Abstract 7550: KB-0742, an oral highly selective CDK9 inhibitor, demonstrates preclinical activity in transcription factor fusion driven adenoid cystic carcinoma patient-derived models

Abstract Adenoid Cystic Carcinoma (ACC) is an aggressive rare type of cancer of the secretory glands with no approved targeted therapy and high unmet clinical need. Deregulated transcription driven by MYB-NFIB or MYBL1-NFIB transcription factor (TF) fusions are a hallmark of ACC pathogenesis. More aggressive disease and resistance to therapy has been associated with co-mutation in NOTCH. Although direct targeting of oncogenic TF fusions has remained challenging, targeting of their activity via transcriptional cofactors has emerged as an attractive and clinically actionable strategy. Cyclin-dependent kinase 9 (CDK9) is a key potentiator of TF activity via its ability to act both as an upstream regulator of TF expression and a downstream cofactor. KB-0742 is a potent, selective, and orally bioavailable inhibitor of CDK9 with a long plasma half-life. KB-0742 is being studied in an ongoing Phase 1/2 study (NCT04718675) in advanced solid tumors including ACC. KB-0742 has demonstrated on-mechanism, single agent anti-tumor activity and a manageable safety profile in heavily pre-treated patients (0-11 prior lines of therapy) with transcriptionally addicted solid tumors. Here we present the preclinical rationale for targeting of oncogenic fusion TF activity in ACC. XenoSTART Patient-Derived Xenograft (XPDX) models were established from viable human tumor tissue for in vivo and ex vivo testing. Cell viability was assessed by CellTiter GLO reagent and phase imaging. Changes in MYB-fusion transcripts were assessed by RNA-Seq. Tumor Fragments (~70 mg) from ACCx5M1, ACCx6, ACCx9, ACCx11 models were implanted into athymic nude mice. Animals were treated with either vehicle (normal saline) or 60 mg/kg KB-0742 on a Q3D/week until tumors reached 2500 mm3 or up to 60 days post start of treatment. In a retrospective analysis using real world data, ACC has a low incidence of genomic instability and high rates of MYB fusions detectable by RNA-seq, making immunotherapy challenging but creating an opportunity for targeting MYB transcription. In primary MYB-fusion positive and NOTCH co-mutated patient-derived spheroid models, KB-0742 treatment induced strong antiproliferative activity and cytotoxicity. Importantly, KB-0742 demonstrated stronger cytotoxic effects when compared to historical control agents. In XPDXs, CDK9 inhibition with KB-0742 resulted in antiproliferative activity and stronger tumor growth inhibition in MYB-fusion positive and NOTCH co-mutated tumor models compared to MYB-fusion positive only models. These data demonstrate KB-0742 is effective in preclinical models of ACC suggesting KB-0742 may be a promising therapeutic option for ACC patients. KB-0742 is currently being evaluated in a phase 1/2 dose-escalation and cohort expansion trial in patients with ACC and other transcriptionally addicted tumors (NCT04718675). Citation Format: Michael R. McKeown, Luis A. Carvajal, Tressa R. Hood, Nicole S. Burr, Jeffrey Kaufman, Adam Boynton, Kameron Mori, Tessa DesRochers, Michael Wick, Charles Y. Lin, Jorge F. DiMartino. KB-0742, an oral highly selective CDK9 inhibitor, demonstrates preclinical activity in transcription factor fusion driven adenoid cystic carcinoma patient-derived models abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 7550.