Abstract B110: A novel BRD4 degrader, MT-4561, degrades the BRD4-NUT fusion protein <i>in vitro</i> and induces complete tumor regression in a subcutaneous NUT carcinoma xenograft model

Abstract Background: NUT carcinoma is a rare and aggressive malignancy driven by the BRD4-NUT fusion protein, which promotes oncogenesis through potent transcriptional dysregulation. Due to its poor prognosis and the lack of effective treatment options, there remains a critical unmet medical need. MT-4561, a novel BRD4 degrader, selectively targets the BRD4-NUT fusion protein and offers a promising therapeutic approach for patients with NUT carcinoma. Methods: To evaluate the therapeutic potential of MT-4561 for NUT carcinoma, a series of in vitro and in vivo studies were conducted. In vitro cytotoxicity assays were performed using NUT carcinoma cell lines to assess its potency to degrade the BRD4-NUT fusion protein. In vivo efficacy was evaluated in both cell line-derived xenograft (CDX) and patient-derived xenograft (PDX) mice models of NUT carcinoma. Additionally, pharmacokinetic study was carried out in CDX mice models to characterize the systemic exposure of MT-4561. Results: In vitro studies: MT-4561 demonstrated potent antitumor activity against Ty-82 cells, a NUT carcinoma cell line harboring the BRD4-NUT fusion protein, and showed greater efficacy than the BET inhibitor JQ1. In addition, MT-4561 effectively degraded the BRD4-NUT fusion protein and suppressed the expression of c-MYC, a downstream target of BRD4, with sustained inhibition observed until 168 hours after treatment.In vivo studies: MT-4561 exhibited dose-dependent antitumor effects in Ty-82 CDX mice model following once-weekly intravenously administration. Similarly, MT-4561 demonstrated potent antitumor activity, including complete tumor regression, in two PDX mice models of NUT carcinoma: J-PDX0537 (treatment-naïve) and J-PDX0196 (treatment-resistant), also with weekly intravenous dosing. Pharmacokinetic analysis in PDX mice models revealed that although MT-4561 was rapidly cleared from the blood, its concentration remained high in tumor tissue until 168 hours post-administration. Besides, MT-4561 demonstrated antitumor activity not only against BRD4-NUT cell lines but also against BRD3-NUT cell lines in in vitro cytotoxic study. This finding suggests that MT-4561 may degrade BRD4 expressed in BRD3-NUT cells, leading to suppression of c-MYC expression. Conclusions: MT-4561 exhibited potent antitumor efficacy against NUT carcinoma in both in vitro and in vivo studies. Notably, MT-4561 demonstrated activity not only against BRD4-NUT cells but also against BRD3-NUT cell lines. This effect is considered to be mediated through suppression of c-MYC (expression). Since approximately 75% of NUT carcinoma patients harbor either the BRD4-NUT or BRD3-NUT fusion gene, MT-4561 may represent a promising therapeutic option for the majority of patients with this disease. Citation Format: Masahiko Tokura, Yasutaka Saito, Shinsuke Ooike, Tsuyoshi Takahashi, Atsushi Toratani, Minoru Tanaka, Ryo Fujita, Ryousuke Ide, Michael Dydo. A novel BRD4 degrader, MT-4561, degrades the BRD4-NUT fusion protein in vitro and induces complete tumor regression in a subcutaneous NUT carcinoma xenograft model [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr XX.