Abstract 935: Inhibition of pancreatic cancer growth via MEK-targeted therapy is significantly enhanced by concomitant inhibition of EGFR and Her2.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Introduction: MAPK and EGFR family signaling promote tumor proliferation in pancreatic ductal adenocarcinoma (PDAC). Previously we have shown that lapatinib (EGFR/Her2 inhibitor) significantly improved the ability of trametinib (MEK1/2 inhibitor) to inhibit PDAC tumor growth. Here, we investigated whether combined panitumumab (EGFR inhibitor), trastuzumab (Her2 inhibitor) and trametinib treatment would more effectively inhibit tumor growth than trametinib monotherapy in a patient-derived (PD), orthotopic, xenograft model of PDAC. Methods: In vitro proliferation assays were performed with PD-PDAC cell line MAD09-366 exposed to trametinib, panitumumab, trastuzumab, and combination therapies. Western blot analysis was performed on treated cell lysates. Athymic, nude mice were orthotopically implanted with 3 different PD-PDAC xenografts (MAD09-366, 08-608, and 08-738). Established murine tumors were treated with control, trametinib (0.3mg/kg,PO, qDay), panitumumab (500ug, IP, BIW), trastuzumab (200ug, IP, BIW) or combination for 2-4 weeks. Interval volumetric MRI was used to measure tumor growth. Phospho-RTK and MAPK arrays were used to assess tumor molecular response. Results: In vitro studies demonstrated improved growth inhibition of MAD09-366 (Kras mut.) cells exposed to combination therapy with all 3 inhibitors relative to control or each inhibitor alone. Western blot analysis revealed that EGF stimulation increased Ras pathway signaling in this Kras mutant cell line. Under conditions of EGF stimulation, the greatest inhibition of Ras pathway signaling was seen in cells exposed to all 3 inhibitors. In vivo studies in all PD-PDAC xenografts revealed that triple inhibitor therapy significantly decreased the rate of tumor growth relative to control, trametinib alone, panitumumab alone, or panitumumab plus trastuzumab. In two of three PD-PDACs assessed, triple therapy was superior to trametinib plus panitumumab. The greatest response was seen in MAD08-738 (Kras wt) triple-therapy-treated mice whose tumor size decreased by 9.3%. Phospho-array analysis demonstrated increased activity of Akt and its downstream effectors in trametinib treated tumors. Relative Akt phosphorylation returned to levels seen in control treated tumors with the addition of panitumumab and trastuzumab under triple therapy. Conclusions: Combination therapy with trametinib, panitumumab, and trastuzumab produced the most complete in vitro Ras signaling blockade and most effective in vivo growth inhibition. Phospho-array data suggests that a compensatory increase in pro-survival PI3K/Akt signaling in response to Ras pathway blockade alone is mitigated by the concomitant inhibition of EGFR/Her2 signaling. This combination treatment strategy should be considered for a future clinical trial in pancreatic cancer patients. Citation Format: James M. Lindberg, Timothy E. Newhook, Sara J. Adair, Clifford A. Cutchins, Alison Kim, J. Thomas Parsons, Todd W. Bauer. Inhibition of pancreatic cancer growth via MEK-targeted therapy is significantly enhanced by concomitant inhibition of EGFR and Her2. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 935. doi:10.1158/1538-7445.AM2013-935