The synthesis and evaluation of new analogues of thieno [2, 3-d] pyrimidin-4-yl hydrazones are described. 2-Pyrdinecarboxaldehyde [6-(tert-butyl) thieno [2, 3-d] pyrimidine-4-yl] hydrazone derivatives have been identified as cyclin-dependent kinase 4 (CDK4) inhibitors. The potency, selectivity profile, and structure–activity relationship of this series of compounds are discussed.