Demonstration of high monoaminoxidase-A levels in neuroendocrine gastroenteropancreatic tumors in vitro and in vivo-tumor visualization using positron emission tomography with 11C-harmine

The highs and lows of monoamine oxidase as molecular target in cancer: an updated review

The global burden of cancer as a major cause of death and invalidity has been constantly increasing in the past decades. Monoamine oxidases (MAO) with two isoforms, MAO-A and MAO-B, are mammalian mitochondrial enzymes responsible for the oxidative deamination of neurotransmitters and amines in the central nervous system and peripheral tissues with the constant generation of hydrogen peroxide as the main deleterious ancillary product. However, given the complexity of cancer biology, MAO involvement in tumorigenesis is multifaceted with different tumors displaying either an increased or decreased MAO profile. MAO inhibitors are currently approved for the treatment of neurodegenerative diseases (mainly, Parkinson's disease) and as secondary/adjunctive therapeutic options for the treatment of major depression. Herein, we review the literature characterizing MAO's involvement and the putative role of MAO inhibitors in several malignancies, and also provide perspectives regarding the potential biomarker role that MAO could play in the future in oncology.

Molecular Imaging of Monoamine Oxidase A Expression in Highly Aggressive Prostate Cancer: Synthesis and Preclinical Evaluation of Positron Emission Tomography Tracers

The role of monoamine oxidase A (MAO-A) in the aggressiveness of prostate cancer (PCa) has been established in recent years. The molecular imaging of MAO-A expression could offer a noninvasive tool for the visualization and quantification of highly aggressive PCa. This study reports the synthesis and preclinical evaluation of 11C- and 18F-labeled MAO-A inhibitors as positron emission tomography (PET) tracers for proof-of-concept studies in animal models of PCa. Good manufacturing practice production and quality control of these radiotracers using an automated platform was achieved. PET imaging was performed in an LNCaP tumor model with high MAO-A expression. The tumor-to-muscle (T/M) uptake ratio of [11C]harmine (4.5 ± 0.5) was significantly higher than that for 2-[18F]fluoroethyl-harmol (2.3 ± 0.7) and [11C]clorgyline (2.0 ± 0.1). A comparable ex vivo biodistribution pattern in all radiotracers was observed. Furthermore, the tumor uptake of [11C]harmine showed a dramatic reduction (T/M = 1) in a PC3 tumor model with limited MAO-A expression, and radioactivity uptake in LNCaP tumors was blocked in the presence of nonradioactive harmine. Our findings suggest that [11C]harmine may serve as an attractive PET probe for the visualization of MAO-A expression in highly aggressive PCa. These radiotracers have the potential for clinical translation and may aid in the development of personalized therapeutic strategies for PCa patients.

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.

The effects of molar activity on [18F]FDOPA uptake in patients with neuroendocrine tumors

Background

6-[¹⁸F]fluoro-l-3,4-dihydroxyphenyl alanine ([¹⁸F]FDOPA) is a commonly used PET tracer for the detection and staging of neuroendocrine tumors. In neuroendocrine tumors, [¹⁸F]FDOPA is decarboxylated to [¹⁸F]dopamine via the enzyme amino acid decarboxylase (AADC), leading to increased uptake when there is increased AADC activity. Recently, in our hospital, a new GMP compliant multi-dose production of [¹⁸F]FDOPA has been developed, [¹⁸F]FDOPA-H, resulting in a higher activity yield, improved molar activity and a lower administered mass than the conventional method ([¹⁸F]FDOPA-L).

Aims

This study aimed to investigate whether the difference in molar activity affects the [¹⁸F]FDOPA uptake at physiological sites and in tumor lesions, in patients with NET. It was anticipated that the specific uptake of [¹⁸F]FDOPA-H would be equal to or higher than [¹⁸F]FDOPA-L.

Methods

We retrospectively analyzed 49 patients with pathologically confirmed NETs and stable disease who underwent PET scanning using both [¹⁸F]FDOPA-H and [¹⁸F]FDOPA-L within a time span of 5 years. A total of 98 [¹⁸F]FDOPA scans (49 [¹⁸F]FDOPA-L and 49 [¹⁸F]FDOPA-H with average molar activities of 8 and 107 GBq/mmol) were analyzed. The SUVmean was calculated for physiological organ uptake and SUVmax for tumor lesions in both groups for comparison, and separately in subjects with low tumor load (1–2 lesions) and higher tumor load (3–10 lesions).

