Abstract PD5-8: <i>HER2/PIK3CAH1047R</i> transgenic mammary tumors develop acquired resistance to triple therapy with trastuzumab, pertuzumab and PI3K inhibitors via multiple mechanisms
Authors: Ariella B Hanker, Benjamin Bulen, Monica Red Brewer, Christian D Young, Kirsten M Farrar, Rebecca S Cook, Thomas P Stricker, Carlos L Arteaga
Journal: Cancer Research (2015)

Abstract HER2 amplification and activating mutations in PIK3CA, the gene encoding the p110α subunit of PI3K, often co-occur in breast cancer. We generated a transgenic mouse model of HER2-overexpressing (HER2+), PIK3CAH1047R-mutant breast cancer. In these mice, PIK3CAH1047R accelerates HER2-mediated mammary epithelial transformation and metastatic progression, confers stem cell-like properties to HER2-overexpressing cancers and generates resistance to the combination of trastuzumab and pertuzumab (Hanker et al. PNAS 2013). HER2+/PIK3CA tumor growth was inhibited by treatment with the HER2 antibodies trastuzumab and pertuzumab in combination with the pan-PI3K inhibitor BKM120 (TPB). We sought to discover mechanisms of acquired resistance to the triple therapy by long-term treatment of established HER2+/PIK3CA tumors. We used tumor transplants derived from two HER2+/PIK3CA transgenic mice, #564 and #635. Tumor transplants from model 564 were initially growth inhibited by TPB, but did not regress. A subset of 564 transplants (3/11) resumed growth in the presence of continuous TPB therapy. All transplants (n=9) from model 635 regressed to a volume of &amp;lt;100 mm3 within 6 weeks of treatment. All tumors recurred and 2 tumors continued growth when re-treated with TPB. Resistance was maintained following passaging in mice and tumors were cross-resistant to trastuzumab/pertuzumab/BYL719, a p110α-specific inhibitor. TPB-resistant tumor 635-2 expressed p95 HER2, which was not detected in untreated tumors. In contrast, HER2 expression was significantly reduced in TPB-resistant tumor 635-3. P-AKT remained suppressed in some resistant tumors, but was restored in others. Short-term TPB treatment strongly suppressed P-S6 in sensitive tumors, whereas P-S6 was no longer inhibited in all TPB-resistant tumors from both models. We are currently performing whole-exome sequencing and RNA-sequencing on TPB-resistant vs. untreated tumors in order to identify additional mechanisms of resistance. In parallel, we established human HER2+, PIK3CA-mutant cell lines (MDA-MB 453, UACC893, and HCC1954) resistant to TPB by long-term treatment (&amp;gt;5 months) in the presence of the three drugs. Similar to the TPB-resistant tumors, P-S6 was no longer inhibited following TPB treatment in the resistant cell lines. Treatment with the TORC1/2 inhibitor MLN0128 abolished levels of P-S6 in HER2+/PIK3CAH1047R tumors. Combined treatment with MLN0128 and TPB inhibited growth of the drug-resistant tumors. Interestingly, Both TPB-resistant HER2+/PIK3CAH1047R tumor lines displayed resistance to the antibody-drug conjugate trastuzumab-DM1 (T-DM1) in vitro and in vivo, despite maintenance of HER2 overexpression. In addition, HCC1954 cells selected for resistance to TPB in culture were 66-fold less sensitive to T-DM1 than parental cells, despite maintaining equal levels of HER2 by western blot. These data suggest that multiple mechanisms may contribute to resistance to dual HER2 and PI3K blockade, including re-activation of mTOR signaling. We speculate that a similar heterogeneity of resistance mechanisms may occur in HER2+/PIK3CA-mutant metastases in patients. Citation Format: Ariella B Hanker, Benjamin Bulen, Monica Red Brewer, Christian D Young, Kirsten M Farrar, Rebecca S Cook, Thomas P Stricker, Carlos L Arteaga. HER2/PIK3CAH1047R transgenic mammary tumors develop acquired resistance to triple therapy with trastuzumab, pertuzumab and PI3K inhibitors via multiple mechanisms [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr PD5-8.