Radiolabeled RGD peptides move beyond cancer: PET imaging of delayed-type hypersensitivity reaction.

The arginine-glycine-aspartic acid (RGD) cell adhesion motif was discovered in fibronectin by Pierschbacher and Ruoslahti 20 years ago (1). Shortly thereafter, Smith and Cheresh (2) identified the vitronectin receptor, or v 3 integrin, as one of the adhesion molecules recognizing this sequence. The v 3 integrin is expressed on the luminal surface of neovasculature but is not found on the endothelial surface of mature capillaries. In addition, v 3 has been shown to be upregulated in tumor blood vessels that undergo continuous angiogenesis (3) and has been implicated in metastasis (4). Synthetic RGD peptide antagonists of v 3 were subsequently shown to inhibit growth of neovasculature and effect tumor regression in animal models (5,6), presumably by starving tumors of their blood supply. Recent reports of an v 3-targeting, cyclic RGD peptide that specifically localized to tumor xenografts after in vivo phage display selection (7) and produced durable responses in tumorbearing mice when coupled to doxorubicin (8) caused great excitement in the nuclear medicine community. These findings raised the possibility that radiolabeled cyclic peptide ligands for v 3 might become powerful new tools for molecular imaging and targeted radiotherapy of tumors undergoing angiogenesis. Haubner et al. (9), who synthesized the first such peptide, 125I-c(RGDyV), demonstrated affinity of 2 nmol/L and high selectivity of this peptide for v 3 in vitro and showed specific tumor uptake in mouse models of melanoma and osteosarcoma. However, tumor targeting was modest in those studies, with high hepatobiliary clearance and intestinal uptake presenting major obstacles to adequate imaging contrast. Subsequently, van Hagen et al. (10) evaluated c(RGDyK) conjugated to diethylenetriaminepentaacetic acid (DTPA) and labeled with either 125I or 111In. These investigators found that the radioiodinated analog exhibited v 3-mediated uptake not only in tumor neovasculature but also in BON human carcinoid and CA20948 rat pancreatic carcinoma cells in culture. In vivo uptake of 111In-DTPA-c(RGDyK) in CA20948 tumors was mediated by v 3 but remained relatively low, with most tumor-to-normal tissue ratios near unity. Although DTPA conjugation reduced liver accumulation of the 111In tracer substantially, uptake and retention were 4–8 times higher in kidney than in any other tissue, also limiting the utility of this compound. A breakthrough improvement in the in vivo distribution and imaging properties of radiolabeled RGD peptides was made by attachment of a sugar amino acid (SAA) moiety to a cyclic v 3 ligand (11). Compared with 125Ic(RGDyV), the second-generation glycosylated derivative 125I-c[RGDyK(SAA)] (125I-gluco-RGD) showed similar blood and renal clearance in melanomaand osteosarcoma-bearing mice, greatly reduced liver uptake, increased tumor retention, and yielded higher tumor-to-blood ratios. It was concluded that glycosylation of the peptide improved its confinement to the vascular space early after injection, allowing improved tumor targeting and reduced liver uptake compared with the hydrophobic parent compound. A third-generation glycosylated peptide, c[RGDfK(SAA)], was labeled with 2-18F-fluoropropionate to yield 18F-galacto-RGD for PET imaging of v 3 (12). High-resolution PET of mice bearing both v 3-positive and v 3negative melanoma xenografts demonstrated integrin-specific tracer uptake with high ratios of tumor to normal tissue at 60–120 min after injection, as well as blocking by v 3 antagonists. Preliminary results on the use of 18Fgalacto-RGD PET to image tumor angiogenesis in cancer patients have been reported (13). As clinical applications of this tracer develop, it may have significant potential utility for noninvasive evaluation of angiogenic and metastatic activity in cancer, as well as for monitoring tumor therapy using specific v 3 antagonists or other angiogenesis inhibitors. Janssen et al. used a dimeric peptide, E-[c(RGDfK)2], conjugated to 111In-1,4,7,10-tetraazacyclododecane1,4,7,10-tetraacetic acid (DOTA) or 99mTc-hydrazinonicotinamide (HYNIC), to target v 3 (14). These authors showed that v 3-specific tumor targeting, normal organ uptake, and tumor-toblood ratios were generally superior for the 111In-labeled compound. Moreover, the relative affinity of the dimeric tracer for v 3 was 10-fold higher than that of the corresponding monomer (15), suggesting that this compound offers the advantage of avidity in integrin binding. In the first reported radiotherapeutic studies of an v 3-targeting peptide, 90YDOTA-E-[c(RGDfK)2] delayed tumor growth in nude mice bearing human ovarian carcinoma xenografts, with minimal toxicity. Received Sep. 22, 2004; revision accepted Sep. 27, 2004. For correspondence or reprints contact: Michael R. Lewis, PhD, Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, 379 E. Campus Dr., University of MissouriColumbia, Columbia, MO 65211. E-mail: [email protected]