Results

Comparable or slightly higher uptake was demonstrated in various physiological uptake sites in subjects scanned with [¹⁸F]FDOPA-H compared to [¹⁸F]FDOPA-L, with large overlap being present in the interquartile ranges. Tumor uptake was slightly higher in the [¹⁸F]FDOPA-H group with 3–10 lesion (SUVmax 6.83 vs. 5.19, p < 0.001). In the other groups, no significant differences were seen between H and L.

Conclusion

[18F]FDOPA-H provides a higher activity yield, offering the possibility to scan more patients with one single production. Minor differences were observed in SUV’s, with slight increases in uptake of [¹⁸F]FDOPA-H in comparison to [¹⁸F]FDOPA-L. This finding is not a concern for clinical practice, but could be of importance when quantifying follow-up scans while introducing new production methods with a higher molar activity of [¹⁸F]FDOPA.

Differential Morphological Diagnosis of Various Forms of Congenital Hyperinsulinism in Children

INTRODUCTION

Congenital hyperinsulinism (CHI) has diffuse (CHI-D), focal (CHI-F) and atypical (CHI-A) forms. Surgical management depends on preoperative [18F]-DOPA PET/CT and intraoperative morphological differential diagnosis of CHI forms. Objective: to improve differential diagnosis of CHI forms by comparative analysis [18F]-DOPA PET/CT data, as well as cytological, histological and immunohistochemical analysis (CHIA).

MATERIALS AND METHODS

The study included 35 CHI patients aged 3.2 ± 2.0 months; 10 patients who died from congenital heart disease at the age of 3.2 ± 2.9 months (control group). We used PET/CT, CHIA of pancreas with antibodies to ChrA, insulin, Isl1, Nkx2.2, SST, NeuroD1, SSTR2, SSTR5, DR1, DR2, DR5; fluorescence microscopy with NeuroD1/ChrA, Isl1/insulin, insulin/SSTR2, DR2/NeuroD1 cocktails.

RESULTS

Intraoperative examination of pancreatic smears showed the presence of large nuclei, on average, in: 14.5 ± 3.5 cells of CHI-F; 8.4 ± 1.1 of CHI-D; and 4.5 ± 0.7 of control group (from 10 fields of view, x400). The percentage of Isl1+ and NeuroD1+endocrinocytes significantly differed from that in the control for all forms of CHI. The percentage of NeuroD1+exocrinocytes was also significantly higher than in the control. The proportion of ChrA+ and DR2+endocrinocytes was higher in CHI-D than in CHI-F, while the proportion of insulin+cells was higher in CHI-A. The number of SST+cells was significantly higher in CHI-D and CHI-F than in CHI-A.

CONCLUSION

For intraoperative differential diagnosis of CHI forms, in addition to frozen sections, quantitative cytological analysis can be used. In quantitative immunohistochemistry, CHI forms differ in the expression of ChrA, insulin, SST and DR2. The development of a NeuroD1 inhibitor would be advisable for targeted therapy of CHI.

Toxicokinetics and Toxicodynamics of Ayahuasca Alkaloids N,N-Dimethyltryptamine (DMT), Harmine, Harmaline and Tetrahydroharmine: Clinical and Forensic Impact

Ayahuasca is a hallucinogenic botanical beverage originally used by indigenous Amazonian tribes in religious ceremonies and therapeutic practices. While ethnobotanical surveys still indicate its spiritual and medicinal uses, consumption of ayahuasca has been progressively related with a recreational purpose, particularly in Western societies. The ayahuasca aqueous concoction is typically prepared from the leaves of the N,N-dimethyltryptamine (DMT)-containing Psychotria viridis, and the stem and bark of Banisteriopsis caapi, the plant source of harmala alkaloids. Herein, the toxicokinetics and toxicodynamics of the psychoactive DMT and harmala alkaloids harmine, harmaline and tetrahydroharmine, are comprehensively covered, particularly emphasizing the psychological, physiological, and toxic effects deriving from their concomitant intake. Potential therapeutic utility, particularly in mental and psychiatric disorders, and forensic aspects of DMT and ayahuasca are also reviewed and discussed. Following administration of ayahuasca, DMT is rapidly absorbed and distributed. Harmala alkaloids act as potent inhibitors of monoamine oxidase A (MAO-A), preventing extensive first-pass degradation of DMT into 3-indole-acetic acid (3-IAA), and enabling sufficient amounts of DMT to reach the brain. DMT has affinity for a variety of serotonergic and non-serotonergic receptors, though its psychotropic effects are mainly related with the activation of serotonin receptors type 2A (5-HT_2A). Mildly to rarely severe psychedelic adverse effects are reported for ayahuasca or its alkaloids individually, but abuse does not lead to dependence or tolerance. For a long time, the evidence has pointed to potential psychotherapeutic benefits in the treatment of depression, anxiety, and substance abuse disorders; and although misuse of ayahuasca has been diverting attention away from such clinical potential, research onto its therapeutic effects has now strongly resurged.

