Abstract 5803: Homoharringtonine, a natural protein synthesis inhibitor, inhibits growth of triple negative breast cancer in vitro and in vivo
Authors: Yakhni Mohamad , Briat Arnaud , Miot-Noirault Elisabeth , Cachin Florent , Penault-Llorca Frederique , Radosevic-Robin Nina 
Published: August 2018

Abstract
Introduction: Triple negative breast cancer (TNBC) is a heterogeneous disease for which new therapeutic approaches are urgently needed. Homoharringtonine (HHT) is a protein synthesis inhibitor clinically approved in chronic myeloid leukemia. Its multi-factor mechanism of action includes downregulation of short-lived cellular proteins like Mcl-1. As increased protein synthesis lies behind several hallmarks of cancer present in TNBC, we investigated the effect of HHT on TNBC in vitro and in vivo.
Methods: TNBC cell lines CAL-51, MDA-MB-231 and MDA-MB-468, representative of aggressive, BRCA1 non-mutated TNBC (Lehmann, J Clin Invest 2011) were used. HHT was provided by LeukePharma, Houston, TX, USA. Cell viability was assessed by sulphorhodamine B assay. Number of cells in different cell cycle phases or in apoptosis was determined by flow cytometry. Protein expression was evaluated by western blot. For in vivo experiments, xenografts of MDA-MB-231 were established in Swiss nu/nu mice, treated for 7 days by subcutaneous HHT (1 mg/kg, bi-daily), and analyzed on day 10. Evaluation of HHT on MDA-MB-468 and CAL-51 xenografts is ongoing.
Results: Already after 24h of exposure to 100 ng/ml HHT (mean plasma HHT concentration after 5 mg/m2/day in humans (Sentenac S et al, Blood 2003)), viability of CAL-51 and MDA-MB-468 cells was strongly reduced (10.4±1.0% and 19.2±2.2% of control, respectively). This inhibition level was obtained in MDA-MB-231 cells only after 72h (18.7±4.8%). After 48h in 100 ng/ml HHT, MDA-MB-231 and -468 cells accumulated in the S phase of the cell cycle, whereas the CAL-51 accumulated in the G1/0 phase. This was associated with an increase of apoptosis in CAL-51 (42.0±2.4% vs 9.2±1.3%, untreated vs treated cells, respectively) and in MDA-MB-468 (24.2±2.8% vs 12.2±3.6%) but not in MDA-MB-231 (9.0±2.2% vs 3.6±1.7%). Concordantly, 100 ng/ml of HHT induced, after only 2h, a marked decrease of the level of anti-apoptotic proteins Mcl-1, Bcl-2, survivin and XIAP in CAL-51 and MDA-MB-468 cells, which preceded by 2h the reduction of caspase-3 level. No change in expression of those proteins was observed in MDA-MB-231 during all 48h of exposure to HHT. On the other side, the quantity of phosphorylated S6 increased in MDA-MB-231 but decreased in CAL-51 and MDA-MB-468 already after 24h of incubation with 100 ng/ml HHT. In vivo, HHT reduced growth of MDA-MB-231 xenografts for 36.5% (tumor volume on day 10: 230.4±27.7% mm3 vs 146.2±15.5% mm3, untreated vs treated, respectively).
Conclusion: HHT exerts cytotoxic action on TNBC cell lines CAL-51 and MDA-MB-468 whereas its effect on MDA-MB-231 is dominantly cytostatic. Because of its capacity to rapidly reduce the levels of anti-apoptotic proteins, HHT is worth further studies as potential sensitizer of TNBC to chemo- and/or targeted therapy.
Citation Format: Mohamad Yakhni, Arnaud Briat, Elisabeth Miot-Noirault, Florent Cachin, Frederique Penault-Llorca, Nina Radosevic-Robin. Homoharringtonine, a natural protein synthesis inhibitor, inhibits growth of triple negative breast cancer in vitro and in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5803.