Demonstration of [11C] 5-hydroxy-L-tryptophan uptake and decarboxylation in carcinoid tumors by specific positioning labeling in positron emission tomography

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

11C- and 18F-labelled tryptophans as PET-tracers for imaging of altered tryptophan metabolism in age-associated disorders

The ageing of the world’s population is the result of increased life expectancy observed in almost all countries throughout the world. Consequently, a rising tide of ageing-associated disorders, like cancer and neurodegenerative diseases, represents one of the main global challenges of the 21st century. The ability of mankind to overcome these challenges is directly dependent on the capability to develop novel methods for therapy and diagnosis of age-associated diseases. One hallmark of age-related pathologies is an altered tryptophan metabolism. Numerous pathological processes including neurodegenerative and neurological diseases like epilepsy, Parkinson’s and Alzheimer’s diseases, cancer and diabetes exhibit marked changes in tryptophan metabolism. Visualization of key processes of tryptophan metabolic pathways, especially using positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI, can be exploited to early detect the aforementioned disorders with considerable accuracy, allowing appropriate and timely treatment of patients. Here we review the published 11C- and 18F-labelled tryptophans with respect to the production and also preclinical and clinical evaluation as PET-tracers for visualization of different branches of tryptophan metabolism.

The bibliography includes 159 references.

PET/MR Imaging of the Pancreas

PET/MR imaging has the potential to markedly alter pancreatic care in both the malignant, and premalignant states with the ability to perform robust, high-resolution, quantitative molecular imaging. The ability of PET/MR imaging to monitor disease processes, potentially correct for motion in the upper abdomen, and provide novel biomarkers that may be a combination of MR imaging and PET biomarkers, offers a unique, precise interrogation of the pancreatic milieu going forward.

Role of biomarker tests for diagnosis of neuroendocrine tumours

Neuroendocrine tumours (NETs) are neoplasms that arise from neuroendocrine cells. Neuroendocrine cells and their tumours can secrete a wide range of amines and polypeptide hormones into the systemic circulation. This feature has triggered widespread investigation into circulating biomarkers for the diagnosis of NETs as well as for the prediction of the biological behaviour of tumour cells. Classic examples of circulating biomarkers for gastroenteropancreatic NETs include chromogranin A, neuron-specific enolase and pancreatic polypeptide as well as hormones that elicit clinical syndromes, such as serotonin and its metabolites, insulin, glucagon and gastrin. Biomarker metrics of general markers for diagnosing all gastroenteropancreatic NET subtypes are limited, but specific hormonal measurements can be of diagnostic value in select cases. In the past decade, methods for detecting circulating transcripts and tumour cells have been developed to improve the diagnosis of patients with NETs. Concurrently, modern scanning techniques and superior radiotracers for functional imaging have markedly expanded the options for clinicians dealing with NETs. Here, we review the latest research on biomarkers in the NET field to provide clinicians with a comprehensive overview of relevant diagnostic biomarkers that can be implemented in dedicated situations.

Preclinical evaluation of 5-([11C]-methyloxy)-L-tryptophan as a potential PET molecular imaging probe

OBJECTIVE

The new tumor probe 5-([C]-methyloxy)-L-tryptophan ([C]-L-CMTP) had been found to show high uptake in tumor tissue in our previous report; however, the pharmacokinetic properties of [C]-L-CMTP have not been characterized. In this present study, we evaluated the potential of [C]-L-CMTP as a PET probe for tumor imaging.

METHODS

The biodistribution of [C]-L-CMTP was determined in healthy mice and the incorporation of [C]-L-CMTP into proteins was investigated. In-vitro competitive inhibition experiments were conducted with Hepa1-6 hepatoma cell lines. [C]-L-CMTP PET imaging was performed on S180 tumor-bearing mice and lung tumor-bearing mice.

RESULTS

Biodistribution studies showed high uptake of [C]-L-CMTP in the liver, kidney, and pancreas, but low uptake in brain. The transport assay studies in Hepa1-6 cells suggested that [C]-L-CMTP was mainly transported by the amino acid transport system B and LAT1. [C]-L-CMTP was not incorporated into proteins in vitro. PET imaging showed that [C]-L-CMTP had high uptake in S180 fibrosarcoma tumor and lung tumor.

CONCLUSION

[C]-L-CMTP was not incorporated into proteins and was mainly transported by the amino acid transport system B and LAT1, PET imaging showed that [C]-L-CMTP had high uptake in tumor models; it seemed to be a promising PET probe for tumor by LAT1 transport. Video abstract: http://links.lww.com/NMC/A70.

