Dopamine receptors in pituitary adenomas: PET visualization with 11C-N-methylspiperone

DRD2 expression based on 18F-fallypride PET/MR predicts the dopamine agonist resistance of prolactinomas: a pilot study

PURPOSE

The dopamine agonists (DA) have been used widely to treat prolactinomas. However, it is difficult to predict whether the patient will be responsive to DA treatment.

METHODS

We aimed to investigate whether the in vivo expression of DRD2 based on ¹⁸F-fallypride PET/MR could predict the therapeutic effect of DA on prolactinomas. Seven patients with prolactinomas completed ¹⁸F-fallypride PET/MR. Among them, three patients underwent surgery and further tumor immunohistochemistry. Imaging findings and immunohistochemical staining were compared with treatment outcomes.

RESULTS

¹⁸F-fallypride PET/MR was visually positive in 7 of 7 patients, and DRD2 target specificity could be confirmed by immunohistochemical staining. A significantly lower tracer standard uptake value (SUV) could be detected in the resistant patients (n = 3) than in the sensitive patients (n = 4; SUV_mean, 4.67 ± 1.32 vs. 13.57 ± 2.42, p < 0.05). DRD2 expression determined by ¹⁸F-fallypride PET/MR corresponded with the DA treatment response.

CONCLUSION

¹⁸F-fallypride PET/MR may be a promising technique for predicting DA response in patients with prolactinoma.

11C-methionine PET aids localization of microprolactinomas in patients with intolerance or resistance to dopamine agonist therapy

PURPOSE

To assess the potential for ¹¹C-methionine PET (Met-PET) coregistered with volumetric magnetic resonance imaging (Met-PET/MR^CR) to inform clinical decision making in patients with poorly visualized or occult microprolactinomas and dopamine agonist intolerance or resistance.

PATIENTS AND METHODS

Thirteen patients with pituitary microprolactinomas, and who were intolerant (n = 11) or resistant (n = 2) to dopamine agonist therapy, were referred to our specialist pituitary centre for Met-PET/MR^CR between 2016 and 2020. All patients had persistent hyperprolactinemia and were being considered for surgical intervention, but standard clinical MRI had shown either no visible adenoma or equivocal appearances.

RESULTS

In all 13 patients Met-PET/MR^CR demonstrated a single focus of avid tracer uptake. This was localized either to the right or left side of the sella in 12 subjects. In one patient, who had previously undergone surgery for a left-sided adenoma, recurrent tumor was unexpectedly identified in the left cavernous sinus. Five patients underwent endoscopic transsphenoidal selective adenomectomy, with subsequent complete remission of hyperprolactinaemia and normalization of other pituitary function; three patients are awaiting surgery. In the patient with inoperable cavernous sinus disease PET-guided stereotactic radiosurgery (SRS) was performed with subsequent near-normalization of serum prolactin. Two patients elected for a further trial of medical therapy, while two declined surgery or radiotherapy and chose to remain off medical treatment.

CONCLUSIONS

In patients with dopamine agonist intolerance or resistance, and indeterminate pituitary MRI, molecular (functional) imaging with Met-PET/MR^CR can allow precise localization of a microprolactinoma to facilitate selective surgical adenomectomy or SRS.

18F-fallypride and 11C-methionine positron emission tomography/computed tomography for the study of prolactinomas and nonfunctioning pituitary adenomas: A case series

