Relationships among tumor burden, tumor size, and the changing concentrations of fibrin degradation products and fibrinolytic factors in the pleural effusions of rabbits with VX2 lung tumors

The VX2 tumor is derived from a papilloma virus-induced rabbit epithelial cell line. If VX2 tumor cells (trapped in a plasma clot) are introduced intravenously into NZW rabbits, the cells lodge in the lung capillary bed and produce tumors. Independently of the tumor burden (ie, the total tumor weight per rabbit), approximately 15% of rabbits with VX2 lung tumors accumulate an effusion in the interpleural space and this pleural effusion contains products of hemostasis. We hypothesized that these products were of intra-tumoral origin and that they changed in concentration as tumor burden increased. Interrelationships among lung-, tumor-weights, and pleural effusion volumes, and the concentrations of fibrinolytic factors, their catabolic products, and other proteins of pleural effusions were measured in rabbits with a wide range of tumor burdens. Positive correlations between tumor burden and total lung weight and between pleural effusion volume and net lung weight suggested that interstitial fluid from the stroma of tumors passed directly into the extravascular space of the lung(s) and into the interpleural space(s). Analyses of pleural effusions indicated that plasminogen-, alpha(2)-antiplasmin-, and plasminogen activator inhibitor-1-related proteins, urokinase-like- and tissue-plasminogen activator activities, and vascular endothelial growth factor increased in concentration up to a tumor burden of approximately 20-25 g. Plasmin activity and intact fibrinogen were absent. The concentration of fibrin(ogen) degradation products did not change significantly up to a tumor burden of approximately 25 g but increased substantially as tumor burdens exceeded 25 g. In conclusion, interstitial fluid from tumors enters the extravascular space of the host and may accumulate with fluid from non-tumor sources as a pleural effusion. The concentrations of fibrinolytic factors and their products in pleural effusions reflect the tumor burden of the rabbit. Conceivably, the components of a malignant effusion contain much information about the extent of tumor growth.