Cathepsin cysteine protease inhibitors and their use

A compound of formula (I):

• R1 is independently hydrogen, C1-6alkyl or C3-6cycloalkyl;
• R2 is independently aryl, heteroaryl or a group C1-6alkylR9, CO(C1-6alkyl)R9 or SO2(C1-6alkyl)R9;
• Het is a heteroaryl ring chosen from pyridine, pyrimidine, pyrazine, pyridazine or triazine and optionally substituted by halogen, amino, hydroxy, cyano, nitro, carboxy, CONR12R13, SO2NR12R13, SO2R12, trifluoromethyl, NHSO2R12, NHCOR12, C1-6alkyl, C1-6alkoxy, SR12 or NR10R11;
• R3 is independently hydrogen, C1-6 alkyl or C3-6cycloalkyl;
• R4 is independently hydrogen, C1-8alkyl, C3-8cycloalkyl, arylC1-5alkyl or heteroarylC1-5alkyl, the latter three groups being optionally substituted by one or more halogen, amino, hydroxy, C1-6alkyl, C1-6 l alkoxy, SR12 or NR10R11;
• R5 is independently hydrogen, C1-6alkyl or C3-6cycloalkyl;
• R6 is independently hydrogen, C1-6alkyl or C3-6cycloalkyl;
• R7 is independently hydrogen, C1-6alkyl or C3-6cycloalkyl;
• R8 is independently hydrogen, aryl, heteroaryl or C1-6alkyl optionally substituted with one or more aryl, heteroaryl, halogen, amino, hydroxy, carboxy, CONR12R13, SO2NR12R13, SO2R12, NHSO2R12, NHCOR12, C1-6alkyl, C3-6cycloalkyl, C1-6 alkoxy, SR12 or NR10R11;
• R9 is aryl or heteroaryl;
• R10 and R11 are independently hydrogen, C1-6alkyl or together with the nitrogen atom to which they are attached form a 5- or 6-membered saturated ring optionally containing a further O, S or NR1 group;
• and
• R12 and R13 are independently hydrogen or C1-6alkyl;
• or a pharmaceutically acceptable salt thereof.