The present invention relates to a cathepsin inhibitor comprising or consisting of Formula (I) (X1)(X2)(X3)(X4)(Y)(X5)(X6)(X7) wherein (X1) is an amino acid selected from K, L, F and M; (X2) is an amino acid selected from L, A, D, E, F, G, H, I, K, M, N, S, W, Y, Q and R; (X3) is an amino acid selected from R, A and L; (X4) is an amino acid selected from M, I, K, V and Y; (X5) is an amino acid selected from P, A, I, L, V, W and Y; (X6) is an amino acid selected from K, A, E, F, G, I, L, M, N, Q, S, T, V, W, Y and Q; (X7) is an amino acid selected from D, A, E, F, G, I, L, M, N, P, T, V, W, Y, Q and R; and (Y) is a Michael acceptor.