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`Printed in U.S.A.
`
`Endocrine Reviews 25(3):458–511
`Copyright © 2004 by The Endocrine Society
`doi: 10.1210/er.2003-0014
`
`The Diagnosis and Medical Management of Advanced
`Neuroendocrine Tumors
`
`GREGORY A. KALTSAS, G. MICHAEL BESSER, AND ASHLEY B. GROSSMAN
`Department of Endocrinology, St. Bartholomew’s Hospital, London EC1A 7BE, United Kingdom
`
`Neuroendocrine tumors (NETs) constitute a heterogeneous
`group of neoplasms that originate from endocrine glands such
`as the pituitary, the parathyroids, and the (neuroendocrine)
`adrenal, as well as endocrine islets within glandular tissue
`(thyroid or pancreatic) and cells dispersed between exocrine
`cells, such as endocrine cells of the digestive (gastroentero-
`pancreatic) and respiratory tracts. Conventionally, NETs may
`present with a wide variety of functional or nonfunctional
`endocrine syndromes and may be familial and have other
`associated tumors. Assessment of specific or general tumor
`markers offers high sensitivity in establishing the diagnosis
`and can also have prognostic significance. Imaging modalities
`include endoscopic ultrasonography, computed tomography
`and magnetic resonance imaging, and particularly, scintig-
`
`raphy with somatostatin analogs and metaiodobenzylguani-
`dine. Successful treatment of disseminated NETs requires a
`multimodal approach; radical tumor surgery may be curative
`but is rarely possible. Well-differentiated and slow-growing
`gastroenteropancreatic tumors should be treated with soma-
`tostatin analogs or ␣-interferon, with chemotherapy being
`reserved for poorly differentiated and progressive tumors.
`Therapy with radionuclides may be used for tumors exhibit-
`ing uptake to a diagnostic scan, either after surgery to erad-
`icate microscopic residual disease or later if conventional
`treatment or biotherapy fails. Maintenance of the quality of
`life should be a priority, particularly because patients with
`disseminated disease may experience prolonged survival.
`(Endocrine Reviews 25: 458–511, 2004)
`
`I. Introduction
`II. Histopathological Classification and Variables Used to
`Predict Biological Behavior
`III. Tumor Biology
`A. Genetic defects
`B. Apoptosis
`C. Growth factors
`IV. Tumor Markers in Neuroendocrine Tumors
`A. Serum and immunohistochemical tumor markers
`B. Amine
`and peptide
`receptor
`expression and
`visualization
`C. Radionuclide imaging
`
`Abbreviations: ADH, Antidiuretic hormone secretion; ASVS, arterial
`stimulation venous sampling; CAG, chronic atrophic gastritis; CCK,
`cholecystokinin; CEA, carcinoembryonic antigen; CgA, chromogranin
`A; CGRP, calcitonin gene-related peptide; CHD, carcinoid heart disease;
`CS, carcinoid syndrome; CT, computed tomography; CVD, cyclophos-
`phamide, vincristine, and dacarbazine; DOTA, 1,4,7,10-tetraazacyclodo-
`decane-1,4,7,10-tetraacetic acid; DTPA, diethylene-triamine-penta acetic
`acid; ECL, enterochromaffin; EUS, endoscopic ultrasound; FDG, 18F-
`labeled deoxyglucose; FMTC, familial MTC; 5-FU, 5-fluorouracil; GC,
`gastric carcinoid(s); GEP, gastroenteropancreatic; GI, gastrointestinal;
`GRP, gastrin-releasing peptide; hCG, human chorionic gonadotropin;
`5-HIAA, 5-hydroxyindoleacetic acid; 5-HT, 5-hydroxytryptamine;
`5-HTP, 5-hydroxytryptophan; INF, interferon; IOUS, intraoperative ul-
`trasound; MEN, multiple endocrine neoplasia; MIBG, metaiodobenzyl-
`guanidine; MRI, magnetic resonance imaging; MTC, medullary thyroid
`carcinoma; NE, neuroendocrine; NET, NE tumor; NF, neurofibromato-
`sis; NIPH, noninsulinoma pancreatogenous hypoglycemia; NME, necro-
`lytic migratory erythema; NSE, neuron-specific enolase; PET, positron
`emission tomography; PP, pancreatic polypeptide; SCLC, small-cell
`lung carcinoma; SDH, succinate dehydrogenase; SPECT, single-photon
`emission CT; SS, somatostatin; STZ, streptozotocin; VEGF, vascular en-
`dothelial growth factor; VHL, von Hippel-Lindau; VIP, vasointestinal
`peptide; ZES, Zollinger-Ellison syndrome.
