achieved), and show IgG-like biophysical properties and pharmacokinetic profiles. We have generated a novel bispecific FynomAb which can simultaneously bind HER2 on tumor cells and CD3 on T cells. The bispecific HER2/CD3 targeting FynomAb COVA420 potently redirected T cells to HER2 expressing tumor cells showing picomolar tumor cell lysis activity. We present for the first time that a tailored architecture of the FynomAb leads to optimal tumor cell killing properties. The activity of COVA420 was found to be highly specific, as no lysis of cells was observed in the absence of HER2 expression. In addition, COVA420 demonstrated an antibody-like pharmacokinetic profile in mice. We anticipate that the increased half-life of T cell recruiting FynomAbs compared to other bispecific formats translates into a significant benefit for patients, since it circumvents the need for continuous infusion and prolongs the intervals between successive treatments. In summary, bispecific T cell recruiting FynomAbs represent a novel platform technology to redirect T cells to tumor cells with optimal biophysical properties, long-half lives and tailored architectures. Citation Format: Ulrich Wuellner, Fabian Buller, Kristina Klupsch, Simon Brack, Irene Zbinden, Roger Santimaria, Isabella Attinger-Toller, Susann König-Friedrich, Julian Bertschinger, Dragan Grabulovski. A bispecific HER2/CD3 targeting FynomAb with excellent tumor killing and favorable pharmacokinetic properties. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 656. doi:10.1158/1538-7445.AM2014-656