Potential value of EUS in pancreatic surveillance of VHL patients

BACKGROUND

Patients with von Hippel-Lindau (VHL) disease are prone to develop pancreatic neuroendocrine tumors (pNETs). However, the best imaging technique for early detection of pNETs in VHL is currently unknown. In a head-to-head comparison, we evaluated endoscopic ultrasound (EUS) and (11)C-5-hydroxytryptophan positron emission tomography ((11)C-5-HTP PET) compared with conventional screening techniques for early detection of pancreatic solid lesions in VHL patients.

METHODS

We conducted a cross-sectional, prospective study in 22 patients at a tertiary care university medical center. Patients with VHL mutation or with one VHL manifestation and a mutation carrier as first-degree family member, with recent screening by abdominal computed tomography (CT) or magnetic resonance imaging (MRI) and somatostatin receptor scintigraphy (SRS), were eligible. Patients underwent EUS by linear Pentax echoendoscope and Hitachi EUB-525, and (11)C-5-HTP PET. Patient-based and lesion-based positivity for pancreatic solid lesions were calculated for all imaging techniques with a composite reference standard.

RESULTS

In 10 of the 22 patients, 20 pancreatic solid lesions were detected: 17 with EUS (P < 0.05 vs CT/MRI+ SRS), 3 with (11)C-5-HTP PET, 3 with SRS, 9 with CT/MRI, and 9 with CT/MRI + SRS. EUS evaluations showed solid lesions with a median size of 9.7 mm (range 2.9-55 mm) and most of them were homogeneous, hypoechoic, isoelastic, and hypervascular. Moreover, EUS detected multiple pancreatic cysts in 18 patients with a median of 4 cysts (range 1-30).

CONCLUSIONS

EUS is superior to CT/MRI + SRS for detecting pancreatic solid lesions in VHL disease.(11)C-5-HTP PET has no value as a screening method in this setting. EUS performs well in early detection of pNETs, but its role in VHL surveillance is unclear.

EUS is superior for detection of pancreatic lesions compared with standard imaging in patients with multiple endocrine neoplasia type 1

BACKGROUND

In multiple endocrine neoplasia type 1 (MEN1), pancreatic neuroendocrine tumors (pNETs) are the leading MEN1-related cause of death.

OBJECTIVE

To evaluate EUS and (11)C-5-hydroxytryptophan positron emission tomography ((11)C-5-HTP PET), compared with the recommended screening techniques in MEN1 patients for early detection of pNETs.

DESIGN

Cross-sectional study.

SETTING

Tertiary-care university medical center.

PATIENTS

This study involved 41 patients with a proven MEN1 mutation or with one MEN1 manifestation and a mutation carrier as a first-degree family member, with recent screening by abdominal CT or magnetic resonance imaging (MRI) and somatostatin receptor scintigraphy (SRS).

INTERVENTIONS

EUS by using a linear Pentax echoendoscope and Hitachi EUB-525 and (11)C-5-HTP PET.

MAIN OUTCOME MEASUREMENTS

Patient-based and lesion-based positivity for pNET was calculated for all imaging techniques. The McNemar test was used to compare the yield of the 4 imaging techniques.

RESULTS

In 35 of 41 patients, 107 pancreatic lesions were detected in total. EUS detected 101 pancreatic lesions in 34 patients, (11)C-5-HTP PET detected 35 lesions in 19 patients, and CT/MRI + SRS detected 32 lesions in 18 patients (P < .001). (11)C-5-HTP PET performed similarly to CT/MRI + SRS and better compared with SRS only (13 lesions in 12 patients), both at a patient-based and lesion-based level (P < .05).

LIMITATIONS

Single-center study.

CONCLUSION

EUS is superior to CT/MRI + SRS for pancreatic lesion detection in patients with MEN1. In this setting, (11)C-5-HTP PET is not useful. We recommend EUS as the first-choice pancreas imaging technique in patients with MEN1. (

CLINICAL TRIAL REGISTRATION NUMBER

NTR1668.).

Radiosynthesis and biological evaluation of 5-(3-[18F]fluoropropyloxy)-L-tryptophan for tumor PET imaging

INTRODUCTION

[(18)F]FDG PET has difficulty distinguishing tumor from inflammation in the clinic because of the same high uptake in nonmalignant and inflammatory tissue. In contrast, amino acid tracers do not accumulate in inflamed tissues and thus provide an excellent opportunity for their use in clinical cancer imaging. In this study, we developed a new amino acid tracer 5-(3-[(18)F]Fluoropropyloxy)-L-tryptophan ([(18)F]-L-FPTP) by two-step reactions and performed its biologic evaluation.

