Abstract 7171: Non-linear IV pharmacokinetics of the ATR inhibitor berzosertib (M6620) in mice
Authors: Deppas Joshua J. , Kiesel Brian F. , Guo Jianxia , Parise Robert A. , Bakkenist Christopher J. , Beumer Jan H. 
Published: March 2024

Abstract
Introduction: The ataxia-telangiectasia and Rad3-related (ATR) protein kinase is an attractive target for cancer therapies as it is an apical initiator of DNA damage response (DDR) pathways that facilitate cell cycle arrest and DNA repair, thus promoting survival of cancer cells. Several ATR inhibitors (ATRi) are in clinical development including berzosertib (M6620, VX-970), which is currently being evaluated in combination with cytotoxic chemotherapies and radiation. Although clinical studies have examined plasma pharmacokinetics (PK) in humans, little is known regarding dose/exposure relationships and tissue distribution which may influence berzosertib’s therapeutic window. To understand these concepts, we extensively described the PK of berzosertib in mouse plasma and tissue.
Methods: Dose proportionality was assessed using single IV doses of berzosertib (2, 6, 20 and 60 mg/kg) administered to female BALB/c mice. Mice were euthanized from 5 min to 24 h post-dose. Using a highly sensitive LC-MS/MS method, berzosertib was quantitated in red blood cells, plasma, and tissues. Non-compartmental analysis (NCA) was performed using Phoenix WinNonlin to determine relevant PK parameters. Dose linearity was assessed by statistical comparison of dose-normalized plasma Cmax and dose-normalized tissue AUC across the four dose levels. Statistical analyses were performed using ANOVA with Tukey’s multiple comparisons test. Berzosertib plasma protein binding was assessed in vitro from 300 ng/mL to 100 μg/mL using rapid equilibrium dialysis. An extensive PK study was also conducted in tumor-bearing mice to assess distribution by quantitating drug in tissues.
Results: The dose proportionality study indicated that both plasma Cmax and AUC increase with dose, and dose-normalized concentration vs time profiles suggested nonlinear increases in exposure based on different terminal slopes. At the two highest doses, dose-normalized AUC in each tissue were greater than the exposure generated at the two lowest doses. However, log transformations of AUC0-360 plotted vs dose generated a slope of 0.886 (95% CI: 0.718 - 1.054), suggesting a linear relationship. The fraction of unbound berzosertib in mouse plasma increased at concentrations above 10 μg/mL, which is a concentration exceeded at the two highest doses and where nonlinearity in exposure is observed. Tissue analysis revealed the following tissues had a tissue/plasma coefficient &gt;1: bone marrow, kidney, liver, spleen, lung, tumor, lymph nodes, thymus, and heart.
Conclusion: IV berzosertib displayed nonlinear PK in mice. Increased doses of berzosertib were associated with greater than proportional increases in plasma and tissue exposure most likely due to saturation of plasma protein binding. Our results will help to understand preclinical pharmacodynamic and toxicity data and subsequent PBPK modeling to inform optimal dosing and clinical deployment of berzosertib.
Citation Format: Joshua J. Deppas, Brian F. Kiesel, Jianxia Guo, Robert A. Parise, Christopher J. Bakkenist, Jan H. Beumer. Non-linear IV pharmacokinetics of the ATR inhibitor berzosertib (M6620) in mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7171.