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duct epithelium of 60%-partial pancreatectomy rats. Diabetologia 44, 2056–2065 49 Holst, L.S. et al. (1998) Protein kinase B is expressed in pancreatic β cells and activated upon stimulation with insulin-like growth factor I. Biochem. Biophys. Res. Commun. 250, 181–186 50 Tuttle, R.L. et al. (2001) Regulation of pancreatic β-cell growth and survival by the serine/threonine protein kinase Akt1/PKBα. Nat. Med. 7, 1133–1137 51 Rondinone, C.M. et al. (1999) Impaired glucose transport and protein kinase B activation by insulin, but not okadaic acid, in adipocytes from

subjects with type II diabetes mellitus. Diabetologia 42, 819–825 52 Krook, A. et al. (1998) Insulin-stimulated Akt kinase activity is reduced in skeletal muscle from NIDDM subjects. Diabetes 47, 1281–1286 53 Nadler, S.T. et al. (2001) Normal Akt/PKB with reduced PI3K activation in insulin-resistant mice. Am. J. Physiol. Endocrinol. Metab. 281, E1249–E1254 54 Kim, Y.B. et al. (2000) Divergent regulation of Akt1 and Akt2 isoforms in insulin target tissues of obese Zucker rats. Diabetes 49, 847–856

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55 Cho, H. et al. (2001) Insulin resistance and a diabetes mellitus-like syndrome in mice lacking the protein kinase Akt2 (PKBβ). Science 292, 1728–1731 56 Birnbaum, M.J. (2001) Diabetes. Dialogue between muscle and fat. Nature 409, 672–673 57 Chen, W.S. et al. (2001) Growth retardation and increased apoptosis in mice with homozygous disruption of the Akt1 gene. Genes Dev. 15, 2203–2208 58 Cho, H. et al. (2001) Akt1/PKBα is required for normal growth but dispensable for maintenance of glucose homeostasis in mice. J. Biol. Chem. 276, 38349–38352

Somatostatin analogs in the diagnosis and treatment of cancer Steven W.J. Lamberts, Wouter W. de Herder and Leo J. Hofland Over the past few years, significant progress has been made in our understanding of the biology and functional significance of somatostatin receptors (sst) on human tumors. Somatostatin analogs, such as octreotide, bind predominantly to sst2 and successfully control hormone hypersecretion in patients with acromegaly, islet cell tumors and carcinoids, and (temporary) control of tumor growth is often also seen. Furthermore, sst2 on tumors can be imaged in vivo after the injection of radionuclide-coupled octreotide. Targeted chemo- and radiotherapy, in which somatostatin analogs coupled to a chemotherapeutic agent or a radionuclide are selectively internalized by sst-positive tumors, are now being studied for their effect on tumor growth. Knowledge about the differential anti-tumor effects of the sst subtypes on tumor cells might have clinical significance after the development of new subtype-specific somatostatin analogs. Published online: 14 October 2002

Steven W.J. Lamberts* Wouter W. de Herder Leo J. Hofland Dept Medicine, Erasmus Medical Centre, 40 Dr. Molewaterplein, 3015 GD Rotterdam, The Netherlands. *e-mail: lamberts@ inw3.azr.nl

Since the discovery of somatostatin (SS) in 1973, knowledge of its roles in neurotransmission, as an inhibitor of endocrine and exocrine secretory processes, and its vasoconstrictor and immunomodulatory properties has increased significantly [1–3]. The diverse biological effects of SS are mediated by a family of G-protein-coupled receptors (GPCRs), the SS receptors (sst), which characteristically comprise a single polypeptide chain with seven transmembrane-spanning domains. At present, five different human sst subtypes (sst1, sst2, sst3, sst4 and sst5) have been cloned and characterized [2]. sst are encoded by five non-allelic genes, located on separate chromosomes [2]. Although the different sst subtypes are 40–60% structurally homologous, each subtype mediates different biological actions of SS. Depending on the cell type, the five sst are coupled to a variety of signal transduction pathways, including adenylate and guanylate cyclase, http://tem.trends.com

phospholipase A2 and C, K+ and Ca2+ channels, Na+–H+ exchanger, Src, Erk1/2, p38 mitogen-activated protein kinases, and tyrosine phosphatases [2]. Although the evidence is preliminary, some specific physiological regulatory roles can be attributed to sst subtypes. For example, in humans, sst2 and sst5 are involved in the control of growth hormone (GH) release, and sst5 appears to be important in the control of insulin and possibly also glucagon release (for Refs see [4]). The early observation that SS might also control cell proliferation in normal and tumorous tissues has not yet resulted in clinically important applications [5]. However, in recent years, important new data have been presented on sst-activated signal transduction pathways that are responsible for inhibition of cell growth and induction of apoptosis: sst3, and to a lesser extent sst2, can induce apoptosis, and sst1, sst4 and sst5 have an inhibitory effect on the cell cycle. For a summary of these effects see Table 1 and for more detailed information see Refs [6–10]. Several other preclinical and clinical advances have brought the potential use of SS analogs in the diagnosis and treatment of patients with cancer much closer to reality. Here, we summarize the current thoughts, in addition to the developments and expectations for practical application in the diagnosis and treatment of cancer in the immediate future. Experimental tumor models

