Peptide receptor expression in GEP-NET

Numerous peptide receptors have recently been reported to be expressed or overexpressed in various human cancers. For instance, somatostatin receptors are particularly frequently expressed in gastroenteropancreatic neuroendocrine tumors (GEP-NET), including both primaries and metastases. The density is often high, and the distribution is usually homogenous. While various somatostatin receptor subtypes can be expressed in these tumors, the sst(2) is clearly predominant. These receptors represent the molecular basis for a number of clinical applications, including symptomatic therapy with octreotide in hormone-secreting GEP-NET, in vivo diagnostic with radiolabeled diethylene triamine pentaacetic acid octreotide (Octreoscan) to evaluate the extend of the disease, and (90)Y- or (177)Lu-[(90)Y-DOTA]-D: -Phe(1)-Tyr(3) octreotide radiotherapy. GEP-NET can, however, express peptide receptors other than somatostatin receptor: Insulinomas have more glucagon-like peptide 1 receptors than somatostatin receptors; gastrinomas express very high levels of secretin receptors. GEP-NET may also express cholecystokinin 2, bombesin, neuropeptide Y, or vasoactive intestinal peptide receptors. Often, several of these peptide receptors are expressed simultaneously in GEP-NET, providing a molecular basis for in vivo multireceptor targeting of those tumors

Novel dimeric DOTA-coupled peptidic Y1-receptor antagonists for targeting of neuropeptide Y receptor-expressing cancers

ABSTRACT

Macrocyclic chelator-coupled gastrin-based radiopharmaceuticals for targeting of gastrin receptor-expressing tumours

Purpose: Diethylenetriamine-pentaacetic acid (DTPA)-coupled minigastrins are unsuitable for therapeutic application with the available β-emitting radiometals due to low complex stability. Low tumour-to-kidney ratio of the known radiopharmaceuticals is further limiting their potency. We used macrocyclic chelators for coupling to increase complex stability, modified the peptide sequence to enhance radiolytic stability and studied tumour-to-kidney ratio and metabolic stability using 111In-labelled derivatives. Methods: Gastrin derivatives with decreasing numbers of glutamic acids were synthesised using 111In as surrogate for therapeutic radiometals for in vitro and in vivo studies. Gastrin receptor affinities of the natIn-metallated compounds were determined by receptor autoradiography using 125I-CCK as radioligand. Internalisation was evaluated in AR4-2J cells. Enzymatic stability was determined by incubating the 111In-labelled peptides in human serum. Biodistribution was performed in AR4-2J-bearing Lewis rats. Results: IC50 values of the natIn-metallated gastrin derivatives vary between 1.2 and 4.8nmol/L for all methionine-containing derivatives. Replacement of methionine by norleucine, isoleucine, methionine-sulfoxide and methionine-sulfone resulted in significant decrease of receptor affinity (IC50 between 9.9 and 1,195nmol/L). All cholecystokinin receptor affinities were >100nmol/L. All 111In-labelled radiopeptides showed receptor-specific internalisation. Serum mean-life times varied between 2.0 and 72.6h, positively correlating with the number of Glu residues. All 111In-labelled macrocyclic chelator conjugates showed higher tumour-to-kidney ratios after 24h (0.37-0.99) compared to 111In-DTPA-minigastrin 0 (0.05). Tumour wash out between 4 and 24h was low. Imaging studies confirmed receptor-specific blocking of the tumour uptake. Conclusions: Reducing the number of glutamates increased tumour-to-kidney ratio but resulted in lower metabolic stability. The properties of the macrocyclic chelator-bearing derivatives make them potentially suitable for clinical purpose

DOTA-NOC, a high-affinity ligand of somatostatin receptor subtypes 2, 3 and 5 for labelling with various radiometals

