B16 melanoma metastasis to an "artificial organ" implant.

The mouse B16 melanoma metastasizes in two stages, first to the lungs and then from lung metastases to systemic organs. Despite widespread dissemination, visible metastases generally occur only in the brain, adrenals, kidneys, ovaries, pancreas, and mesentery. As a novel approach to investigate the basis of metastatic patterning in this system, the possibility was explored that an implantable "artificial organ" could serve as a site for the occurrence and experimental modulation of secondary-stage metastasis. Each implant consisted of a cellulose disc 4 mm in diameter, with a central 1-mm polymer pellet to effect local sustained release of angiogenic or growth factors in a s.c. environment. During the secondary spread of tumors initiated with the B16 melanoma clone G3.12 and with the more metastatic variant G3.12/BM2, metastatic involvement of implants containing angiogenic factors was mainly as invisible micrometastases demonstrable by bioassay; visible metastases were rare and were located in implant blood vessels. Metastasis occurred in about 30% (G3.12) and 50% (G3.12/BM2) of implants with vasculature induced by ethylene-vinyl acetate copolymer alone. Endothelial cell growth factor and heparin promoted greater vascularization but did not significantly alter metastatic involvement of implants. Release of tumor cell mitogenic activity from pellets containing a crude extract of mouse lungs increased the incidence of G3.12/BM2 metastasis in implants to over 70% and stimulated growth of visible metastases within the cellulose matrix. In contrast, liver extract inhibited metastasis growth. Colonization of implants following intracardiac injection of G3.12/BM2 cells was generally similar to metastasis, but visible colonies formed more readily and were less dependent on the influence of lung extract. These results indicate that metastasis and colonization can occur regularly in implants and that the relative favorability of the implant environment for secondary tumor growth can be altered by incorporation of tumor cell growth modulators.