PLANETTE: A modular phase IIa multicenter open-label study evaluating the ATR inhibitor ceralasertib (AZD6738) in ATM mutant advanced solid tumors.

TPS189 Background: Ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) are kinases that orchestrate cellular responses to DNA damage. ATM is primarily activated by DNA double strand breaks, and ATR is recruited to regions of single stranded DNA that arise due to collapsed or stalled replication forks. Although ATM and ATR are activated by distinct pathways, their downstream targets and effects partially overlap to activate cell cycle checkpoints and DNA damage repair. Ceralasertib is a potent, oral, selective inhibitor of ATR. Pre-clinical studies have consistently demonstrated synthetic lethality of ATR inhibitors, including ceralasertib, in ATM-deficient models across multiple tumor types (Kwok et al 2017, Min et al 2017, Lloyd et al 2020, Hustedt et al 2019). Early clinical evidence of efficacy is from a phase I study of ceralasertib in combination with olaparib in relapsed, refractory cancer (NCT02576444). Amongst 5 participants with a range of solid tumors harboring deleterious ATM mutations, there was 1 complete response, 3 stable diseases (1 with minor response 20-30%) and 1 patient with disease progression. Two participants had clinical benefit ongoing for more than 12 months (Eder et al 2019). ATM deficiency may be detected through genomic testing for loss-of-function alterations in the gene, or through immunohistochemical methods to detect loss of protein expression. Genomic alterations in ATM occur across multiple solid tumor types, including approximately 4% of advanced prostate cancers. Methods: PLANETTE (clinicaltrials.gov identifier (NCT 04564027) is a modular phase 2a multicenter open-label study investigating DNA-damage response agents in patients with advanced cancers that harbor molecular alterations. Module 1 will study the ATR inhibitor ceralasertib in tumors with deleterious or suspected deleterious mutations in ATM. Patients will be identified at cancer centers which routinely perform molecular profiling to detect ATM mutations. Central confirmation of ATM mutation by NGS and ATM IHC testing will be conducted retrospectively. Cohort A will enroll ~25 patients with advanced solid tumors (except NSCLC and prostate cancer), cohort B will include ~27 patients with metastatic CRPC who have previously progressed on a novel hormonal agent. The primary endpoints are investigator assessed ORR in Cohort A and composite response that includes radiographic, PSA, and CTC response per PCWG criteria in Cohort B. Secondary endpoints include duration of response, and progression free survival. Pharmacodynamic and other biomarkers will be explored. Enrolment is planned to start in November 2020. Clinical trial information: 04564027.