Receptor tyrosine kinases as rational targets for prostate cancer treatment: Platelet-derived growth factor receptor and imatinib mesylate

Over the past 15 years, numerous signal transduction pathways have been elucidated whose dysregulation may play an important role in the growth and survival of cancer cells. The success of imatinib mesylate (Gleevec; Novartis Pharmaceuticals, East Hanover, NJ), a small molecule that inhibits the activation of the BCR-Abl oncogene in the treatment of chronic myelogenous leukemia, has demonstrated how effective targeted strategies can be when properly applied. With the hope of selectively targeting other critical components of cancer growth and survival while minimizing toxicity to the host, numerous strategies have been developed to inhibit receptor tyrosine kinases for various growth factors commonly expressed by cancer cells. Success of targeted inhibitors is inherently dependent on the proper selection of patients whose tumors are dependent on these growth factor pathways. Unfortunately, in prostate cancer, such selection has been a difficult-to-impossible task to date. Because of the vast number of mutational events, it is difficult to demonstrate that any particular growth factor signaling pathway is critical. In addition, because of the type (mostly bone only) and nature (usually small foci) of metastases, limited access to tumor tissue in the advanced cancer population has hampered attempts to characterize patients by their molecular features or phenotype. This article will focus on defining alternative criteria for a rational drug target and novel study designs for testing these agents in prostate cancer. In particular, the neoadjuvant setting represents a unique opportunity for new drug development in prostate cancer. An example of a neoadjuvant study testing, imatinib mesylate, is presented to display the advantages and limitations of this study design.