Background: A-CE has demonstrated improved overall survival in ES-SCLC, but there are currently no approved therapies targeting genomic alterations in SCLC. Mutations in acetyltransferases CREBBP and EP300 occur at a frequency of 15% and 13%, respectively, in SCLC, and PDX models with CREBBP mutations were demonstrated to be susceptible to targeting with HDAC inhibitors. Ent, a class I selective HDAC inhibitor, has also demonstrated clinical activity in a combination with pembrolizumab in patients with NSCLC and uveal melanoma with minimal hematologic toxicity at RP2D of 5 mg PO weekly. We conducted a phase I trial to evaluate the combination of Ent with A-CE for ES-SCLC, NCI ETCTN 10399. Methods: Patients (pts) with treatment-naïve ES-SCLC, stable or treated brain metastases, ECOG ≤2 were enrolled and treated on up to 4 dose levels of Ent. Allocation to cohorts was determined using Bayesian optimal interval (BOIN) design targeting a MTD with a DLT rate of 20%. Dose levels (DL) included Ent 2 mg, 3 mg, or 5 mg PO weekly on day (d) 1 in addition to 4 cycles of A-CE (A 1200 mg, C AUC 5, E 100 mg/m2 d 1-3) followed E+A for 1 year. Pre-treatment tissue and plasma collected for WES. Results: 3 pts were enrolled and treated at DL1 with Ent 2 mg. Pts were age 69-83, 2 male, 1 female, 2 were ECOG 1, 1 was ECOG 0, and 1 with prior SRS radiation for brain metastases. 2 of 3 experienced DLTs during cycle 1: (1) Grade (Gr) 4 febrile neutropenia after 2 doses of Ent and (1) Gr 5 sepsis after 1 dose of Ent. BOIN design required stopping accrual to dose level 1 and the trial was closed to further accrual. The pt without DLT experienced grade 3 thrombocytopenia after 2 doses of Ent, but recovered after holding cycle 1, day 15 Ent. This patient experienced Gr 3 neutropenia during cycle 2. The most common adverse events were anemia (3), neutropenia (3), thrombocytopenia (2), leukopenia (2), hypocalcemia (2). Of these, most were Gr 3-4: anemia (1), neutropenia (3), thrombocytopenia (2), leukopenia (1), hypocalcemia (1). Conclusions: The combination of low dose Ent 2 mg PO weekly + AC-E is unsafe and resulted in early onset and severe neutropenia, thrombocytopenia in the first 1-2 weeks and ≥Gr 3 neutropenia and thrombocytopenia prior to completing 2 cycles of treatment for all pts. There is no role for further exploration of entinostat with carboplatin, etoposide, and atezolizumab. Clinical trial information: NCT04631029.