Abstract LB-086: ATR inhibition broadly sensitizes TP53-deficient soft-tissue sarcomas to chemotherapy independent of alternative lengthening telomere (ALT) status: Moving forward to personalized medicine
Authors: Laroche-Clary Audrey , Chaire Vanessa , Malykh Andrei , Algeo Marie Paule , Loarer François Le , Italiano Antoine 
Published: August 2018

Abstract
Background: Despite optimal local treatment, 40 to 50% of patients with soft-tissue sarcomas (STS) will develop metastatic disease. Only few drugs have shown activity in the advanced setting and the median overall survival is poor (12 to 18 months). Germline variants in several DNA damage sensing and repair genes, including BRCA2, ATM, ATR, and ERCC2, contributed greatly to STS risk. Moreover, next-generation sequencing of large cohorts of STS have shown frequent alterations of genes involved in the DNA damage repair pathways. The aim of this study is to assess the impact of ATR inhibition in pre-clinical models of STS.
Methods: We have performed a screening of a panel of 10 STS cell lines encompassing several histologies and established from surgical specimens resected at our institution. Impact of ATR inhibition was assessed by using VE-822 (a specific ATR inhibitor) as a single agent and in combination with chemotherapy (gemcitabine). Cell proliferation and cell death analysis were evaluated with high throughput assay. Effect on cell cycle was evaluated by using flow cytometry after PI DNA staining. DNA strand breaks were evaluated by testing for the presence of intra-nuclear foci γH2ax after 48h of treatment with drugs alone or in combination by immunofluorescence and microscopy confocal analysis. Anti-tumor activity of VE-822 alone and in combination was correlated with the TP53 status as well as the alternative lengthening telomere (ALT). In vivo experiments were performed by using 2 PDX models of undifferentiated pleomorphic sarcoma (UPS), the first with P53wt and the second with P53 mut.
Results: VE-822 induced modest proliferation decrease in all the 10 cell lines of STS, IC50 were ranging from 1.3µM to 12µM. We observed synergy or additive effect with gemcitabine in 70% of cell lines. VE-822 as a single agent increased the G1-phase-fraction and induced a significantly increase of S-phase fraction when combined with gemcitabine. The combination also induced a significantly higher intra-nuclear accumulation of γ-H2ax expression than each drug used alone. ALT did not render cells more sensitive to ATR inhibition. Moreover, ATR inhibition induced strong compensatory ATM activation in TP53-wild type but not in TP53 mutated cells. In vivo, the combination of gemcitabine and VE-822 reduced tumor growth rate and increased significantly progression-free-survival significantly in comparison with each drug used alone only in TP53mut but not in TP53 wt PDX models.
Conclusion: ATR inhibition in combination with chemotherapy is a promising strategy in TP53 deficient STS and is unrelated to ALT status. Co-targeting of ATM should be investigated in TP53-proficient tumors. A randomized phase II study investigating the safety and efficacy of an ATR inhibitor in combination with gemcitabine in TP53 deficient STS will be launched soon. Its design will be presented at the meeting.
Citation Format: Audrey Laroche-Clary, Vanessa Chaire, Andrei Malykh, Marie Paule Algeo, François Le Loarer, Antoine Italiano. ATR inhibition broadly sensitizes TP53-deficient soft-tissue sarcomas to chemotherapy independent of alternative lengthening telomere (ALT) status: Moving forward to personalized medicine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-086.