SS analogs inhibit the growth of several transplantable tumors in rodents [5]: (transplantable) osteo- and chondrosarcomas, acinar and ductal pancreatic adenocarcinomas, pituitary tumors, insulinomas and various mammary and prostate

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Table 1. Signal transduction mechanisms of somatostatin receptor subtypes controlling cell growth

Effector coupling

Effector

Effect Refs

a,b

sst1

sst2

sst3

sst4

sst5

PTP (SHP2) ↑ MAPK (Erk) ↑ ACL ↓ 2+ Ca ↓ Ras ↑ Rb ↑ p21 ↑ Cell-cycle arrest [6,7]

PTP (SHP1) ↑ MAPK ↓ ACL ↓ 2+ Ca ↓ Rb ↑ p27 ↑ Cyclin E–cdk2 ↓ Cell-cycle arrest [6,8]

PTP ↑ MAPK ↑↓ ACL ↓

PTP ↑ MAPK ↑ ACL ↓

PTP ↑ MAPK (Erk) ↓ ACL ↓

p53 ↑ Bax ↑

Rb ↑

Rb ↑ p21 ↑

Apoptosis [9]

Cell-cycle arrest [6]

Cell-cycle arrest [10]

a

Abbreviations: ACL, adenylyl cyclase; MAPK, mitogen-activated kinase; PTP, protein tyrosine phosphatase; Rb, retinoblastoma tumor b suppressor; sst, somatostatin receptor. Schematic representation of the anti-tumor effects of somatostatin mediated via the five known sst. Data from Ref. [6].

carcinomas are inhibited in growth, and the survival of the tumor-bearing animals is prolonged. Furthermore, the growth of transplanted human adenocarcinomas of the colon and pancreas and human small-cell lung cancers transplanted into nude mice was shown to be inhibited by the administration of SS analogs [11]. Most of the tumor models used express sst. Because direct antiproliferative effects of SS and its analogs have been demonstrated in many tumor cell models in vitro (for potential mechanisms see Table 1), SS analog-mediated control of tumor growth in vivo was hypothesized to occur mainly via a direct effect. However, sst were not expressed in all these experimental tumor models, suggesting indirect mechanisms of SS analog-mediated tumor control also. These could be accomplished via inhibition of GH, gastrin or insulin release, and also via the inhibition of tumor-growth factors, such as insulin-like growth factor I (IGF-I). Other potential mechanisms through which SS analogs might exert their anti-tumor effect are through inhibition of tumor angiogenesis or immune modulation. Several experimental studies suggest that SS analogs inhibit angiogenesis in vitro and in vivo [12], whereas overexpression of peritumoral vascular sst has been demonstrated in several human cancers [13]. Growing vascular endothelial cells express sst2 [14]. The potential significance of a direct or indirect SS action on tumor cell angiogenesis is supported in two studies: in human glioma cell lines expressing sst2, SS analogs suppressed vascular endothelial growth factor synthesis by 50% [15], whereas SS controls Kaposi sarcoma growth through inhibition of angiogenesis by inhibiting endothelial cell growth and monocyte migration [16]. The antiproliferative role of sst2 in pancreatic adenocarcinomas was studied by Delesque et al. [17]. sst2 expression was shown to be lost in most pancreatic adenocarcinomics, and was not inhibited by SS analogs. Stable transfection of such tumor cells with cDNA encoding human sst2 resulted in the induction of a negative autocrine loop in the absence of exogenous ligand, as a result of sst2-induced expression and secretion of endogenous SS [17]. Experiments http://tem.trends.com

conducted in athymic mice demonstrated a dramatic decrease in tumor growth after sst2 gene therapy of the tumor, in addition to both local and distant anti-tumor bystander effects [18]. These observations suggest an important tumor suppressor role for sst2 in pancreatic cancer and raise the possibility that in vivo, sst2 gene transfer could represent a novel therapeutic approach to human pancreatic cancer, which does not express sst2 receptors [19]. Clinical experience in patients with pituitary and neuroendocrine tumors

Most human neuroendocrine (NE) tumors express large numbers of sst2 receptors, which are homogeneously, densely distributed over the tumor cells [20]. Long-term therapy with the mainly sst2-specific, long-acting SS analogs octreotide and lanreotide suppresses GH release by GH-secreting pituitary adenomas, and this control of hormone release also normalizes IGF-I levels in two-thirds of patients with acromegaly [4]. Other than instant clinical improvement, notable tumor shrinkage also occurs in most patients, based on a decrease in the size of individual pituitary tumor cells, which no longer synthesize and secrete hormone. In addition, the proliferation marker Ki-67 is lowered in octreotide-treated GH-secreting tumors, but there is no change in the apoptotic index [21]. In most patients with metastatic carcinoid disease and islet cell tumors, octreotide therapy also improves clinical symptoms. Control of diarrhea and flushing attacks, caused by an overproduction of serotonin or tachykinin(s), was reported in 70–90% of patients with metastatic carcinoid tumors. Diarrhea, dehydration and hypokalemia in patients with tumors secreting vasoactive intestinal peptide, and peptic ulceration, hypoglycemic attacks and necrolytic skin lesions in patients with tumors secreting gastrin, insulin and glucagon, respectively, were also well controlled in 50–80% of patients treated with octreotide [3]. Results from studies also suggest a temporary stabilization of (metastatic) tumor growth during SS analog therapy in one- to two-thirds of patients with carcinoids and/or islet cell tumors [22,23]. The observed prolonged survival in

