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Repurposing disulfiram for the treatment of Merkel cell carcinoma

      Background and aims: Merkel cell carcinoma (MCC) is an aggressive neuroendocrine skin cancer. Although two immune checkpoint inhibitors are approved for the treatment of advanced stage MCC, less than half of patients achieve durable benefit. Therefore, we sought to identify new and effective treatments for MCC. Methods: To identify novel treatments for MCC, we performed high-throughput screening of ∼4,000 small molecules on MCC cell lines. Additionally, we conducted mechanistic studies using molecular profiling, cell viability assays, and antioxidant assays. Results: Our drug screen identified the alcoholism drug disulfiram (DSF) as a promising treatment for MCC, as DSF selectively reduced MCC viability compared to control cell lines. Additionally, we determined copper increased the potency of DSF against MCC cells. DSF inhibits ALDH1A1 and ALDH2, which leads to an increase in reactive oxygen species (ROS). We found that DSF does not increase ROS in MCC cells. Moreover, the antioxidant N-acetyl-l-cysteine (NAC) did not rescue MCC viability in cells treated with DSF plus copper, suggesting that ALDH inhibition is not a relevant target for DSF in MCC. Treatment with DSF plus copper was cytostatic, induced autophagy, and caused non-apoptotic cell death in MCC cells. Interestingly, this treatment also increased the expression of immunogenic cell death markers and PD-L1 at the cell membrane. Finally, we observed that DSF plus copper synergized with the topoisomerase II inhibitor, etoposide, to reduce MCC cell viability by enhancing DNA damage as evidenced by gamma H2A.X foci in the nucleus. Conclusions: Taken together, our data suggest that DSF plus copper can be repurposed for the treatment of advanced MCC. This treatment may synergize with immune checkpoint inhibitors through the induction of immunogenic cell death in MCC tumors without inducing neutropenia. Moreover, combining DSF plus copper with etoposide could serve as a new treatment regimen for MCC in patients where immunotherapy is not appropriate. One strong advantage of this combination is that all agents can be given orally and at dosages with minimal adverse effects. Our pre-clinical findings provide rational to conduct needed clinical trials testing DSF combinations in MCC patients.