Some studies have assessed the expression of dopaminergic dopamine 2 (D2)/3 receptors in prolactinomas and nonfunctioning pituitary adenomas (NFPA) by positron emission tomography/computed tomography (PET/CT) with ¹¹C-raclopride, proving that this modality can be useful to predict the response to treatment with dopamine agonists. However, the use of ¹¹C-labeled radiotracers is limited, as it requires a cyclotron in the PET center. ¹⁸F-fallypride is a radiotracer that has proven useful in assessing the expression of D2/3 receptors. As it is labeled with ¹⁸F, it can be produced and transported to distant PET centers. There are no studies on the usefulness of ¹⁸F-fallypride for the evaluation of patients with prolactinomas and NFPA. The aim of this study was to describe the first case series of patients with prolactinomas and NFPA studied with ¹⁸F-fallypride and ¹¹C-methionine PET/CT to reveal D2/3 expression and amino acid (AA) metabolism. ¹⁸F-fallypride and ¹¹C-methionine uptake were assessed in a case series of six patients, five with prolactinomas and one with a NFPA, and compared with clinical presentation and follow-up at 6–18 months. All patients presented with macroadenomas, with a wide range of AA metabolism, as revealed by ¹¹C-methionine PET/CT. ¹⁸F-fallypride PET/CT identified low to moderate/high D2/3 expression in the tumors. The patient that presented low expression of D2/3 in the tumor and high AA metabolism showed a poor response to DA therapy. ¹⁸F-fallypride was able to reveal D2/3 receptor expression in prolactinomas and NFPA, with the advantage of been a more accessible radiotracer in comparison with previous ¹¹C labeled analogs.

Using Molecular Imaging to Enhance Decision Making in the Management of Pituitary Adenomas

In most patients with suspected or confirmed pituitary adenomas (PAs), MRI, performed using T1- (with or without gadolinium enhancement) and T2-weighted sequences, provides sufficient information to guide effective clinical decision making. In other patients, additional MR sequences (e.g., gradient recalled echo, fluid-attenuation inversion recovery, MR elastography, or MR angiography) may be deployed to improve adenoma detection, assess tumoral consistency, or aid distinction from other sellar/parasellar lesions (e.g., aneurysm, meningioma). However, there remains a small but important subgroup of patients in whom primary or secondary intervention (e.g., first or redo transsphenoidal surgery, stereotactic radiosurgery) is limited by the inability of MRI to accurately localize the site(s) of de novo, persistent, or recurrent PA. Emerging evidence indicates that hybrid imaging, which combines molecular (e.g. 11C-methionine PET) and cross-sectional (MRI) modalities, can enable the detection and precise localization of sites of active tumor to guide targeted intervention. This not only increases the likelihood of achieving complete remission with preservation of remaining normal pituitary function but may mitigate the need for long-term (even lifelong) high-cost medical therapies. Here, we review published evidence supporting the use of molecular imaging in the management of PAs, including our own 10-y experience with 11C-methionine PET.

Use of micro-positron emission tomography with (18)F-fallypride to measure the levels of dopamine receptor-D2 and (18)F-FDG as molecular imaging tracer in the pituitary glands and prolactinomas of Fischer-344 rats

Dopamine receptor-D2 (DRD2) is the most important drug target in prolactinoma. The aim of this current study was to investigate the role of using micro-positron emission tomography (micro-PET) with (18)F-fallypride and (18)F-fluorodeoxyglucose ((18)F-FDG) as molecular imaging tracer in the pituitary glands and prolactinomas of Fischer-344 (F344) rats and detect the difference of the levels of DRD2 in the pituitary glands and prolactinomas of F344 rat prolactinoma models. Female F344 rat prolactinoma models were established by subcutaneous administration of 15 mg 17β-estradiol for 8 weeks. The growth of tumors was monitored by the small-animal magnetic resonance imaging and micro-PET. A series of molecular biological experiments were also performed 4 and 6 weeks after pump implantation. The micro-PET molecular imaging with (18)F-fallypride revealed a decreased expression of DRD2 in F344 rat prolactinoma models, but the micro-PET molecular imaging with (18)F-FDG presented an increased uptake in the prolactinoma compared with the pituitary gland. A decreasing trend of levels of DRD2 in F344 rat prolactinoma models was also detected by molecular biological experiments. From this, we can conclude that micro-PET with (18)F-fallypride and (18)F-FDG can be used to assess tumorigenesis of the prolactinomas in vivo and molecular imaging detection of DRD2 level in prolactinoma may be an indication of treatment effect in the animal experiment.

