Abstract CT272: Pharmacodynamic and immunophenotyping analyses of ATR inhibitor M1774 in a Phase I study in patients with solid tumors (DDRiver Solid Tumors 301)

Abstract 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 in Part A1 of an open-label, single-arm study (NCT04170153) for safety, tolerability, maximum tolerated dose, pharmacokinetics (PK) and pharmacodynamics (PD). M1774 monotherapy in patients with advanced solid tumors was well-tolerated and 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 Here, we report findings of the M1774 PD and immunophenotyping analyses. Methods: M1774 PD was explored by assessing phosphorylation by ATR of CHK1 (p-CHK1) in tumor and of H2AX (γ-H2AX) in serial blood samples, stimulated ex vivo with the radiomimetic 4-Nitroquinoline N-oxide or Dimethyl sulfoxide as control. A flow cytometry quantitative assay was used to measure γ-H2AX in the CD45+ lymphocytes fraction. The effect of M1774 on the immunophenotype was explored by flow cytometry. Blood samples were collected at baseline, 3 and 24 hours after first M1774 administration on day 1 of cycle 1 for the γ-H2AX analysis, and on Days 1 and 15 of Cycles 1 and 2 before treatment for immunophenotyping. Results: Preclinical tumor tissue-blood bridging PD analyses in a mouse model demonstrated that the inhibition of γ-H2AX in lymphocytes highly correlated with inhibition of p-CHK1 in tumor. Clinical data of γ-H2AX levels and immunophenotyping were generated for the blood samples collected from the 55 participants of Part A1 of the study. Exploratory PK-PD analysis using γ-H2AX levels 3 h post-dose on day 1 across the doses predicted target inhibition >80% for doses ≥130 mg, suggesting target engagement. The levels of ɣ-H2AX at 24 h after first dose intake were variable and mean levels rebounded to baseline value. M1774 treatment did not cause any significant and consistent change in the levels of all explored immune cell subsets at the tested dose levels, including myeloid-derived suppressor cells, T and B lymphocytes, monocytes, and natural killer cells. Conclusions: PD analyses showed that M1774 efficiently inhibited ATR at the RDE without impacting the immunophenotype. 1TA Yap, et al. Ann Oncol. 2022; 33(suppl_7): S197-S224. Citation Format: Ruth Plummer, Anthony W. Tolcher, Timothy A. Yap, Giuseppe Sessa, Jatinder K. Mukker, Annick Seithel-Keuth, Christine Hicking, Zoltan Szucs, Ioannis Gounaris, Giuseppe Locatelli, Johann S. de Bono. Pharmacodynamic and immunophenotyping 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 CT272.