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octreotide-treated patients with these metastasized gastroenteropancreatic (GEP) tumors seems to be related, at least in part, to this temporary inhibition of tumor growth, but might also be attributed to the improvement in the quality of life of these patients. The acceptance of the use of SS analogs like octreotide and lanreotide by the patients further improved as monthly long-acting depot formulations of these compounds became available. An important aspect of the long-term successful control of hormone secretion and tumor cell growth during SS analog treatment is a loss of effect (tachyphylaxis) [24]. Whereas normal hormone secretion shows tachyphylaxis following continuous receptor activation within hours to days after the start of SS analog administration, hormone secretion by sst-positive NE tumors can be inhibited for prolonged periods. Octreotide controls hormone secretion effectively in most acromegalics for many years, and escape from therapy has not been observed. In striking contrast, the initial rapid improvement of clinical symptoms in the first weeks to months of SS analog therapy in patients with GEP tumors gradually escapes, in spite of an increase in the dose administered. The potential mechanisms responsible for this loss of control, in addition to the considerable variability in the duration of the responses to octreotide, are not yet known [24]. The relatively long time frame of this escape suggests that mechanisms other than G-protein uncoupling or internalization are involved. This loss of sensitivity of NE cancers to octreotide is probably associated with the outgrowth of clones of tumor cells that lack sst or that have developed mutations in it [24]. sst scintigraphy

The dense and homogeneous distribution of sst2 on most NE tumors also forms the basis of the development of sst scintigraphy (SRS), in which sst2-positive tumors can be imaged in vivo. IIIIn- diethylenetriamine pentaacetic acid (DPTA)-octreotide is injected into patients who are suspected of having sst2-positive tumors, while planar and emission computed tomography images are made with a γ camera [25]. This scanning procedure reveals the localization of the primary tumor and, in most instances, its previously unknown metastases in nearly 90% of patients with neuroendocrine tumors. A close relationship exists between the in vitro detection of sst2 in these tumors with autoradiography, the positive γ camera pictures obtained after injection of radionuclide-coupled octreotide in vivo, and the inhibitory effect of octreotide on hormone release by these tumors in vitro and in vivo [26]. This suggests that a positive scan predicts a suppressive effect of octreotide treatment of patients with GEP tumors on hormonal hypersecretion. SRS images virtually all NE tumors (including islet cell tumors, carcinoids, small-cell lung cancers, pheochromocytomas, http://tem.trends.com

453

L

S

K

K

Fig. 1. Octreoscan in a patient with untreated primary breast cancer. Gamma camera pictures are taken 24 h after IIIIn-DTPA octreotide administration. In an anterior view of the chest and abdomen, the normal liver, spleen and kidneys are imaged. In the right lower margin of the right breast a somatostatin receptor-positive breast tumor is imaged (indicated by arrow). Abbreviations: DTPA, diethylenetriamine pentaacetic acid; K, kidney; L, liver; S, spleen.

paragangliomas, and medullary thyroid cancers). The use of this technique in the initial analysis of patients with NE tumors is mainly to image multiple or metastatic tumors, which is important to determine the optimal treatment [25–27]. SRS does not image NE tumors only. Most well-differentiated human brain tumors, such as meningiomas, low-grade astrocytomas and neuroblastomas, Merkel cell tumors, thyroid cancers, Hodgkin and non-Hodgkin lymphomas, metastatic prostate cancers and more than 50% of primary breast cancers (Fig. 1) can be imaged with SRS [25,26,28]. All these tumor types variably express sst2. The sensitivity of SRS to image sst2-positive processes is very high, but because of the variable expression of sst2 both in NE and non-NE tumors, in addition to most immune diseases, its specificity is relatively low [29]. Apart from IIIIn-DTPA octreotide, other ligands for SRS are also currently under study [30,31]. It is expected that ligands that specifically bind to sst other than sst2 will be developed for SRS shortly [32–34]. A new development is the technique of the intra-operative detection of small sst2-positive carcinoids or metastases of medullary thyroid cancer with a hand-held γ camera after the preoperative administration of IIIIn-DTPA-octreotide [35,36]. The treatment of cancer

It has been recognized increasingly that human cancers often show mixed or divergent differentiation. Microscopically, regions with NE features are frequently seen upon submicroscopic evaluation in many different types of tumor. Most cancers

