BAY1143572 is a highly selective, potent and orally available inhibitor of PTEFb/CDK9 currently in Phase I. BAY1143572 inhibits MYC and shows convincing anti-tumor activity in multiple xenograft models by the induction of apoptosis. BAY 1143572 had potent and highly selective PTEFb-kinase inhibitory activity in the low nanomolar range against PTEFb/CDK9 and an at least 50-fold selectivity against other CDKs in enzymatic assays. BAY 1143572 also showed single agent in vivo efficacy at tolerated doses in various xenograft tumor models in mice and rats upon once daily oral administration.