The present invention relates to substituted xanthines of general formulawherein R1to R3are defined as in claims 1 to 16, the tautomers, the stereoisomers, the mixtures, the prodrugs thereof and the salts thereof which have valuable pharmacological properties, particularly an inhibiting effect on the activity of the enzyme dipeptidylpeptidase-IV (DPP-IV).本發明係關於以下通式之經取代黃嘌呤其中R1至R3均如申請專利範圍第1至16項中之定義,其互變異構物、立體異構物、混合物、前體藥物及其鹽,其具有有價值之藥理學性質,特別是對於酵素二肽基肽酶-IV(DPP-IV)活性之抑制作用。