Vasopressin-2 receptor agonists

A compound of formula I:

• or a pharmaceutically acceptable salt thereof, wherein:
• R2 is H, C1-C4 alkyl, halogen, —OH or —O—C1-C4 alkyl;
• R3 is H or —CH2—OH or —C(O)—NR5R6;
• R4 is H or —C(═NH)—NH2;
• R5 and R6 are independently H, C1-C6 alkyl, —CH2-cyclopropyl, -cyclopropyl, or arylalkyl, with the proviso that R5 and R6 are not both H;
• X and Y are independently —CH2— or —S— with the proviso that if X is —CH2—, Y is not —CH2—;
• Z is —CHR7— or —S— and R7 is H or C1-C4 alkyl, halogen, —OH or —O—C1-C4 alkyl;
• R8 is H or —CH3;
• and
• Ar is heteroaryl or phenyl optionally substituted with one C1-C4 alkyl, halogen, —OH or —O—C1-C4 alkyl.