METHODS

[(18)F]-L-FPTP was prepared by [(18)F]fluoropropylation of 5-hydroxy-L-tryptophan disodium salt and purification on C18 cartridges. The biodistribution of [(18)F]-L-FPTP was determined in normal mice and the incorporation of [(18)F]-L-FPTP into tissue proteins was investigated. In vitro competitive inhibition experiments were performed with Hepa1-6 hepatoma cell lines. [(18)F]-L-FPTP PET imaging was performed on tumor-bearing and inflammation mice and compared with [(18)F]-L-FEHTP PET.

RESULTS

The overall uncorrected radiochemical yield of [(18)F]-L-FPTP was 21.1 ± 4.4% with a synthesis time of 60 min, the radiochemical purity was more than 99%. Biodistribution studies demonstrate high uptake of [(18)F]-L-FPTP in liver, kidney, pancreas, and blood at the early phase, and fast clearance in most tissues over the whole observed time. The uptake studies in Hepa1-6 cells suggest that [(18)F]-L-FPTP is transported by the amino acid transport system B(0,+), LAT2 and ASC. [(18)F]-L-FPTP displays good stability and is not incorporated into proteins in vitro. PET imaging shows that [(18)F]-L-FPTP can be a better potential PET tracer for differentiating tumor from inflammation than [(18)F]FDG and 5-(3-[(18)F]fluoroethyloxy)-L-tryptophan ([(18)F]-L-FEHTP), with high [(18)F]-L-FPTP uptake ratio (2.53) of tumor to inflammation at 60 min postinjection.

CONCLUSIONS

Using [(18)F]fluoropropyl derivatives as intermediates, the new tracer [(18)F]-L-FPTP was achieved with good yield and radiochemical purity, and the biological evaluation results of [(18)F]-L-FPTP showed that it was a hopeful tracer for PET tumor imaging.

11C-5-hydroxytryptophan positron emission tomography after radiofrequency ablation of neuroendocrine tumor liver metastases

AIM

The aim was to assess the feasibility of (11)C-5-hydroxy-tryptophan positron emission tomography ((11)C-5-HTP-PET) in the follow-up after radiofrequency ablation (RFA) of liver metastases from neuroendocrine tumors (NETs).

BACKGROUND

Contrast-enhanced computed tomography (CECT) and contrast-enhanced ultrasound (CEUS) are commonly used to evaluate the liver after RFA of NETs. In general, (11)C-5-HTP-PET is more sensitive in the visualization of NETs, but no studies have investigated its role after RFA.

METHODS

Six consecutive patients with liver metastases from NETs were subjected to RFA treatment. All patients underwent baseline imaging before RFA and on two occasions (1-2 and 6-11 months) after RFA. The imaging consisted of (11)C-5-HTP-PET, CEUS and CECT on all three occasions.

RESULTS

Thirty RFA areas were evaluated, and residual tumors (RTs) were depicted in eight areas (22%). (11)C-5-HTP-PET depicted RTs after RFA with maximum sensitivity (100%) and specificity (100%), using radiological follow-up as the gold standard. (11)C-5-HTP-PET detected five out of eight RTs earlier than CECT or CEUS. In general, the sensitivity of (11)C-5-HTP-PET exceeded that of CECT and CEUS for early visualization of NET liver metastases.

CONCLUSION

(11)C-5-HTP-PET can be used in the follow-up after RFA for the purpose of detecting RT, and it provides additional information to CEUS and CECT by detecting new lesions.

PET-Guided Surgery - High Correlation between Positron Emission Tomography with 11C-5-Hydroxytryptophane (5-HTP) and Surgical Findings in Abdominal Neuroendocrine Tumours

Positron emission tomography (PET) with 11C-labeled 5-hydroxytryptophane (5-HTP) is a sensitive technique to visualize neuroendocrine tumours (NETs), due to high intracellular uptake of amine-precursors like L-dihydroxyphenylalanine (L-DOPA) and 5-HTP. NETs are often small and difficult to localize in spite of overt clinical symptoms due to hormonal excess. In our study, 38 consecutive NET patients underwent 11C-5-HTP-PET and morphological imaging by CT within 12 weeks prior to surgery. Surgical, histopathological and 5-HTP PET findings were correlated. 11C-5-HTP-PET corresponded to the surgical findings in 31 cases, was false negative in six, and true negative in one case resulting in 83.8% sensitivity and 100% specificity. Positive predicted value was 100%. In 11 patients 11C-5-HTP-PET was the only imaging method applied to localize the tumour. Thus, we could demonstrate that functional imaging by 11C-5-HTP-PET in many cases adds vital preoperative diagnostic information and in more than every fourth patient was the only imaging method that will guide the surgeon in finding the NET-lesion. Although the present results demonstrates that 11C-5-HTP may be used as an universal NET tracer, the sensitivity to visualize benign insulinomas and non functioning pancreatic NETs was lower.

