Detection ranges were set to eliminate crosstalk between fluorophores: 409485 nm for AMCA, 494553 nm for GFP, and 564712 nm for Cy3. == In Vitro Study == CAM expression was also analyzed on both cell lines (4T1-GFP and MDA231BR-GFP) in vitro. in vivo. Two of these ligands showed particularly high tumor cell expression (ALCAM and VLA-4), and 4-Butylresorcinol consequently their functional role in tumor seeding was determined. Antibody neutralization of either ALCAM or VLA-4 significantly reduced tumor seeding within the brain (>60% decrease in tumor number/mm2brain;P< .050.01). == 4-Butylresorcinol Conclusions == These findings suggest that ALCAM/ALCAM and VLA-4/VCAM-1 interactions play an important functional role in the early stages of metastasis seeding in the brain. Moreover, this work identifies a specific subset of ligand-receptor interactions that may yield new therapeutic and diagnostic targets for brain metastasis. Keywords:brain, cellular adhesion molecules, endothelium, metastasis, mouse, neutralizing antibody Brain metastasis is estimated to occur in 10%30% of all cancer patients and most commonly originates from one of 3 primary cancers: lung (40%50%), breast (15%25%), and melanoma (5%20%).1Importantly, the brain is the only site of tumor relapse in 60% of lung cancer patients, 25% of breast cancer patients, and 55% of melanoma patients2and is a frequent site of therapeutic failure. The process of brain metastasis depends on the success of several steps including cancer cell dissociation from the primary tumor, dissemination and survival within the circulation, adhesion and subsequent penetration of the blood brain barrier, and proliferation within the brain microenvironment. The initial stages of brain metastasis are difficult to detect in vivo, and PECAM1 this is one of the primary reasons for poor prognosis.3 Critically, our understanding of metastatic progression in the brain remains incomplete. The distinct steps of tumor cell extravasation and subsequent metastatic colonization are mediated by a variety of receptor-ligand pairs on opposing cell type; therefore, interactions of tumor cells with components of the brain microenvironment are crucial determinants in their progression towards metastasis, dormancy, or clearance.4Central to these interactions is the expression of cell adhesion molecules (CAMs), cell surface receptors, ligands, and growth factors, which can influence both progression and tumor phenotype as metastases develop.5At the same time, in vitro studies have indicated that functional activation of tumor cell surface markers may promote metastasis through a combination of altered adhesive and migratory cell functions.6 Previous work at other metastatic sites, such as lung and liver, have suggested a role for several CAMs in tumor cell adhesion and extravasation;4,7however, little is known about their role in brain metastasis. The primary reason for our poor understanding of the role of CAMs in brain metastasis is the unique nature of the brain microenvironment, which means that processes in other organs cannot simply be extrapolated to the brain. For example, vessels in bone marrow and liver are fenestrated, 8while the endothelial cells in the 4-Butylresorcinol brain exhibit special tight junctions and are therefore largely impermeable to many molecules. Moreover, the endothelial cells are closely surrounded by a double layer of basal lamina, and astrocyte end-feet closely ensheath these layers, forming the glial limitans. This blood brain barrier confers a significant and unique challenge to extravasating metastatic cells that is not present elsewhere in the body. At the same time, it has become clear that the inflammatory response of the brain is markedly different to that of systemic organs,9and thus even the CAM profile of the brain endothelium cannot be considered to follow the pattern seen in liver, lung, or bone. In vitro studies of metastatic tumor cells have provided some information on tumor cell adhesion to brain endothelial cells and subsequent transendothelial migration,10but these are limited by their nature in vitro. At the same time, in vivo studies, although scarce, have demonstrated that metastatic extravasation into the brain takes significantly longer than it does in other organs, 11supporting the concept that this process may require different mechanisms to those involved at.