Abstract P6-02-01: Identification of subgroups of triple negative breast cancer cells with selective responses to mTOR, CDK, mitotic and proteasome inhibitors
Authors: Gautam Prson , Karhinen Leena , Szwajda Agnieszka , Jha Sawan Kumar , Yadav Bhagwan , Aittokallio Tero , Wennerberg Krister 
Published: May 2015

Abstract
Triple negative breast cancer (TNBC) is characterized by the lack of of estrogen, progesterone and HER2/ErbB2 receptors. It is a highly heterogeneous class of breast cancer and transcriptomics has recently been used to define 6 major subtypes of TNBC. We studied a panel of 15 TNBC cell lines using a chemical biology approach where we measure the responses to 306 approved and investigational oncology drugs. Clustering of cell lines based on their overall drug responses resulted in a strikingly different grouping compared to the gene expression derived one, highlighting that the current TNBC subtyping is not easily converted to differential sensitivities to drugs.
To further evaluate the nature of the drug responses and to differentiate between their cell growth and cytotoxic effects, we multiplexed the standard cell viability readout in the cell line screening with detection of cytotoxicity. This simple multiplexed readout identified several drug classes that previously had been assumed to be cytotoxic based on strong effects on cell viability (cell numbers) while they in fact showed no or a very heterogeneous effect on cytotoxicity. Drug classes exhibiting this type of response included mTOR inhibitors, cyclin-dependent kinase inhibitors (eg. alvociclib), mitotic inhibitors (eg. paclitaxel) as well as proteasome inhibitors (eg. bortezomib) and RNA synthesis inhibitors (eg. dactinomycin).
Further investigation of these drug classes showed that their static effects were reversible and in some cases the cells even overcame the inhibitory effect in the presence of the drug in a matter of a few days.
Given the non-toxic responses to major classes of anticancer compounds such as mTOR inhibitors, we performed combination screens with these compounds to identify other drugs with which they may synergize to promote cancer cell specific killing. Surprisingly, we instead found that mTOR inhibitors had an antagonistic effect on the activity of many other cancer drugs such as different cytotoxic and antimitotic drugs, tyrosine kinase inhibitors, HDAC inhibitors and PARP inhibitors, suggesting that combining these classes of drugs may be counterproductive also in the clinic. We also found out that accessing a cytotoxic readout allowed us to identify effective synergistic drug combination concentrations that were not seen in cell viability readouts. For example, these synergistic toxic combination responses were seen in DU4475 cells when the MEK inhibitor trametinib was combined either with the PARP inhibitor iniparib or with the broad spectrum tyrosine kinase inhibitor ponatinib.
In conclusion, multiplexed cell viability cell death readouts in drug sensitivity testing yields novel critical information on single drug and drug combination activities and liabilities. With this we were able to conclude that antimitotic, mTOR, CDK, proteasome and metabolic inhibitors have a heterogeneous cytotoxic effect across the panel of TNBC cell lines in contrast to their homogenous effect on metabolic inactivation.
Citation Format: Prson Gautam, Leena Karhinen, Agnieszka Szwajda, Sawan Kumar Jha, Bhagwan Yadav, Tero Aittokallio, Krister Wennerberg. Identification of subgroups of triple negative breast cancer cells with selective responses to mTOR, CDK, mitotic and proteasome inhibitors [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-02-01.