Abstract P4-12-30: A bivalent Her2 targeting DARPin with high efficacy against Her2-low and Her2-positive tumors
Authors: Dolado I , Fiedler U , Strobel H , Metz C , Stumpp MT , Rojkjaer L 
Published: August 2014

Abstract
The approval of Herceptin, Perjeta and Kadcyla has dramatically improved the outcome of the 15% of breast cancer patients with high expression of Her2 (Her2-positive, IHC3+). In contrast, up to 50% of women with breast cancer express low levels of Her2 (Her2-low, IHC+2 or +1) at diagnosis, and these patients do not derive benefit from approved Her2 targeted therapies. It is estimated that 5-10% of Her2-positive patients will also develop Her2-low metastases refractory to treatment. Therefore, new therapeutic strategies are needed to alleviate the high tumor burden of Her2-low patients.
We generated a bivalent Designed Ankyrin Repeat Protein (DARPin) containing two anti-Her2 moieties that target unique non-overlapping epitopes on Her2 (biparatopic). The biparatopic DARPin exerts a strong pro-apoptotic response on a panel of breast and gastric cancer cell lines, which cannot be fully recapitulated by treatment with Herceptin, Perjeta or both combined. DARPin treatment of Her2-positive breast cancer BT474 cells induces apoptosis (IC50 &lt; 1nM) in virtually all cells within 24 hours, whereas single or combined treatment with Herceptin and/or Perjeta reduces proliferation by 50% (IC50 = 2-4nM). The biparatopic DARPin inhibits as well the viability (via apoptosis) of Her2-low MDAMB175 cells with a 100-fold higher potency than Herceptin. Mechanistically, treatment with the anti-Her2 biparatopic DARPin inhibits both Her2 and Her3 signaling, which in turn results in the induction of apoptosis. Interestingly, cell treatment with the mix of the two individual anti-Her2 DARPin moieties does not recapitulate the apoptotic effect of the biparatopic molecule, indicating that both moieties must be linked for maximum efficacy. In vivo, the biparatopic DARPin shows efficient anti-tumor activity in established breast cancer Her2-positive xenografts and Her2-low PDX tumor models. DARPin treatment of Her2-positive BT474 xenograft in BALB/c mice results in rapid tumor regression (50% tumor shrinkage after one dose) that is sustained at least during one month after the last dose. The biparatopic DARPin strongly inhibits tumor growth as well in a Her2-low breast cancer PDX in NMRI mice, where it demonstrates superior efficacy compared to Herceptin (DARPin/Herceptin tumor volume ratio &lt; 30%).
In summary, biparatopic targeting of Her2 enables inhibition of both Her2 and Her3, and blocks a tumor driver and its resistance mechanism. As a result, the DARPin interferes with both the proliferation and survival of tumor cells and triggers durable anti-tumor responses in vivo. DARPin treatment of Her2 positive tumors in mice show equivalent efficacy to Herceptin with faster kinetics; an effect that is magnified in Her2-low PDX tumors where the DARPin provides superior tumor control to Herceptin. Altogether, our data demonstrate the potential for the biparatopic DARPin to surpass some of the limitations of approved Her2 targeted agents, and warrants clinical investigation.
Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-12-30.