`Endocrine Reviews is published bimonthly by The Endocrine Society
`(http://www.endo-society.org), the foremost professional society serv-
`ing the endocrine community.
`
`458
`
`tumors (pheochromocytomas and
`
`V. Natural History of Neuroendocrine Tumors
`VI. Clinical Presentation, Biochemical Confirmation, and Im-
`aging of NETs
`A. Carcinoid tumors
`B. Islet cell tumors
`C. Chromaffin cell
`paragangliomas)
`D. Medullary thyroid carcinoma
`VII. Medical Management of Advanced Neuroendocrine
`Tumors
`A. Pretreatment considerations
`B. Symptomatic treatment of GEP tumors
`C. Systemic treatment of GEP tumors
`D. Multidisciplinary approach—quality of life
`E. Symptomatic and systemic treatment of benign and
`malignant chromaffin cell tumors (pheochromocy-
`toma and paraganglioma)
`F. Treatment of medullary thyroid carcinoma
`VIII. Summary and Final Conclusions
`
`I. Introduction
`
`ENDOCRINE TUMORS CONSTITUTE a heterogeneous
`
`group of neoplasms that have been postulated to orig-
`inate from a common precursor cell population (1). The sys-
`tem includes endocrine glands, such as the pituitary, the
`parathyroids, and the [neuroendocrine (NE)] adrenal, as well
`as endocrine islets within glandular tissue (thyroid or pan-
`creatic) and cells dispersed between exocrine cells, such as
`endocrine cells of the digestive and respiratory tracts, the
`diffuse endocrine system (2–4). Because these cells share a
`number of antigens with nerve elements, the term “neuroen-
`docrine” is also used to connote such cell types and will be
`adopted in this review (4). Traditionally, this classification
`
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`Kaltsas et al. (cid:127) Advanced NET Diagnosis
`
`Endocrine Reviews, June 2004, 25(3):458–511 459
`
`has tended to exclude pituitary and parathyroid tissue, and
`these will not be further discussed in this review. NE tumors
`(NETs) originating from the gastrointestinal (GI) tract, along
`with similar tumors originating from the lungs and thymus,
`have traditionally been defined as “carcinoid tumors”; this
`term will still be used in this review because most of the
`literature regarding the diagnosis, management, and prog-
`nosis of these tumors uses the previously established clas-
`sification (5). Some NETs may occasionally show very ag-
`gressive behavior and become highly malignant (poorly
`differentiated NETs), but the great majority tend to be rel-
`atively slow growing (well-differentiated NETs) and retain
`many multipotent differentiation capacities (3). Such fea-
`tures include the ability to produce and secrete a variety of
`metabolically active substances (amines and peptides) and
`cause distinct clinical syndromes (6). In addition, NETs pos-
`sess neuroamine uptake mechanisms and/or specific recep-
`tors at the cell membrane, such as somatostatin (SS) recep-
`tors, which can be of great value in identifying and localizing
`these tumors as well as being useful in their therapy (7). NETs
`may occur either sporadically or as part of familial syn-
`dromes; the latter are associated with particular genetic de-
`fects, a number of which have recently been delineated at the
`molecular level (8).
`This review will focus on the gastroenteropancreatic (GEP)
`NETs, NETs originating from chromaffin cells, and NETs
`originating from C (parafollicular) cells of the thyroid. The
`particular features of NETs that have recently been incor-
`porated into their classification will be covered in an attempt
`to combine accurate diagnosis with biological behavior and
`prognosis. Recent developments involving the pathogenesis,
`earlier diagnosis, and screening for NETs, particularly in
`familial forms, will also be discussed. Classical symptoms of
`specific syndromes related to humoral secretion or other
`clinical presentation of such tumors and recent advances in
`their biochemical confirmation and localization methods will
`be presented, and a diagnostic algorithm will be formulated.
