e14654 Background: Metastatic colorectal cancer (mCRC) is currently considered incurable. Recent evidences support the importance of micro-environment and angiogenesis on survival and propagation of mCRC. Lenalidomide is a derivative of thalidomide, effective in several hematologic disorders, whose activity includes modulation on tumor microenvironment and inhibition of angiogenesis. We investigated the preclinical activity of lenalidomide against mCRC in vitro and in vivo. Methods: CRC cell lines and primary mCRC cells were treated with scalar concentrations of lenalidomide (10 to 150 µM). Two cohorts of NOD/SCID mice (n=8) implanted with mCRC xenografts derived from two different patients were treated orally for 21 consecutive days with Lenalidomide (50 mg/Kg/die). Results: In vivo a significant reduction of tumor growth was observed in one cohort compared to untreated controls (n=8), 247±118 mm³ vs. 567±216 mm³ (P=0.02), while no significant differences occurred in the second cohort. Pathology review on tumor samples removed from the mice revealed a trend toward the restoration of normal colon tissue morphology and architecture in lenalidomide treated samples compared to untreated controls. Quantitative evaluation of tumor angiogenesis on the responsive cohort, by immunohistochemistry for factor VIII, revealed a tumor-vessel reduction of 49% with lenalidomide compared to controls. Moreover, necrotic-hypoxic areas were reduced by 6 fold in lenalidomide treated xenografts compared to untreated controls (P<0.001). Treatment in vitro did not result in any direct antitumor effect. The average rate of cell death in treated samples was 15.5±2.1%, 15.3±6.7%, and 11.5±3.4% at lenalidomide doses of 10, 40 and 150 µM respectively, compared to 14.6±2.6% of untreated controls. Conclusions: Our data indicate a potential activity of lenalidomide against mCRC. The beneficial effect was limited in-vivo, supporting the hypothesis that the main activity of lenalidomide is exerted through modulation of tumor microenvironment and angiogenesis rather than a direct antitumor effect. Our findings may be of clinical relevance and support further investigations to explore the benefit of lenalidomide in the challenging setting of mCRC.