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Tumor growth and angiogenesis are interdependent. Cyclooxygenase (COX) catalyzes the synthesis of prostaglandins from arachidonic acid. Nonsteroidal antiinflammatory drugs (NSAIDs) inhibit COX-mediated synthesis of prostaglandins. COX-1 is constitutively expressed in a wide range of tissues, whereas COX-2 is cytokine inducible. Enhanced COX-2 expression has been attributed a key role in the development of inflammation and related processes observed in pathologically altered disease states. Two specific COX-2 inhibitors, namely rofecoxib (Vioxx) and celecoxib (Celebrex), both oral agents and U.S. Food and Drug Administration approved, have been shown preclinically and clinically to have efficacy comparable to that of NSAIDs for relief of pain and inflammation in osteoarthritis, with decreased risk of gastrointestinal damage. Little is known about how angiogenesis is affected by the combination of rofecoxib and radiation. We have evaluated the combination of rofecoxib, at various concentrations, and radiation on cytokine-induced angiogenesis in vitro. We have found that rofecoxib inhibited endothelial cell proliferation, migration, and tube formation (differentiation) at clinically relevant doses. In combination with radiation, inhibition of endothelial cell function further increased twofold. The combination of rofecoxib and radiation suggests a complementary strategy with clinical ramifications to target angiogenesis- dependent malignancies.