Abstract 2561: Contextual combination of PARP inhibitors with p110beta inhibitors: Functional logistics to tame PTEN null tumors
Authors: Jennifer H. Carlson, Yuliang Sun, Xiaoqian Lin, Pradip De, Brian Leyland-Jones, Nandini Dey
Journal: Cancer Research (2015)

Abstract Background: Hyper-activation of the PI3K pathway due to a functional loss of PTEN is a characteristic feature of many tumors including TNBC and GBM. Although p110b rarely undergoes activating mutations in these cancers, loss of PTEN function makes these tumor cells dependent specifically on p110b. In a mechanism-based study we have recently established an integral role of the PI3K pathway in DNA damage repair (DDR)-mediated anti-tumor efficacy of PARP inhibitor wherein we have demonstrated that PTEN-nullness confers in vivo sensitivity to the combination as compared to tumors carrying PTEN WT (De et al., Neoplasia, 2014). Thus here we hypothesize that PTEN-null tumors will be more susceptible to a combination of p110b inhibitor (p110b i) plus PARP inhibitor (PARP i). Method: To test our hypothesis we used PTEN-null TNBC and GBM models. Isogenic cell lines as well as SiRNA-directed PTEN-knockdown cells were tested for the (a) IC50 (MTT) (b) real-time confluence assay (Incucyte) (c) cell cycle (flow cytometry) (d) clonogenecity (ON-TOP assay) (e) cytosolic proliferation and apoptotic signals and (f) nuclear HRD signals for DDR using 3 p110b i (TGX221, AZD6482, GSK2636771) and 5 PARPi (BMN673, Niraparib, Olaparib, Rucaparib, Veliparib) (procured from Selleckchem, USA). Results: Individually (1) AZD6482 was the most effective among p110b i in controlling the cell cycle when tested in PTEN null U87MG GBM cells and (2) AZD6482 had the lowest dose (1-10 uM) for achieving the minimum effective dose (MED) in the confluence studies in TNBC and GBM models as compared to other p110b i. The MED of 5 PARPi for TNBC and GBM cells were determined by clonogenic assay (7-10 days). The most potent among 5 PARPi was BMN673 with a dose range of 100 nM-500 nM. Based on the effectiveness of individual drugs, we used a combination dose range of 500 nM-10 uM of 5 PARPi with 10 uM fixed dose of p110b i, AZD6482, and tested the effect of this combination using (1) real time apoptosis assay (Incucyte) and (2) 3D clonogenic assay. The combination of each PARPi with AZD6482 was more effective in perturbing clonogenic growth of PTEN-null TNBC and GBM cells as compared to the individual agent(s) which can be corroborated with the increased apoptosis in the cells treated with the combination and their effect on cell cycle. Significance: Better understanding of the mechanistic involvement of the combination of p110b i and PARPi in BRCA-competent PTEN null tumors may provide a novel target for future therapeutic intervention and provide insight to PTEN-mediated carcinogenesis in these tumors which eventually will extend the realm of PARP i use in cancer therapy. Citation Format: Jennifer H. Carlson, Yuliang Sun, Xiaoqian Lin, Pradip De, Brian Leyland-Jones, Nandini Dey. Contextual combination of PARP inhibitors with p110beta inhibitors: Functional logistics to tame PTEN null tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2561. doi:10.1158/1538-7445.AM2015-2561