Abstract P6-21-08: Synergistic anti-cancer activity of cyclin-dependent kinase 4/6 inhibitor palbociclib and dual mTOR kinase inhibitor MLN0128 in pRb-expressing triple negative breast cancer
Authors: Yamamoto T , Kanaya N , Somlo G , Chen S 
Published: May 2019

Abstract
Background
No targeted therapies have been approved for triple negative breast cancer (TNBC) thus far. Retinoblastoma protein (pRb), a major substrate of cyclin-dependent kinase (CDK) 4/6, might be a potential target especially in chemoresistant TNBC. Palbociclib is a first approved oral CDK4/6 inhibitor for treatment of patients with estrogen receptor (ER) -positive and human epidermal growth factor receptor 2 (HER2) -negative breast cancers. Nevertheless, the usefulness of CDK4/6 inhibitors has not been established in patients with TNBC although, pRb is expressed in approximately 60% of this subtype of breast cancer. In addition, pRb expression has been shown to be associated with poor prognosis after chemotherapy. This pre-clinical study investigated the combination effects of palbociclib with oral dual mTOR kinase inhibitor MLN0128 in TNBC in vitro and in vivo.
Methods
Four TNBC cell lines (MB231, MB453, MB468, and CAL148) were tested with the combination of two drugs in vitro. The combination effects on cell proliferation were investigated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and colony formation assay. Cell cycle analysis and level changes of molecules related to G1/S transition and mTOR pathway were examined. Importantly, a pRb-expressing TNBC patient-derived xenograft (PDX) model was used for confirming the combination effect in vivo.
Results
Palbociclib suppressed cell proliferation in pRb-expressing cell lines (MB231 and MB453), not pRb-deficient lines (MB468 and CAL148). The combination of palbociclib with MLN0128 showed synergistic inhibition of proliferation of MB231 and MB453 cells. Western blot analysis revealed that CDK4/6-pRb and mTOR-p70S6K pathways were inhibited by palbociclib or MLN0128 alone, but considerably more effective by the combination treatment. Cell cycle analysis showed that this combination induced G1 cell cycle arrest. The combined effect of palbociclib and MLN0128 were investigated further in vivo. In pRb-expressing TNBC PDX, the combination treatment drastically inhibited pRb phosphorylation and tumor growth compared to control or single treatment. In addition, effective reduction of PDX tumors was also demonstrated by major suppression of Ki67-positive cells by the combination treatment compared to control or single treatment.
Conclusions
In this pre-clinical study, we discovered that the combination treatment of CDK4/6 inhibitor palbociclib and dual mTOR kinase inhibitor MLN0128 had synergistic anti-cancer activity in pRb+ TNBC cell lines and a PDX model. Our results prove that such combination therapy is earnest to be further investigated in a clinical setting.
Citation Format: Yamamoto T, Kanaya N, Somlo G, Chen S. Synergistic anti-cancer activity of cyclin-dependent kinase 4/6 inhibitor palbociclib and dual mTOR kinase inhibitor MLN0128 in pRb-expressing triple negative breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-21-08.