Expression of the Platelet-Derived Growth Factor Receptor in Prostate Cancer and Treatment Implications with Tyrosine Kinase Inhibitors

The platelet-derived growth factor receptor (PDGFR) is a receptor tyrosine kinase overexpressed in a subset of solid tumors and therefore is the target of drugs inhibiting this function such as imatinib mesylate (Gleevec). Thus far, drug therapy has played a limited role in the treatment of localized prostate cancer (PCa). This study characterizes PDGFR-beta expression in a wide spectrum of PCa samples to provide empirical data as part of a rational treatment strategy. A survey of five published prostate expression array studies, including 100 clinically localized PCa, did not identify tumors with increased PDGFR-beta expression level. Protein expression of PDGFR-beta, as determined by immunohistochemistry, revealed 5% of clinically localized PCa and 16% of metastatic PCa cases to show moderate or strong expression. To develop a strategy to detect patients most likely to profit from Gleevec treatment, we analyzed cDNA expression array data from 10,000 transcripts for PDGFR-beta expression and divided tumors in groups based on PDGFR-beta expression level. Performing a supervised analysis to identify potential comarkers of PDGFR-beta in PCa, we identified a set of genes whose expression was associated with PDGFR-beta status including early growth response 1 (Egr1), an upstream effector of PDGF (4.2-fold upregulation), alpha-methylacyl-CoA racemase, as well as v-Maf and neuroblastoma suppressor of tumorigenicity (both with a 2.2-fold downregulation). Taken together, this study suggests that only a small subset of PCas may be amenable to tyrosine kinase inhibitors specific for PDGFR.