Targeted Notch1 inhibition with a Notch1 antibody, OMP‐A2G1, decreases tumor growth in two murine models of prostate cancer in association with differing patterns of DNA damage response gene expression

Abstract Notch plays a protumorigenic role in many cancers including prostate cancer (PCa). Global notch inhibition of multiple Notch family members using γ‐secretase inhibitors has shown efficacy in suppressing PCa growth in murine models. However, global Notch inhibition is associated with marked toxicity due to the widespread function of many different Notch family members in normal cell physiology. Accordingly, in the current study, we explored if specific inhibition of Notch1 would effectively inhibit PCa growth in a murine model. The androgen‐dependent VCaP and androgen‐independent DU145 cell lines were injected subcutaneously into mice. The mice were treated with either control antibody 1B7.11, anti‐Notch1 antibody (OMP‐A2G1), docetaxel or the combination of OMP‐A2G1 and docetaxel. Tumor growth was measured using calipers. At the end of the study, tumors were assessed for proliferative response, apoptotic response, Notch target gene expression, and DNA damage response (DDR) expression. OMP‐A2G1 alone inhibited tumor growth of both PCa cell lines to a greater extent than docetaxel alone. There was no additive or synergistic effect of OMP‐A2G1 and docetaxel. The primary toxicity was weight loss that was controlled with dietary supplementation. Proliferation and apoptosis were affected differentially in the two cell lines. OMP‐A2G1 increased expression of the DDR gene GADD45α in VCaP cells but downregulated GADD45α in Du145 cells. Taken together, these data show that Notch1 inhibition decreases PCa xenograft growth but does so through different mechanisms in the androgen‐dependent VCaP cell line vs the androgen‐independent DU145 cell line. These results provide a rationale for further exploration of targeted Notch inhibition for therapy of PCa.