Abstract 3586: CP-868,596, a highly potent PDGFR inhibitor, inhibits phosphorylation of the imatinib-resistant PDGFRA D842V activating mutation associated with advanced GIST
Authors: Heinrich Michael , Griffith Diana , McKinley Arin , Presnell Ajia , Ramachandran Abhijit , Chaudhary Neha 
Published: June 2012

Abstract
Background: 8-10% of gastrointestinal stromal tumors (GIST) have activating mutations of the platelet-derived growth factor receptor alpha (PDGFRA) kinase. The most common PDGFRA mutation is the D842V substitution that is encoded by exon 18. This gain-of-function mutation results in auto-phosphorylation and constitutive activation of PDGFRA kinase activity. Type II receptor tyrosine kinase (RTK) inhibitors, such as imatinib and sunitinib, which only bind to the inactive conformation of the RTK, have little or no in vitro activity against this mutation. Clinically, these drugs are not effective for the treatment of GIST patients with D842V mutation. In addition, the D842V mutation can develop as a secondary imatinib resistant mutation during treatment of GISTs with primary imatinib-sensitive PDGFRA mutations (e.g. primary exon 12 mutations), or treatment of patients with hypereosinophillic syndrome with translocations involving PDGFRA.
CP-868,596 is an orally bioavailable, highly potent and selective PDGFR TKI. CP-868,596 is a benzimidazole compound that has IC50s of 0.9 nM and 1.8 nM for PDGFRA and PDGFRB, respectively. Phase I trials of CP-868,596 have shown good oral bioavailability, a favorable toxicity profile, and achievable serum concentrations as high as 2000 nM.
Methods: Mutant PDGFRA isoforms were expressed by transient transfection of Chinese Hamster ovary cells. The transfected cells were treated with various concentrations of CP-868,596 before preparation of protein lystates. PDGFRA protein was assayed for activation status (phosphorylation) by immunoprecipitation using an anti-PDGFRA antibody, followed by sequential immunoblotting for phospho PDGFRA (using antiphosphotyrosine antibody) or total PDGFRA (anti-PDGFRA monoclonal antibody). IC50 was measured by densitometry of the phospho PDGFRA bands and normalization using total PDGFRA expression (to correct for variations in loading of PDGFRA protein in the various lanes). Results: CP-868,596 inhibited the phosphorylation of wild type PDGFRA at an IC50 of 10 nM and PDGFRA D842V with an IC50 between 10 to 30 nM. Imatinib was ineffective in blocking PDGFRA D842V phosphorylation in these experiments (IC50 &gt; 1000 nM). Profiling of CP-868,596 against other GIST-relevant primary and secondary PDGFRA mutations is ongoing and will be reported.
Discussion: GISTs due to D842V activating mutations of PDGFRA gene are clinically resistant to imatinib, sunitinib, and other commercially available tyrosine kinase inhibitors. CP-868,596 blocks phosphorylation of the PDGFRA D842V mutant at clinically achievable concentrations, providing a potentially new therapeutic modality for GIST patients. A clinical trial in GIST patients with primary or secondary PDGFRA D842V mutations is being initiated.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3586. doi:10.1158/1538-7445.AM2011-3586