Background Small‐cell lung cancer (SCLC) is the deadliest form of lung cancer but lacks targeted therapies. Methods We studied the effect of the natural product mistletoe lectin (ML) in pre‐clinical models of SCLC, focusing on cell lines with amplification of the myc family oncogenes C‐myc and N‐myc. Results We found that ML treatment inhibits growth of SCLC cell lines in culture and induces apoptosis. ML treatment also decreases the expression of the amplified myc proteins. Over‐expression of either C‐myc or N‐myc results in enhanced SCLC cell sensitivity to ML. In a mouse xenograft model of SCLC, treatment with ML results in decreased tumor growth over 4 weeks with evidence of increased apoptosis in tumors from treated animals. Conclusion Overall, our results demonstrate that ML exhibits therapeutic potential in SCLC, that is at least partially dependent on myc protein expression.

c-Myc is part of the Myc family proteins associated with cellular proliferation, differentiation, and apoptosis. Dysregulated c-Myc expression is associated with tumorigenesis, and cancers with c-Myc overexpression are often associated with poor prognosis, partly due to difficulty in targeting c-Myc. Mistletoe lectin (ML), derived from the European mistletoe plant, has shown variable efficacy in cancer patients. One case series following 12 patients with advanced hepatocellular carcinoma (HCC) showed 2 patients with &gt30% reduction in the tumor marker alpha-fetoprotein and 1 patient who had stable disease for 9 months. These cases indicate promising results. ML consists of a heteroprotein complex of an A- and B-chain connected by a disulfide bond. The A-chain was previously described to inhibit translation by 28s rRNA cleavage, while the B-chain binds cell surface glycoproteins to trigger internalization of the lectin. We found that purified mistletoe extract, consisting primarily of ML, downregulates c-Myc protein and induces apoptosis in in vitro and in vivo models of human-derived cancer, and that the efficacy of ML treatment is strongly correlated with MYC gene expression and c-Myc protein abundance in in vitro human models of HCC, ovarian cancer, small cell lung cancer, and lymphoma. MTS assay data demonstrated that exogenous MYC expression can sensitize low-MYC cell lines to ML treatment, suggesting that anti-cancer effects of ML are dependent on its ability to downregulate c-Myc. While ML may be able to downregulate many proteins via general translational inhibition, we found that of 296 proteins profiled by RPPA, c-Myc was one of 11 proteins decreased by ML with at least a 2-fold change (p&lt0.01). We also found that ML promotes c-Myc ubiquitylation, suggesting that ML regulates c-Myc protein stability, rather than synthesis. In summary, here we describe a novel mechanism of action for ML in downregulating c-Myc and promoting apoptotic cell death in various models of MYC-driven cancer, and provide evidence for MYC gene expression as a predictive marker for response. This research supports the development of ML-based therapies that may provide a c-Myc-targeting treatment in multiple cancer types. Citation Format: Gabrielle Kennelley, Eric Yuan, Mohammad Shatat, Betsy Gauthier, Peiying Yang, Analisa Difeo, Richard Lee. Mistletoe lectin induces apoptosis in MYC-driven cancers through post-translational downregulation of c-MYC protein [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1831.