Citations

Abstract Background: Endemic nasopharyngeal carcinoma (NPC) refers to non-keratinizing (NK) subtypes harboring Epstein Barr virus (EBV) that frequently express programmed death-ligand 1 (PD-L1). We recently reported ≥30% of NPC harbor MHC class 1 gene alterations in a whole genome study. Method: We investigated the activity of nivolumab (NIVO) & relationship with PD-L1 & HLA expression (exp) in tumor cells (TC) & tumor infiltrating cells (TIL). Eligible patient (pt)s had to fail ≥ 1 prior line of platinum-based chemo. NIVO (3mg/kg IV, q2 weeks) was given until disease progression (PD). Archived tumors were analyzed for PD-L1 (DAKO) & HLA mutations (indicated by loss of HLA-A &/or B exp). Result: A total of 45 pts were enrolled across 11 sites from Oct 2015 to June 2016. Overall, 43 and 44 pts were evaluable respectively for response rate (RR, RECIST) & adverse events (AE). The median age was 57 years, 77% pts were males & 84% were Asians. The median number of prior chemo was 3 (range 1-9 lines). Of the 43 evaluable pts, there were 8 confirmed (19%, 95% CI: 8-33%) & 1 unconfirmed partial response (PR), 17 PD (40%), 14 stable disease (SD, 33%) & 3 pts were not assessed. One pt (MD015-027) had an unconfirmed clinical CR in an occipital metastasis (met). Pts received a median of 3 cycles of NIVO (range 1 - 14). Median overall survival has not been reached yet. Of the 44 pts evaluable for AE, 8 (18.2%) had grade 3 or higher treatment-related AEs & 1 pt died of sepsis. The most common grade 3 or higher AEs were increase in alanine/aspartate aminotransferase (n=3), & 1 pt respectively for hyponatremia, neutropenia, fatigue & diarrhea. Of the 21 tumor samples analyzed, 11/21 (52%) had >1% PD-L1 expressed mainly in TC. Table 1 outlines relationship between RR & PD-L1 exp. All pts with PR had >5% PD-L1 exp & one pt (34003-022) with CR (nasopharynx) & PR (lung mets) had the highest exp (80%). Majority of pts with PD had <1% PD-L1 exp tumors. One primary progressor (34003-032) had a PD-L1 +ve tumor harboring a complete loss of HLA-B signal (TC), but intact signal in TILs & stromal cells. Conclusion: NIVO is active in this heavily pre-treated group of pts with NK-NPC and longer follow-up is needed for survival endpoints. Preliminary result suggests PD-L1 exp may predict benefit from NIVO. The analytical result of all 45 tumors will be presented. Table 1.PD-L1 exp and RECIST response in 21 ptsPD-L1%espRR%PRSDPD0-11<1%-265%24-10%--120%-1-30%11-80%1--Total498 Citation Format: Brigette B. Ma, Boon Cher Goh, Wan T. Lim, Kwok W. Lo, Edwin P. Hui, Jonathan W. Riess, Mark Agulnik, Alex Y. Chang, Julie A. Kish, Dean W. Lim, Douglas R. Adkins, Kevin J. Cullen, Barbara J. Gitlitz, Nathan R. Foster, Adam M. Pettinger, Sanna Mckinzie, Ka F. To, Brian Costello, Howard Streicher, Anthony T. Chan. Multicenter phase II study of nivolumab in previously treated patients with recurrent and metastatic non-keratinizing nasopharyngeal carcinoma - Mayo clinic Phase 2 Consortium P2C-MN026, NCI9742, NCT02339558 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr CT076. doi:10.1158/1538-7445.AM2017-CT076

Abstract Introduction: Axicabtagene ciloleucel (axi-cel) is an autologous anti-CD19 chimeric antigen receptor (CAR) T cell therapy. ZUMA-1 is a multicenter registrational trial of axi-cel in patients with refractory, aggressive B-cell non-Hodgkin lymphoma. In a pre-specified interim analysis, ZUMA-1 met its primary endpoint with a 76% objective response rate and 47% complete response. The incidence of Grade 3 and higher (Gr 3+) cytokine release syndrome (CRS) and neurologic events (NE) was 13% and 29%, respectively, in 93 patients with 1 month follow-up (Neelapu ASH 2016). We present novel immune signatures of Gr 3+ CRS and NE by analyzing the axi-cel-related biomarker profile in association with clinical outcomes. Methods: In this interim analysis of 62 patients, 44 serum analytes pre- and post-axi-cel treatment were measured via ELISA at multiple timepoints during the first month. The number of CAR+ cells in blood was determined by qPCR. Kinetics and association of these markers with CRS and NE were