Compounds of the formula I wherein R1a is H; and R1b is C1-C6 alkyl, Carbocyclyl or Het; or R1a and R1b together define a saturated cyclic amine with 3-6 ring atoms; R2a and R2b are H, halo, C1-C4alkyl, C1-C4haloalkyl, 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-C10alkyl chain, C2-C4haloalkyl or C3-C7cycloalkylmethyl, R4 is Het, Carbocyclyl, optionally substituted as defined in the specification and pharmaceutically acceptable salts, hydrates and N-oxides thereof; are inhibitors of cathepsin S and have utility in the treatment of psoriasis, autoimmune disorders and other disorders such as asthma, arteriosclerosis, COPD and chronic pain.