Atrt-06. Pediatric Ini1-Negative Atypical Teratoid Rhabdoid Tumors Treated With Tazemetostat as Maintenance Therapy in the Relapse Setting: An Italian Single-Centre Experience

Abstract BACKGROUND Rhabdoid tumors are aggressive malignancies, primarily affecting children ≤3 years. Common sites of primary localization are kidney, central nervous system (atypical teratoid rhabdoid tumor -AT/RT-) or soft tissues. Although high-dose chemotherapy (HD-CT) with radiotherapy (RT) has significantly increased EFS and OS rates, more than 50% of patients diagnosed with AT/RT still die of tumor progression. METHODS From February 2021 to January 2024 three patients with AT/RT were treated with tazemetostat after first-line treatment (complete resection, CT, RT). RESULTS At diagnosis, patient 1 presented with a lesion in the lamina quadrigemina and a renal mass; patient 2 and 3 with a lesion in the posterior cranial fossa. Median age at diagnosis was 8 months (3-34 months). Patient 1 died due to the progression of the disease (OS 24 months); patient 2 ended treatment with tazemetostat on December 2023; for patient 3 the treatment is still ongoing. Patient 1 started tazemetostat at the end of first-line treatment, as maintenance therapy. Before starting treatment with tazemetostat, patient 2 performed a re-irradiation on the tumor bed; patient 3 performed surgery and then RT on a new-onset spinal lesion. Median time of treatment with tazemetostat was 9 months (6-12 months). Patient 1 showed an EFS of 9 months. EFS of patients 2 and 3 has exceeded this timepoint. For patient 2 the treatment is ongoing (in total 6 months of treatment), with a complete response of the disease (CR); patient 3 ended treatment after 12 months, with a CR. Tazemetostat was overall well tolerated. CONCLUSIONS Despite the small number of patients, we feel able to state that the use of tazemetostat as a single agent in the relapse setting could represent a therapeutic promise as maintenance therapy. Future studies with a larger number of patients are needed to confirm this hypothesis.