Noninvasive imaging of osteoclasts in parathyroid hormone-induced osteolysis using a 64Cu-labeled RGD peptide

Bone diseases are often a result of increased numbers of osteoclasts, or bone-resorbing cells. Bone metastases are a significant cause of morbidity in many types of cancer. An imaging agent targeting osteoclasts, which are upregulated in osteolytic lesions, may facilitate earlier follow-up in patients with osteolytic or mixed bone metastases. Osteoclasts express high levels of alpha(v)beta(3) integrin, to which peptides containing the Arg-Gly-Asp (RGD) sequence are known to bind. We proposed that radiolabeled RGD peptides could be used to detect osteoclasts in lytic bone lesions. Methods: The cross-bridged macrocyclic chelator 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-TE2A) was conjugated to c(RGDyK) for radiolabeling with Cu-64 (t(1/2), 12.7 h; beta(+), 17.4%; E beta+max, 656 keV; beta(-), 39%; E beta-max, 573 keV). The in vitro affinity of Cu(II)-CB-TE2A-c(RGDyK) for alpha(v)beta(3) and alpha(v)beta(5) was evaluated in a heterologous competitive binding assay. Ex vivo uptake was examined in osteoclasts prepared from bone marrow macrophages. As a proof of principle, biodistribution and imaging studies were performed on parathyroid hormone (PTH)-induced osteolysis in the calvarium. Results: Cu-CB-TE2A-c(RGDyK) was shown to have a 30-fold higher affinity for alpha(v)beta(3) than for alpha(v)beta(5). Osteoclasts were shown to specifically take up Cu-64-CB-TE2A-c(RGDyK). However, bone marrow macrophages showed only nonspecific uptake. PTH treatment increased calvarial uptake of Cu-64-CB-TE2A-c(RGDyK), compared with uptake in mice receiving a sham treatment. In addition, calvarial uptake correlated linearly with the number of osteoclasts on the bone surface. Conclusion: These results suggest that Cu-64-CB-TE2A-c(RGDyK) selectively binds alpha(v)beta(3) on osteoclasts and may potentially be used to identify increased numbers of osteoclasts in osteolytic bone diseases such as osteolytic bone metastasis and inflammatory osteolysis.