`Finally, evidence-based current medical therapeutic ap-
`proaches aiming at humoral control and prevention of fur-
`ther tumor growth will be reviewed, and a possible thera-
`peutic algorithm for each of these tumors will be proposed.
`
`II. Histopathological Classification and Variables
`Used to Predict Biological Behavior
`
`The major function of NE cells is to elaborate, store, and
`secrete small peptides and biogenic amines (6, 9). Their his-
`topathological examination aims at classifying the tumors
`according to their tissue origin, biochemical behavior, and
`prognosis (10). The assessment of endocrine differentiation
`in tumors has traditionally been obtained using light mi-
`croscopy, silver impregnation methods (histochemistry), and
`electron microscopy (1, 4). Currently, the diagnosis of NETs
`mainly relies on the positive assessment of markers of NE
`differentiation by immunohistochemistry (3, 4, 11). The most
`commonly used markers are general NE markers (applicable
`to all NE cells), either in the cytosol such as neuron-specific
`enolase (NSE) and the protein gene product 9.5 (12, 13) or
`granular markers such as chromogranin A (CgA) and syn-
`
`aptophysin (2, 4, 14). The cell-specific characterization of
`NETs requires hormone immunohistochemistry (4, 11).
`NETs associated with hyperfunctional syndromes are de-
`fined as functioning, whereas NETs exhibiting immunopos-
`itivity for endocrine markers and/or elevated serum markers
`but unassociated with a distinct clinical syndrome are called
`nonfunctioning tumors (3). Histological and hormonal fea-
`tures of specific cell types are integrated in a so-called “mor-
`phofunctional” classification, in an attempt to predict the
`natural history of the tumor (2, 3, 15). In the recent World
`Health Organization (WHO) classification, the following
`types of NETs have been recognized, at least for GEP tumors,
`but this is probably applicable to all NETs (1, 3, 4, 15–17): 1)
`well-differentiated endocrine tumor (benign or low grade
`malignant); 2) well-differentiated endocrine carcinoma; 3)
`poorly differentiated endocrine carcinoma (small cell carci-
`noma); and 4) mixed exocrine-endocrine carcinoma.
`The differentiation is based on histomorphology, tumor
`size (in general larger tumors are more aggressive), and the
`presence or absence of gross local invasion and/or metas-
`tasis, thus reflecting biological behavior (2, 3). Most NETs are
`well-differentiated tumors that are characterized by a solid
`trabecular or glandular structure, tumor cell monomorphism
`with absent or low cytological atypia, and a low mitotic (⬍2
`mitoses/mm2) and proliferative status (⬍2% Ki-67 positive
`cells) (3). Such tumors are slowly growing but can occasion-
`ally exhibit more aggressive behavior (⬎2 mitoses/mm2
`and/or proliferation index ⬎2% Ki-67 positive cells); how-
`ever, only in the presence of metastasis and/or invasiveness
`is the tumor defined as a well-differentiated NE carcinoma
`(2, 18). Poorly differentiated NETs are invariably malignant,
`are defined as poorly differentiated NE carcinomas, and are
`characterized by a predominantly solid structure with abun-
`dant necrosis, cellular atypia with a high mitotic index (ⱖ10
`mitoses/mm2) and proliferative status (⬎15% Ki-67 positive
`cells), diffuse reactivity for cytosolic markers, and scant or
`weak reactivity for granular markers or neurosecretory prod-
`ucts (3). Mixed exocrine-endocrine carcinomas are epithelial
`tumors with a predominant exocrine component admixed
`with an endocrine component comprising at least one third
`of the entire tumor cell population. Their biological behavior
`is essentially dictated by the exocrine component, which may
`be acinar or ductal type (18). It is hoped that, in the future,
`other factors such as the angiogenic capacity of tumor cells
`and specific genetic changes may prove to be valuable tools
`in determining prognosis, biological behavior, and response
`to therapy (10).
