Compounds of formula (I) where: X
1
is OH or SH or NH
2
or -L-Z; X
2
and X
3
are the same or different and each is H, C
1-4
alkyl, benzyl a protecting group or -L-Z, X
4
is H or C
1-4
alkyl, L is a linker comprising a chain of 0-10 atoms, Z is a group comprising a detectable moiety which comprise at least one detectable moiety are useful in the diagnosis and prognosis and radiotherapy of metastatic bone disease.
1