Repurposing of plant alkaloids for cancer therapy: Pharmacology and toxicology

Drug repurposing (or repositioning) is an emerging concept to use old drugs for new treatment indications. Phytochemicals isolated from medicinal plants have been largely neglected in this context, although their pharmacological activities have been well investigated in the past, and they may have considerable potentials for repositioning. A grand number of plant alkaloids inhibit syngeneic or xenograft tumor growth in vivo. Molecular modes of action in cancer cells include induction of cell cycle arrest, intrinsic and extrinsic apoptosis, autophagy, inhibition of angiogenesis and glycolysis, stress and anti-inflammatory responses, regulation of immune functions, cellular differentiation, and inhibition of invasion and metastasis. Numerous underlying signaling processes are affected by plant alkaloids. Furthermore, plant alkaloids suppress carcinogenesis, indicating chemopreventive properties. Some plant alkaloids reveal toxicities such as hepato-, nephro- or genotoxicity, which disqualifies them for repositioning purposes. Others even protect from hepatotoxicity or cardiotoxicity of xenobiotics and established anticancer drugs. The present survey of the published literature clearly demonstrates that plant alkaloids have the potential for repositioning in cancer therapy. Exploitation of the chemical diversity of natural alkaloids may enrich the candidate pool of compounds for cancer chemotherapy and -prevention. Their further preclinical and clinical development should follow the same stringent rules as for any other synthetic drug as well. Prospective randomized, placebo-controlled clinical phase I and II trials should be initiated to unravel the full potential of plant alkaloids for drug repositioning.

Toxicological Aspects and Determination of the Main Components of Ayahuasca: A Critical Review

Ayahuasca is a psychoactive beverage prepared traditionally from a mixture of the leaves and stems of Psychotria viridis and Banisteriopsis caapi, respectively, being originally consumed by indigenous Amazonian tribes for ritual and medicinal purposes. Over the years, its use has spread to other populations as a means to personal growth and spiritual connection. Also, the recreational use of its isolated compounds has become prominent. The main compounds of this tea-like preparation are N,N-dimethyltryptamine (DMT), β-Carbolines, and harmala alkaloids, such as harmine, tetrahydroharmine, and harmaline. The latter are monoamine-oxidase inhibitors and are responsible for DMT psychoactive and hallucinogenic effects on the central nervous system. Although consumers defend its use, its metabolic effects and those on the central nervous system are not fully understood yet. The majority of studies regarding the effects of this beverage and of its individual compounds are based on in vivo experiments, clinical trials, and even surveys. This paper will not only address the toxicological aspects of the ayahuasca compounds but also perform a comprehensive and critical review on the analytical methods available for their determination in biological and non-biological specimens, with special focus on instrumental developments and sample preparation approaches.

Systematic screening of imaging biomarkers for the Islets of Langerhans, among clinically available positron emission tomography tracers

INTRODUCTION

Functional imaging could be utilized for visualizing pancreatic islets of Langerhans. Therefore, we present a stepwise algorithm for screening of clinically available positron emission tomography (PET) tracers for their use in imaging of the neuroendocrine pancreas in the context of diabetes.

METHODS

A stepwise procedure was developed for screening potential islet imaging agents. Suitable PET-tracer candidates were identified by their molecular mechanism of targeting. Clinical abdominal examinations were retrospectively analyzed for pancreatic uptake and retention. The target protein localization in the pancreas was assessed in silico by -omics approaches and the in vitro by binding assays to human pancreatic tissue.

RESULTS

Six putative candidates were identified and screened by using the stepwise procedure. Among the tested PET tracers, only [(11)C]5-Hydroxy-tryptophan passed all steps. The remaining identified candidates were falsified as candidates and discarded following in silico and in vitro screening.