`
`III. Tumor Biology
`
`A. Genetic defects
`
`NETs can occur sporadically or in a familial context of
`autosomal dominant inherited syndromes such as multiple
`endocrine neoplasia (MEN) (8, 19). Four major MEN syn-
`dromes, MEN I, MEN II, von Hippel-Lindau (VHL) disease,
`and Carney complex, represent the most common forms of
`inherited predisposition to NETs with variable but high pen-
`etrance in various NE tissue; early screening can be used for
`presymptomatic diagnosis (8, 19). Less commonly, endocrine
`
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`460 Endocrine Reviews, June 2004, 25(3):458–511
`
`Kaltsas et al. (cid:127) Advanced NET Diagnosis
`
`tumors of the pancreas, parathyroids, and adrenal glands
`have been observed in phacomatoses, such as neurofibro-
`matosis (NF) type 1 and tuberous sclerosis (19). In addition,
`familial occurrence of single endocrine lesions such as pri-
`mary hyperparathyroidism, pituitary adenomas, medullary
`thyroid carcinoma (MTC), or pheochromocytomas have been
`identified as putative genetic diseases for most of which the
`genetic pathways remain to be identified (20).
`Most NET-predisposing diseases have been related to in-
`activation of tumor growth suppressor genes, except in MEN
`II and the inherited form of MTC, which occur through
`dominant activation of the RET protooncogene (19, 21). The
`RET protooncogene encodes a transmembrane tyrosine-
`kinase receptor that causes cellular proliferation, differenti-
`ation, and increased cell motility (8, 21). MEN II comprises
`three clinical subtypes, MEN IIA, MEN IIB, and familial MTC
`(FMTC) (21); in MEN IIA, all patients develop MTC, about
`50% pheochromocytoma, and about 15% primary hyper-
`parathyroidism (21, 22). Patients with MEN IIB may have a
`marfanoid habitus and mucosal neuromas but not hyper-
`parathyroidism; in these patients, MTC occurs at a younger
`age and behaves more aggressively compared with MEN IIA
`(19, 21). Approximately 95% of MEN II cases are accounted
`for by germline RET mutations (⬃98% of MEN IIA cases, 97%
`of MEN IIB cases, and 85% of FMTC cases) (21, 23). MEN I
`is an autosomal dominant syndrome characterized mainly
`by hyperplasia and/or multiple tumors of the parathyroid,
`endocrine pancreas, anterior pituitary, foregut-derived NE-
`tissues, and adrenocortical glands (24). Somatic mutations of
`the MEN I gene have been reported in sporadic forms of
`endocrine tumors with a variable incidence of 20–30% in
`parathyroid (25), endocrine pancreas (33% gastrinomas, 17%
`insulinomas) (26), 25% of lung carcinoids (27), but less than
`1% in pituitary and adrenocortical tumors (4). In clinical
`practice, genetic analysis is useful to assess the syndromic
`diagnosis of MEN I, but the diagnosis cannot be excluded
`with certainty when a mutation is not found (8). Therefore,
`the clinical screening of patients remains a prerequisite for
`genetic analysis. The three major features of VHL disease are
`retinal angiomas, central nervous system hemangioblasto-
`mas, and clear cell renal cell carcinomas; the lifetime risk for
`each of these tumors has been estimated as greater than 70%
`(21, 28, 29). Other VHL-related lesions include pheochromo-
`cytomas, pancreatic islet cell tumors, and papillary cystade-
`nomas of the pancreas, epididymis, the broad ligament, and
`the lymphatic sac of the middle ear (29). However, the in-
`cidence of specific tumors depends on the phenotypic class
`of VHL, of which four have been described (type 1 and types
`2A, 2B, and 2C). The Carney complex is an autosomal dom-
`inant disease predisposing to various types of tumors, in-
`cluding cardiac and cutaneous myxomas, spotty pigmenta-
`tion of
`the skin, and nonneoplastic hyperfunctioning
`endocrine states, such as nodular adrenocortical hyperplasia
`associated with Cushing’s syndrome and pituitary and thy-
`roid adenomas (30, 31). Approximately 1% of patients diag-
`nosed with pheochromocytomas may have NF1, a domi-
`nantly inherited disorder with complete penetrance but
`highly variable expressivity (32). Diagnostic criteria for NF1
`include cutaneous or sc neurofibromas, cafe´-au-lait spots
`appearing early in life, optic glioma, benign iris hamartomas
`
`(Lisch nodules), and specific dysplastic bone lesions (32).
