Dynamic transition of the blood-brain barrier in the development of non-small cell lung cancer brain metastases

Invasion of the brain by non-small cell lung cancer (NSCLC) results in a shift of the blood-brain barrier (BBB) to the insufficiently characterized blood-tumor barrier (BTB). Effective drug delivery through the BTB is one of the greatest therapeutic obstacles in treating brain metastases. Using an experimental model, we defined key changes within the BTB and the BBB in the brain around the tumor (BAT) region over time. Brain-seeking NSCLC cells were delivered into the circulation of athymic-nude mice via intracardiac injection and developing brain metastases were evaluated over six-weeks. Components of the BBB and BTB were analyzed using immunofluorescence microscopy and compared using a mixed model of regression. Our results demonstrate a dynamic time-dependent BTB phenotype. Capillaries of the BAT and BTB were dilated with increased CD31 expression compared to controls. Expression of collagen IV, a pan-basement membrane component, was significantly decreased in the BTB compared to the BBB. There was also a significant increase in the desmin-positive pericyte subpopulation in the BTB compared to the BBB. The most striking changes were identified in astrocyte water channels with a 12.18-fold (p < 0.001) decrease in aquaporin-4 in the BTB; the BAT was unchanged. Analysis of NSCLC brain metastases from patient samples similarly demonstrated dilated capillaries and loss of both collagen IV and aquaporin-4. These data provide a comprehensive analysis of the BTB in NSCLC brain metastasis. Astrocytic endfeet, pericytes, and the basement membrane are potential therapeutic targets to improve efficacy of chemotherapeutic delivery into NSCLC brain metastases.