Abstract
BACKGROUND: Brain metastases (BM) occur in up to 50% of patients suffering from metastatic malignant melanoma (MM), and up to 30% of those suffering from HER2 positive and triple negative breast cancer. BM are associated with high morbidity and mortality due to limited treatment strategies. Therefore, a deeper understanding of the key factors involved in the brain metastatic cascade are needed to identify targets for preventive strategies. Platelets and the coagulation system are interesting candidates in this respect. However, the impact of thrombus formation and von Willebrand (vWF) factor fibers on the brain metastatic cascade has not been investigated so far. METHODS: Multiphoton laser scanning microscopy (MPLSM) via a chronic cranial window in mice was used to investigate the single steps of BM formation after intracardial injection of A2058 and H1 (human melanoma) and Jimt1 br (human breast cancer), stably expressing green or red fluorescent protein. I.v. Rhodamin 6G dye injection before imaging was used to visualize thrombocytes in thrombus formation and FITC labeled anti-vWF was used to visualize vWF fibers in vivo. RESULTS: It was possible to establish the first long-term imaging method that allows to study the entire brain metastatic cascade and platelet / vWF accumulation simultaneously. More than 60% of tumor cells demonstrated platelet aggregation at the site of their arrest in brain microvessels, particularly just before the time point of extravasation. The ability to successfully extravasate and grow to a micrometastasis in the brain parenchyma was increased in cells with surrounding thrombus formation, indicating that early clot formation fosters several steps of the brain metastatic cascade. Ongoing experiments investigate the impact of vWF on extravasation and micrometastasis outgrowth. Finally, we will present data of how anticoagulants (heparin) and dual platelet inhibition can prevent extravasation into the brain, and later outgrowth to a clinically relevant macrometastasis. CONCLUSION: Thrombus formation seems to facilitate extravasation of brain metastasis initiating cells and is therefore an early key factor of the brain metastatic cascade. Further experiments are on-going to investigate the potential of clot-inhibiting agents for BM prevention.