`Digestive tract carcinoid tumors have rarely been described
`in patients with NF1 and tuberous sclerosis (8, 33). Knowl-
`edge of the particular genetic defects in these familial syn-
`dromes is essential for the early screening and counseling of
`other family members.
`
`B. Apoptosis
`
`The protein product of the bcl-2 oncogene is an important
`modulator of apoptosis because it blocks programmed cell
`death without affecting cell proliferation (34–36), whereas
`the c-myc protooncogene, which inactivates key tumor sup-
`pressors such as p53 and retinoblastoma gene product, also
`plays a central role in some forms of apoptosis (36, 37).
`Coexpression of bcl-2 and c-myc leads to a synergism that
`may result from the ability of bcl-2 to directly interfere with
`the apoptotic cell death resulting from the dysregulated ex-
`pression of c-myc (34–36). Such an association has recently been
`described for a number of NETs including MTC, pheochromo-
`cytomas, carotid body tumors, and some carcinoids (34).
`
`C. Growth factors
`
`Malignant progression of NETs may also be triggered by
`overexpression of growth factors involved in endocrine and
`endothelial cell proliferation such as TGF␣, endothelial
`growth factor, nerve growth factor, and vascular endothelial
`growth factor (VEGF)/VEGF-related factors (19). Among
`various growth factors promoting angiogenesis, VEGF was
`found to be overexpressed, mainly in midgut carcinoid and
`some pancreatic tumors, suggesting that it may be involved
`indirectly in the growth of these tumors (38).
`The genetic markers so far identified in various sporadic
`types of NETs are not specific enough to be used for diag-
`nostic purposes, but they provide some clues as to the genetic
`mechanism of tumor development.
`
`IV. Tumor Markers in Neuroendocrine Tumors
`
`A. Serum and immunohistochemical tumor markers
`
`The various cell types of the NE cell system can secrete
`specific products, such as peptides and biogenic amines, that
`are tumor-specific and may serve as markers for the diag-
`nosis and follow-up of treatment (see Section IV.A.1); it is also
`probable that some tumor markers may have prognostic
`implications (6, 39) (Table 1). A number of other components
`specific for all NE cells and associated with secretory gran-
`ules or cytosolic proteins can also be used as tumor markers;
`among these, the chromogranin family is the one most com-
`monly used (see Section IV.A.2) (6, 39).
`
`1. Specific tumor markers. Peptide hormones are synthesized
`as precursors, which are cleaved in a sequence- and tissue-
`specific manner to yield the biologically active peptides;
`however, their fine processing is usually deficient in NET
`cells (6). Therefore, direct measurement of these peptides,
`and when necessary of their precursors, establishes the di-
`agnosis and occasionally also provides information regard-
`ing the size of the tumor (39). In addition, there are cases in
`
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`Kaltsas et al. (cid:127) Advanced NET Diagnosis
`
`Endocrine Reviews, June 2004, 25(3):458–511 461
`
`TABLE 1. Common tumor markers and distribution of SS receptors in patients with GEP tumors, chromaffin cell tumors, and MTCs
`
`Tumor types
`
`Specific serum tumor markers
`
`Thymus
`C-thyroid cells
`Lung
`GI tract
`
`Pancreatic islet cells
`Ovary
`Chromaffin cells
`
`SS, serotonin
`Calcitonin, CGRP, ACTH, SS, serotonin
`GRP, CT, SS, POMC, ACTH, ADH, serotonin, ␤-hCG
`Gastrin, CCK, GIP, VIP, motilin, glucagon, GRP, PP, GHRH,
`POMC, ACTH, serotonin
`Insulin, gastrin, VIP, glucagon, SS, serotonin
`Serotonin, hCG, PTHrP, POMC, CGRP
`Noradrenaline, adrenaline, dopamine, POMC, calcitonin,
`neuropeptide Y, neurotensin, SS
`POMC, CGRP
`
`Nonspecific serum
`tumor markers
`
`CgA, NSE
`CgA, CEA
`CgA, NSE
`CgA, NSE, hCG
`
`CgA, NSE, hCG
`CgA, NSE
`CgA, NSE
`
`SS receptors
`(positive scintigraphy
`with 111In-octreotide)
`50–80%
`70–75%
`80%
`80–90%
`
`60–95%
`
`85–95%
`
`Adenocarcinomas with NE
`differentiation
`Derived from Krenning et al. (53); Olsen et al. (70); Lamberts et al. (6); Nobels et al. (40); Oberg (407); Norheim et al. (211); and Tomassetti
`et al. (213). POMC, Proopiomelanocortin; GIP, gastric inhibitory peptide.
