Selection of Radiolabeled Gastrin Analogs for Peptide Receptor–Targeted Radionuclide Therapy

The gastrin/cholecystokinin-2 (CCK-2) receptor has been identified as a possible target for peptide receptor radionuclide imaging and therapy. Levels of expression of the receptor are increased in several tumor types, including medullary thyroid cancer (1), neuroendocrine tumors (2), small cell lung cancer, and others (3). CCK-2 receptor–targeted peptides originate from 2 main families. Human gastrin is a 34-amino-acid peptide that also exists in several C-terminal truncated forms, including minigastrin, a 13-residue peptide with the sequence LEEEEEAYGWMDF-NH2. Full-length CCK is also a 34-residue peptide but again exists in several shorter bioactive forms, including CCK-8, which has the structure DYMGWMDF-NH2. The ability of both gastrin- and CCK-derived peptides to target the gastrin receptor in vivo has been studied by Behr et al. (4,5), who found that the labeled minigastrin analogs showed the greatest uptake in receptor-positive tissues but were also associated with extremely high uptake in the kidney, whereas CCK-8 exhibited low kidney uptake and tumor uptake. Recently published data suggest that renal uptake of gastrin analogs is mediated through a receptor that recognizes the pentaglutamic acid sequence (6). Behr explored the clinical utility of radiolabeled minigastrin in patients (7,8) and, for these studies, used the novel bifunctional agent DTPA-d-Glu (DTPA is diethylenetriaminepentaacetic acid), which showed improved kinetic stability when compared with conventional monofunctional DTPA (9). They were able to show a sensitivity of 91% in 75 patients with medullary thyroid cancer imaged with 111In-DTPA d-Glu-minigastrin. Eight patients were also treated with 90Y-labeled minigastrin and, although some evidence of efficacy was seen, hematologic and renal toxicities were observed. Reubi et al. explored the potential of a series of radiolabeled CCK-8 peptides. They identified a modified analog of CCK-8, dYNleGWNleDF-NH2 with high affinity and selectivity for the CCK-2 receptor (10). When labeled with 111In using tetraazacyclododecane tetraacetic acid (DOTA) as a bifunctional chelating agent, the radioligand showed high specific internalization rates in the receptor-positive cell line AR42J, and biodistribution studies in tumor-bearing rats showed a modest but specific uptake into receptor-positive tissues (11). In 7 patients with medullary thyroid cancer imaged with the 111In-DTPA analog, some lesions were visualized but there was relatively low uptake both at these sites and in the receptor-positive stomach. Aloj et al. evaluated the potential of 111In-DTPA-Glu-Gly-CCK-8 using models based on cells transfected with the CCK-2 receptor. Rapid internalization of the radioligand by cells was observed in vitro and uptake was seen in tumor-bearing nude mice, although levels of kidney uptake were greater than those seen in tumor (12). The potential of 99mTc-labeled analogs for imaging gastrin receptors has also been recognized. von Guggenberg et al. have showed very high tumor uptake of 99mTc-HYNIC-minigastrin complexes (HYNIC is hydrazinonicotinamide) in AR42J-bearing nude mice (13), and Nock et al. have recently described the characterization of a series of tetraamine-conjugated gastrin analogs in the same animal model (14). The aim of our research presented in this article was to identify a radioligand that combined the relatively high tumor uptake shown by peptides belonging to the gastrin family with the low renal uptake seen with CCK derivatives. Although 111In was selected as a suitable isotope for in vitro, biodistribution, and imaging studies, the ultimate goal was to explore the radiotherapeutic properties of this peptide and to develop an analog that could be labeled with a β-emitting radionuclide such as 90Y or 177Lu. A small library of peptide-DOTA and -DTPA conjugates based on the C-terminal structure of minigastrin was therefore prepared, and the biodistribution of these labeled peptides was compared with that of some of the radioligands described in nude mice bearing a variety of CCK-2–expressing tumors.