analyzed. The treatment-related profile was defined by analytes with at least double an increase over baseline in at least 50% of patients. These analytes were selected and evaluated for association with Gr 3+ CRS or NE. Immune signatures were determined based on p-values applied to peak or cumulative levels of analytes and adjusted for multiplicity (stepdown method). Results: Out of 44 analytes measured, 12 demonstrated an increase by double over baseline in at least 50% of patients. This axi-cel-related biomarker profile includes immune proliferative/modulating cytokines, pro-inflammatory cytokines, markers of myeloid activation, and chemokines. Markers with the highest median-fold increase were IFNγ (44x), IL-10 (31x), IL-6 (26x), IL-15 (20x), and GRZB (17x). These peaked within 7-14 days and generally returned to baseline within 1 month post-treatment. Within this panel of 12 analytes elevated post-axi-cel treatment, markers associated with Gr 3+ CRS (p<0.05) were IL-10, IL-6, IL-2Rα, IL-8, MCP-1, IFNγ, IL-1Rα, and IP-10. Markers associated with Gr 3+ NE (p<0.05) were IL-6, IL-15, IL-8, and MCP-1. Peak levels of CAR T cells in blood, occurring within 14 days post-infusion, were associated with Gr 3+ NE (p=0.02) but not Gr 3+ CRS (p=0.6). A majority of patients with Gr 3+ NE had only Gr 0-2 CRS (16/22) while nearly 50% of patients with Gr 3+ CRS had Gr 0-2 NE (5/11). Conclusions: Biomarker and clinical evaluation suggests mechanistic differences underlying CRS and NE following axi-cel treatment. While Gr 3+ NE was associated with CAR T cell expansion, Gr 3+ CRS was instead associated with a broader array of serum markers, many linked to systemic myeloid activation. Further understanding of these immune signatures may provide insight into managing AEs associated with anti-CD19 CAR T cell therapy. Citation Format: Frederick L. Locke, John Rossi, Xiaodong Xue, Sattva S. Neelapu, Daniel H. Ryan, Armin Ghobadi, Lazaros J. Lekakis, David Miklos, Caron A. Jacobson, Ira Braunschweig, Olalekan Oluwole, Tanya Siddiqi, Yi Lin, John Timmerman, Patrick M. Reagan, Marika Sherman, Janice Nagatani, Xiao Zhang, Lynn Navale, William Y. Go, Jeff Wiezorek, Adrian Bot. Immune signatures of cytokine release syndrome and neurologic events in a multicenter registrational trial (ZUMA-1) in subjects with refractory diffuse large B cell lymphoma treated with axicabtagene ciloleucel (KTE-C19) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr CT020. doi:10.1158/1538-7445.AM2017-CT020

Abstract Introduction: Outcomes for pts with refractory aggressive NHL are poor with current therapies (Crump, ASCO 2016). Results from the interim analysis of (n=62) of ZUMA-1, the 1st multicenter trial of an anti-CD19 chimeric antigen receptor (CAR) T cell, axi-cel, in refractory aggressive NHL, showed an objective response rate (ORR) of 79% (complete response [CR] 52%; Blood 2016;128:LBA-6). Here we present results from the primary analysis of ZUMA-1. Methods: Pts received a target dose of 2 × 106 anti-CD19 CAR T cells/kg after lowdose conditioning with cy/flu. Eligible pts (≥18 y) had diffuse large B cell lymphoma (DLBCL), primary mediastinal B cell lymphoma (PMBCL) or transformed follicular lymphoma (TFL); an ECOG performance status (PS) 0-1; and refractory disease (progressive or stable disease as best response to last prior therapy, or relapsed ≤12 m of autologous stem cell transplant [ASCT]). The primary endpoint for this analysis was ORR in the combined DLBCL, PMBCL, and TFL population. Key secondary endpoints were duration of response (DOR), overall survival (OS), and frequency of adverse events (AEs). The primary analysis was triggered when 92 pts had at least 6 m of follow-up. Results: As of January 27, 2017, 111 pts from 22 institutions were enrolled; 101 pts (91%) received axi-cel. Median age was 58 y (range, 23-76), 67% male, 85% stage IIIIV, 47% IPI 3-4, 77% refractory to ≥2nd line of therapy, and 21% relapsed ≤12 m of ASCT. Axi-cel was successfully manufactured in 110/111 (99%) pts with an average turnaround time from apheresis to the clinical site of 17 d. With an ORR of 82% (n = 92; P<.0001) the