`
`CgA, NSE
`
`20–35%
`
`which multiple hormone production is evident, which can
`also fluctuate throughout the course of the disease (39). The
`measurement of serum hormone concentrations can also be
`useful in the diagnosis of clinically nonfunctioning tumors in
`which the hormonal products may not be associated with
`clinical syndromes (6, 40). More recently, the ␣- and ␤-sub-
`units of human chorionic gonadotropin (␣- and ␤-hCG) have
`been shown to be markers of nonfunctioning GEP tumors, as
`well as MTC and small-cell lung carcinoma (SCLC) (6, 40).
`
`2. Nonspecific tumor markers. In addition to specific hormones
`secreted by NE cells, other proteins that exert regulatory
`activities on the packaging, processing, and secretion of hor-
`mones are increasingly recognized as NET markers (6, 39,
`41). CgA, CgB, and CgC form a group of acidic monomeric
`soluble proteins that are localized within secretory granules
`in which they are costored and cosecreted with the locally
`present peptides (39, 42). CgA is the granin mostly used in
`clinical practice, although the other chromogranins are rel-
`evant, particularly as CgA-negative, but CgB-positive tu-
`mors are increasingly being recognized (39, 43). Plasma CgA
`levels may be elevated in a variety of NETs, including pheo-
`chromocytomas (43–45), paragangliomas (40, 46), carcinoid
`and pancreatic islet cell tumors (43, 46, 47), MTC (43), para-
`thyroid and pituitary adenomas (48), although much less
`(⬍60%) in SCLC (40, 44). The highest CgA levels have been
`found in metastatic carcinoids and GEP tumors (44, 45, 49,
`50). Both tumor burden and secretory activity should be
`considered when interpreting CgA results, with a sensitivity
`and specificity varying between 10–100% and 68–100%, re-
`spectively (5, 43, 50). Renal insufficiency and hypergastrine-
`mia are the main causes of false-positive CgA results (40, 43).
`Several assays for the measurement of intact CgA and the
`different cleavage products have been developed using ei-
`ther monoclonal or polyclonal antibodies, and thus exhibit-
`ing substantial differences in sensitivities and specificities
`(51). This must be taken into consideration until a recognized
`international standard for CgA is established (51). Compar-
`ative studies have shown that the sensitivity of CgA in re-
`lation to the reference biological specific markers is higher in
`foregut carcinoids, comprising bronchial, thymic, head and
`neck primaries (5, 40, 43), and comparable to specific tumor
`marker sensitivities in patients with ileal carcinoids and
`pheochromocytomas (43, 50). In addition, CgA has been
`
`shown to be an independent prognostic factor for midgut
`carcinoids because it correlates not only with tumor burden
`but also with biological activity (46, 47).
`Synaptophysin and NSE are present diffusely in the cyto-
`plasm of NETs, so they are consistently positive in most NETs
`(6). NSE is only present in neurons and NE cells and can also
`serve as a circulating marker for NETs (6). NSE is most fre-
`quently elevated in patients with SCLC (74%) but has also been
`found to be elevated in 30–50% of patients with carcinoids,
`MTC, islet cell tumors, and pheochromocytomas (40). Elevated
`levels of NSE are also roughly correlated with tumor size, al-
`though the specificity is lower than that of CgA; however, the
`combination of both CgA and NSE has a higher sensitivity than
`either parameter separately (40). Some oncogenic proteins are
`not specific for NETs but are frequently synthesized in these
`tumors, i.e., carcinoembryonic antigen (CEA) in MTC (6).