The value of 11C-5-hydroxy-tryptophan positron emission tomography in neuroendocrine tumor diagnosis and management: experience from one center

Many neuroendocrine tumors (NET) are small and may escape localization by conventional imaging techniques. In such cases, 11C-5-hydroxy-tryptophan (11C-5-HTP) positron emission tomography (PET) has been tested as an additional diagnostic tool. Nine patients with clinically, biochemically and/or histologically confirmed NET and negative computerized tomography (CT) or magnetic resonance imaging (MRI), and 111In-pentetreotide (Octreoscan) scintigraphy underwent imaging with 11C-5-HTP-PET/CT in order to: 1) detect the primary tumor lesion in three patients; 2) detect residual disease in two patients with appendiceal carcinoid, one with rectal carcinoid, one with midgut carcinoid, and one with ectopic ACTH secretion (EAS) due to residual pulmonary carcinoid; and 3) restage a patient with medullary thyroid carcinoma (MTC) and hepatic metastases. 11C-5-HTP-PET/CT detected lesions in the mediastinum in a patient with EAS due to a pulmonary carcinoid, further hepatic metastases in a patient with carcinoid syndrome (CS) from a NET of unknown primary, further hepatic metastases in the patient with MTC, and hepatic metastases in the patient with midgut carcinoid. The 11C-5-HTP-PET/CT findings contributed to radical cure of the patient with recurrent EAS, and pointed towards bilateral adrenalectomy in the patient with EAS without evident primary tumor. In addition, 11C-5- HTP-PET/CT directed towards combined surgical and medical treatment in the patient with CS and multiple rather than single hepatic metastases and in the patient with midgut carcinoid, and towards continuation of medical treatment in the patient with MTC. 11C-5-HTP-PET/CT is a useful imaging technique, providing additional information for the diagnosis, staging and decision-making regarding management of patients with NET.

Clinical value of 18-fluorine-fluorodihydroxyphenylalanine positron emission tomography/computed tomography in the follow-up of medullary thyroid carcinoma

BACKGROUND

18-Fluorine-fluorodihydroxyphenylalanine positron emission tomography ((18)F-DOPA PET) is a sensitive method for detecting medullary thyroid carcinoma (MTC). The advent of PET/computed tomography (CT) has enabled more sensitive and specific lesion identification using various tracers in many other tumors. The aim of this study was therefore to test the hypothesis that combined (18)F-DOPA PET/CT more accurately detects MTC lesions than each modality does alone.

METHODS

Twenty-eight consecutive (18)F-DOPA PET, CT, and (18)F-DOPA PET/CT scans of patients followed up for sporadic MTC or multiple endocrine neoplasia 2 syndrome-associated MTC were reviewed retrospectively in randomized sequence by two blinded readers, one a nuclear medicine physician and the other a radiologist, with extensive experience interpreting such images.

RESULTS

Of 18 lesions detected concurrently by the three modalities, PET identified all as positive for MTC, but was unable to definitively localize 4 (22%) lesions. CT could definitively localize all 18 lesions, but could not definitively diagnose or exclude MTC in 6 (33%) lesions. Further, CT falsely identified as MTC-negative one lesion that was judged to be MTC-positive by both PET and PET/CT. Only PET/CT scans accurately characterized and localized all 18 lesions. On a per patient basis, the sensitivity of (18)F-DOPA PET/CT for MTC was 74% and the specificity, 100%. In the present series, no truly MTC-positive (18)F-DOPA PET/CT case was found in patients with basal human calcitonin (hCt) levels under 60 pg/mL, and conversely, no truly MTC-negative PET/CT case was found in patients with basal hCt over 120 pg/mL. Between hCt concentrations of 60 and 150 pg/mL true-negative, false-negative, and true-positive scans all were obtained. (18)F-DOPA PET/CT had 100% sensitivity and specificity when hCt at the time of scanning was over 150 pg/mL, the threshold at which the 2009 American Thyroid Association guidelines recommend performing additional imaging including (18)F-DOPA PET/CT.

CONCLUSIONS

(18)F-DOPA PET/CT allows for a more accurate diagnosis and localization of MTC lesions than do (18)F-DOPA PET or CT alone. Supporting the recent American Thyroid Association recommendations on additional imaging in MTC, (18)F-DOPA PET/CT appears to have 100% sensitivity in patients with hCt over 150 pg/mL.

