This invention relates to substituted pyrido(3,2-d)pyrimidine derivatives, their pharmaceutically acceptable salts, N-oxides, solvates, pro-drugs and enantiomers, possessing unexpectedly desirable pharmaceutical properties, in particular which are highly active immunosuppressive agents, and as such are useful in the treatment in transplant rejection and/or in the treatment of certain inflammatory diseases. These derivatives are also useful in preventing or treating cardiovascular disorders, disorders of the central nervous system, TNF-.alpha. related disorders, viral diseases (including hepatitis C), erectile dysfunction and cell proliferative disorders.