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      Somatostatin and Other Peptide Receptors as Tools for Tumor Diagnosis and Treatment

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          Somatostatin receptors are expressed in selected human cancers. They are particularly frequently expressed in gastroenteropancreatic neuroendocrine tumors (GEP NET), including both primaries and metastases. The density is often high, the distribution is usually homogeneous. While various somatostatin receptor subtypes can be expressed in these tumors, sst<sub>2</sub> is clearly predominant. These receptors represent the molecular basis for a number of clinical applications, including symptomatic therapy with cold octreotide in hormone-secreting GEP NET, in vivo diagnostic with Octreoscan to evaluate the extend of the disease, and <sup>90</sup>Y-DOTATOC radiotherapy. GEP NET can, however, express peptide receptors other than somatostatin receptors: insulinomas have more glucagon-like peptide 1 receptors than somatostatin receptors, gut NET (carcinoids) may also express cholecystokinin 2, bombesin 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.

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            Concomitant expression of several peptide receptors in neuroendocrine tumours: molecular basis for in vivo multireceptor tumour targeting.

            Peptide receptors have been found to represent excellent targets for in vivo cancer diagnosis and therapy. Recent in vitro studies have shown that many cancers can overexpress not only one but several peptide receptors concomitantly. One of the challenges for nuclear medicine in this field in the coming decade will be to take advantage of the co-expression of peptide receptors for multireceptor tumour targeting. In vitro receptor studies can reveal which peptide receptor is overexpressed in which tumour and which receptors are co-expressed in an individual tumour; such knowledge is a prerequisite for successful in vivo development. One group of tumours of particular interest in this respect is the neuroendocrine tumours, which have previously been shown often to express peptide receptors. This review summarises our investigations of the concomitant expression of 13 different peptide receptors, in more than 100 neuroendocrine tumours of the human intestine, pancreas and lung, using in vitro receptor autoradiography with subtype-selective ligands. The incidence and density of the somatostatin receptors sst(1)-sst(5), the VIP receptors VPAC(1) and VPAC(2), the CCK(1) and CCK(2) receptors, the three bombesin receptor subtypes BB(1) (NMB receptor), BB(2) (GRP receptor) and BB(3), and GLP-1 receptors were evaluated. While the presence of VPAC(1) and sst(2) was detected in the majority of these neuroendocrine tumours, the other receptors, more differentially expressed, revealed a characteristic receptor pattern in several tumour types. Ileal carcinoids expressed sst(2) and VPAC(1) receptors in virtually all cases and had CCK(1), CCK(2), sst(1) or sst(5) in approximately half of the cases; they were the only tumours of this series to express NMB receptors. Insulinomas were characterised by a very high incidence of GLP-1, CCK(2) and VPAC(1) receptors, with the GLP-1 receptors expressed in a particularly high density; they expressed sst(2) in two-thirds and sst(1) in approximately half of the cases and lacked CCK(1) and NMB receptors. All gastrinomas had sst(2) and GLP-1 receptors; they expressed GRP receptors in three-quarters of the cases and CCK(1) or VPAC(1) in approximately half of the cases. Most bronchial carcinoids had VPAC(1), while sst(1), sst(2) and CCK(2) were found in two-thirds of the cases and BB(3) in one-third of the cases. These data provide evidence for the vast biological diversity of these neuroendocrine tumours. Moreover, the results represent a basis for starting and/or optimising the in vivo targeting of these tumours by selecting the suitable radiopeptides for tumour diagnosis and/or therapy. Finally, the data strongly encourage concomitant application of several radiopeptides to permit more efficient targeting of these tumours.
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              Treatment of patients with gastro-entero-pancreatic (GEP) tumours with the novel radiolabelled somatostatin analogue [177Lu-DOTA0,Tyr3]octreotate

              Medical treatment and chemotherapy are seldom successful in achieving objective tumour reduction in patients with metastatic neuroendocrine tumours. Treatment with the radiolabelled somatostatin analogue [90Y-DOTA0,Tyr3]octreotide may result in partial remissions in 10–25% of patients. The newer analogue [DOTA0,Tyr3]octreotate (octreotate) has a ninefold higher affinity for the somatostatin receptor subtype 2 as compared with [DOTA0,Tyr3]octreotide. Also, labelled with the beta- and gamma-emitting radionuclide 177Lu, it has proved very successful in achieving tumour regression in animal models. The effects of 177Lu-octreotate therapy were studied in 35 patients with neuroendocrine gastro-entero-pancreatic (GEP) tumours who underwent follow-up for 3–6 months after receiving their final dose. Patients were treated with doses of 100, 150 or 200 mCi 177Lu-octreotate, to a final cumulative dose of 600–800 mCi, with treatment intervals of 6–9 weeks. Nausea and vomiting within the first 24 h after administration were present in 30% and 14% of the administrations, respectively. WHO toxicity grade 3 anaemia, leucocytopenia and thrombocytopenia occurred after 0%, 1% and 1% of the administrations, respectively. Serum creatinine and creatinine clearance did not change significantly. The effects of the therapy on tumour size were evaluable in 34 patients. Three months after the final administration, complete remission was found in one patient (3%), partial remission in 12 (35%), stable disease in 14 (41%) and progressive disease in seven (21%), including three patients who died during the treatment period. Tumour response was positively correlated with a high uptake on the octreoscan, limited hepatic tumour mass and a high Karnofsky Performance Score. Because of the limited efficacy of alternative therapies, many physicians currently adopt an expectant attitude when dealing with patients with metastatic GEP tumours. However, in view of the high success rate of therapy with 177Lu-octreotate and the absence of serious side-effects, we advocate its use in patients with GEP tumours without waiting for tumour progression.

                Author and article information

                S. Karger AG
                October 2004
                15 October 2004
                : 80
                : Suppl 1
                : 51-56
                Division of Cell Biology and Experimental Cancer Research, Institute of Pathology, University of Berne, Berne, Switzerland
                80742 Neuroendocrinology 2004;80(suppl 1):51–56
                © 2004 S. Karger AG, Basel

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                Page count
                Figures: 5, Tables: 1, References: 19, Pages: 6


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