CONCLUSIONS

Of the six clinically available PET tracers identified, [(11)C]5-HTP was found to be a promising candidate for beta cell imaging, based on intensity of in vivo pancreatic uptake in humans, and islet specificity as assessed on human pancreatic cell preparations. The flow scheme described herein constitutes a methodology for evaluating putative islet imaging biomarkers among clinically available PET tracers.

In vivo biodistribution of no-carrier-added 6-18F-fluoro-3,4-dihydroxy-L-phenylalanine (18F-DOPA), produced by a new nucleophilic substitution approach, compared with carrier-added 18F-DOPA, prepared by conventional electrophilic substitution

A novel synthetic approach to 6-(18)F-fluoro-3,4-dihydroxy-L-phenylalanine ((18)F-DOPA), involving the nucleophilic substitution of a diaryliodonium salt precursor with non-carrier-added (18)F-fluoride, yielded a product with a specific activity that was 3 orders of magnitude higher than the product of the conventional synthesis method, involving an electrophilic substitution of a trialkylstannane precursor with (18)F2. We performed a direct comparison of high- and low-specific-activity (18)F-DOPA in a neuroendocrine tumor model to determine whether this difference in specific activity has implications for the biologic behavior and imaging properties of (18)F-DOPA.

METHODS

(18)F-DOPA was produced via the novel synthesis method, yielding (18)F-DOPA-H with a high specific activity (35,050 ± 4,000 GBq/mmol). This product was compared in several experiments with conventional (18)F-DOPA-L with a low specific activity (11 ± 2 GBq/mmol). In vitro accumulation experiments with the human pancreatic neuroendocrine tumor cell line BON-1 were performed at both 0 °C and 37 °C and at 37 °C in the presence of pharmacologic inhibitors of proteins involved in the uptake mechanism of (18)F-DOPA. Small-animal PET experiments were performed in athymic nude mice bearing a BON-1 tumor xenograft.

RESULTS

At 37 °C, the uptake of both (18)F-DOPA-H and (18)F-DOPA-L did not differ significantly during a 60-min accumulation experiment in BON-1 cells. At 0 °C, the uptake of (18)F-DOPA-L was significantly decreased, whereas the lower temperature did not alter the uptake of (18)F-DOPA-H. The pharmacologic inhibitors carbidopa and tetrabenazine also revealed differential effects between the 2 types of (18)F-DOPA in the 60-min accumulation experiment. The small-animal PET experiments did not show any significant differences in distribution and metabolism of (18)F-DOPA-H and (18)F-DOPA-L in carbidopa-pretreated mice.

CONCLUSION

The advantages of the novel synthesis of (18)F-DOPA, which relies on nucleophilic fluorination of a diaryliodonium salt precursor, lie in the simplicity of the synthesis method, compared with the conventional, electrophilic approach and in the reduced mass of administered, pharmacologically active (19)F-DOPA. (18)F-DOPA-H demonstrated comparable imaging properties in an in vivo model for neuroendocrine tumors, despite the fact that the injected mass of material was 3 orders of magnitude less than (18)F-DOPA-L.

Chemotherapy-induced monoamine oxidase expression in prostate carcinoma functions as a cytoprotective resistance enzyme and associates with clinical outcomes

To identify molecular alterations in prostate cancers associating with relapse following neoadjuvant chemotherapy and radical prostatectomy patients with high-risk localized prostate cancer were enrolled into a phase I-II clinical trial of neoadjuvant chemotherapy with docetaxel and mitoxantrone followed by prostatectomy. Pre-treatment prostate tissue was acquired by needle biopsy and post-treatment tissue was acquired by prostatectomy. Prostate cancer gene expression measurements were determined in 31 patients who completed 4 cycles of neoadjuvant chemotherapy. We identified 141 genes with significant transcript level alterations following chemotherapy that associated with subsequent biochemical relapse. This group included the transcript encoding monoamine oxidase A (MAOA). In vitro, cytotoxic chemotherapy induced the expression of MAOA and elevated MAOA levels enhanced cell survival following docetaxel exposure. MAOA activity increased the levels of reactive oxygen species and increased the expression and nuclear translocation of HIF1α. The suppression of MAOA activity using the irreversible inhibitor clorgyline augmented the apoptotic responses induced by docetaxel. In summary, we determined that the expression of MAOA is induced by exposure to cytotoxic chemotherapy, increases HIF1α, and contributes to docetaxel resistance. As MAOA inhibitors have been approved for human use, regimens combining MAOA inhibitors with docetaxel may improve clinical outcomes.

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.

