Purpose: Metastatic melanomas are generally resistant to chemotherapy and radiation, even when wild-type for p53. These tumors often grow in small nests where many of the cells have little contact with extracellular matrix (ECM). Previous work showed that M21melanomas undergo apoptosis in response to chemotherapy when cells are adherent to ECM but not in suspension. Thus, reduced integrin-dependent adhesion to ECM could mediate therapy resistance. The goal of this study was to test whether stimulation of integrin signaling could increase chemotherapeutic efficacy. Experimental Design: Colony forming assays and survival assays were used to test the responses of melanoma lines in vitro. Severe combined immunodeficient mice with subcutaneous human melanomas received chemotherapy with or without reagents that stimulate integrin signaling; tumor volume was then monitored over time. Results: Clonal growth assays confirmed that M21 cells showed reduced sensitivity to the chemotherapeutic drug 1-h-D-arabinofuranosylcytosine (araC). When five additional primary melanoma lines were screened, 80% showed higher sensitivity when adherent compared with suspended. Subcutaneous M21 tumors in vivo showed minimal ECM between tumor cells. To evaluate the importance of integrin signaling in chemoresistance in this model, mice were treated with araC, with or without the multivalent snake venom disintegrin contortrostatin or the activating anti-h1 integrin antibody TS2/16. Although araC, TS2/16, or contortrostatin alone had little effect on M21 tumor growth, combining araC with either integrin signaling reagents strongly reduced growth (P = 0001). Conclusions: Loss of integrin-mediated adhesion is rate limiting for therapeutic response in this model. Combining chemotherapy with reagents that stimulate integrin signaling may therefore provide a new approach to therapy.Melanomas are usually resistant to chemotherapy and radiation even at early stages (1). Surgery is therefore the major mode of treatment, which, if unsuccessful, leaves few options. As a consequence, 5-year survival rates for patients with unresectable or metastatic disease are <10% (2). Because melanomas often have wild-type p53 genes and lack known defects in other DNA damage and apoptosis pathways, the reasons for chemotherapy resistance are poorly understood.Integrin-mediated adhesion promotes the transmission of many signaling pathways initiated by growth factor receptors, including Erk mitogen-activated protein kinase, phosphatidylinositol-3-OH-kinase, and Rho family GTPases (3). As a result of these synergies, many cell types require integrin-mediated adhesion to extracellular matrix (ECM) for survival (4). This mechanism, however, is generally lost in metastatic cancers. Among cells that survive well in suspension, a subset exhibits a synergy between adhesion and DNA damage pathways (5, 6). These include mouse embryo fibroblasts, human fibrosarcoma, and human melanoma. In these systems, including melanomas, loss of adhesion results in decreased p53 level...