Background: Elimusertib is a highly selective oral inhibitor of the Ataxia telangiectasia and Rad3 related (ATR) kinase, which is required for efficient end-joining repair of double-stranded breaks, particularly during DNA replication. Many pediatric cancers involve potential ATR susceptibilities such as increased replication stress due to oncogenic fusions, specific DNA damage repair (DDR) deficiencies, oncogenic activation of DNA damage and nucleases such as PGBD5, as well as alternative lengthening of telomeres. We report the initial phase 1 results of PEPN2112 (NCT05071209), an ongoing pediatric phase 1/2 clinical trial of elimusertib monotherapy. Methods: Eligible patients (pts) included children, adolescents, and young adults with relapsed or refractory tumors with: (1) Ewing sarcoma or related EWSR1 fusion positive tumors (EWS), (2) alveolar rhabdomyosarcoma, PAX3-FOXO1 fusion positive (aRMS), or (3) non-CNS primary tumors exhibiting specific DDR pathway defects anticipated to sensitize to ATR inhibition. For pts < 18 years (yrs), a rolling-six design was used to evaluate elimusertib administered BID x3 days/week of a 28-day cycle starting at the adult equivalent recommended phase 2 dose (RP2D) of 24 mg/m²/dose (max 40 mg) with a potential de-escalation to 18 mg/m²/dose (max 30 mg). The pediatric RP2D determination was based upon dose limiting toxicity (DLT) criteria (CTCAEv5) during cycle 1. Pharmacokinetic (PK) studies were performed. Pts > 18 yrs could enroll onto the phase 2 cohorts at the adult RP2D simultaneously with the pediatric phase 1. Once the pediatric RP2D was defined, pts < 18 yrs accrued to the phase 2 cohorts and a PK expansion for pts < 12 yrs was opened. Results: 8 eligible pts were enrolled (4 males) on the phase 1 part of the study. 2 pts were inevaluable for DLT as they received < 75% of protocol therapy. Median (range) age was 13 (10-17) yrs. Diagnoses included: EWS (n = 4) and 1 pt each with aRMS, adrenocortical carcinoma, clear cell sarcoma, and osteosarcoma. At 24 mg/m²/dose, no DLTs were observed. Toxicities were hematologic, with grade 3 toxicities including anemia (n = 3), neutropenia (n = 4), and decreased white blood cells (n = 1). One pt experienced grade 4 lymphopenia. There were no grade 3/4 non-hematologic toxicities. Day 1 mean ± SD values of T_max, C_max, AUC_0-8h and t_1/2 were 1.6 ± 0.5 h, 1532 ± 385 ng/mL, 5863 ± 1841 h*ng/mL and 3.6 ± 0.9 h, respectively. Conclusions: Elimusertib-related toxicities were primarily hematologic. The RP2D for elimusertib monotherapy in pediatric pts is 24 mg/m²/dose (max 40 mg) orally BID x3 days/week continuously, analogous to dosing in adults with similar PK. Phase 2 and PK expansion cohorts as well as pharmacodynamic studies are ongoing. Clinical trial information: NCT05071209.