Cancer is the second leading cause of death in the United States, exceeded only by heart disease. (Cancer Facts and Figures 2004, American Cancer Society, Inc.). Despite recent advances in cancer diagnosis and treatment, surgery and radiotherapy may be curative if a cancer is found early, but current drug therapies for metastatic disease are mostly palliative and seldom offer a long-term cure. Even with new chemotherapies entering the market, the need continues for new drugs effective in monotherapy or in combination with existing agents as first line therapy, and as second and third line therapies in treatment of resistant tumors.
Cancer cells are by definition heterogeneous. For example, within a single tissue or cell type, multiple mutational “mechanisms” may lead to the development of cancer. As such, heterogeneity frequently exists between cancer cells taken from tumors of the same tissue and same type that have originated in different individuals. Frequently observed mutational “mechanisms” associated with some cancers may differ between one tissue type and another (e.g., frequently observed mutational “mechanisms” leading to colon cancer may differ from frequently observed “mechanisms” leading to leukemia). It is therefore often difficult to predict whether a particular cancer will respond to a particular chemotherapeutic agent (Cancer Medicine, 5th edition, Bast et al., B. C. Decker Inc., Hamilton, Ontario).
Components of cellular signal transduction pathways that regulate the growth and differentiation of normal cells can, when dysregulated, lead to the development of cellular proliferative disorders and cancer. Mutations in cellular signaling proteins may cause such proteins to become expressed or activated at inappropriate levels or at inappropriate times during the cell cycle, which in turn may lead to uncontrolled cellular growth or changes in cell-cell attachment properties. For example, dysregulation of receptor tyrosine kinases by mutation, gene rearrangement, gene amplification, and overexpression of both receptor and ligand has been implicated in the development and progression of human cancers.