Integrin Agonists as Adjuvants in Chemotherapy for Melanoma

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 M21 melanomas 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 integrinsignaling 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 micewith subcutaneous human melanomas received chemotherapy with or without reagents that stimulate integrinsignaling; tumor volume was then monitored over time. Results: Clonal growth assays confirmed that M21 cells showed reduced sensitivity to the chemotherapeutic drug 1-β-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 integrinsignaling in chemoresistance in this model, mice were treated with araC, with or without the multivalent snake venom disintegrincontortrostatin or the activating anti-β1 integrinantibody TS2/16. Although araC, TS2/16, or contortrostatin alone had little effect on M21 tumor growth, combining araC with either integrinsignaling 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 integrinsignaling may therefore provide a new approach to therapy.