The biology of cancer metastasis.

The formation of a metastasis entails a complex sequence of events with the end result dependent on the interaction of malignant cells with host factors. Intrinsic properties of the metastatic tumor cells, including production of proteolytic enzymes, cell surface properties, adhesiveness, and the ability to grow in a distant organ environment, act in concert to influence the tumor cells' interactions with host cells in forming metastases. Life-threatening metastases are formed only by those tumor cells that have survived all steps in a process that has been shown in many experimental studies to be a highly selective event. The results of studies on the distribution of radiolabelled mouse melanoma cells injected into syngeneic mice support the concept that the fate of tumor cells released into the bloodstream is determined by sequential and selective events, and introduces a third regulatory factor. Cells endowed with metastatic properties, isolated by cloning a heterogeneous tumor or selected from a metastasis, have a higher probability of forming metastases than cells not so endowed, yet this probability is not 100%. Metastasis should thus be considered as a selective, sequential and stochastic process. Interruption of the process at any stage will prevent the formation of metastatic disease. Hence, a better understanding of the metastatic process will provide the basis for rational approaches for the prevention or destruction of this most fatal aspect of cancer.