Phase IB study of tovorafenib for children with relapsed/recurrent low-grade gliomas and other MAPK pathway activated tumors.

10001 Background: LGGs present a therapeutic challenge often occurring in areas of brain and spine not amenable to resection and carrying high morbidity with currently available therapies. The majority harbor alterations in the MAPK pathway. Our study (NCT03429803) aimed to establish the recommended phase 2 dose (RP2D) and assess the safety, tolerability and preliminary efficacy of type II RAF-inhibitor tovorafenib in such tumors. Methods: Eligible patients were > 1 year and 1.5 m 2 based on preliminary exposure-efficacy data. Toxicities were graded according to CTCAE version 5. Patients with complete response (CR), partial response (PR) or stable disease (SD) were designated responders. Results: We treated 35 eligible patients, including 20 KIAA1549:BRAF-, 9 BRAF V600E- and 1 FGFR1-altered tumors; 5 possessed novel RAF fusions. Histologically, there were 27 grade 1 gliomas, 3 anaplastic pleomorphic xanthoastrocytomas, 3 pilomyxoid astrocytomas, 1 high-grade glioma and 1 soft tissue sarcoma. There were 6 grade 3 DLTs (2 fatigue, 3 rash, 1 menorrhagia) in 30 evaluable patients: 3 in each BSA subgroup, all at 530 mg/m 2 /dose. Using AAP guidelines for minimum pubertal peak growth velocities (GV) in males (7 cm/year) and females (6 cm/year), decreased GV (median 0.09 cm/year; range 0.02-0.3) was an unexpected, related AE observed in all 13 of 13 pre-pubertal females 6 months of protocol 1B therapy. Three of 13 patients with decreased GV were treated at 420 mg/m 2 . Three of the remaining 10 patients with decreased GV originally treated at 530 mg/m 2 were dose reduced for other toxicities and hence, 6 of 13 patients with decreased GV were treated at the RP2D. For patients with follow-up data available, GV appears to show recovery off drug. The 2S-Sub-TITE model recommended 530 mg/m 2 for BSA ≤1.5m 2 and 420 mg/m 2 for BSA > 1.5m 2 . Dose modifications were required in 8/22 versus 1/13 patients treated at 530 and 420 mg/m 2 , respectively. Twenty-seven of 35 patients were evaluable for objective response performed by an independent single reader. Overall, 25 patients had SD, PR or CR with median duration of response of 11.3 months (range 0.03-35.4) at data cutoff. Conclusions: Oral weekly tovorafenib is well tolerated and shows preliminary efficacy. Decreased GV was observed in pre-pubertal patients on drug and warrants further investigation to understand the mechanism of action. Based on the number of dose modifications required for toxicity, the chosen RP2D for weekly oral dosing of tovorafenib was 420 mg/m 2 . Clinical trial information: NCT03429803 .