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(a)

(b)

Normal hormoneproducing cells

sst positive

Normal eptithelial cells

sst (sst2) negative

Well differentiated neuroendocrine tumor

sst positive

Primary adenocarcinomas

sst (sst2) negative

Undifferentiated (bad prognosis)

sst negative

Loss of differentiation acquiring dispersed neuroendocrine cells (bad prognosis)

Regions of tumor sst positive

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Fig. 3. Time-dependent internalization of two different preparations of Tyr3-octreotide to which DTPA (squares; [DTPA0,125I-Tyr3] octreotide) and DOTA (circles; [DOTA0,125I-Tyr3] octreotide) are chelated. Studies were carried out in AtT20 cells. Abbreviations: DOTA, tetra-azacyclododecane tetraacetic acid; DTPA, diethylenetriamine pentaacetic acid.

demonstrate a high degree of cellular heterogeneity, which is evident among different cells of the same tumor at a given time (intratumor heterogeneity), and at different points in time (tumor progression). The growth of tumor cell clones that express NE markers (e.g. sst, chromogranins and carcinoembryonic antigen) during the process of dedifferentiation is a common feature of colon, breast and prostate cancer [26]. Colorectal adenocarcinomas containing numerous dispersed NE tumor cells seem to have a bad prognosis, whereas prostatic adenocarcinomas containing NE cells in general have become androgen independent [26]. Primary human prostate cancer cells often express sst1, whereas the surrounding stromal cells express sst2 [32]. The metastases of hormone-refractory prostate adenocarcinomas can frequently be imaged by SRS and express sst2, but at a low density and heterogeneously [37]. Studies of (metastatic) prostate and breast cancers using measurements of mRNA have demonstrated the simultaneous expression of three, four or all five sst subtypes in many tumor samples [38,39]. However, most studies using breast cancer specimens displayed considerable heterogeneity of sst expression, with the percentage of tumors expressing, for example, mRNA encoding the sst2 receptor far exceeding the percentage of tumors also expressing sst2 protein. This might be because of the high sensitivity of techniques 30

Radioactivity specifically internalized (%)

Fig. 2. A hypothetical representation of somatostatin receptor (sst) expression during the course of (a) neuroendocrine tumors and (b) adenocarcinomas.

25 20 15 10 5 0 0

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120 Time (min)

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like RT–PCR, which easily detect mRNA from normal surrounding non-malignant cells; that is, endothelial, immune, stromal or other cells surrounding the tumor, which express particular sst subtypes [40]. An alternative explanation is that there is a translational block, in which mRNA encoding sst2 does not lead to the formation of sst2 protein. A study of 52 patients with primary breast cancer showed that 39 (75%) tumors could be imaged by SRS. In the follow up after initial therapy, SRS was very sensitive in detecting recurrences of sst-positive breast cancer, and sst expression by cancer cells was a poor prognostic measure [41]. Clinical experience of the treatment of patients with metastasized solid cancers with SS analogs is limited and disappointing. The largest study was a randomized trial of tamoxifen, alone or in combination with octreotide (450 µg d−1), in 135 postmenopausal women with metastatic breast cancer [42]. No indication was found that the combination therapy was better than tamoxifen alone on progression free survival times or overall survival. Interestingly, recently treatment of patients with hepatocellular carcinoma with octreotide was reported to improve symptoms and probably survival [43]. The promising data from animal models that respond to SS analog therapy, even if they do not express sst, have thus far not been duplicated in humans. Human cancers differ, however, in many aspects from the experimental tumor models that respond so well to SS analog administration: (1) most human cancers comprise a mixture of stromal tissue and different clones of epithelial tumor cells that do no uniformly express sst. This contrasts sharply with the mostly monoclonal tumor models in animals, which, in most instances, homogeneously express sst on all tumor cells; (2) sst expression in parts of human breast, prostate and colonic cancers often indicates loss of differentiation of the tumors. In general, these undifferentiated tumors with NE cell differentiation have a poor prognosis at that stage of development; (3) because of the nature of new clinical trials in oncology, often it is mainly those patients who are late in the onset of their disease who are included in the studies. In addition, there is preliminary evidence that previous chemotherapy might decrease the number of sst; and (4) it remains unclear whether the sst2-specific analogs applied in clinical trials so far are the best compounds to use. The simultaneous expression by most tumors of other sst, which induce apoptosis more actively (possible via sst3?), might have hampered or masked potentially more beneficial effects of SS analogs with a broader sst affinity. These observations in classic NE tumors, which initially respond well to SS analogs and eventually escape from therapy, and epithelial cancers that acquire NE cells expressing sst later are summarized in Fig. 2. This summary is important against the background of the development of targeted sst-mediated chemo- and radiotherapy.

