– AY-1251152 is a potent and highly selective PTEF/CDK9 inhibitor. BAY1251152 binds to and blocks the phosphorylation and kinase activity of CDK9, thereby preventing PTEFb-mediated activation of RNA Pol II and leading to the inhibition of gene transcription of various anti-apoptotic proteins. This may cause cell cycle arrest and induce apoptosis, which may lead to a reduction in tumor cell proliferation.