Platelet-derived growth factor receptors: a therapeutic target in solid tumors

Platelet-derived growth factor (PDGF) was one of the first polypeptide growth factors identified that signals through a cell surface tyrosine kinase receptor (PDGF-R) to stimulate various cellular functions including growth, proliferation, and differentiation. Since then, several related genes have been identified constituting a family of ligands (primarily PDGF A and B) and their cognate receptors (PDGF-R alpha and beta). To date, PDGF expression has been shown in a number of different solid tumors, from glioblastomas to prostate carcinomas. In these various tumor types, the biologic role of PDGF signaling can vary from autocrine stimulation of cancer cell growth to more subtle paracrine interactions involving adjacent stroma and even angiogenesis. The tyrosine kinase inhibitor imatinib mesylate (formerly STI571, [Gleevec]; Novartis Pharmaceuticals Corp, East Hanover, NJ) blocks activity of the Bcr-Abl oncoprotein, and was recently approved for several indications in the treatment of chronic myeloid leukemia. Imatinib mesylate is also a potent inhibitor of the PDGF-R kinase and is currently being evaluated for the treatment of PDGF-responsive tumors such as prostate cancer. More clinical trials that investigate both established clinical endpoints of response and benefit, as well as surrogate endpoints that may describe the biologic significance of PDGF-R inhibition in vivo are needed to expand the applications that target the PDGF axis.