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(a) (i)

(ii)

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K

K

K

(b) (i)

(ii)

L

L K

K K

K

(c)

180 160

% of baseline

140 120 100 80 60 40 20 –260

–130

0

130

260

390

Days from start of treatment TRENDS in Endocrinology & Metabolism

[90Y-DOTA-Tyr3]

Fig. 4. Treatment of liver metastases of a gastrinoma with octreotide. (a) Anterior abdominal octreoscan at (i) baseline and (ii) six weeks after the fourth administration. (b) Computer-aided tomography at (i) baseline and (ii) four weeks after the third administration. Before the start of therapy, the patient had severe diarrhea and (c) increasing serum concentrations of gastrin and chromogranin A. After three cycles of treatment (each indicated by an arrow), the patient had a partial tumor response (>50% decrease of tumor volume on computer tomography, which was accompanied by lower uptake of [IIIIn-DTPA°] octreotide on scintigraphy, normalization of serum gastrin levels (circles; with continued use of 40 mg d−1 omeprazole) and normalization of chromogranin (squares). Diarrhea disappeared after the second cycle. This response has continued for more than two years. Abbreviations: DOTA, tetra-azacyclododecane tetraacetic acid; DTPA, diethylenetriamine pentaacetic acid; K, kidney; L, liver; S, spleen.. Reproduced, with permission, from Ref. [52], courtesy of Bioscientifica Ltd and with the kind permission of E.P. Krenning.

Targeted sst-mediated chemo- and radiotherapy

Most polypeptide hormones and neuropeptides are internalized after binding to specific high-affinity membrane receptors. Several preclinical studies strongly support the concept that binding of SS to sst is followed by rapid internalization of the receptor–ligand complex [44] (Fig. 3). However, the degree of internalization of SS might be cell type and as sst subtype specific. In CHO-K1 cells stably expressing one of the five human sst subtypes, it was found that sst2, sst3, sst4 and sst5 receptors displayed http://tem.trends.com

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rapid agonist-dependent internalization of 125I-SS28 ligand within minutes [45]. The demonstration of the efficient internalization of receptor–ligand complexes into sst-positive tumor cells formed the basis for the concept of targeted sst-mediated chemo- or radiotherapy of sst-expressing metastatic human cancer [3,5,25,46]. Internalization brings the cytotoxic SS analog or the radionuclide-coupled analog closer to the nucleus of the cell, resulting in prolonged cellular retention and exposure to radioactivity or the cytotoxic agent. The adverse reactions in patients with advanced metastatic tumors treated with chemotherapeutic agents are caused by the severe toxicity of these agents to normal cells. The ‘magic bullet’ approach of developing targeted hybrids directed against sst was studied extensively by Schally et al. [46], who synthesized two different cytotoxic SS analogs, AN-51 and AN-238 (comprising methotrexate and a metabolite of doxorubicin, respectively, coupled to the SS analog RC-121). Low doses of these analogs inhibit the growth of several experimental tumor models, in addition to human pancreatic, breast, prostate and renal cancer [46,47]. sst-targeted chemotherapy is effective in these preclinical tumor models, and is a highly promising approach for treating sst-positive human cancers. This approach uses lower dosages of targeted chemotherapeutic agents, thus causing lower toxicity. Clinical trials are under way with these new compounds [48]. For the further development of targeted radiotherapy, radionuclides such as 90Y and 177Lu have been proposed as coupling to SS analogs [25]. 90Y, with a half-life of 2.7 days, is a pure β emitter with a tissue range up to 1 cm, and 177Lu, with a half-life of 6.7 days, emits α radiation (suitable for imaging) and intermediate β particles with 2-mm tissue penetration. For tumors with a heterogeneous distribution of sst, 90Y- and 177Lu-labeled SS analogs might have additional beneficial characteristics because of an effect known as crossfire. A tumor cell lacking sst might be hit by an electron coming from a neighboring cell that has internalized the radioligand [49]. Recently tetra-azacyclododecane tetraacetic acid (DOTA), Tyr3-octreotide has been developed [50]. The use of DOTA as a chelator allows a stable binding of 90Y. Internalization of iodinated DOTA, Tyr3-octreotide was fivefold greater than that of DTPA, Tyr3-octreotide [44] (Fig. 3). A single intravenous administration of 10 mCi kg−1 [90Y-DOTA,Tyr3] octreotide to rats bearing the sst2-positive pancreatic cancer CA 20948 resulted in complete remission in five of seven tumor-bearing rats, with no tumor regrowth until eight months after injection [50]. Similar preclinical and promising clinical observations were reported with [177Lu-DOTA, Tyr3] octreotide [51]. Currently, several phase 1 and phase 2 clinical studies are being performed with [90Y-DOTA, Tyr3]

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Table 2. Somatostatin receptors on tumors Membrane receptors Receptor-coupled activation Internalization

a

Visualization Intraoperative detection Control hormonal hypersecretion Control tumor growth Radiotherapy Chemotherapy

a

As an example of diagnostic and/or therapeutic use of the different aspects of the biology of G-protein-coupled membrane receptors.

