Disclosed is a Compound of formula (I) e.g. [(2-Hydroxy-4-oxo-5,6,7,8-tetrahydro-4H-9-thia-1,4a-diaza-fluorene-3-carbonyl)-amino]acetic acid;their pharmaceutically acceptable salts and their Isomers, stereoisomers, conformers, tautomers, polymorphs, hydrates, and solvates wherein the variables are defined in the specification for treating anemia.