CDK9, or cyclin dependent kinase 9, is a serine/threonine kinase and a key component of the P-TEFb complex. It phosphorylates RNA polymerase (RNAP) II and other transcription factors, regulating gene transcription elongation in numerous physiological processes such as development, differentiation, and cell fate responses [6,7]. Aberrations in its activity have been linked to various cancers, making it an attractive therapeutic target [2].

In hepatocellular carcinoma (HCC), inhibition of CDK9 blocks PINK1-PRKN-mediated mitophagy by regulating the SIRT1-FOXO3-BNIP3 axis, leading to mitochondrial dysfunction and cell death. A novel CDK9 inhibitor, oroxylin A (OA), shows strong therapeutic potential against HCC and can overcome drug resistance [1]. In cancer research in general, there is intensive development of small-molecule CDK9 inhibitors and new strategies like PROTACs [2]. Pharmacological perturbation using selective CDK9 inhibition or degradation reveals that kinase degradation can have distinct effects compared to inhibition [3]. The PP2A-Integrator-CDK9 axis fine-tunes transcription, and targeting this axis can be a therapeutic strategy in cancer [4]. In solid tumors, CDK9 overexpression correlates with cancer development, and many solid cancers depend on its activity for oncogenic signaling [5]. In multiple myeloma, CDK9 inhibitors can overcome resistance and prolong survival, and in acute myeloid leukemia, targeting the CDK9 pathway is an attractive approach [8,9].

In conclusion, CDK9 is essential for regulating transcriptional elongation in key biological processes. Its dysregulation is associated with multiple cancers. Studies using inhibitors and other functional perturbation methods in various cancer models, including in vivo-like patient-derived tumor xenograft (PDX) models in HCC [1], have shown its potential as a therapeutic target in cancer treatment. These findings provide insights into the biological functions of CDK9 and open up new avenues for developing effective cancer therapies.