octreotide, [90Y-DOTA] lanreotide and [177Lu-DOTA, Tyr3] octreotate. Problems of renal toxicity are counteracted by simultaneous administration of amino acids. Apart from several abstracts, a more extensive chapter in a recent book describes in some detail the phase 1 trial with [90Y-DOTA,Tyr3] octreotide [52]. In this dose-finding study, 32 patients with NE tumors (mostly progressive) were included. Escalating single doses of 25, 50, 75 and 100 mCi m−2 (0.9–1.8–2.8–3.7 GBq) were given every six to nine weeks, with a median follow up of 15 months. Renal toxicity, thrombocytopenia and leukopenia in four, five and 14 patients, respectively, were mild. Tumors in these patients were progressive in nine, stable in 17, whereas three had minor (25–50%) and three partial (50–100%) tumor reduction. Sixteen (50%) patients had symptomatic improvement, and hormone levels and symptoms normalized in two of two insulinoma patients and one with gastrinoma (Fig. 4). New developments in sst physiology

Using CHO-K1-transfected cells, Rocheville et al. demonstrated ligand-induced sst dimerization [53]. Both natural SS and SS analogs can produce homo- or heterodimerization of sst1 and sst5 [54], which results in increased binding affinity and modified sst subtypes. This means that protein interactions between different members of the sst subfamily are involved in a molecular crosstalk, which suggests new possibilities for regulating cellular hormonal sensitivity, such as different sst subtypes in target organs and/or tumors operating in concert, rather than as individual members. The sst2 and sst5 bispecific compound BIM 23244 suppresses synergistically GH and prolactin (PRL) release by primary cultures of somatotroph tumors [55]. However, a sst2-preferential agonist antagonized a sst5-selective agonist on the growth of cultured medullary thyroid cancer cells. [56]. Crosstalk between different related GPCR families resulting in References 1 Guillemin, R. (1978) Peptides in the brain: the new endocrinology of the neuron. Science 202, 390–402 2 Patel, Y.C. (1999) Somatostatin and its receptor family. Front. Neuroendocrinol. 20, 157–198 3 Lamberts, S.W. et al. (1996) Octreotide. N. Engl. J. Med. 334, 246–254 http://tem.trends.com

enhanced functional activity was also demonstrated: a SS–dopamine hybrid molecule, BIM 23A387, showed in vitro an enhanced effect on PRL release by cultured rat pituitary cells*. The simultaneous expression of multiple sst subtypes on the same cells in target organs, and on most human sst-expressing tumors, indicates that a differential coupling via the five sst subtypes, which activate simultaneously a variety of intracellular signaling systems, is an essential part of SS function, which should be explored further. The interactive, multireceptor responses, which now include homo- and heterodimerization and crosstalk at the membrane level with other G-protein-coupled systems, provide a tantalizing new concept for drug designers [4]. In this evolving picture, a new SS analog SOM 230 was recently introduced, which is a universal ligand to all sst (except sst4). In animals, the long-term administration of SOM 230 potently and dose-dependently decreased circulating IGF-I levels for prolonged periods of up to 120 days [57]. Conclusions

The potential role of SS in the treatment of cancer is an exciting but unfinished story. In an unprecedented manner, it takes advantage of the physiology of GPCRs (Table 2). The presence of sst on human cancers allows their imaging and their intra-operative detection. Ligand–receptor activation results in control of hormonal hypersecretion in patients with GH- and thyrotropin (TSH)-secreting pituitary tumors, in addition to inoperable islet cell tumors and carcinoids. Control of tumor growth during SS analog administration remains difficult to prove, but new observations on the physiology and interaction of various sst subtypes might make the long-term use of new SS analogs with a different activity profile in the treatment of human cancer more successful. Making use of the process of ligand–receptor internalization, sst-targeted chemo- or radiotherapy of inoperable sst-positive cancer is an exciting new possibility, with promising early clinical observations. Almost 30 years after the characterization of SS, contributions from basic and clinical science bring the story of this peptide full circle, back to the predictions of Guillemin in his Nobel lecture about its potential use in treating a variety of disease processes, including cancer [1]. *S Ren et al. (2000) Functional association of sstr2 and sstr5 in suppression of growth hormone secretion. In The Endocrine Society Denver, p. 339

4 Lamberts, S.W. et al. (2002) New somatostatin analogs: will they fulfil old promises? Eur. J. Endocrinol. 146, 701–705 5 Lamberts, S.W. et al. (1991) The role of somatostatin and its analogs in the diagnosis and treatment of tumors. Endocr. Rev. 12, 450–482 6 Ferjoux, G. et al. (2000) Signal transduction of somatostatin receptors negatively controlling cell proliferation. J. Physiol. Paris 94, 205–210

7 Florio, T. et al. (1999) Somatostatin activation of mitogen-activated protein kinase via somatostatin receptor 1 (SSTR1). Mol. Endocrinol. 13, 24–37 8 Pages, P. et al. (1999) sst2 somatostatin receptor mediates cell cycle arrest and induction of p27(Kip1). Evidence for the role of SHP-1. J. Biol. Chem. 274, 15186–15193 9 Sharma, K. et al. (1996) Subtype-selective induction of wild-type p53 and apoptosis,

