Fused cyclic pyrimidine compounds, including tautomers thereof, and pharmaceutically acceptable salts, prodrugs, solvates and hydrates thereof, are disclosed having the general Formula I: These compounds are useful in methods for treating cancer, selectively targeting cancerous cells via the proton coupled folate transporter, folate receptor alpha, and/or folate receptor beta pathways, inhibiting GARFTase in cancerous cells, and selectively targeting activated macrophages in a patient having an autoimmune disease, such as rheumatoid arthritis.