Abstract #3239: Enhanced in vitro and in vivo activity of trastuzumab-DM1 antibody-drug conjugate combined with GDC-0941, a small molecule inhibitor of PI3 kinase

Molecularly targeted therapeutics continue to offer great promise in the treatment of human cancers. The receptor tyrosine kinase, HER2/ErbB2, is a validated clinical target for HER2-amplified breast cancer, as evidenced by the U.S.F.D.A approval of the humanized HER2 antibody trastuzumab (Herceptin®) and the dual HER2/EGFR small molecule tyrosine kinase inhibitor lapatinib (Tykerb®). Although many patients respond to trastuzumab, innate resistance or acquired resistance upon prolonged treatment also occur. Mechanisms of resistance to trastuzumab include upregulation of compensatory signaling pathways through activation of receptor tyrosine kinases such as EGFR, IGF-1R and met, or through mutational activation of the PI3 kinase (PI3K) pathway. An alternative approach for targeting HER2 is the direct covalent coupling of a cytotoxic drug to trastuzumab. We have previously reported the potent in vitro and in vivo efficacy of trastuzumab linked to the microtubule polymerization inhibitor mertansine (trastuzumab-DM1) in trastuzumab-sensitive and -refractory breast tumor models. Inhibition of signaling through PI3K, which is hyperactivated in HER2-amplified breast cancer due to constitutive activity of overexpressed HER2 and/or through mutation of the p110-\#945; subunit of PI3K, offers an additional therapeutic approach. We describe here the enhanced anti-tumor activity of trastuzumab-DM1 combined with the PI3K inhibitor GDC-0941, in HER2-amplified breast cancer lines with mutated PI3K: BT474 (K111N), MDA-361.1 (E545K), and KPL4 (H1047R). Combination treatment in vitro resulted in additive or synergistic inhibition of cell proliferation, as well as increased apoptosis. Similarly, in vivo efficacy was augmented with combined drug treatment compared to single agent activityin the MDA-MB-361.1 and Fo5 HER2-amplified xenograft models. Biochemical analyses of biomarkers for each agent showed inhibition of phospho-Akt and phospho-ERK by both trastuzumab-DM1 and GDC-0941, decreased phosphorylation of Rb and PRAS40 by GDC-0941, and increased levels of the mitotic markers phospho-histone H3 and cyclin B1 after treatment with trastuzumab-DM1. In addition, trastuzumab-DM1 treatment resulted in apoptosis in these breast cancer models as determined by appearance of the 23 kDa PARP-cleavage fragment, decreased levels of Bcl-XL, as well as activation of capases 3 and 7. Addition of GDC-0941 to trastuzumab-DM1 further enhanced apoptosis induction. These studies provide evidence for the use of rational drug combinations in HER2-amplified breast cancer and offer additional therapeutic approaches for patients whose disease progresses on trastuzumab or lapatinib-based therapy. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3239.