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but not cell cycle arrest, by human somatostatin receptor 3. Mol. Endocrinol. 10, 1688–1696 Sharma, K. et al. (1999) C-terminal region of human somatostatin receptor 5 is required for induction of Rb and G1 cell cycle arrest. Mol. Endocrinol. 13, 82–90 Schally, A.V. et al. (2001) Hypothalamic hormones and cancer. Front. Neuroendocrinol. 22, 248–291 Woltering, E.A. et al. (1997) Somatostatin analogs: angiogenesis inhibitors with novel mechanisms of action. Invest. New Drugs 15, 77–86 Reubi, J.C. et al. (2001) Somatostatin receptor sst1-sst5 expression in normal and neoplastic human tissues using receptor autoradiography with subtype-selective ligands. Eur. J. Nucl. Med. 28, 836–846 Watson, J.C. et al. (2001) Growing vascular endothelial cells express somatostatin subtype 2 receptors. Br. J. Cancer 85, 266–272 Mentlein, R. et al. (2001) Somatostatin inhibits the production of vascular endothelial growth factor in human glioma cells. Int. J. Cancer 92, 545–550 Albini, A. et al. (1999) Somatostatin controls Kaposi’s sarcoma tumor growth through inhibition of angiogenesis. FASEB J. 13, 647–655 Delesque, N. et al. (1997) Sst2 somatostatin receptor expression reverses tumorigenicity of human pancreatic cancer cells. Cancer Res. 57, 956–962 Rochaix, P. et al. (1999) Gene therapy for pancreatic carcinoma: local and distant antitumor effects after somatostatin receptor sst2 gene transfer. Hum. Gene Ther. 10, 995–1008 Benali, N. et al. (2000) Inhibition of growth and metastatic progression of pancreatic carcinoma in hamster after somatostatin receptor subtype 2 (sst2) gene expression and administration of cytotoxic somatostatin analog AN-238. Proc. Natl. Acad. Sci. U. S. A. 97, 9180–9185 Reubi, J.C. et al. (1990) Distribution of somatostatin receptors in normal and tumor tissue. Metabolism 39, 78–81 Losa, M. et al. (2001) Effects of octreotide treatment on the proliferation and apoptotic index of GH-secreting pituitary adenomas. J. Clin. Endocrinol. Metab. 86, 5194–5200 Arnold, R. et al. (2000) Treatment of neuroendocrine GEP tumours with somatostatin analogues. Digestion 62 (Suppl. 1), 84–91 Shojamanesh, H. et al. (2002) Prospective study of the antitumor efficacy of long-term octreotide treatment in patients with progressive metastatic gastrinoma. Cancer 94, 331–343 Hofland, L.J. and Lamberts, S.W. (2002) The physiological and pathophysiological consequences of somatostatin receptor internalization, desensitization and resistance. In New Developments in Endocrinology: The Expanding Role of Octreotide (Lamberts, S.W.J. and Dogliotti L., eds), pp. 89–98, Bioscientifica Kwekkeboom, D. et al. (2000) Peptide receptor imaging and therapy. J. Nucl. Med. 41, 1704–1713 Lamberts, S.W. et al. (2001) Neuroendocrine tumor markers. Front. Neuroendocrinol. 22, 309–339

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27 Termanini, B. et al. (1997) Value of somatostatin receptor scintigraphy: a prospective study in gastrinoma of its effect on clinical management. Gastroenterology 112, 335–347 28 Lugtenburg, P.J. et al. (2001) Somatostatin receptor scintigraphy in the initial staging of low-grade non-Hodgkin’s lymphomas. J. Nucl. Med. 42, 222–229 29 Jensen, R.T. (2000) Editorial: somatostatin receptor-based scintigraphy and antitumor treatment – an expanding vista? J. Clin. Endocrinol. Metab. 85, 3507–3508 30 Traub, T. et al. (2001) 111In-DOTA-lanreotide scintigraphy in patients with tumors of the lung. J. Nucl. Med. 42, 1309–1315 31 Blum, J. et al. (2000) A multicenter trial with a somatostatin analog (99m)Tc depreotide in the evaluation of solitary pulmonary nodules. Chest 117, 1232–1238 32 Reubi, J.C. et al. (2000) Affinity profiles for human somatostatin receptor subtypes SST1-SST5 of somatostatin radiotracers selected for scintigraphic and radiotherapeutic use. Eur. J. Nucl. Med. 27, 273–282 33 Papotti, M. et al. (2001) Expression of somatostatin receptor types 2, 3 and 5 in biopsies and surgical specimens of human lung tumours. Correlation with preoperative octreotide scintigraphy. Virchows Arch. 439, 787–797 34 Virgolini, I. et al. (2001) Comparative somatostatin receptor scintigraphy using in-111-DOTA-lanreotide and in-111-DOTA-Tyr3octreotide versus F-18-FDG-PET for evaluation of somatostatin receptor-mediated radionuclide therapy. Ann. Oncol. 12, S41–45 35 Wangberg, B. et al. (1996) Intraoperative detection of somatostatin-receptor-positive neuroendocrine tumours using indium111-labelled DTPA-D-Phe1-octreotide. Br. J. Cancer 73, 770–775 36 Schirmer, W.J. et al. (1993) Intraoperative localization of neuroendocrine tumors with 125I-TYR(3)-octreotide and a hand-held γ-detecting probe. Surgery 114, 745–752 37 Nilsson, S. et al. (1995) Metastatic hormone-refractory prostatic adenocarcinoma expresses somatostatin receptors and is visualized in vivo by [111In]-labeled DTPA-D[Phe1]-octreotide scintigraphy. Cancer Res. 55, 5805s–5810s 38 Sinisi, A.A. et al. (1997) Different expression patterns of somatostatin receptor subtypes in cultured epithelial cells from human normal prostate and prostate cancer. J. Clin. Endocrinol. Metab. 82, 2566–2569 39 Halmos, G. et al. (2000) High expression of somatostatin receptors and messenger ribonucleic acid for its receptor subtypes in organ-confined and locally advanced human prostate cancers. J. Clin. Endocrinol. Metab. 85, 2564–2571 40 Hofland, L.J. and Lamberts, S.W. (2001) Somatostatin receptor subtype expression in human tumors. Ann. Oncol. 12, S31–36 41 van Eijck, C.H. et al. (1994) Somatostatinreceptor scintigraphy in primary breast cancer. Lancet 343, 640–643 42 Ingle, J.N. et al. (1999) A randomized trial of tamoxifen alone or combined with octreotide in the treatment of women with metastatic breast carcinoma. Cancer 85, 1284–1292

