The present invention relates to a series of prodrugs of inhibitors of DP-IV with improved properties. The compounds can be used for the treatment of a number of human diseases, including impaired glucose tolerance and type II diabetes. The compounds of the invention are described by general formula (1); wherein R1 is H or CN; R2 is selected from CH2R5, CH2CH2R5 and C(R3)(R4)—X2—(CH2)aR5; R3 and R4 are each independently selected from H and Me; R5 is selected from CON(R6)(R7), N(R8)C(═O)R9, N(R8)C(═S)R9, N(R8)SO2R10 and N(R8)R10; R6 and R7 are each independently R11(CH2)b or together they are —(CH2)2—Z—(CH2)2— or CH2—o—C6H4—Z—CH2—; R8 is H or Me; R9 is selected from R11(CH2)b, R11(CH2)bO and N(R6)(R7); R10 is R11(CH2)b; R11 is selected from H, alkyl, optionally substituted aryl, optionally substituted aroyl, optionally substituted arylsulphonyl and optionally substituted heteroaryl; R12 is selected from H2NCH(R13)CO, H2NCH(R14)CONHCH(R15)CO, C(R16)═C(R17)COR18 and R19OCO; R13, R14 and R15 are selected from the side chains of the proteinaceous amino acids; R16 is selected from H, lower alkyl (C1–C6) and phenyl; R17 is selected from H and lower alkyl (C1–C6); R18 is selected from H, lower alkyl (C1–C6), OH, O-(lower alkyl (C1–C6)) and phenyl; R19 is selected from lower alkyl (C1–C6), optionally substituted phenyl and R20C(═O)OC(R21)(R22); R20, R21 and R22 are each independently selected from H and lower alkyl (C1–C6); Z is selected from a covalent bond, —(CH2)c—, —O—, —SOd— and —N(R10)—; X1 is S or CH2; X2 is O, S or CH2; a is 1, 2 or 3; b is 0–3; c is 1 or 2; and d is 0, 1 or 2.