`
`3. Tumor markers and stimulation tests. When patients present
`with a high clinical suspicion of a functional syndrome but
`with normal basal measurements of specific tumor markers,
`a dynamic test can be used to increase sensitivity (39). Al-
`though the rationale of employing such tests has recently
`been questioned, several dynamic tests have traditionally
`been used (52). The dynamic tests that are still in use will be
`discussed later with reference to individual tumor types.
`
`B. Amine and peptide receptor expression and visualization
`
`The demonstration of the presence of amine uptake mech-
`anisms and a high density of peptide receptors on several
`NETs, as well as their metastases, has been used for both
`diagnosis and monitoring of these tumors using radionuclide
`techniques (6, 53).
`Metaiodobenzylguanidine (MIBG) is a guanidine deriva-
`tive that exploits the specific type 1 amine uptake mechanism
`at the cell membrane and the subsequent uptake from the
`cytoplasm and storage within the intracellular storage ves-
`icles (54). It shows little binding to postsynaptic receptors
`and has minimal or no intrinsic pharmacological effect (54,
`55). MIBG localizes to adrenomedullary tumors, hyperplastic
`adrenal medulla and, to a lesser degree, in the healthy ad-
`renal medulla (54, 56). In addition, several other NETs in-
`cluding carcinoids and MTC exhibit this specific uptake
`mechanism and can thus accumulate MIBG (54).
`
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`462 Endocrine Reviews, June 2004, 25(3):458–511
`
`Kaltsas et al. (cid:127) Advanced NET Diagnosis
`
`SS is a 14-amino acid peptide that is widely expressed
`throughout the central nervous system as well as in peripheral
`tissues including the endocrine pancreas, gut, thyroid, adrenals,
`and kidneys (57, 58). SS acts mostly as an inhibitory factor on
`neurotransmission, intestinal mobility, absorption of nutrients
`and ions, vascular contractility, and cell proliferation (57). Ow-
`ing to its short half-life (1–2 min), many SS long-acting analogs
`have been synthesized, among which octreotide and lanreotide
`are the ones most commonly used in clinical practice (59, 60).
`These analogs are cyclic octapeptides that have a more pro-
`longed half-life (1.5–2 h), and thus, biological activity (6, 59–61).
`The biological effects of SS are mediated by five specific SS
`receptors (1–5) that all bind the native peptide but show major
`differences in their affinities for SS analogs; the currently used
`analogs exhibit a very low affinity for SS receptors 1 and 4 but
`bind with high affinity to SS receptors 2 (predominantly) and
`5 and with moderate affinity to SS receptor 3 (6, 57, 62, 63). Each
`receptor subtype is coupled to multiple intracellular transduc-
`tion pathways, but all five are functionally coupled to inhibition
`of adenylate cyclase and decreased calcium influx, and thus
`generally inhibit hormonal secretion and intestinal mobility
`(57). SS also inhibits the proliferation of both normal and tu-
`moral cells as a result of hypophosphorylation of the retino-
`blastoma gene product and G1 cell cycle arrest (64). The anti-
`proliferative effects of SS can also result from apoptosis through
`SS receptor 3 induced by p53 and Bax (39). The SS effect on
`tumor growth may also be the result of indirect effects through
`the inhibition of growth factors (65) and angiogenesis (66, 67).
`SS receptors are found mainly in well-differentiated rather
`than poorly differentiated tumors and thus may exert prog-
`nostic significance as markers of differentiation (Table 1)
`(67–69). The high frequency of SS receptor 2 mRNA in NETs
`allows the localization of various human tumors and me-
`tastases using 111In-labeled octreotide (57, 66); there is a close
`correlation between the presence of SS receptor 2 mRNA,
`tracer uptake using SS receptor autoradiography, and the
`therapeutic response to SS analog treatment (6, 39, 70). In
`addition, specific polyclonal antibodies against SS receptor 2
`have been developed that correlate with 111In-labeled oct-
`reotide uptake (71). Tumors and metastases that harbor up-
`take mechanisms and/or peptidic receptors can be visual-
`ized in vivo using a ␥-camera after the injection of 123I-MIBG
`and/or 111In-pentetreotide (72, 73). In addition, other small
`peptidic receptors that are expressed in cell membranes of
`NE tissues include vasointestinal peptide (VIP), bombesin,
`cholecystokinin (CCK), gastrin and/or substance P (6, 67–
`69). Labeled analogs/peptides can also be used as markers
`for putative receptors for in vivo tumor visualization (69, 74).
