LAUSR

Background: Ataxia telangiectasia and Rad3-related (ATR) protein kinase plays a critical role in the DNA damage response. M1774, a potent, selective, orally administered ATR inhibitor with antitumor activity in preclinical models, was evaluated as monotherapy in Part A1 of the first-in-human open-label, single-arm study (NCT04170153). M1774 was well-tolerated and pharmacodynamic analyses showed that maximum target engagement was reached from the dose of 130 mg QD. The totality of evidence, including quantitative model-based analyses, suggested the recommended dose for expansion (RDE) as 180 mg QD 2 weeks on/1 week off.1 One patient with platinum and PARP inhibitor-resistant BRCAwt ovarian cancer and ATRX mutation on local tumor testing achieved an unconfirmed RECIST v1.1 partial response. Retrospective analyses were conducted to explore the molecular portrait and evolution of the underlying disease of patients treated with M1774. Methods: Archival tumor biopsies, baseline and serial on-treatment circulating tumor DNA (ctDNA) samples collected from patients enrolled in the study were analyzed by next generation sequencing. Somatic putative CHIP mutations and cases with mutation variant allele frequency (VAF) < 0.3% in circulating free DNA at baseline were excluded. Molecular response (MR) was defined as at least 50% reduction of VAF. Results: Molecular data were generated from archival biopsies collected for 33/55 patients and ctDNA samples from 55/55 patients enrolled in the study. High impact mutations were detected in ARID1A (N=10), ATM (N=5), ATRX (N=3), BRCA1/2 (N=13), and other homologous recombination-related genes (N=5). MRs were observed in 11/34 (32%) patients treated with more than 130 mg QD, none were observed in the 10 patients treated with lower doses. The MRs were enriched in patients with ovarian (3/8, 38%), prostate (4/8, 50%), and breast cancer (1/3, 33%), while less frequent (3/15, 25%) in other indications. TP53 mutations were significantly associated with MR, independently of tumor type. Complete MRs were achieved for any mutations in ARID1A (2/7), ATRX (2/5), DAXX (1/3), BRCA1/2 (1/10). A complete MR was seen in a patient with ovarian cancer with prolonged stable disease by RECIST (200 days), and ARID1A mutation in ctDNA. Conclusions: M1774 induced MRs in patients treated with doses in the predicted efficacious concentration range. TP53 alterations were significantly associated with MR. Complete MRs were detected for mutations in the genes ARID1A, ATRX, DAXX that are being tested for participant selection in the ongoing biomarker expansion cohorts of the DDRiver 301 study. 1TA Yap, et al. Ann Oncol. 2022; 33(suppl_7): S197-S224. Citation Format: Anthony W. Tolcher, Timothy A. Yap, Ruth Plummer, Thomas Grombacher, Danyi Wang, Corinna Schaffroth, Christine Hicking, Zoltan Szucs, Giuseppe Locatelli, Johann S. de Bono. Translational analyses of ATR inhibitor M1774 in a Phase I study in patients with solid tumors (DDRiver Solid Tumors 301) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT271.