Radio-guided surgery in neuroendocrine tumors

The majority of Neuroendocrine tumors (NET) express somatostatin (SS) receptors, and thus can be successfully targeted with radiolabeled SS analogs in vivo. Somatostatin receptor scintigraphy (SRS) with (111)In-DTPA Octreotide is the main imaging technique for evaluation of NETs. Radio-guided surgery for NETs, therefore, primarily utilizes (111)In-DTPA Octreotide tumor localization mechanism and kinetics. Somatostatin analog radiopharmacy continues to evolve to include the development of more selective and higher affinity analogs and PET tracers. These changes are expected to open new venues for radio-guided surgery technology.

Imaging of neuroendocrine tumors

Neuroendocrine tumors (NETs) are rare neoplasms, which are characterized by the presence of neuroamine uptake mechanisms and/or peptide receptors at the cell membrane and these features constitute the basis of the clinical use of specific radiolabeled ligands, both for imaging and therapy. Radiolabeled metaiodobenzylguanidine (MIBG) was the first radiopharmaceutical used to specifically depict and localize catecholamine-secreting tumors (pheochromocytomas, paragangliomas, and neuroblastomas) and is still regarded as a first-choice imaging technique for diagnosis and follow-up; in patients with malignant disease, MIBG scintigraphy is an essential step to select patients for (131)I-MIBG therapy. Scintigraphy with (111)In- or (99m)Tc-labeled somatostatin analogs has become the main imaging technique for NETs, particularly those expressing a high density of somatostatin receptors, such as gastroenteropancreatic tumors; this procedure is used routinely for localizing the primary tumor, evaluating disease extension, monitoring the effect of treatment and for selecting patients for radioreceptor therapy. Since the recent development of hybrid machines, it has been possible to obtain images that simultaneously hold both anatomic (computed tomography [CT]) and functional (single-photon emission computed tomography [SPECT] or positron emission tomography [PET]) information, with great impact on diagnostic accuracy. Significant improvements have been made during the past few years with the development of highly specific radiopharmaceuticals for PET studies that reflect the different metabolic pathways of NETs, such as glucose metabolism ((18)F-fluorodeoxyglucose), the uptake of hormone precursors ((11)C-5-hydroxytryptophan, (11)C- or (18)F-dihydroxyphenylalanine, (18)F-fluorodopamine), the expression of receptors ((68)Ga-labeled somatostatin analogs), as well as the synthesis, storage, and release of hormones ((11)C-hydroxyephedrine and others). Among these radiopharmaceuticals, (68)Ga-labeled somatostatin analogs are increasingly used in specialized centers in Europe for PET and PET/CT imaging and show very promising results with high diagnostic sensitivity. New somatostatin analogs with different receptor affinity as well as other peptides are currently under investigation and will further improve our diagnostic and therapeutic capabilities in the future.

A new application of pre-normalized principal component analysis for improvement of image quality and clinical diagnosis in human brain PET studies—Clinical brain studies using [11C]-GR205171, [11C]-l-deuterium-deprenyl, [11C]-5-Hydroxy-l-Tryptophan, [11C]-l-DOPA and Pittsburgh Compound-B

Principal component analysis (PCA) is one of the most applied multivariate image analysis tool on dynamic Positron Emission Tomography (PET). Independent of used reconstruction methodologies, PET images contain correlation in-between pixels, correlations in-between frame and errors caused by the reconstruction algorithm including different corrections, which can affect the performance of the PCA. In this study, we have investigated a new approach of application of PCA on pre-normalized, dynamic human PET images. A range of different tracers have been used for this purpose to explore the performance of the new method as a way to improve detection and visualization of significant changes in tracer kinetics and to enhance the discrimination between pathological and healthy regions in the brain. We compare the new results with the results obtained using other methods. Images generated using the new approach contain more detailed anatomical information with higher quality, precision and visualization, compared with images generated using other methods.

Functional imaging of neuroendocrine tumours with PET

Several pathophysiological attributes of neuroendocrine tumours (NET) can be addressed by specific radiolabelled probes. This paper provides an overview on the different radiopharmaceuticals that have been developed for Positron Emission Tomography (PET) of neuroendocrine tumours. A review of the literature on 18F-fluordeoxyglucose (FDG), biogenic amine precursors, somatostatin analogues and hormone syntheses markers is presented. Due to the highly specific tracers that lack any clear anatomical landmarking the advantages of integrated PET/CT are obvious. Amine precursors should be employed in most gastroenteropancreatic NET, FDG should be preserved for more aggressive, less differentiated NETs. Somatostatin analogues are the most promising tracers, since they can improve dosimetry in cases in which radiopeptide therapies are planned. In conclusion, the individual diagnostic approach using PET or the integrated PET/CT should be tailored depending on the histological classification and the differentiation of the tumour.

