The present invention is directed to sarcophagine based compounds of formula (I). The compounds can complex a radioisotope, such as 64-copper and hence are useful in radioimaging and radiotherapy of cancers, e.g. colorectal cancer. The compounds work via a pre-targeting imaging approach in which the tetrazine reacts via click chemistry with a functional group such as a trans- cyclooctene which is pre-bound to a ligand such as a tumour specific antibody.