`
`C. Radionuclide imaging
`
`Radionuclides provide a diagnostic modality in which
`radiolabeled amines or peptide analogs, based on their abil-
`ity to bind to suitable ligands, are used for the identification
`and localization of NETs (7, 62, 63, 75).
`
`1. Scintigraphy with MIBG (123I-MIBG). The prolonged storage
`of MIBG within secretory vesicles permits high specific up-
`take and imaging after labeling with both 131I- and 123I-MIBG;
`however, imaging quality with 123I-MIBG is superior, and it
`
`is currently the radiopharmaceutical of choice (76–79). The
`efficiency of 123I-MIBG is excellent for the visualization of
`intraadrenal and extraadrenal sites of benign and malignant
`pheochromocytomas, showing a diagnostic sensitivity and
`specificity above 80 and 90%, respectively (80). Radiolabeled
`MIBG facilitates in the diagnosis of multiple tumors and
`paragangliomas, in the detection of suspected malignant
`chromaffin tumors, for the screening of individuals at risk in
`familial forms of the disease, and for the selection of patients
`for therapeutic MIBG based on a positive diagnostic scan (72,
`78). It also has a complementary role in the diagnosis of other
`NETs such as carcinoids and MTC (78–80) (Fig. 1); its sen-
`sitivity is said to be enhanced with the preimaging admin-
`istration of MIBG, but this remains controversial (81).
`
`2. Scintigraphy with SS analogs (111In-octreotide). Octreotide
`(Sandostatin, Novartis, Basel, Switzerland) was the first SS
`analog to be used in clinical practice, although considerable
`experience has also been obtained with lanreotide (Somatu-
`line/Ipstyl, Ipsen, Paris, France) (73). These compounds have
`been conjugated with DTPA (diethylene-triamine-pentaace-
`tic acid) (63, 64, 82), but more recently with DOTA (1,4,7,10-
`tetraazacyclododecane-1,4,7,10-tetraacetic acid) as a way of
`coupling SS analogs with various radionuclides (83, 84).
`There is a predominance of renal clearance of the analog (73,
`85, 86), although the uptake of 111In-DTPA octreotide (pen-
`tetreotide) shows a bell-shaped function of the injected mass,
`explaining the increased uptake that follows prior adminis-
`tration of the unlabeled peptide (73, 87). Planar and single-
`photon emission CT (SPECT) lesions are performed 24 and
`48 h after the injection of the radiopharmaceutical; normal
`visualization includes the thyroid, spleen, liver, kidneys, and
`part of the pituitary (73, 86) (Fig. 2). Scintigraphy with 111In-
`octreotide has been shown to have a detection rate of 67–91%
`for all NETs and is used both for diagnosis and staging, and
`also in the follow-up of patients (7, 53, 59, 70, 83). In addition,
`it also exhibits high specificity (88–92), although occasional
`false-positive localizations may occur because uptake is also
`demonstrable in many other tumors, granulomas, and au-
`toimmune diseases (7, 53, 54, 83, 86). A recent systematic
`study prospectively assessing the specificity of scintigraphy
`with 111In-octreotide in patients with gastrinomas revealed
`an overall specificity of 86% (93, 94). Evidence from in vitro
`studies has shown increased uptake of radiolabeled oct-
`reotide in the presence of low concentrations of unlabeled
`octreotide (95, 96). During octreotide treatment, the uptake
`of 111In-octreotide in SS receptor-positive tumors and the
`spleen is diminished (95). In general, NETs remain visible
`during treatment with octreotide, although tumor uptake
`may be less than without octreotide treatment (95).
`The detection of an unsuspected lesion in a patient with a
`single known lesion is important in that it may affect the se-
`lection of curative surgery, which remains the treatment of
`choice in patients with NETs (83, 97–99); however, there are no
`clinical or biochemical predictors of a positive scan (7, 99).
`False-positive results have been reported, although this may be
`a misnomer because they may actually r