PATH-45. TREATMENT OF FGFR1 – ALTERED WHO GRADE 1 GLIONEURONAL TUMORS WITH FGFR INHIBITOR ERDAFITINIB

Abstract INTRODUCTION Low-grade mitogen-activated protein kinase (MAPK) pathway-altered glioneuronal tumors carry distinct molecular alterations, which may represent therapeutic opportunities to avoid or delay toxic treatments like radiation. Erdafitinib is a pan-FGFR tyrosine kinase inhibitor, regulatorily approved in recurrent urothelial carcinoma, and is under clinical investigation for recurrent FGFR-altered gliomas (NCT05859334). METHODS We report two patients with FGFR1-altered rosette-forming glioneuronal tumors (RGNT) treated with erdafitinib. RESULTS Patient 1 is a 32-year-old woman who presented with obstructive hydrocephalus and a non-enhancing pineal tumor. She underwent a subtotal resection, with histopathology initially consistent with a low grade dysembryoplastic neuroepithelial tumor. She was under observant management for 7 years, and developed worsening symptoms with slow non-enhancing growth, although stable per RANO low grade criteria. The initial tissue was sent for next-generation sequencing (NGS) and DNA methylation profiling, revealing an FGFR1 pathogenic variant with DNA methylation profiling matching with RGNT. Patient began Erdafitinib 8 mg daily, and had stable disease per RANO at 6 months. She developed CTCAE grade 1 toxicities of subretinal fluid, fatigue, dry skin, and paronychia. Patient 2 is a 34-year-old man diagnosed 1 year prior with a tectal tumor, who underwent biopsy due to non-measurable/ non-enhancing progression. Histopathology was consistent with RGNT. NGS revealed FGFR1 mutation, PIK3CA mutation, and NF1 pathogenic variant, described in the literature as characteristic of RGNT. There was insufficient tissue for DNA methylation profiling. Patient started erdafitinib 8 mg daily and remains clinically stable for 1 month. He developed asymptomatic hyperphosphatemia (CTCAE grade 1). Both patients have tolerated treatment without interruptions or dose reductions. CONCLUSION Erdafitinib is a well-tolerated and potentially efficacious treatment of FGFR1- altered RGNT, and may avoid the need for radiation. Diagnosis of MAPK- altered low-grade glioneuronal tumors, specifically RGNT, is increasingly reliant on molecular data, and can significantly impact treatment options.