study met the primary endpoint. The ORR in the mITT analysis set of 101 pts was 82% (CR 54%, PR 28%), and was consistent across key covariates including disease subtype, refractory status, stage, and IPI score. At a median follow up of 8.7 m, 44% of pts were in response and 39% were in CR. The median DOR was 8.2 m overall and not reached for pts who achieved a CR. Median OS was not reached; 80% of pts remained alive at 6 m. The most common Gr ≥3 treatmentemergent AEs were neutropenia (66%), leukopenia (44%), anemia (43%), febrile neutropenia (31%), and encephalopathy (21%). Gr ≥3 cytokine release syndrome (CRS) and neurologic events (NE) occurred in 13% and 28% of pts, respectively. All CRS and all NE resolved except 1 Gr 1 memory impairment. As previously reported, there were 3 Gr 5 AEs (3%). Peak and cumulative CAR T levels post-axi-cel were associated with durable responses. Conclusions: Axi-cel significantly improved ORR in patients with refractory aggressive NHL. The CR rate was 7-fold higher compared to historical controls (Crump, ASCO 2016) and nearly half the patients have an ongoing response. Axi-cel demonstrated significant clinical benefit with a manageable safety profile in pts lacking curative treatment options.[F.L.L. and S.S.N. contributed equally to this study.] Funding source: Kite Pharma and in part by funding from the Leukemia & Lymphoma Society (LLS) Therapy Acceleration Program treatment options. Citation Format: Frederick L. Locke, Sattva S. Neelapu, Nancy L. Bartlett, Lazaros J. Lekakis, David Miklos, Caron A. Jacobson, Ira Braunschweig, Olalekan Oluwole, Tanya Siddiqi, Yi Lin, John Timmerman, Jonathan W. Friedberg, Adrian Bot, John Rossi, Lynn Navale, Yizhou Jiang, Jeff Aycock, Meg Elias, Jeff Wiezorek, William Y. Go. Primary results from ZUMA-1: a pivotal trial of axicabtagene ciloleucel (axicel; KTE-C19) in patients with refractory aggressive non-Hodgkin lymphoma (NHL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr CT019. doi:10.1158/1538-7445.AM2017-CT019

Abstract Introduction: MutHER is a phase II trial that demonstrated the anti-tumor activity of the pan-HER inhibitor neratinib in HER2mut, non-amplified MBC. The major challenges to accrue to this trial were the large number of pts to screen for the 2-3% HER2mut population and the high rate (24%) of poor quality tumor DNA for sequencing. The goals of this ctDNA study were: 1) the concordance of HER2mut detected by ctDNA versus tumor testing; 2) the incidence of ctDNA HER2mut in HER2 non-amplified MBC; 3) changes in HER2mut variant allele frequency (VAF) on neratinib therapy. Methods: A sample size of 30 negative (neg) controls was needed to ensure 90% confidence if ctDNA testing has >90% specificity in detecting HER2mut. Thus, plasma from MBC pts obtained at screening for MutHER trial (Neg control: 40 pts without HER2mut on tumor testing; Positive (pos) control: 14 pts with known HER2mut who received neratinib) were subjected to Guardant360 ctDNA 70-gene panel sequencing (all exons of HER2 included). ctDNA from the 14 neratinib treated pts were also analyzed at week (wk) 4 and upon progression. ctDNA data from MBC pts clinically tested at Guardant Health were interrogated for HER2mut incidence. Results: Among the 14 pts with tumor pos for HER2mut, ctDNA sequencing identified the same HER2mut in 11, discrepant HER2mut in 1, and neg in 2. The 2 pts with ctDNA neg for HER2mut had progressive disease (PD) and stable disease (SD > 6 months) on neratinib, respectively. Among the 40 neg controls, 8 were not evaluable (no detectable ctDNA or assay unsuccessful) and all 32 successfully sequenced cases were neg for HER2mut. The sensitivity and specificity of ctDNA for HER2mut detection was 11/14 (79%, 90% CI: 53-94%) and 32/32 (100%, 90% CI: 91-100%), respectively. Among the 11 paired baseline and wk 4 samples, 9 (82%) had lower HER2mut VAFs at wk 4 than at baseline, with 1 complete response (CR), 1 partial response (PR), 5 SD, and 4 PD at wk 8 as best tumor response. Two pts had higher wk 4 ctDNA HER2mut VAFs and both had radiographic PD at wk 8. The absolute HER2mut VAF levels at wk 4 were significantly