Carbidopa pretreatment improves image interpretation and visualisation of carcinoid tumours with 11C-5-hydroxytryptophan positron emission tomography

PURPOSE

Positron emission tomography (PET) with 11C-5-hydroxytryptophan (5-HTP) as tracer is a promising imaging instrument in the management of patients with neuroendocrine tumours (NETs). However, high radioactivity concentrations in the urinary collecting system sometimes produce image reconstruction artefacts that can make detection of small NETs difficult. As a means to decrease urinary excretion of radioactivity and thereby improve image quality, we examined the effect of pretreatment with carbidopa (CD), a peripheral inhibitor of aromatic amino acid decarboxylase (AADC), which converts 5-HTP to serotonin (5-hydroxytryptamine, 5-HT).

METHODS

Six patients with midgut carcinoid metastases were examined with 11C-5-HTP PET before and 1 h after oral administration of 100 or 200 mg of CD.

RESULTS

There was a fourfold significant reduction of tracer uptake in the urinary collecting system after CD administration (p=0.0277, n=6), with a mean standard uptake value (SUV) of 155+/-195 before CD and 39+/-14 after CD. In tumour lesions there was a significant increase in SUV after CD administration (p<0. 0001, n=18), with a mean SUV of 11+/-3 before CD and 14+/-3 after CD. There was no difference between the doses (100 and 200 mg) of CD in this respect. In all patients, image interpretation and tumour detection were markedly improved after CD administration.

CONCLUSION

We conclude that CD premedication improves 11C-5-HTP PET image quality and facilitates detection of NET lesions. Because of the similarity of metabolic pathways, this method could probably be applied to improve PET imaging using other tracers like 18F-DOPA and 11C-DOPA.

Whole-body (11)C-5-hydroxytryptophan positron emission tomography as a universal imaging technique for neuroendocrine tumors: comparison with somatostatin receptor scintigraphy and computed tomography

Neuroendocrine tumors (NETs) can be small and situated almost anywhere throughout the body. Our objective was to investigate whether whole-body (WB) positron emission tomography (PET) with (11)C-5-hydroxytryptophan (5-HTP) can be used as a universal imaging technique for NETs and to compare this technique with established imaging methods. Forty-two consecutive patients with evidence of NET and a detected lesion on any conventional imaging (six bronchial, two foregut, 16 midgut, and two thymic carcinoids; one ectopic Cushing's syndrome; four gastrinomas; one insulinoma; six nonfunctioning endocrine pancreatic tumors; one gastric carcinoid, one paraganglioma; and two endocrine-differentiated pancreatic carcinomas) were studied. The WB-(11)C-5-HTP-PET examinations were compared with WB-computed tomography (CT) and somatostatin receptor scintigraphy (SRS). Tumor lesions were imaged with PET in 95% of the patients. In 58% of the patients, PET could detect more lesions than SRS and CT and equal numbers in 34%, whereas in three cases, SRS or CT showed more lesions. In 84% (16 of 19 patients), PET could visualize the primary tumor compared with 47 and 42% for SRS and CT, respectively. The surgically removed PET-positive primary tumor sizes were 6-30 mm. To conclude, this study indicates that WB-(11)C-5-HTP-PET can be used as a universal imaging method for detection of NETs. This study also shows that WB-(11)C-HTP-PET is sensitive in imaging small NET lesions, such as primary tumors, and can in a majority of cases image significantly more tumor lesions than SRS and CT.

Nuclear medicine in the detection, staging and treatment of gastrointestinal carcinoid tumours