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43 Kouroumalis, E. et al. (1998) Treatment of hepatocellular carcinoma with octreotide: a randomised controlled study. Gut 42, 442–447 44 Hofland, L.J. et al. (1999) Internalization of [DOTA,125I-Tyr3]octreotide by somatostatin receptor-positive cells in vitro and in vivo: implications for somatostatin receptor-targeted radio-guided surgery. Proc. Assoc. Am. Phys. 111, 63–69 45 Hukovic, N. et al. (1996) Agonist-dependent regulation of cloned human somatostatin receptor types 1-5 (hSSTR1-5): subtype selective internalization or upregulation. Endocrinology 137, 4046–4049 46 Schally, A.V. and Nagy, A. (1999) Cancer chemotherapy based on targeting of cytotoxic peptide conjugates to their receptors on tumors. Eur. J. Endocrinol. 141, 1–14 47 Plonowski, A. et al. (2000) Inhibition of metastatic renal cell carcinomas expressing somatostatin receptors by a targeted cytotoxic analogue of somatostatin AN-238. Cancer Res. 60, 2996–3001 48 Nagy, A. and Schally, A.V. (2001) Targeted cytotoxic somatostatin analogs: a modern approach to the therapy of various cancers. Drugs of the Future 26, 261–270 49 Breeman, W.A. et al. (2001) Somatostatin receptor-mediated imaging and therapy: basic science, current knowledge, limitations and future perspectives. Eur. J. Nucl. Med. 28, 1421–1429 50 Stolz, B. et al. (1998) The somatostatin receptor-targeted radiotherapeutic [90Y-DOTA-DPhe1, Tyr3]octreotide (90Y-SMT 487) eradicates experimental rat pancreatic CA 20948 tumours. Eur. J. Nucl. Med. 25, 668–674 51 Kwekkeboom, D.J. et al. (2001) [177Lu-DOTAOTyr3]octreotate: comparison with [111In-DTPAo]octreotide in patients. Eur. J. Nucl. Med. 28, 1319–1325 52 Krenning, E. et al. (2002) Somatostatin receptor targeted-radiotherapy of tumors, preclinical and clinical findings. In New Developments in Oncology: the Expanding Role of Octreotide (Lamberts, S.W.J. and Dogliotti, L., eds), pp. 211–227, Bioscientifica 53 Rocheville, M. et al. (2000) Receptors for dopamine and somatostatin: formation of hetero-oligomers with enhanced functional activity. Science 288, 154–157 54 Rocheville, M. et al. (2000) Subtypes of the somatostatin receptor assemble as functional homo- and heterodimers. J. Biol. Chem. 275, 7862–7869 55 Saveanu, A. et al. (2001) Bim-23244, a somatostatin receptor subtype 2- and 5-selective analog with enhanced efficacy in suppressing growth hormone (GH) from octreotide-resistant human GH-secreting adenomas. J. Clin. Endocrinol. Metab. 86, 140–145 56 Zatelli, M.C. et al. (2001) Somatostatin receptor subtypes 2 and 5 differentially affect proliferation in vitro of the human medullary thyroid carcinoma cell line tt. J. Clin. Endocrinol. Metab. 86, 2161–2169 57 Bruns, C. et al. (2002) SOM230: a new somatostatin peptidomimetic with a broad SRIF receptor binding and a unique inhibitory profile. Eur. J. Endocrinol. 146, 707–716