A compound of formula (I), or a pharmaceutically acceptable salt, hydrate, complex or pro-drug thereof,wherein: one of R1 and R2 is H, and the other is selected from OR6, SR6, NR6R7, N3, Me, Et, CF3, SOR8 and SO2R8; orR1 and R2 are both H;one of R3 and R4 is H, and the other is selected from tert-butylmethyl, iso-propylmethyl, sec-butyl, tert-butyl, cyclopentyl and cyclohexyl; orR3 and R4 are joined together with the adjacent backbone carbon atom to form a spiro-C5-C6 cycloalkyl group;R6 and R7 are each independently selected from H, C1-8-alkyl and C3-8-cycloalkyl; orR6 and R7 are linked to form a cyclic group together with the nitrogen to which they are attached;R8 is C1-8-alkyl or C3-8-cycloalkyl;R9 is a para-substituted 6-membered monocyclic aryl or heteroaryl ring which includes up to five heteroatoms.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.