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)beta3 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 64Cu (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)beta3 and alpha(v)beta5 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)beta3 than for alpha(v)beta5. Osteoclasts were shown to specifically take up (64)Cu-CB-TE2A-c(RGDyK). However, bone marrow macrophages showed only nonspecific uptake. PTH treatment increased calvarial uptake of 64Cu-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.


CONCLUSIONS
These results suggest that 64Cu-CB-TE2A-c(RGDyK) selectively binds alpha(v)beta3 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.
Sprague
Jennifer E
JE
Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Kitaura
Hideki
H
Zou
Wei
W
Ye
Yunpeng
Y
Achilefu
Samuel
S
Weilbaecher
Katherine N
KN
Teitelbaum
Steven L
SL
Anderson
Carolyn J
CJ
D000818:Animals; D001710:Biotin; D015153:Blotting, Western; D001854:Bone Marrow Cells / Q000378:metabolism; D003301:Copper Radioisotopes; D006207:Half-Life; D016023:Integrins / Q000378:metabolism; D008264:Macrophages / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009842:Oligopeptides; D010010:Osteoclasts / Q000000981:diagnostic imaging*; D010014:Osteolysis / Q000139:chemically induced* / Q000000981:diagnostic imaging*; D010281:Parathyroid Hormone; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals; D014018:Tissue Distribution; D019096:Vitronectin
D003301:Copper Radioisotopes; D016023:Integrins; D009842:Oligopeptides; D010281:Parathyroid Hormone; D019275:Radiopharmaceuticals; D019096:Vitronectin; D001710:Biotin; C047981:arginyl-glycyl-aspartic acid