Carcinoid tumours belong to the family of neuroendocrine tumours with a capacity to take up and concentrate amines and precursors as well as peptides, and can thereby be detected by nuclear medicine techniques. These rare tumours are difficult to diagnose at earlier stages because of small size and multiplicity. Computed tomography (CT) and magnetic resonance imaging (MRI) are mostly of benefit for detection of larger primary tumours (1-3 cm) and liver and lymph-node metastases. A majority of carcinoid tumours express somatostatin receptors, particularly receptor type 2, and thus somatostatin receptor scintigraphy (SRS) can be used for detection and staging of carcinoid tumours. The detection rate of carcinoid tumours has been reported to be somewhere between 80 and 100% in different studies. The scintigraphy gives a good staging of the disease and detection of unexpected tumour sites, which were not determined by conventional imaging. This method also indicates content of somatostatin receptors, which might indicate efficacy of treatment with octreotide or other somatostatin analogues. Another new non-invasive technique for detection of carcinoid tumours is positron emission tomography (PET). The biological substance for study can be labelled for radioactive imaging with radionuclears, such as (11)C, (15)O and (18)F, with emission of positrons. More than 95% of patients studied displayed high tracer uptake from PET with (11)C-5HTP (5-hydroxytryptophan), which is significantly higher compared to both computer tomography and somatostatin receptor scintigraphy. MIBG has been used for decades to visualize carcinoid tumours, because MIBG is concentrated in the endocrine cells. It was initially developed to detect phaeochromocytomas of the adrenal with reported high sensitivity (87%) and specificity as high as 99%. The method can be used when other methods fail to localize carcinoid tumours and particularly when treatment with (131)I-MIBG is being considered. Tumour-targeted treatment for malignant carcinoid tumour is still investigational, but has become of significant interest with the use of radiolabelled somatostatin analogues. Since a majority of carcinoid tumours present somatostatin receptors and can therefore be visualized in vivo by using radiolabelled somatostatin analogues, it seems logical to try to target these tumours with radioactive substances, not only for visualization but also for treatment. (111)Indium-DTPA-octreotide has been used as the first tumour-targeted treatment, with rather low response rates (in the order of 10-20%) and no significant tumour shrinkage. The second radioactive analogue which has been applied in the clinic is (90)yttrium-DOTA-Tyr3-octreotide, which has given partial and complete remissions in 20-30% of patients. The most significant side-effects have been kidney dysfunction, thrombocytopenia and liver toxicity. The most recent compound is (177)lutetium-DOTA-Tyr3-octreotate, which has been applied by the Rotterdam group and has been reported to give partial remission in about 40% of the patients. In the near future, combined treatment with both (90)yttrium and (177)lutetium coupled to a somatostatin analogue might come into clinical trials. (177)Lutetium may be more effective for smaller tumours whereas (90)yttrium may be more effective for larger tumours.

Functional imaging of endocrine tumors: role of positron emission tomography

This article provides an update on functional imaging approaches for diagnostic localization of endocrine tumors, with emphasis on positron emission tomography (PET). [18F]Fluorodeoxyglucose PET scanning is now a widely accepted imaging approach in clinical oncology. Benefits include improved patient outcome facilitated by staging and monitoring of disease and better treatment planning. [18F]Fluorodeoxyglucose PET is also useful in some endocrine tumors, particularly in recurrent or metastatic thyroid cancer where the degree of accumulation of the radionuclide has prognostic value. However, this imaging approach does not take full advantage of the unique characteristics of endocrine tumors. Endocrine tumor cells take up hormone precursors, express receptors and transporters, and synthesize, store, and release hormones. These characteristics offer highly specific targets for PET. Radiopharmaceuticals developed for such approaches include 6-[18F]fluorodopamine, and [11C]hydroxyephedrine for localization of pheochromocytomas, [11C]5-hydroxytryptophan and [11C]L-dihydroxyphenylalanine for carcinoid tumors, and [11C]metomidate for adrenocortical tumors. These functional imaging approaches are not meant to supplant conventional imaging modalities but should be used conjointly to better identify specific characteristics of endocrine tumors. This represents a relatively new and evolving approach to imaging that promises to answer specific questions about the behavior and growth of endocrine tumors, their malignant potential, and responsiveness to different treatment modalities.

PET in the diagnosis of neuroendocrine tumors

For general oncological imaging, positron emission tomography (PET) using [18F]fluoro-deoxy-glucose (FDG) has evolved as a powerful functional imaging modality. Unfortunately, FDG-PET has not been as advantageous for imaging gastropancreatic neuroendocrine tumors, and only tumors with high proliferative activity and low differentiation have shown an increased FDG uptake. Therefore, the 11C-labeled amine precursors L-dihydroxyphenylalanine and 5-hydroxy-L-tryptophan (5-HTP) were developed for PET imaging of these tumors. Because of the higher tumor uptake of the latter tracer in a study of patients with endocrine pancreatic tumors, 11C-5-HTP was chosen for further evaluation. In comparative studies of patients with carcinoids and endocrine pancreatic tumors, 5-HTP-PET proved better than CT and somatostatin receptor scintigraphy for tumor visualization, and many small, previously overlooked lesions were diagnosed by 11C-5-HTP-PET. The strong correlation found during medical treatment between the changes in the transport rate constant at repeated PET and those of U-HIAA indicates the possible use of 11C-5-HTP-PET also for therapy monitoring. By premedication of patients with Carbidopa orally before PET examination, in order to block the aromatic amino acid decarboxylase enzyme, the decarboxylation rate of 11C-5-HTP was decreased, leading to a higher tumor uptake and a considerably lower urinary radioactivity concentration.

