Abstract 6976: Identification of novel KAT6A/B inhibitors with enhanced antitumor activity and reduced hematologic toxicity

Abstract Introduction: KAT6A/B are promising targets for breast cancer therapy. A phase 1 study of the KAT6A/B inhibitor PF-07248144 showed encouraging clinical results; however, target-associated hematologic toxicity, particularly neutropenia, may hinder dose escalation. KAT6A/B inhibitors that exhibit suboptimal specificity towards other members of the KAT family may lead to increased hematologic toxicity. This study aims to identify potent and highly selective KAT6A/B inhibitors with "fast-on, fast-off" pharmacokinetic profiles in vivo, enabling sustained antitumor efficacy while minimizing hematologic toxicity. Methods: Cytotoxicity of the candidates was evaluated in ZR-75-1 cells using the CellTiter-Glo (CTG) assay. An in vitro acetyltransferase assay, employing radiolabeled acetyl-CoA as a substrate, was conducted to assess the inhibitory effects of the candidates. In vivo efficacy using mono-agent was tested in a ZR-75-1 breast cancer model, while synergy studies were performed in the patient-derived xenograft (PDX) model LD1-0009-362016, which was resistant to CDK4/6 inhibitors. Top candidates were screened in the WuXi Mini 44 safety panel at a concentration of 10 µM to identify potential off-target effects. Additionally, the inhibition of H3K23 acetylation in ZR-75-1 cells was analyzed by Western blotting. Results: HLX97-069/053 emerged as the top two candidates, exhibiting superior enzymatic inhibition and enhanced selectivity against KAT5/7/8, along with more potent cytotoxic effects in ZR-75-1 cells, in head-to-head comparisons with PF-07248144. The cytotoxicity of each compound was correlated with their potency to inhibit H3K23 acetylation. Both compounds displayed favorable ADME characteristics, such as high oral bioavailability and excellent pharmacokinetics. In a ZR-75-1 xenograft model, they demonstrated dose-dependent antitumor efficacy with minimal weight loss over a five-week treatment period. Notably, HLX97-069 resulted in less significant reductions in leukocyte, neutrophil, lymphocyte, and monocyte counts compared to PF-07248144 after five weeks of treatment. We propose that the more robust KAT6A/B inhibition and “relatively faster” clearance of HLX97-069 may contribute to its broader therapeutic window. Both compounds were well tolerated at 100 mg/kg in a single-dose acute toxicity study in rats. Additionally, HLX97-069 showed no off-target inhibition in safety panel screening. Compared to PF-07248144, HLX97-053 exhibited significantly stronger synergy with fulvestrant and palbociclib in the LD1-0009-362016 model. Conclusion: In summary, these preclinical data provide compelling evidence that we have identified novel KAT6A/B inhibitors with best-in-class potential. Confirmation of the final candidate will depend on the outcomes of the forthcoming pilot toxicity studies, with an IND application anticipated by the end of 2025. Citation Format: Rui Liu, Yushi Chi, Xiayan Zhang, Xuemei Tian, Chenqiang Jia, Zhiliang Lv, Huixin Yan, Qian Zou, Liping Han, Wan-Jen Yang, Jijun Yuan, Xiaoling Yuan, Zhiyu Ma, Yi Zhang, Yongqiang Shan, Chen Hu. Identification of novel KAT6A/B inhibitors with enhanced antitumor activity and reduced hematologic toxicity [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 6976.