associated with TTP (Spearman rho=-0.69, p=0.02) and tumor size change (rho=0.67, p=0.05). The HER2mut VAFs were significantly higher at progression than wk 4 in all pts (p<0.01). One pt acquired a new HER2mut T798I, which is analogous to the gate-keeper mutation EGFR T790M. The incidence of HER2mut without amplification in unselected consecutive MBC clinically tested by Guardant360 was 3% (48/1,584), with mutation pattern similar to published tumor testing data. Conclusions: ctDNA sequencing is sensitive and highly specific in detecting HER2mut, offering a non-invasive method to identify pts for trials of HER2mut-targeted therapy. Decreased HER2mut VAFs at wk 4 was observed in 82% of cases, consistent with the on-target effect of neratinib. Increased HER2mut VAFs at wk 4 is a potential early marker of progression. Citation Format: Cynthia Ma, Ron Bose, Feng Gao, Rachel Freedman, Melinda Telli, Gretchen Kimmick, Eric Winer, Michael Naughton, Matthew Goetz, Christy Russell, Debu Tripathy, Melody Cobleigh, Andres Forero, Timothy Pluard, Carey Anders, Shana Thomas, Jill Anderson, Caroline Bumb, Kimberly Banks, Richard Lanman, Richard Bryce, Alshad Lalani, John Pfeifer, Daniel Hays, Mark Pegram, Kimberly Blackwell, Philippe Bedard, Hussam Al-Kateb, Matthew Ellis. Circulating tumor DNA (ctDNA) sequencing for HER2 mutation (HER2mut) screening and response monitoring to neratinib in metastatic breast cancer (MBC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr CT011. doi:10.1158/1538-7445.AM2017-CT011

Abstract Metastatic melanoma (MM) is the most aggressive type of skin cancer contributing to approximately 80% of all skin cancer related deaths. In 2016, it was estimated that more 76,000 cases would be diagnosed in the United States. The high metastatic potential and aggressive clinical behavior of this disease makes it a major health problem. As a consequence there has been notable development in for novel targeted (i.e. BRAF inhibitors) and immune therapies (i.e. anti-PD1 and CTLA4 inhibitors). However, despite improvements in patient outcomes, most patients develop resistance within 6-11 months on dual BRAF/MEK inhibition, and 5 months on immune therapy, highlighting the need to identify new therapies that improve disease management and patient survival. Increased expression of CyclinD1 has been reported to occur as a mechanism of resistance to BRAF inhibition, resulting in reactivation of the canonical activating pathway in melanoma, MAPK. Thus, we hypothesize that combined inhibition of CDK4/6 and MAPK signaling will provide a novel anti-tumor synergy drug combination superior to current standard of care in BRAF mutant patient-derived preclinical models. Using MM human tissue from patients we developed 3 BRAF mutant preclinical models investigating a novel triple therapy drug combination BRAFi/MEKi/CDK4/6i. Protein analysis and drug proliferation assays were used to design the novel combination. In vivo, we found statistically significant differences in tumor inhibition between the novel combination and the current standard of care. The triple therapy also showed promising results, in vivo, when assessed as a salvage therapy following tumor escape from BRAFi/MEKi combination treatment. Citation Format: Antoneicka L. Harris, Michael Thompson, Svetomir Markovic, William Durham, Louis Dawson, Daniel Small, John A. Copland. Tumor regression in novel triple therapy BRAF mutant metastatic melanoma patient-derived preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-130. doi:10.1158/1538-7445.AM2017-LB-130