The diagnosis and medical management of advanced neuroendocrine tumors

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 (gastroenteropancreatic) 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, scintigraphy with somatostatin analogs and metaiodobenzylguanidine. 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 somatostatin analogs or alpha-interferon, with chemotherapy being reserved for poorly differentiated and progressive tumors. Therapy with radionuclides may be used for tumors exhibiting uptake to a diagnostic scan, either after surgery to eradicate 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.

Synthesis of no-carrier-added C-11 labeled D- and L-enantiomers of phenylalanine and tyrosine for comparative PET Studies

The natural amino acids tyrosine (Tyr) and phenylalanine (Phe) were labeled with carbon-11 via a modified Bucherer-Strecker synthesis. A rapid reaction of the sodium bisulfite adduct of the aldehyde precursor with ammonia provided the precursor for radiosynthesis. [(11)C]Cyanide displacement followed by base hydrolysis afforded the corresponding (11)C-labeled amino acids. The purification and chiral separation were simply achieved by using a combination of solid-phase extraction and chiral HPLC to afford individual enantiomers of each amino acid. The decay corrected radiochemical yields for each of the enantiomers were 12-16% with respect to the [(11)C]cyanide after 40-45 min of radiosynthesis. Radiochemical purity of the products was >97% (typically >99%), enantiomeric excess was >98% with the specific radioactivity 2-3 Ci/micromol at the end of bombardment. Because of its simplicity and wide applicability, the described procedure could be the method of choice to produce [(11)C]amino acids for PET studies.

The role of PET in localization of neuroendocrine and adrenocortical tumors

Positron emission tomography (PET) supplies a range of labeled compounds to be used for the characterization of tumor biochemistry. Some of these have proved to be of value for clinical diagnosis, treatment follow up, and clinical research. The first routinely used PET tracer in oncology, (18)F-labeled deoxyglucose (FDG), was successfully used for diagnosis of cancer, reflecting increased expression of glucose transporter in cancerous tissue. This tracer, however, usually does not show sufficient uptake in well-differentiated tumors such as neuroendocrine tumors. We developed a tracer more specific to neuroendocrine tumors-the serotonin precursor 5-hydroxytryptophan (5-HTP) labeled with (11)C-and demonstrated increased uptake and irreversible trapping of this tracer in carcinoid tumors. The uptake was so selective and the resolution was so high that we could detect more liver and lymph node metastases with PET than with CT or octreotide scintigraphy. To further improve the method, especially to reduce the high renal excretion of the tracer producing streaky artifacts in the area of interest, we introduced premedication by the decarboxylase inhibitor carbidopa, leading to a six-fold decreased renal excretion while the tumor uptake increased three-fold, hence improving the visualization of the tumors. (11)C-labeled l-DOPA was evaluated as an alternative tracer, especially for endocrine pancreatic tumors, which usually do not demonstrate enhanced urinary serotonin metabolites. However, only half of the EPTs, mainly functioning tumors, could be detected with l-DOPA. Instead 5-HTP seems to be a universal tracer for EPT and foregut carcinoids. With new, more sensitive PET cameras, larger field of view and procedures for whole-body coverage, the PET examination with 5-HTP is now routinely performed as reduced whole-body PET examinations with coverage of the thorax and abdomen. With this method we have been able to visualize small neuroendocrine lesions in the pancreas and thorax (e.g., ACTH-producing bronchial carcinoids) not detectable by any other method, including octreotide scintigraphy, MRI, and CT. Another tracer, the 11beta-hydroxylase inhibitor, metomidate labeled with (11)C, was developed to simplify diagnosis and follow-up of patients with incidentalomas. A large series of patients with incidentally found adrenal masses have been investigated and so far all lesions of adrenocortical origin have been easily identified because of exceedingly high uptake of (11)C-metomidate, whereas noncortical lesions showed very low uptake. In addition, adrenocortical cancer shows high uptake, suggesting that this PET tracer can be used for staging purposes.

Recent advances in carcinoid pathogenesis, diagnosis and management

Carcinoid tumors usually present as diagnostic dilemmas due to obscure or nonspecific symptomatology. Advances in molecular biology are allowing the investigation of molecular markers of aggressiveness, better serum tumor markers, as well as the molecular pathogenesis of carcinoid heart disease. Somatostatin receptor scintigraphy (SRS) and whole body positron emission tomography (PET) are providing much improved sensitivity in localization of both primary and metastatic tumors. Long acting depot somatostatin analogues are combining effectiveness and ease of use for medical management of carcinoid syndrome. An additional benefit may be tumor growth suppression.