Shaping future strategies for the pharmacological control of tumor cell metastases

The eradication of established metastases in patients with malignant tumors is the single most important objective in clinical oncology. The current panel of antineoplastic agents discovered through random and semiempirical screening procedures has proven largely ineffective in treating disseminated disease and there is a clear and urgent need for more efficient antimetastatic drugs. Unfortunately, although progress has been made in examining the biology of metastatic spread, our understanding of the pharmacology, biochemistry and molecular genetics of this process is meager and insufficient to provide a rational foundation for the design of mechanism-based antineoplastic agents. Faced on the one hand with the failure of existing drugs to control metastatic spread and on the other with a dearth of alternative pharmacological approaches, the prospect of offering significantly improved therapy to the cancer patient of the 1990's is poor. The challenge of the coming decade lies in obtaining better insights into the molecular mechanisms of metastasis and using this information to identify pharmacological opportunities to curtail the proliferation of secondary tumor growths. As a first step toward this goal we need to define more rigorously what constitutes a therapeutic target in malignant disease and what steps in the pathogenesis of cancer metastasis represent the gravest risk to the patient and thus are most eligible for direct pharmacological intervention. In addressing these issues and developing future strategies for antimetastatic drugs, Paget's 100 year-old 'seed and soil' hypothesis continues to offer a useful conceptual framework for analysis of metastatic behavior. Although Paget's proposal has been validated by a century of clinical observation, efforts to define the 'seed and soil' theory in molecular terms have not been attempted. With the advent of more efficient methodologies for culturing human normal and neoplastic cells coupled with the availability of microanalytical technologies it now becomes possible to investigate and identify the complementary biochemical components of the tumor cell 'seed' and organ 'soil' that combine to encourage the proliferation of metastases. With this information the design of specific pharmacological strategies to uncouple the 'seed and soil' relationship may emerge as a potential therapeutic approach for antagonizing the growth of disseminated malignant tumors.