Pathogenesis and vascular integrity of breast cancer brain metastasis

Dogma dictates that brain metastasis originate from the proliferation of extravasated tumor cells and that the blood-brain barrier (BBB) prevents the delivery of chemotherapeutic drugs to the tumors. The purpose of this study was to clarify the relationship between tumor localization and progression and the involvement of BBB function in a murine model of breast cancer brain metastasis. Green fluorescent protein expressing MDA-MB435 breast cancer cells were injected into the left ventricle of nude mice. At various time points, the entire vasculature was labeled with rhodamine-conjugated albumin. The tumors and vasculature were then imaged by laser-scanning confocal and stereo fluorescence microscopy. About 75% of the cells that reached the brain extravasated and grew perivascularly. Twenty five percent of the cells, however, proliferated within the vasculature and ultimately led to thrombosis-like infarction of the brain parenchyma. The tumorigenic "embolus" served as a sustained release source of tumor cells to downstream sites. Continuing intravascular tumor expansion led to disruption of the BBB and to overflow of cells that progressed along the vessels perivascularly to distant sites that regained protection of the BBB. Breast cancer brain metastases involve both extravascular and intravascular growth of tumor cells. These distinct pathways contribute to different pathological phenotypes that generate a heterogeneous BBB that facilitates or inhibits the delivery of chemotherapeutic drugs to the tumor.