Abstract 3610: Pan-ErbB inhibition by ARRY-334543 is superior to selective ErbB inhibition in a preclinical model that signals through multiple ErbB receptors
Authors: Blackwell Ryan , Bouhana Karyn , Winski Shannon , Lee Patrice , Winkler Jim , Walker Duncan , Garrus Jennifer 
Published: February 2011

Abstract
Activation of redundant receptor tyrosine kinases (RTKs) in tumors can confer resistance to selective RTK inhibitors. In particular, activation of multiple ErbB-family kinases is associated with diminished activity of selective EGFR or ErbB2 inhibitors. ErbB-family ligands are commonly expressed in cancers and can activate multiple redundant ErbB dimers. We hypothesize that ligand activation of multiple ErbB-family kinases in tumors can confer resistance to selective ErbB inhibitors, but not pan-ErbB inhibitors, suggesting a setting where pan-ErbB inhibitors may be advantageous over selective inhibitors. One such candidate currently in Phase 1/2 clinical trials is ARRY-334543, an oral, reversible pan-ErbB inhibitor with potent in vivo activity in both EGFR- and ErbB2-dependent human tumor xenografts.
We tested this hypothesis in vitro with N87 gastric carcinoma cells that express high levels of constitutively active ErbB2 and low levels of EGFR. In the absence of EGF, ErbB2 homodimers are predominant and cells are highly sensitive to ErbB2 inhibition. ARRY-334543 and the ErbB2-selective inhibitor, ARRY-380, strongly inhibited N87 proliferation (IC50 = 60 nM and 5 nM, respectively) while the EGFR-selective inhibitor erlotinib was active only at high concentrations (IC50 = 860 nM). No benefit was observed by combining ARRY-380 and erlotinib.
EGF-stimulation of N87 cells caused the formation of EGFR homodimers and EGFR/ErbB2 heterodimers which significantly alters cellular responses to selective ErbB inhibitors. The cellular IC50 for ARRY-380 increased 57-fold while erlotinib activity was relatively unchanged, suggesting a loss of dependence on ErbB2 signaling alone. The dual inhibition of EGFR and ErbB2, either by ARRY-334543 or the combination of ARRY-380 and erlotinib, largely maintained potency with only a modest increase in IC50 (9- to 10-fold). These data suggest that activation of EGFR can significantly diminish the activity of selective ErbB inhibitors, but not pan-ErbB inhibitors.
Dual-responsiveness was also observed in vivo where both EGFR and ErbB2 homodimers were detected in N87 xenografts and dual EGFR/ErbB2 treatment resulted in superior tumor growth inhibition (TGI). ARRY-334543 was highly active with 90% TGI (100 mg/kg BID). Treatment with the combination of ARRY-380 and erlotinib (50 mg/kg QD each, MTD when given in combination) had similar activity with 82% TGI. Treatment with either erlotinib (100 mg/kg QD) or ARRY-380 (50 mg/kg QD) alone was less efficacious.
These results suggest that the pan-ErbB inhibitor, ARRY-334543, can be differentiated from selective ErbB inhibitors by targeting tumors that signal through multiple ErbB-family receptors. A wide range of tumor types has been shown to activate multiple ErbB-family members and clinical evaluation of this hypothesis is ongoing.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3610.