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The ditosylated salt of A51, an orally bioavailable inhibitor of casein kinase 1alpha (CK1alpha) and cyclin-dependent kinases 7 and 9 (CDK7 and CDK9), with potential antineoplastic activity. Upon administration, BTX-A51 binds to and inhibits the activity of CK1alpha, CDK7, and CDK9. Blocking the phosphorylation and kinase activity of CK1alpha prevents the enhanced binding of murine double minute X (MDMX) to p53, the formation of CK1alpha and MDM2 complex, and the resulting inhibition of p53. This induces p53-mediated cell cycle arrest, slowing tumor cell proliferation. Blocking the phosphorylation and kinase activity of CDK7 and CDK9 prevents the positive transcription elongation factor b (PTEFb)-mediated activation of RNA polymerase II (RNA Pol II) and leads to the inhibition of gene transcription of various anti-apoptotic proteins. This also induces cell cycle arrest and apoptosis, slowing tumor cell proliferation. CK1alpha, a serine/threonine kinase and a leukemic stem cell target, acts as a tumor suppressor in several cancers through the negative regulation of Wnt/beta-catenin signaling and p53. It negatively regulates p53 by phosphorylating MDMX, thus enhancing binding of MDMX to p53, as well as by forming a complex with MDM2. CDK7, a serine/threonine kinase, plays a role in controlling cell cycle progression, transcriptional regulation, and promotes the expression of key oncogenes such as c-Myc through the phosphorylation of RNA Pol II. CDK9, also a serine/threonine kinase, regulates elongation of transcription through phosphorylation of RNA Pol II at serine 2 (p-Ser2-RNAPII). It is upregulated in various tumor cell types and plays a key role in the regulation of RNA Pol II-mediated transcription of anti-apoptotic proteins. Tumor cells are dependent on anti-apoptotic proteins for their survival.
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