Introduction: Small cell lung cancer (SCLC) is the most lethal form of lung cancer characterized by early metastasis and rapid development of drug resistance. The protein kinase ataxia telangiectasia mutated and Rad3 related (ATR) is a vital regulator of DNA damage response (DDR) pathway and commonly upregulated in cancers with high background replication stress (RS) such as SCLC. Inhibition of ATR has shown activity in prior preclinical studies of SCLC and is being invested in a Phase 1 clinical trial. We hypothesize that ATR inhibition will have greatest activity in models with DDR deficiency and will act synergistically with PARP inhibitors, another class of DDR inhibitors which our group previously identified as a promising targeted therapy for SCLC.

Methods: Effect of VE821 +/- olaparib on viability of SCLC cell lines (n = 11) was determined by Cell Titer-Glo. Drug sensitivity (IC50) was correlated with baseline expression level of >200 proteins measured by reverse phase protein array (RPPA) to identify potential predictive markers. siRNA-mediated knockdown of ATM was done in select human SCLC lines. The effect of VE821 +/- olaparib in wild type versus ATM knockdown SCLC lines was determined by cell viability and immunoblot analyses.

Results: Gene expression of ATR in SCLC tumor samples (n = 23) is about 1.5 fold higher than normal lung (n = 42) (p = 0.01). Proteomic analysis revealed ATM protein expression to be the top biomarker of VE821 resistance out of >195 proteins (total and phospho) measured by RPPA (by t-test, p<0.0001). Specifically, SCLC cells with lower ATM protein levels (n = 5) were more sensitive to single agent VE821 (median IC50 4uM) as compared to those with higher basal expression (n = 6) (median IC50 >10uM). Phosphorylated ATM (p = 0.005) and CHK2 (p = 0.006) and total PLK1 levels (p = 0.01) were also associated with VE821 resistance. The combination of VE821 and olaparib was more potent than either drug alone, particularly in cells with low basal expression of ATM. ATM-/-knockout enhanced activity of the VE821 with and without olaparib as compared to controls.

Conclusion: Our study demonstrates that high basal expression of ATM is a biomarker of ATR inhibitor resistance and knockdown of ATM improves the efficacy of VE821+/- olaparib. The data in this study is consistent with our prior observation that low ATM levels correspond to greater sensitivity to PARP inhibitor, talazoparib, in patient derived xenograft (PDX) SCLC models. DNA damage response in cancer cells is mainly governed by the ATM/CHK2 and ATR/CHK1 pathway and deficiency in one of the pathways makes cancer cells more reliant on the other pathway for survival. Our present data thus provide support for a synthetic lethal interaction between ATR and the ATM- pathway in SCLC models responding to DNA damage and that combination of ATR and PARP inhibition may be particularly effective in ATM deficient SCLC models.

Citation Format: Triparna Sen, Pan Tong, Jing Wang, Lauren A. Byers. Proteomic profiling identifies ATM expression level as a predictive biomarker to ATR and PARP inhibition in small cell lung cancer (SCLC). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-132.