Abstract P4-14-28: XMT-1522 induces tumor regressions in pre-clinical models representing HER2-positive and HER2 low-expressing breast cancer

XMT-1522 is an anti-HER2 antibody-drug conjugate (ADC) comprised of a novel anti-HER2 antibody (HT-19) and the Dolaflexin ADC platform, which allows for conjugation of 12-15 proprietary auristatin drug payload molecules per antibody without aggregation or detrimental impact on pharmacokinetics. The HT-19 antibody binds to a novel HER2 extracellular domain epitope and does not compete for HER2 binding with trastuzumab or pertuzumab. In vitro, XMT-1522 has sub-nanomolar potency in cell lines expressing as few as 25,000 HER2 antigens per cell, and is ∼100X more potent than ado-trastuzumab emtansine (T-DM1) across a panel of 25 cell lines representing a range of tumor indications and HER2 expression levels. In the BT-474 HER2-positive breast cancer model, treatment with the HT-19 antibody alone at a single 5 mg/kg dose is inactive, while XMT-1522 ADC at a single 2 mg/kg or 5 mg/kg dose induces durable complete tumor regression, indicating that the primary mechanism of XMT-1522 is cytotoxic payload delivery, not inhibition of HER2 signaling. T-DM1 at a single 5 mg/kg dose in the same model is inactive, consistent with the significant improvement in potency of XMT-1522 compared to T-DM1. Biodistribution studies using a HER2-targeted Dolaflexin ADC in the BT-474 model demonstrate accumulation of intact ADC and released active drug payload in tumor at levels significantly higher than normal tissues. In a HER2-positive patient-derived xenograft (PDX) model, XMT-1522 induces durable complete tumor regression after a single 1 mg/kg dose, while T-DM1 at a 10 mg/kg dose achieves tumor growth delay without regression. In a HER2 1+ PDX model without HER2 gene amplification, a single 3 mg/kg dose of XMT-1522 achieves partial tumor regression where a 10 mg/kg dose of T-DM1 is inactive. Combination of XMT-1522 with trastuzumab does not block the ability of the ADC to bind HER2 or efficiently internalize, and in vivo the combination of low dose XMT-1522 with full dose trastuzumab and pertuzumab is synergistic in a HER2-driven model. The exposure achieved with XMT-1522 at well-tolerated doses in cynomolgus monkey is several fold higher than the exposure needed in mice to achieve complete tumor regressions across models representing a range of HER2 expression and tumor indications. Based on these data, XMT-1522 will soon enter clinical testing in breast cancer patients with both HER2-positive tumors and HER2 low-expressing (IHC 1+ and 2+/FISH-) tumors. Citation Format: Bergstrom DA, Bodyak N, Park PU, Yurkovetskiy A, DeVit M, Yin M, Poling L, Thomas JD, Gumerov D, Xiao D, Ter-Ovanesyan E, Qin L, Uttard A, Johnson A, Lowinger TB. XMT-1522 induces tumor regressions in pre-clinical models representing HER2-positive and HER2 low-expressing breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-14-28.