Efficacy and pharmacodynamic effects of bosutinib (SKI-606), a small molecule inhibitor of Src/Abl, in human pancreatic tumor xenografts

A250 Purpose: Recently,Src tyrosine kinase has emerged as an attractive target for antitumortherapy and several small molecule inhibitors are currently under intensive investigation. Src overexpression (70%) and increased activity have been reportedin pancreatic adenocarcinomas. The purpose of the study was to investigate the in vivo efficacy and pharmacodynamic effects of bosutinib (SKI-606), a Src/Abl inhibitor, using a panel of human pancreatic tumor xenografts. Experimental Procedure: Surgical non-diagnostic pancreatic tumors operatively removed from patients at the Johns Hopkins Hospital were subcutaneously implanted to separate pathogen-free female nude mice for each patient and kept as a live PancXenoBank according to an IRB-approved protocol. Fifteen patient tumors in the PancXenoBank were further expanded and cohort tumors were allowed to grow until reaching ~200 mm3, at which time mice were randomized to two groups with 5-6 mice (10 evaluable tumors) in 1) Control and 2) bosutinib. Bosutinib was administered at 100 mg/kg/day (PO) for 28 days. Tumor size was measured twice per week and relative tumor growth index was calculated by relative tumor growth of treated mice divided by relative tumor growth of control mice since the initiation of therapy (T/C). Frozen tumors at the time of harvest were used for western blot analysis of Src kinase pathway proteins and apoptotic markers. Western blots were quantitated by densitometry,and the results are shown as percentage of control. Formalin-fixed, paraffin-embedded tissues for tissue microarray (TMA) were constructed for immunohistochemistry (IHC). Global gene expression profile with gene set enrichment analysis and Affymetrix U133 Plus 2.0 gene arrays are in progress to interrogate relative pathway relevance. Summary: Three patient tumors (Panc 281, 247 and 294) out of fifteen were found to be sensitive to bosutinib, defined as tumor growth 80%) and the remainder were intermediate (tumor growth 40-80%) to bosutinib. There was no definite difference between sensitive, intermediate, and resistant tumors in the baseline Src kinase pathway protein expression among patient tumors selected for the study. In vivo administration of bosutinib resulted in the significant inhibition of Src (46%) coupled with reduced tumor growth (32%), and increased apoptosis as evidenced by the elevated expression of cleaved PARP. Phosphorylation of two key signaling molecules downstream of Src and associated with oncogenesis, signal transducers and activators of transcription (STAT-3 and STAT-5), were significantly inhibited by bosutinib. In addition, bosutinib downregulated the levels of Erk 44/42, EGFR and phospho Akt. There were no changes in protein expression in the resistant cases. Conclusion: Baseline Src expression was not predictive of growth inhibition in this limited set of human tumors. Administration of bosutinib downregulated some members of Src signaling pathways. The data obtained in the present study support a potential role for Src kinaseinhibitors in the treatment of pancreatic cancer.