Earlier studies have shown that modification of the octapeptide octreotide in positions 3 and 8 may result in compounds with increased somatostatin receptor affinity that, if radiolabelled, display improved uptake in somatostatin receptor-positive tumours. The aim of a recent research study in our laboratory was to employ the parallel peptide synthesis approach by further exchanging the amino acid in position 3 of octreotide and coupling the macrocyclic chelator DOTA(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to these peptides for labelling with radiometals like gallium-67 or -68, indium-111, yttrium-90 and lutetium-177. The purpose was to find radiopeptides with an improved somatostatin receptor binding profile in order to extend the spectrum of targeted tumours. A first peptide, [111In,90Y-DOTA]-1-Nal3-octreotide (111In,90Y-DOTA-NOC), was isolated which showed an improved profile. InIII-DOTA-NOC exhibited the following IC50 values (nM) when studied in competition with [125I][Leu8, d-Trp22, Tyr25]somatostatin-28 (values for YIII-DOTA-NOC are shown in parentheses): sstr2, 2.9±0.1 (3.3±0.2); sstr3, 8±2 (26±1.9); sstr5, 11.2±3.5 (10.4±1.6). Affinity towards sstr1 and 4 was very low or absent. InIII-DOTA-NOC is superior to all somatostatin-based radiopeptides having this particular type of binding profile, including DOTA-lanreotide, and has three to four times higher binding affinity to sstr2 than InIII,YIII-DOTA-Tyr3-octreotide (InIII,YIII-DOTA-TOC). In addition, [111In]DOTA-NOC showed a specific and high rate of internalization into AR4-2J rat pancreatic tumour cells which, after 4h, was about two times higher than that of [111In]DOTA-TOC and three times higher than that of [111In]DOTA-octreotide ([111In]DOTA-OC). The internalized radiopeptides were externalized intact upon 2h of internalization followed by an acid wash. After 2-3h of externalization a plateau is reached, indicating a steady-state situation explained by reactivation of the receptors followed by re-endocytosis. Biodistribution studies in CA 20948 tumour-bearing rats showed rapid clearance from all sstr-negative tissues except the kidneys. At 4h the uptake of [111In]DOTA-NOC in the tumour and sstr-positive tissues, such as adrenals, stomach and pancreas, was three to four times higher than that of [111In]DOTA-TOC. Differential blocking studies indicate that this is at least partially due to the uptake mediated by sstr3 and sstr5. These very promising preclinical data justify the use of this new radiopeptide for imaging and potentially internal radiotherapy studies in patient

Somatostatin receptor imaging: The presence of somatostatin receptors in rheumatoid arthritis

Objective. To investigate the in vivo and in vitro expression of somatostatin receptors (SS-R) on synovial membranes of patients with rheumatoid arthritis (RA). Methods. The joints of 14 consecutive patients with active RA, 4 patients with severe osteoarthritis (OA), and 30 control patients were studied. The somatostatin analog [111In-DTPA-D-Phe1]-octreotide was used for in vivo SS-R scintigraphy, and the somatostatin analog [125I-Tyr3]-octreotide for in vitro SS-R autoradiography. Results. Seventy-six percent (220 of 290) of the painful joints and 76% (207 of 274) of the swollen joints of the patients with RA were visualized by SS-R scintigraphy. The degree of pain and swelling correlated well with positive scintigraphy findings in the joints (P < 0.0001). In 2 of the RA patients who underwent scintigraphy, as well as in 4 of 5 other patients, in vitro studies of the synovial membranes showed the presence of specific SS-R. In patients with OA, uptake of radioactivity in the affected joints was significantly lower than that in patients with RA. None of the joints of the control patients demonstrated uptake of radioactivity. Conclusion. SS-R are present in the synovial tissue of p

The use of somatostatin receptor scintigraphy in the differential diagnosis of pancreatic duct cancers and islet cell tumors

OBJECTIVE: In the present study, the diagnostic value of somatostatin receptor scintigraphy (SRS) was evaluated in the preoperative workup in patients with pancreatic duct cancers and islet cell tumors, as well as in the follow-up of these patients. METHODS: Twenty-six patients with suspected primary pancreatic duct cancers and 48 patients with islet cell tumors were studied. The SRS was performed using the radionuclide-labeled somatostatin analogue 111In-octreotide. Another group of 12 patients who were still alive more than 3 years after pancreaticoduodenectomy for pancreatic duct adenocarcinomas also underwent SRS. RESULTS: In 31 (65%) of 48 patients, the primary pancreatic islet cell tumor as well as its often previously not yet recognized metastases could be visualized. In contrast, none of the 26 pancreatic adenocarcinomas or their metastases could be seen. In 5 of 12 patients who were alive more than 3 years after pancreaticoduodenectomy for pancreatic duct adenocarcinomas, metastatic lesions were visualized at SRS. In retrospect, these patients were not operated on for adenocarcinomas but for "nonfunctioning" islet cell tumors. CONCLUSIONS: The present study supports the concept that SRS has a place in the preoperative differential diagnosis of islet cell tumors and pancreatic duct cancers as well as in the follow-up, especially in those cases in which no tumor histologic analysis was obtained, or the pathologic examination of the tumor tissue had not included special staining procedures for neuroendocrine characteristics. Our results also indicate that the evaluation of the results of investigations on the role of surgery or radiation therapy and chemotherapy or both in pancreatic duct cancer have to be interpreted with caution, if no histologic analysis and staining for neuroendocrine characteristics was performed