Role of 18F-DOPA PET/CT imaging in congenital hyperinsulinism

Congenital hyperinsulinism is a leading cause of severe hypoglycaemia in the newborn period. There are two (diffuse and focal) histological subtypes of congenital hyperinsulinism. The diffuse form affects the entire pancreas and if medically unresponsive will require a near total (95%-98%) pancreatectomy. The focal form affects only a small region of the pancreas (with the rest of the pancreas being normal in endocrine and exocrine function) and only requires a limited pancreatectomy. This limited section of the focal lesion has the potential for curing the patient. Thus the pre-operative differentiation of these two subgroups is extremely important. Recent advances in Fluorine-18-L-dihydroxyphenylalanine positron emission tomography ((18)F-DOPA PET/CT) have radically changed the clinical approach to patient with congenital hyperinsulinism. In most patients this novel imaging technique is able to offer precise pre-operative localisation of the focal lesion, thus guiding the extent of surgical resection.

Identification of novel neuroendocrine-specific tumour genes

Neuroendocrine tumours (NETs) comprise a heterogenous group of malignancies with an often unpredictable course, and with limited treatment options. Thus, new diagnostic, prognostic, and therapeutic markers are needed. To shed new lights into the biology of NETs, we have by cDNA transcript profiling, sought to identify genes that are either up- or downregulated in NE as compared with non-NE tumour cells. A panel of six NET and four non-NET cell lines were examined, and out of 12 743 genes examined, we studied in detail the 200 most significantly differentially expressed genes in the comparison. In addition to potential new diagnostic markers (NEFM, CLDN4, PEROX2), the results point to genes that may be involved in the tumorigenesis (BEX1, TMEPAI, FOSL1, RAB32), and in the processes of invasion, progression and metastasis (MME, STAT3, DCBLD2) of NETs. Verification by real time qRT–PCR showed a high degree of consistency to the microarray results. Furthermore, the protein expression of some of the genes were examined. The results of our study has opened a window to new areas of research, by uncovering new candidate genes and proteins to be further investigated in the search for new prognostic, predictive, and therapeutic markers in NETs.

[18F]Fluoro-L-DOPA PET/CT in Congenital Hyperinsulinism

Congenital hyperinsulinism can be divided into diffuse or focal form. The treatment and outcome depend on distinguishing between the 2 forms. Pancreatic venous sampling was the only method available to localize the insulin secretion. [F]Fluoro-levodopa, 3,4-dihydroxy-L-phenylalanine positron emission tomography/computed tomography is a noninvasive imaging investigation and increasingly used to determine the type of hyperinsulinism preoperatively. We present a case of diffuse form of congenital hyperinsulinism demonstrated by the [F]levodopa, 3,4-dihydroxy-L-phenylalanine positron emission tomography/computed tomography preoperatively and review the literature.

Biodistribution and radiation dosimetry of 11C-harmine in baboons

OBJECTIVE

Monoamine oxidase A is a mitochondrial enzyme which is responsible for the metabolism of catecholamines such as dopamine, norepinephrine, as well as serotonin. This study describes the biodistribution and dosimetry of 11C-harmine, a tracer designed to specifically bind to monoamine oxidase A for positron emission tomography imaging.

METHODS

Three baboon studies were acquired using a Seimens ECAT camera. Dynamic whole-body emission scans were collected in two-dimensional mode over a 2 h period after 223-255 MBq of 11C-harmine was injected. Regions of interest were drawn on transmission corrected images to encompass the entire activity in visible organs at each time point. Time-activity data were used to obtain residence times and absorbed radiation dose to various organs and to the entire body.

RESULTS

Tracer uptake was greatest in the lungs, followed by kidney, small intestine, liver and brain. The largest absorbed dose based on averaged residence times was found in the lungs (reference adult/female 3.99x10(-2)/5.03x10(-2) mSv x MBq(-1)).

CONCLUSION

The lungs are the critical organs for administration of 11C-harmine. For example, in the United States, the absorbed dose to the lungs would limit a single 11C-harmine administration for a research subject with the approval of a Radioactive Drug Research Committee to 1258/999 MBq (34/27 mCi) in the adult male/female. Quantitative measurement of monoamine oxidase A activity in the brain and elsewhere may aid in understanding the pathophysiology of several disease processes including neuroendocrine neoplasms and depression.

