The present invention relates to compounds of formula (I), and pharmaceutically acceptable salts thereof, wherein: R.sup.3 is tert-butylmethyl, sec-butyl or tert-butyl; X is CH or N; and R.sup.4 is optionally substituted C.sub.1-8 alkyl or optionally substituted C.sub.3-8 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. ##STR00001##