BACKGROUND: Gastroesophageal cancer (GEAC) remains a major health burden and urgently needs novel therapeutic targets. The inhibition of CDK9's activity holds the potential to be a highly effective anti-cancer therapeutic. However, the functional role of CDK9, and its potential targeting in GEAC, remain largely unknown. OBJECTIVE: We aim to evaluate the potential of degradation CDK9 in GEAC treatment and explore its mechanisms. DESIGN: We evaluated the expression and distribution of CDK9 in GEAC tissue. We designed and synthesized novel CDK9 degraders using proteolysis targeting chimeras (PROTACs) strategy and selected the promising one for further anti-tumor activity evaluation both in vitro and in vivo. We evaluated the effects of CDK9 degradation on epigenetic reactivation, gene expression and chromatin accessibility. We evaluated the co-targeting of CDK9 and YAP/Tead signaling for GEAC treatment, especially for radiation-resistant tumor treatment. RESULTS: We demonstrated significantly elevated CDK9 expression in primary GEAC tumor tissues compared to normal tissues, in association with poor survival. We developed a novel CDK9 degrader, YX0597, reducing RNA Pol II Serine 2 phosphorylation, and inhibition MCL-1
this was accompanied by potent inhibition of GEAC cell growth, especially in radiation-resistant tumor cells. Mechanistically, YX0597 strongly enhanced chromatin accessibility, activating epigenetically silenced genes
and dramatically inhibited YAP/TEAD signaling. CDK9 closely interacts with YAP/TEAD signaling, and co-targeting these two mediators could be a novel treatment strategy for the treatment of GEAC. CONCLUSION: Our studies reveal a new avenue for targeting CDK9-hyperactivated GEAC tumors, especially in combination with YAP1/TEAD inhibition in radiation-resistant GEAC tumors. WHAT IS ALREADY KNOWN ON THIS SUBJECT?: Gastroesophageal cancer (GEAC), the third-leading cause of global cancer death and urgently needs novel targeted therapies. Targeted protein degradation has emerged as an attractive strategy to fight cancer, complementing the activity of traditional small-molecule inhibitors. Cyclin-dependent kinase 9 (CDK9) has been implicated in various cancers. Recently, CDK9 degraders have now been developed for future clinical benefit in targeting tumors with highly activated CDK9. WHAT ARE THE NEW FINDINGS?: In this study, we show that CDK9 expression was significantly elevated in primary GEAC tumor tissues including PDXs compared to normal tissues, in association with poor survival. We developed a novel CDK9 degrader XY0597 and proved that it inhibits the progression of GEAC by increasing chromatin accessibility and down-regulating YAP/Tead signaling. We also proved a strong connection between CDK9 and YAP/TEAD signaling and Co-targeting them synergistically inhibits GEAC tumor cell growth, especially in tumors with high YAP signaling activity, such as radiation-resistant cancer. HOW MIGHT IT IMPACT ON CLINICAL PRACTICE IN THE FORESEEABLE FUTURE?: Our data suggests that CDK9 is a viable anticancer target in GEAC. We demonstrate that a novel CDK9 degrader, YX0597, has high potential clinical application for GEAC treatment, especially in radiation-resistant cancer. We reveal the crosstalk of CDK9 and YAP/TEAD signaling, and that co-targeting these two mediators could be a new avenue for targeting CDK9-hyperactivated GEAC primary and radiation-resistant tumors.