The design, synthesis, and evaluation of novel thieno [2, 3-d] pyrimidin-4-yl hydrazone analogues as cyclin-dependent kinase 4 (CDK4) inhibitors are described. In continuing our program aim to search for potent CDK4 inhibitors, the introduction of a thiazole group at the hydrazone part has led to marked enhancement of chemical stability. Furthermore, by focusing on the optimization at the C-4′ position of the thiazole ring and the C-6 position ...