The present invention relates to compounds of formula (I), and pharmaceutically acceptable salts thereof, wherein: X is CH or N; one of R1 and R2 is H, and the other is selected from OR6, SR6, NR6R7, N3, Me, Et, CF3, SOR8 and SO2R8; R3 is selected from tert-butylmethyl, iso-propylmethyl, sec-butyl, tert-butyl, cyclopentyl and cyclohexyl; R4 is optionally substituted C1-8 alkyl or optionally substituted C3-8 cycloalkyl; R6 and R7 are each independently selected from H, C1-8-alkyl and C3-8-cycloalkyl, or R6 and R7 are linked to form a cyclic group together with the nitrogen to which they are attached; and R8 is C1-8-alkyl or C3-8-cycloalkyl. The invention further relates to pharmaceutical compositions comprising compounds of formula (I), and the use of such compounds in the treatment of a disease selected from osteoporosis, Paget's disease, Chagas's disease, malaria, gingival diseases, hypercalaemia, metabolic bone disease, diseases involving matrix or cartilage degradation, and bone cancer disorders such as bone metastases and associated pain.