Bisphosphonic acid compound

A bisphosphonic acid compound of formula (1) or a pharmaceutically acceptable salt thereof:

• wherein - - - - - - represents a single bond or a double bond;
• A represents a C3-8 saturated cyclic hydrocarbon, wherein the saturated cyclic hydrocarbon is optionally substituted by 1 to 6 groups selected from the group consisting of a C1-6 alkyl group, a C2-6 alkenyl group, a C2-6 alkynyl group, a C1-6 alkoxy group, a C6-10 aryloxy group, a C1-6 haloalkoxy group, a C1-6 haloalkyl group and a halogen atom;
• and
• R1 and R2 each independently represent a C1-6 alkyl group, a C2-6 alkenyl group, a C2-6 alkynyl group, a C1-6 alkoxy group, a C6-10 aryloxy group, a C1-6 haloalkoxy group, a C1-6 haloalkyl group, a halogen atom or a hydrogen atom, wherein when - - - - - - is a double bond, R2 is absent.