Abstract 1727: Sequencing MEK inhibitor therapy after tovorafenib in <i>BRAF</i> fusion-driven cancers: Preclinical evidence of sustained tumor regression

Abstract BRAF fusions and BRAF V600 point mutations are oncogenic drivers found in many pediatric and adult cancers, and constitutively activate the downstream MAPK pathway (Jones et al 2009, Yaeger and Corcoran 2019). Tovorafenib is the first and only type II RAF inhibitor approved for the treatment of BRAF-altered relapsed/refractory pediatric low-grade glioma (pLGG) in patients ≥ 6 months of age. In the ongoing ph 2 study, FIREFLY-1, tovorafenib demonstrated robust clinical activity in patients with BRAF-altered pLGG who progressed on systemic therapy, which may have included MEK inhibitors (Kilburn et al 2024). This preclinical study evaluated if tovorafenib-sensitive BRAF fusion tumors that have regrown during a drug holiday would also respond to a MEK inhibitor. The potential emergence of resistance with long-term exposure to tovorafenib was also explored. Due to the lack of availability of pLGG KIAA1549::BRAF fusion (the most commonly occurring BRAF fusion in pLGG) PDX models, in vivo experiments were conducted using a AGK::BRAF fusion melanoma PDX model. Mice bearing AGK::BRAF fusion melanoma tumors were treated with a clinically relevant dose (25 mg/kg oral, once daily) of tovorafenib for 14 days or until complete regression of tumors was observed. An off-treatment period was then initiated, during which tumors regrew. Once individual tumors regrew to 100-200 mm3 size (within approximately 2 weeks), mice were treated for 14 days with a MEK inhibitor (trametinib; 0.3 mg/kg oral, once daily), or tovorafenib was resumed. Complete tumor regression was observed in all tumors in response to either trametinib or tovorafenib retreatment. Further evaluation of other MEK inhibitors and different dosing sequences of MAPK inhibitors is ongoing. In addition, the efficacy of tovorafenib in a CDC42SE2::BRAF anaplastic pleomorphic xanthoastrocytoma (APXA) PDX model is also being investigated. To explore potential mechanisms of acquired resistance, we also evaluated whether resistant tumors would emerge with continuous dosing of tovorafenib. Tovorafenib was administered (25 mg/kg oral, once daily) to mice bearing AGK::BRAF fusion tumors for 90 days. Over the entire course of treatment, significant tumor regression was maintained with no emergent tumors. In conclusion, our study provides evidence for the efficacy of sequencing MEK inhibitor treatment after tovorafenib in a PDX model of AGK::BRAF fusion melanoma. Furthermore, long-term efficacy was maintained with continuous dosing of tovorafenib in vivo, with no signs of emerging resistance preclinically, supporting the hypothesis that acquired resistance may be less common in BRAF fusion driven tumors treated with tovorafenib. Citation Format: Samantha Perino, Yujin Wang, Shubhra Rastogi, Nur P. Damayanti, Karen E. Pollok, Sandya Govinda Raju, Mark W. Kieran, Samuel C. Blackman, Eleni Venetsanakos. Sequencing MEK inhibitor therapy after tovorafenib in BRAF fusion-driven cancers: Preclinical evidence of sustained tumor regression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1727.