Abstract Estrogen receptor positive (ER+) breast cancer is treated with endocrine therapy but intrinsic resistance occurs in ~1/3 of patients and acquired resistance in ~1/5 of the remainder. While many resistance mechanisms have been explored, therapeutic strategies to overcome resistance in the clinical setting have seen mixed outcomes, and appear most effective in the acquired resistance setting. Understanding mechanisms of resistance and finding therapeutic strategies to target them, therefore, remain important challenges facing breast cancer researchers. In this study we systematically examine the role of DNA damage repair defects in inducing endocrine therapy resistance, a relatively understudied question of recent interest. We use in silico analysis of clinical datasets, in vitro experiments evaluating endocrine therapy resistance in response to DDR dysregulation in multiple breast cancer celllines, and in vivo validation using cellline xenograft and patient-derived xenograft models. We also use gene expression microarrays and RPPA data from cell lines, patient-derived xenografts and primary ER+ breast tumors to uncover therapeutic options that are validated in vitro and in vivo and corroborated by clinical trial data. The results of this study uncover an intriguing link between mismatch repair (MMR) deficiency, specifically of the MutL complex (MLH1/3, PMS1/2), and poor prognosis in ER+ disease. We find a direct role for MutL loss in endocrine therapy resistance in vitro and in vivo by knocking down multiple MutL genes using CRISPR and stable shRNA approaches validated using standard rescue experiments. We identify the underlying mechanism: MutL deficiency in ER+ breast cancer abrogates Chk2-mediated feedback inhibition of CDK4/6 that appears necessary for endocrine therapy responsiveness. Consequently, pharmacological targeting of CDK4/6 in vitro and in vivo significantly inhibits growth of endocrine therapy resistant MutL-deficient ER+ breast cancer cells. These results are corroborated by data from a neoadjuvant clinical trial demonstrating that cell cycle regulation of MutL-mutant tumors tends to be estrogen-independent but sensitive to CDK4/6 inhibitors. The results of this study provide important biological and clinically relevant insights. 1) MMR deficiency is unexpectedly causal to intrinsic endocrine therapy resistance 2) This causal effect appears to be mediated by abrogation of cell cycle checkpoint activation in response to endocrine therapy 3) MMR deficiency in a subset of ER+ tumors explains why CDK4/6 inhibition is effective against some de novo endocrine therapy resistant tumors. While there are currently no biomarkers to guide the use of CDK4/6 inhibitors for ER+ breast cancer, markers of MMR dysregulation could identify patients in whom CDK4/6 inhibition should be used to prevent disease recurrence. Citation Format: Svasti Haricharan, Jacob Schmelz, Cheryl Schmidt, Purba Singh, Kimberly R. Holloway, Meenakshi Anurag, Shunqiang Li, Shyam M. Kavuri, Shixia Huang, Dean P. Edwards, Vera Suman, Kelly Hunt, John A. Olson, Jeremy Hoog, Cynthia X. Ma, Matthew N. Bainbridge, Matthew J. Ellis. Mismatch repair defects and endocrine therapy resistance in estrogen receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 489. doi:10.1158/1538-7445.AM2017-489

Abstract Chimeric antigen receptor (CAR) T cell immunotherapy has demonstrated profound results in hematologic malignancies but clinical efficacy in the solid tumor setting has not been observed. Barriers to T cell entry and function may partially explain this observation. As solid tumors actively recruit myeloid cells, we hypothesized that macrophages have the potential to be a powerful cellular immunotherapeutic agent in this setting if properly activated and redirected. We here describe the development of CAR macrophages (CARMA), demonstrating the feasibility, mechanism, and efficacy of this platform. To examine the function of CARs in macrophages, first generation anti-CD19, anti-mesothelin, or anti-HER2 CARs with a CD3ζ intracellular domain were introduced into the THP1 macrophage model. In vitro function was assessed via quantitative phagocytosis and luciferase-based specific killing assays. CARMA selectively phagocytosed and cleared cognate antigen-bearing tumor cells. To demonstrate the requirement for CAR-mediated intracellular signaling for activity, a CD3ζ-null CAR construct was tested in vitro. The deletion of CD3ζ significantly reduced the phagocytic and killing capacity (p<0.01) of CARMA. We identified Ad5f35, a chimeric adenovirus, as a novel and highly efficient viral vector for the transduction of normal donor and cancer patient macrophages (>70% CAR