4544 Background: Src and other Src-family kinases (SFKs) promote prostate cancer (PC) cell growth, survival, invasion, migration and the transition from castration-sensitive to castration-resistant PC (CRPC). Dasatinib, an oral inhibitor of SFKs and other kinases, is active in preclinical CRPC models and in a subset (∼20%) of CRPC patients. These results support the use of dasatinib as a targeted therapy for CRPC, but highlight an unmet need to qualify predictive biomarkers of sensitivity to the drug to guide treatment selection. Methods: Anovel statistical approach for determining characteristically expressed genes (biomarkers) and optimal clustering of discretized expression profiles was applied to a published gene expression dataset of PC cell lines with variable sensitivity to dasatinib (GSE9633) and to a panel of 150 banked primary and metastatic PC specimens. Results: We identified 68 unique transcripts, each predicting in vitro sensitivity or resistance to dasatinib with 100% accuracy. Many of the transcripts identified regulate transcription, TGFβ, Myc, Akt/PTEN and hormone-independent signalling, and have established roles in CRPC pathophysiology, supporting the biological relevance of the signature. Moreover, 56/131 (43%) primary and 5/19 (26%) metastatic tumor specimens were predicted to be sensitive to dasatinib. Applying this PC-derived signature to a panel of breast cancer cell lines predicted in vitro sensitivity to the drug with 78% accuracy, suggesting the presence of common biological pathways. Conclusions: The predicted frequency of sensitivity to dasatinib in metastatic PC specimens was comparable to the PSA decline rate in phase II trials enrolling unselected CRPC patients. We are currently quantifying the expression of these transcripts by multiplex qRT-PCR in circulating tumor cells harvested from CRPC patients that will be enrolled in a clinical trial to generate evidence to qualify the signature as a noninvasive predictive biomarker of sensitivity to dasatinib in patients with CRPC. No significant financial relationships to disclose.