Background: CCAAT/enhancer-binding protein β (C/EBPβ) is an important transcription factor in GBM cell maintenance and is associated with worse outcomes and resistance to chemotherapy through transcription of genes associated with cell cycle and DNA repair. C/EBPβ is also required for maintenance of immunosuppressive tumor-associated macrophages. ST101 is a first-in-class antagonist of C/EBPβ that promotes selective GBM cell death without impacting normal cell viability (Darvishi, 2022), and repolarization of macrophages toward an immunostimulatory program. In an all comers P1 study, ST101 was well-tolerated, showed significant tumor uptake, ~40 hr plasma half-life, and signal of efficacy (1PR +8SD). With strong rationale targeting C/EBPβ in GBM and evidence that ST101 crosses the BBB in preclinical biodistribution studies, cohorts in rGBM were explored. Methods: Pts with ≤2 recurrences of GBM were enrolled in a P2 cohort (n=30) and treated with 500mg QW. A WoO cohort of 6 pts with rGBM received 2-4 doses of ST101 500mg QW prior to surgery and resumed ST101 after surgery until progression (NCT04478279). Endpoints for the two cohorts included response, PFS and OS while the WoO cohort also included tissue analyses (GBM subtypes, tumor microenvironment (TME) characterization, ST101 tumor penetration, and C/EBPβ target engagement). Clinical data is available for both cohorts and tissue analysis is available for the WoO cohort. Results: Data is a snapshot as of February 1, 2024. ST101 was safe and well-tolerated in both cohorts with no discontinuations due to AEs. Pts in the P2 study show 30% DCR (1PR + 8SD), 20% 6-mo PFS and 37% OS at 1 year. Median duration of treatment is 3.4 mos with 2 pts stable and still on study for 10+ and 16+ mos. In the WoO study, efficacy from post-surgery MRI showed 67% DCR (2 PR + 2 SD) and ~50% 6-mo PFS. Median OS cannot be assessed at this early stage. The median duration of treatment is 6.02 mos with 2 pts still on study for 7+ and 10+ mos. Tissue compared before and after ST101 exposure shows ST101 uptake and C/EBPβ target engagement by IHC, and pathologic evidence of treatment effect, such as geographic necrosis, in 5/6 pts. Evaluation of tumor and TME characteristics in post-ST101 treatment tissue will be presented. Conclusions: Monotherapy ST101 in pts with rGBM has an excellent safety profile and results in comparable outcomes to current 2^nd line treatments. ST101 crosses the BBB, engages its target, and potentially induces treatment-related necrosis. These encouraging clinical activity, safety, and tissue findings, as well as preclinical data showing additive activity in combination with lomustine, support advancing ST101 to a randomized, placebo-controlled study comparing ST101 + lomustine vs. lomustine + placebo in rGBM. We aim to initiate this study in Q3 2024. Clinical trial information: NCT04478279.