Two series of pyrazolo[4,3-d]pyrimidin-7-ones and pyrido[2,3-d]pyrimidin-4-ones were designed, synthesised, and evaluated for their antibacterial activities and CDKs inhibitory activities. The pyridazine derivative: 6-phenyl-5-phenylhydrazono-2,3,4,5-tetrahydropyridazine-3,4-dione (3a) revealed activity against Staphylococcus aureus as Gram-positive bacteria while compound 2-(2- Ethoxyphenyl-5-Phenylpiperazinosulfonamido)-3H-pyrido [2,3-d]pyrimidin-4-one (13c) was showing moderate antifungal activity against Candida albicans.