Abstract P074: STAT3 pathway in palbociclib resistance in breast cancer cell lines and the role of inhibitors such as tocilizumab and silymarin
Authors: Doello Kevin , Mesas Cristina , Amecua Víctor , Fuel Marco , Cabeza Laura , Ortiz Raúl 
Published: December 2021

Abstract
Introduction Resistance to cyclin inhibitors such as palbociclib is a challenge in the treatment of hormone-dependent metastatic breast cancer. Likewise, triple negative tumors are resistant to these inhibitors. The role of the present research is to identify the role that the transcription factor STAT3 (antiapoptotic transcription factor) and the inhibitors of the STAT3 pathway, such as tocilizumab (interleukin 6 receptor inhibitor) and silymarin (extract of milk thistle containing silibinin, a STAT3 inhibitor), could have in this situation. Methods Breast cancer cell lines MCF-7 (positive for estrogen receptors) and MDA-MB-231 (triple negative) were cultured in DMEM + 10% FBS for 72 hours with different concentrations of palbociclib, tocilizumab and silymarin in order to obtain cell growth inhibition values ​​(IC50) in each compound and combinations of palbociclib with tocilizumab or silymarin. In these cases, the combination indexes (CI &lt; 1 (synergy) and CI &gt; 1 (antagonism)) were calculated using CompuSyn software. Likewise, an RNA extraction was carried out from the untreated cell lines to determine the basal expression levels of STAT3 by RT-qPCR. Results The IC50 of palbociclib in MCF-7 was 3.14 µM and in MDA-MB-231, 29.69 µM. The results of RT-qPCR for the basal expression of STAT3 revealed its absence of expression in MCF-7 and the presence of expression in MDA-MB-231, which could be related to the greater resistance to palbociclib of this last cell line. Tocilizumab did not present cytotoxicity per se and silymarin did not reach IC50 value in the cell lines studied (both compounds in doses up to 100 µM). Regarding the combinations of silymarin (50 µM and 75 µM) and palbociclib (0.1 µM and 1 µM), a synergistic effect (CI &lt; 1) was detected in all the combinations studied in MCF-7 and in most of the same in MDA-MB-231 (except 75 µM silymarin and 0.1 µM palbociclib). However, in the case of the combination of tocilizumab (10 µM and 50 µM) and palbociclib (0.1 µM, 1 µM and 5 µM in MCF-7, and 5 µM, 10 µM and 25 µM in MDA-MB-231) synergy was only obtained in half of the combinations studied in both cell lines. Conclusion STAT3 factor could be related to resistance to cyclins in hormone-dependent breast cancer and to the lower sensitivity to these drugs observed in triple-negative breast cancer. The synergistic combination of palbociclib with silymarin, a STAT3 inhibitor, in both the hormone-sensitive and triple negative cell lines indicates that silymarin could help to enhance the effect of palbociclib in breast cancer and, even, help to reverse the resistance to this drug both in the hormone dependent and triple negative, although further studies would be needed to confirm this fact.
Citation Format: Kevin Doello, Cristina Mesas, Víctor Amecua, Marco Fuel, Laura Cabeza, Raúl Ortiz. STAT3 pathway in palbociclib resistance in breast cancer cell lines and the role of inhibitors such as tocilizumab and silymarin [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P074.