PROJECT SUMMARY/ABSTRACT This application is being submitted in response to the Notice of Special Interest (NOSI) identified as “NOT-CA- 24-044.” In recent years, antibody-drug conjugates (ADCs) such as the HER2 ADC trastuzumab deruxtecan (T- DXd) have emerged as a powerful therapeutic strategy across solid tumors, with dozens of new ADCs in development. Thus, we have strong interest in developing ADCs as a therapeutic strategy for uterine serous carcinoma (USC), in line with our long-term goal to optimize biomarker-driven therapy in gynecologic diseases. To this end, we propose two specific aims: Aim 1. To determine the expression pattern of emerging ADC targets, using a tissue microarray of 100 endometrial cancers containing at least 50 USCs. We will perform immunostaining for emerging ADC targets using well-established assays, several of which are already deployed in the CLIA environment. We hypothesize that many ADC targets are expressed in USC, and better understanding of the relative expression pattern in USC may seed better therapeutic trials. Aim 2. To determine the antitumor activity of T-DXd or datopotamab deruxtecan (Dato-DXd), alone and in combination with the WEE1 inhibitor azenosertib in vitro and in vivo, using well-characterized models of USC. We hypothesize that USC will be sensitive to many ADCs such as T-DXd and Dato-DXd, and that azenosertib may enhance their efficacy, based on preliminary work that WEE1 inhibition can enhance activity of T-DXd. Of note, the WEE1 inhibitors adovasertib and azenosertib have shown intriguing activity in USC, with molecular alterations such as TP53 mutation status thought to contribute to increase sensitivity. Taken together, we will demonstrate the frequency of expression of different ADC targets and their overlap in USC. We will also test whether we can enhance the activity of T-DXd and Dato-DXd with WEE1 inhibitor combinations. If so, this work will provide preliminary data not only for planning of clinical trials with T-DXd and azenosertib, but will also allow us to pursue the same combination strategy using other ADCs with topoisomerase payloads.