Abstract Background: Wee1 tyrosine kinase phosphorylates and inactivates cyclin dependent kinase (Cdk) 1 as part of DNA damage response (DDR) signaling, resulting in G2 cell cycle arrest to enable DNA repair. We are conducting a Phase I study of oral Wee1 inhibitor AZD1775 in patients (pts) with advanced solid tumors using twice-daily (Arm A) and once-daily (Arm B) dosing. Results for Arm A were published previously. Here, we report updated pharmacodynamic (PD), pharmacokinetic (PK), and clinical response results for Arm B. Methods: In Arm B, AZD1775 was administered once daily on days 1-5 and 8-12 of each 21-day cycle in a 3+3 design. Primary objectives were to establish safety, tolerability, PK, and MTD. Secondary objectives included determining PD effects on DDR markers in tumor tissue and circulating tumor cells (CTCs), and tumor response (RECIST 1.1). Post-treatment PD effects were established in Arm A. Given the importance of sustained molecular target control in maximizing targeted therapy efficacy, paired tumor biopsies in Arm B were obtained at baseline and day 8 (pre-dose) to assess durability of Wee1 inhibition and downstream PD effects. Immunofluorescence assays were used to measure target inhibition (pY15-Cdk1/2) and DDR markers (e.g., γH2AX). Results: Arm B included 42 pts; of the 34 evaluable for response, 6 (18%) had a partial response (PR; 4 ovarian, 2 endometrial), and 20 (59%) had stable disease (SD; 2-26 cycles, mean 7). Of 10 evaluable BRCA-deficient pts, 2 had a PR (20%) and 6 had SD (60%; mean: 7.17 cycles). AZD1775 was well-tolerated, as previously reported. Three of 11 paired biopsies had elevated baseline pY15-Cdk (8-23% nuclear area positive [NAP]) and showed persistence of target suppression following the dosing break during days 6-8. Of the 8 pts with low baseline pY15-Cdk (0-3% NAP), 5 had elevated pY15-Cdk levels at day 8 (range: 192-1094%), suggesting a target rebound effect. Of the 8 pts with elevated pY15-CdK at baseline or day 8, 4 (50%) achieved a PR. Day 8 γH2AX activation was detected for 3 pts, all who progressed within 2 cycles. Preliminary PK data revealed low plasma drug concentrations at day 8 pre-dose (55 ± 50 nM; n = 4) relative to peak levels at day 5 (1697 ± 1062 nM; n = 9). Conclusions: Daily AZD1775 exhibits antitumor activity in pts with both BRCA-proficient and -deficient ovarian and endometrial cancer. Responses amongst pts with elevated pY15-Cdk levels at baseline or day 8 provide additional clinical/PD evidence for the presumed mechanism of action of AZ1775 on the cell cycle and suggest that levels of pY15-Cdk may be an important correlate of response. In contrast, γH2AX activation on day 8 demonstrates the presence of drug-induced DNA double strand breaks that are not associated with clinical drug activity. PK data confirmed substantially reduced plasma AZD1775 concentrations at day 8. Ongoing genomic and CTC analyses may uncover additional molecular determinants of response. NCT01748825. Supported in part by NCI Contract HHSN261200800001E. Citation Format: Arjun Mittra, Geraldine O'Sullivan Coyne, Shivaani Kummar, Khanh Do, Ashley Bruns, Lamin Juwara, Richard Piekarz, Larry Rubinstein, Sheila Prindiville, Elad Sharon, Howard Streicher, Tiffaney Hsia, Ganesh Mugundu, Jiuping Ji, Deborah Wilsker, Angie B. Dull, Brandon Miller, Robert J. Kinders, Ralph Parchment, James H. Doroshow, Alice P. Chen, Naoko Takebe. DNA damage response and therapeutic activity following once-daily administration of the Wee 1 inhibitor AZD1775 (adavosertib) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT099.