Abstract 4216: Antitumor effect of Nexrutine, a phellodendron amurense bark extract, in multiple myeloma

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Multiple myeloma (MM), a clonal malignancy of plasma cells, is the second most common adult hematological malignancy with approximately 15,000 new cases diagnosed annually. Despite recent advances in its clinical management, morbidity and mortality associated with MM remain high and there is a need for continuing development of novel anti-myeloma agents. Nexrutine, a non-toxic herbal extract from the bark of Chinese tree Phellodendron amurense, has been shown to exert significant anti-tumor effects in animal models of prostate cancer and is currently being evaluated in phase I/II clinical trials in patients with prostate cancer. In this study, we investigated the potential anti-tumor effect of nexrutine in MM. Exposure to nexrutine (3-100µg/ml) reduced viability of murine 5TGM1 and human RPMI 8226 MM cells in a dose- and time-dependent manner, independent of cell cycle disruption. Loss of cell viability was due primarily to apoptosis as indicated by >2-fold increase in annexin V+, 7AAD- cells and increased DNA fragmentation detectable by 24 hours. Previous studies showed that down-regulation of short-lived anti-apoptotic protein myeloid cell leukemia-1 (Mcl-1), in part by caspase-dependent cleavage of the full-length protein (Mcl-1L) to generate smaller (pro-apoptotic) Mcl-1 fragments, plays a major role in execution of apoptosis in MM cells. Nexrutine-induced apoptosis in MM cells was associated with reduced levels of Mcl-1L, caspase activation as indicated by a large increase in the active 17kDa cleavage product of procaspase 3 and an increase in pro-apoptotic Mcl-1 cleavage product. Nexrutine also inhibited the mammalian target of rapamycin (mTOR) and this was accompanied by accumulation of LC3-II, a marker of autophagosomes, in autophagy-competent MM cells. However, treatment of MM with cells with either pepstatin A and E64D that target lysosomal proteases involved in autophagy or with chloroquine, a lysosomotrophic autophagy inhibitor also resulted in cytotoxicity and these autophagy inhibitors did not rescue nexrutine-induced loss of viability in MM cells suggesting that autophagy plays a role constitutively in MM cell survival. Finally, we evaluated potential in vivo anti-tumor efficacy of nexrutine in the well-characterized preclinical model of MM in which 5TGM1 cells are inoculated intravenously in syngeneic naive female C57BL/KaLwRij mice. Administration of nexrutine (400µg/mouse p.o. by gavage 6 days/week for 5 weeks) to tumor-bearing mice reduced serum levels of the monoclonal IgG2bκ paraprotein, a surrogate for tumor progression and overall tumor burden. Collectively, our results indicate an anti-myeloma effect of nexrutine and support further extensive preclinical testing and early phase clinical trial of this non-toxic herbal extract in MM. Our data also indicate that a combination of nexrutine and one or more autophagy inhibitors would result in enhanced anti-tumor effect in MM. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4216. doi:10.1158/1538-7445.AM2011-4216