MULTISTEP NATURE OF METASTATIC INEFFICIENCY - DORMANCY OF SOLITARY CELLS AFTER SUCCESSFUL EXTRAVASATION AND LIMITED SURVIVAL OF EARLY MICROMETASTASES

In cancer metastasis, only a small percentage of cells released from a primary tumor successfully form distant lesions, but it is uncertain at which steps in the process cells are lost. Our goal was to determin e what proportions of B16F1 melanoma cells injected intraportally to t arget mouse liver 1) survive and extravasate, 2) form micrometastases (4 to 16 cells) by day 3, 3) develop into macroscopic turners by day 1 3, and 4) remain as solitary dormant cells. Using in vivo videomicrosc opy, a novel cell accounting assay, and immunohistochemical markers fo r proliferation (Ki-67) and apoptosis (TUNEL), we found that 1) 80% of injected cells survived in the liver microcirculation and extravasate d by day 3, 2) only a small subset of extravasated cells began to grow , with 1 in 40 forming micrometastases by day 3, 3) only a small subse t of micrometastases continued to grow, with 1 in 100 progressing to f orm macroscopic tumors by day 13 tin fact, most micrometastases disapp eared), and 4) 36% of injected cells remained by day 13 as solitary ca ncer cells, most of which were dormant (proliferation, 2%; apoptosis, 3%; in contrast to cells within macroscopic tumors: proliferation, 91% ; apoptosis/necrosis, 6%). Thus, in this model, metastatic inefficienc y is principally determined by two distinct aspects of cell growth aft er extravasation: failure of solitary cells to initiate growth and fai lure of early micrometastases to continue growth into macroscopic tumo rs.