The added value of [18F]fluoro-L-DOPA PET in the diagnosis of hyperinsulinism of infancy: a retrospective study involving 49 children

PURPOSE

Neuroendocrine diseases are a heterogeneous group of entities with the ability to take up amine precursors, such as L-DOPA, and convert them into biogenic amines, such as dopamine. Congenital hyperinsulinism of infancy (HI) is a neuroendocrine disease secondary to either focal adenomatous hyperplasia or a diffuse abnormal pancreatic insulin secretion. While focal hyperinsulinism may be reversed by selective surgical resection, diffuse forms require near-total pancreatectomy when resistant to medical treatment. Here, we report the diagnostic value of PET with [(18)F]fluoro-L-DOPA in distinguishing focal from diffuse HI.

METHODS

Forty-nine children were studied with [(18)F]fluoro-L-DOPA. A thoraco-abdominal scan was acquired 45-65 min after the injection of 4.2 +/- 1.0 MBq/kg of [(18)F]fluoro-L-DOPA. Additionally, 12 of the 49 children were submitted to pancreatic venous catheterisation for blood samples (PVS) and 31 were also investigated using MRI.

RESULTS

We identified abnormal focal pancreatic uptake of [(18)F]fluoro-L-DOPA in 15 children, whereas diffuse radiotracer uptake was observed in the pancreatic area in the other 34 patients. In children studied with both PET and PVS, the results were concordant in 11/12 cases. All patients with focal radiotracer uptake and nine of the patients with diffuse pancreatic radiotracer accumulation, unresponsive to medical treatment, were submitted to surgery. In 21 of these 24 patients, the histopathological results confirmed the PET findings. In focal forms, selective surgery was followed by clinical remission without carbohydrate intolerance.

CONCLUSION

These data demonstrate that PET with [(18)F]fluoro-L-DOPA is an accurate non-invasive technique allowing differential diagnosis between focal and diffuse forms of HI.

Translational neuroimaging: positron emission tomography studies of monoamine oxidase

Positron emission tomography (PET) using radiotracers with high molecular specificity is an important scientific tool in studies of monoamine oxidase (MAO), an important enzyme in the regulation of the neurotransmitters dopamine, norepinephrine, and serotonin as well as the dietary amine, tyramine. MAO occurs in two different subtypes, MAO A and MAO B, which have different substrate and inhibitor specificity and which are different gene products. The highly variable subtype distribution with different species makes human studies of special value. MAO A and B can be imaged in the human brain and certain peripheral organs using PET and carbon-11 (half-life 20.4 minutes) labeled mechanism-based irreversible inhibitors, clorgyline and L -deprenyl, respectively. In this article we introduce MAO and describe the development of these radiotracers and their translation from preclinical studies to the investigation of variables affecting MAO in the human brain and peripheral organs.

Enteroendocrine tumors other than carcinoid: a review of clinically significant advances

Only relatively recently has there been an increased clinical recognition and characterization of the heterogeneous group of rare gastroenteropancreatic neuroendocrine neoplasms. Most have endocrine function and exhibit varying degrees of malignancy. This review summarizes the derivation of these tumors and the advances in their diagnosis and treatment over the past decade and a half. They are varied in their biological behavior and clinical courses and, depending on their cell type, can produce different hormones causing distinct clinical endocrine syndromes (insulinoma [hypoglycemia], gastrinoma [Zollinger-Ellison syndrome (ZES)], vasoactive intestinal peptideoma [VIPoma], watery diarrhea, hypokalemia-achlorhydria [WDHA], glucagonoma [glucagonoma syndrome], and so forth). In addition to surgery for cure or palliation (by excision and a variety of other cytoreductive techniques), they each are treated with anti-hormonal agents or drugs targeted to each tumor's specific product or its effects. The majority have benefited from the gut hormone-inhibiting action of somatostatin analogs. Because of their usual slow rate of growth it is recommended that, even when they are advanced and incurable, unlike in patients with common and more malignant cancers, patients with neuroendocrine tumors often can be palliated and appear to survive longer when managed with an active approach using sequential multimodality treatment. Advances in these various therapies are reviewed and the beneficial emergence of global self-help patient support groups is noted.

Current concepts in functioning endocrine tumors of the pancreas

Duodenal and pancreatic endocrine tumors are uncommon and their surgical treatment is often difficult. The management of these tumors has changed with recent advancements in tumor localization, intraoperative hormone measurements, standardized surgical techniques, and a better understanding of the genetic basis of multiple endocrine neoplasia syndrome. We present our experience with 191 endocrine tumors and elaborate the contemporary management of functioning duodenopancreatic endocrine tumors.

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.