CDK9-Cyclin T1 PPI-IN-1 (Compound B19) is a selective inhibitor of the CDK9-Cyclin T1 protein-protein interaction (PPI), suppressing cell proliferation in TNBC MDA-MB-231 cells with an IC50 of 0.044 μM and inducing apoptosis. This compound diminishes CDK9 transcriptional activity and downregulates phosphorylation of RNA Pol II CTD ser2. In vivo, CDK9-Cyclin T1 PPI-IN-1 effectively impedes tumor growth in a TNBC 4T1 mouse model [1].