Abstract: The invention is directed to pyrido[4,3-d]pyrimidin-5(6H)-one derivatives. Specifically, the invention is directed to compounds according to Formula I: wherein R1, R2, R3, and R4 are defined below. The compounds of the invention are inhibitors of PDK1 and can be useful in the treatment of disorders characterized by constitutively activated ACG kinases such as cancer and more specifically cancers of the breast, colon, and lung. Accordingly, the invention is further directed to pharmaceutical compositions comprising a compound of the invention. The invention is still further directed to methods of inhibiting PDK1 activity and treatment of disorders associated therewith using a compound of the invention or a pharmaceutical composition comprising a compound of the invention.