expression). Ad5f35 transduction polarized human macrophages toward a durable immunostimulatory M1 phenotype and rendered CARMA resistant to subversion toward the immunosuppressive M2 phenotype, as defined by surface markers and metabolomics. CARMA enhanced the proliferative capacity of CD8+ T cells in phytohemagglutinin activation assays and secreted factors that activated by-stander macrophages. Primary human anti-HER2 CARMA demonstrated targeted phagocytosis and killing of HER2 expressing ovarian and breast cancer cell lines, and exhibited a six-fold higher luciferase-based killing capacity of SKOV3 cells compared to trastuzumab in vitro (p=0.002). Anti-HER2 CARMA was evaluated in vivo in an intraperitoneal (IP) SKOV3 ovarian cancer xenograft model. Mice that received IP CARMA had a decrease in tumor burden of approximately two orders of magnitude and had a 30-day survival benefit relative to untreated or control macrophage treated mice (p=0.018). In a systemically disseminated SKOV3 model, a single dose of IV CARMA led to a durable anti-tumor response (38-fold reduction relative to control on day 31 post-treatment; p=0.016) Lastly, we demonstrated that the blockade of the anti-phagocytic CD47/SIRPα axis enhanced the phagocytic capacity of CARMA. In summary, we here demonstrate that human macrophages engineered with a CAR exhibit targeted anti-tumor function in both in vitro and in vivo preclinical models. This novel cellular immunotherapeutic approach has a clear translational potential for the treatment of solid tumors. Citation Format: Michael Klichinsky, Marco Ruella, Olga Shestova, Saad S. Kenderian, Miriam Y. Kim, Roddy O'Connor, John Scholler, Carl June, Saar Gill. Chimeric antigen receptor macrophages (CARMA) for adoptive cellular immunotherapy of solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4575. doi:10.1158/1538-7445.AM2017-4575

Abstract Mismatch repair (MMR) is essential for temozolomide (TMZ) sensitivity, and MMR deficiency causes TMZ resistance and disease recurrence. However, the role MMR in TMZ response has been linked with Msh2/Msh6 (MutSα), while role of Msh3, a component of MutSβ remains unclear. To delineate role of MutSα and MutSβ in TMZ response, we used siRNA to disrupt MSH6 and MSH3 expression. As expected, silencing of MSH6 led to resistance in a TMZ-sensitive U251 cell line. In contrast, silencing of MSH3 enhanced TMZ sensitivity as compared to the control cells. These results support established role of MutSα in TMZ response via replication stress and DNA double strand breaks (DSB), and reveal that MutSβ may potentially influence TMZ response by modulating DSB repair. Consistently, host cell reactivation (HCR) assays in U251 expressing DRGFP reporter showed decreased homologous recombination (HR) after MSH3 knockdown (KD), while Msh6 KD had no effect on HR efficiency. Supporting the concept of synthetic lethality of HR with PARP inhibition, clinically relevant dose of veliparib (1 µM) showed a modest but consistent increase in TMZ sensitization measured in terms of potentiation factor (PF, ratio of relative growth with TMZ alone/ TMZ and veliparib) in MSH3 KD (PF 1.6±0.2) as compared to control cells (PF 1.1±0.2). Sensitizing effect of 1 µM veliparib in MSH6 KD (PF 1.7±0.5) was comparable to that in MSH3 KD cells. Interestingly, supra-therapeutic dose of veliparib (10 µM) induced more pronounced sensitization in MSH6 KD cells (PF 2.6±0.3) as compared to MSH3 KD (PF 2.4±0.3) or control cells (PF 1.6±0.2). Consistently, veliparib (10 µM) alone had no impact on DNA damage signaling, while TMZ induced prominent increase in phosphorylation of KAP1, Chk1 and Chk2 in control and MSH3 KD cells, but not in MSH6 KD cells; veliparib/TMZ co-treatment led a robust signaling in all 3 types of cells. Increased sensitization by veliparib after MSH3 KD was also examined in a TMZ-resistant (GBM22TMZ) PDX model carrying resistance due to a homozygous mutation in MSH6, and a 4-fold increase in HR efficiency. In clonogenic assays silencing of MSH3 or BRCA1 had no effect on TMZ sensitivity in GBM22TMZ cells. However, veliparib alone (10 µM) decreased colony formation