The present invention is directed to novel pyrido[4,3-d]pyrimidin-4(3H)-one derivatives and pharmaceutically acceptable salts thereof of structural formula I wherein the variables R1, R2, R3, R4 and R5 are as described herein. Also provided are pharmaceutical compositions comprising the compounds of formula I as well as methods of treatment employing compounds of formula I to treat a disease or disorder characterized by abnormal bone or mineral homeostasis such as hypoparathyroidism, osteoporosis, osteopenia, periodontal disease, Paget's disease, bone fracture, osteoarthritis, rheumatoid arthritis, and humoral hypercalcemia of malignancy.