Compounds which are inhibitors of Notum

A compound of formula (I):

• where Ar is C5 heteroaryl, optionally substituted by a group selected from
• a) OH, SH, NH2, OMe, SMe, NHMe, NMe2, OEt, NHCN, Me;
• b) CN, CO2H;
• c) SO2Me, SO3−Na+;
• d) CH2Q1, where Q1 is selected from OH, OMe, Cl, CN, NHCOMe, SO3H, NRN1RN2, where RN1 and RN2 are either H or Me;
• e) C2-3 alkyl substituted by OH;
• where a N-ring heteroatom may additional bear a methyl group;
• and
• X1 is CR1;
• X2 is CR2;
• X3 is CR3;
• X5 is CR5;
• two of R1, R2, R3, R4 and R5 are H, two of R1, R2, R3, R4 and R5 are Cl and one of R1, R2, R3, R4 and R5 is CF3.