Compounds of Formula II wherein R1a is H; and R1b is C1-C6alkyl, Carbocyclyl or Het; or R1a and R1b together define a saturated cyclic amine with 3-6 ring atoms; R2a and R2b are independently H, halo, C1-C4alkyl, C1-C4haloalkyl or C1-C4alkoxy, or R2a and R2b together with the carbon atom to which they are attached form a C3-C6cycloalkyl; R3 is a branched C5-C10 alkyl chain, C2-C4haloalkyl or —CH2C3-C7 cycloalkyl; R4′ is C1-C6alkyl, C1-C6haloalkyl or oxetany-3-yl. for use in the prophylaxis or treatment of a disorder characterised by inappropriate expression or activation of cathepsin S.