Background: AZD1775 is an inhibitor of the WEE1 kinase, leading to loss of genome integrity. Pathogenic SETD2 mutations lead to genome-wide loss of histone H3K36 trimethylation (H3K36me3) and hypersensitivity to WEE1 inhibition with a synthetic lethal effect. AZD1775 was studied in 2 cohorts of patients: A) Patients (pts) with locally advanced/metastatic solid tumor malignancy other than clear cell renal cell carcinoma (ccRCC) and B) pts with locally advanced/metastatic ccRCC. Methods: Eligible pts had a locally advanced/metastatic cancer with progression on ≥ 1 prior systemic therapy (including prior tyrosine kinase inhibitor or checkpoint inhibitor for cohort B), and a pathogenic SETD2 mutation by CLIA-certified next generation sequencing panel. Pts received AZD1775 at a starting dose of 300 mg PO daily on days 1-5, 8-12 of each 21-day cycle. The primary objective was to determine the objective response rate (ORR) by RECIST 1.1 criteria in each cohort. Planned sample size was 9 pts in each cohort in stage 1 of a Simon two-stage design. Secondary objectives were to determine the clinical benefit rate (CBR) (defined as partial response or stable disease [SD] > 4 months), duration of response (DOR), and safety. Correlative assays include the evaluation of H3K36me3 mark by immunohistochemistry (IHC) in archival tumor tissue. Results: Between 05/2019 and 10/2021, 18 pts whose tumor harbored a pathogenic mutation in SETD2 were enrolled (9/cohort). The median age was 60 years (range 45 – 74), ECOG PS 0 in 8 (44.4%). One patient remains on treatment; 17 have discontinued (disease progression, n = 9; adverse event, n = 4; patient withdrawal, n = 2; patient death, n = 1). The median duration of treatment was 1.89 months (range 0.75 – 13.5). There were no confirmed objective responses and therefore enrollment was halted following accrual to stage 1. Tumor regression of any magnitude was observed in 5/17 (29%) evaluable pts. Stable disease was the best overall response in 10/17 (59%) evaluable pts, including 3 pts with SD for > 4 months. One patient with RCC remains on treatment with SD for 14+ months duration. The most common adverse events of any grade were nausea (59%), anemia (41%), diarrhea (41%), and neutropenia (41%). Twelve pts (67%) experienced a Grade ≥ 3 adverse event. Evaluation of H3K36me3 using IHC as an exploratory biomarker is underway. Conclusions: AZD1775 given as monotherapy failed to achieve an objective response in SETD2-altered RCC and other solid tumor malignancies. However, a subset of pts derived clinical benefit as manifested by tumor regressions and/or durable stable disease. Evaluation of AZD1775 in combination with other DNA damage-inducing agents that cause replication stress may be warranted, potentially with the integration of the H3K36me3 mark as a biomarker for patient selection. Clinical trial information: NCT03284385.