with relative colony formation 0.54±0.03 and 0.32±0.06 in MSH3 and BRCA1 KD cells, respectively as compared to 0.82±0.04 in controls. TMZ/veliparib co-treatment in GBM22TMZ led to further decrease in colony formation in MSH3 KD and BRCA1 KD cells with relative colony formation of 0.30±0.04 and 0.19±0.02, respectively versus 0.46±0.03 in control. Taken together, our results suggest that MutSα and MutSβ differentially regulate cytotoxic effects of TMZ. Decreased HR efficiency and enhanced sensitizing effects of veliparib in MSH3 KD cells partially define synthetic lethality of MMR deficiency with PARP, and may help development of PARP inhibitors as sensitizer of TMZ therapy in GBM. Citation Format: Shiv K. Gupta, Emily J. Smith, Ann M. Tuma, Brett L. Carlson, Mark A. Schroeder, Gasper J. Kitange, Jann N. Sarkaria. MutSα and MutSβ mismatch repair complexes differentially regulate temozolomide efficacy & sensitizing effects of veliparib in glioblastoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1109. doi:10.1158/1538-7445.AM2017-1109

Abstract Dysregulation of estrogen receptor gene (ESR1) is an established mechanism of inducing endocrine therapy resistance. We previously discovered a chromosomal translocation event generating an estrogen receptor gene fused in-frame to C-terminal sequences of YAP1 (ESR1-YAP1) that contributed to endocrine therapy resistance in estrogen receptor positive (ER+) breast cancer models. This current study compares functional and pharmacological properties of additional ESR1 gene fusion events of both early stage (ESR1-NOP2) and advanced endocrine therapy resistant (ESR1-YAP1 and ESR1-PCDH11x) breast cancers. The YAP1 and PCDH11x fusions conferred estrogen-independent and fulvestrant-resistant growth in T47D, an ER+ breast cancer cell line in vitro and in vivo, in contrast to the NOP2 fusion which was sensitive to hormone deprivation. Immunohistochemical (IHC) staining of mouse lungs revealed significantly higher numbers of micrometastatic ER+ cells from the T47D tumors expressing the YAP1 and PCDH11x fusions than YFP control and NOP2 fusion. Estrogen response element (ERE) reporter and pull down assays revealed that although all ESR1 fusions studied bound EREs, only the YAP1 and PCDH11x caused ERE activation. Cell lines containing these “canonical” ESR1 fusions upregulated expression of ER responsive genes such as TFF1 and GREB1 in hormone deprived conditions. In contrast, the NOP2 fusion neither induced ERE activity nor upregulated TFF1 and GREB1 gene expression. The proliferative ability of canonical fusion-containing T47D cells was inhibited by palbociclib, a CDK4/6 inhibitor, in a dose-dependent manner. In vivo growth of patient-derived xenograft tumors naturally harboring the ESR1-YAP1 fusion (WHIM18) was significantly reduced in mice fed palbociclib-containing chow. Mice transplanted with WHIM18 also formed lung micrometastases, with an ER IHC staining pattern similar to lungs from YAP1 and PCDH11x fusion expressing T47D xenografts. In conclusion, in-frame ERE activating canonical fusions occur in end-stage, drug resistant, advanced breast cancer and can be added to ESR1 point mutations as a class of somatic mutation that may cause acquired resistance. Endocrine therapy resistant growth induced by these fusions can be treated with CDK4/6 inhibition, using an FDA approved drug, palbociclib, which could potentially improve outcomes in patients with ESR1 translocated tumors. Citation Format: Jonathan T. Lei, Jieya Shao, Jin Zhang, Michael Iglesia, Doug W. Chan, Ryoichi Matsunuma, Xiaping He, Purba Singh, Yoshimasa Kosaka, Robert Crowder, Svasti Haricharan, Shyam Kavuri, Jeremy Hoog, Chanpheng Phommaly, Rodrigo Goncalves, Susana Romalho, Wei-Chu Lai, Oliver Hampton, Anna Rogers, Ethan Tobias, Poojan Parikh, Sherri Davies, Cynthia Ma, Vera Suman, Kelly Hunt, Mark Watson, Katherine A. Hoadley, Aubrey Thompson, Charles Perou, Chad J. Creighton, Chris Maher, Matthew J. Ellis. Estrogen receptor gene fusions drive endocrine therapy resistance in estrogen receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1033. doi:10.1158/1538-7445.AM2017-1033