Affinity Labeling of Membrane Receptors Using Tissue-Penetrating Radiations

Photoaffinity labeling, a useful in vivo biochemical tool, is limited when applied in vivo because of the poor tissue penetration by ultraviolet (UV) photons. This study investigates affinity labeling using tissue-penetrating radiation to overcome the tissue attenuation and irreversibly label membrane receptor proteins. Using X-ray (115 kVp) at low doses (<50 cGy or Rad), specific and irreversible binding was found on striatal dopamine transporters with 3 photoaffinity ligands for dopamine transporters, to different extents. Upon X-ray exposure (115 kVp), RTI-38 and RTI-78 ligands showed irreversible and specific binding to the dopamine transporter similar to those seen with UV exposure under other conditions. Similarly, gamma rays at higher energy (662 keV) also affect irreversible binding of photoreactive ligands to peripheral benzodiazepine receptors (by PK14105) and to the dopamine (D2) membrane receptors (by azidoclebopride), respectively. This study reports that X-ray and gamma rays induced affinity labeling of membrane receptors in a manner similar to UV with photoreactive ligands of the dopamine transporter, D2 dopamine receptor (D2R), and peripheral benzodiazepine receptor (PBDZR). It may provide specific noninvasive irreversible block or stimulation of a receptor using tissue-penetrating radiation targeting selected anatomic sites.

A review of imaging techniques for systems biology

This paper presents a review of imaging techniques and of their utility in system biology. During the last decade systems biology has matured into a distinct field and imaging has been increasingly used to enable the interplay of experimental and theoretical biology. In this review, we describe and compare the roles of microscopy, ultrasound, CT (Computed Tomography), MRI (Magnetic Resonance Imaging), PET (Positron Emission Tomography), and molecular probes such as quantum dots and nanoshells in systems biology. As a unified application area among these different imaging techniques, examples in cancer targeting are highlighted.

Diagnostic imaging of dopamine receptors in pituitary adenomas

Dopamine D2 receptor scintigraphy of pituitary adenomas is feasible by single-photon emission computed tomography using (123)I-S-(-)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-2-hydroxy-3-iodo-6-methoxybenzamide ((123)I-IBZM) and (123)I-epidepride. (123)I-epidepride is generally superior to (123)I-IBZM for the visualization of D2 receptors on pituitary macroadenomas. However, (123)I-IBZM and (123)I-epidepride scintigraphy are generally not useful to predict the response to dopaminergic treatment in pituitary tumour patients. These techniques might allow discrimination of non-functioning pituitary macroadenomas from other non-tumour pathologies in the sellar region. Dopamine D2 receptors on pituitary tumours can also be studied using positron emission tomography with (11)C-N-raclopride and (11)C-N-methylspiperone.

Positron emission tomography in acromegaly and other pituitary adenoma patients

BACKGROUND

Positron emission tomography (PET) is a well-recognized technique used in research, especially for intracranial studies, as well as for clinical practice, and has contributed to the fast development in neuroscience during the last decades.

PROCEDURES

We have used PET in pituitary tumors for in vivo characterization with respect to metabolism, 11C-L-methionine and 18F-fluorodeoxyglucose, receptor properties, 11C-N-methylspiperone and 11C-raclopride, and monoamine oxidase B enzyme content, 11C-L-deprenyl; further, for diagnosing and outlining the tumors in differential diagnostic perspectives and in the follow-up of treatment.

OBSERVATIONS

11C-raclopride, a specific dopamine antagonist, demonstrated high amounts of dopamine D2 binding in prolactinomas and some growth hormone-secreting adenomas. There was a significant correlation between high amounts of D2 receptors and the positive treatment effect of dopamine agonist therapy. When 11C-L-methionine and 18F-fluorodeoxyglucose were used for metabolic mapping, the highest metabolic activity was found in the prolactinomas, which correlated well with the serum prolactin levels. The growth hormone adenomas also showed high metabolic rates. At treatment follow-up, a considerable decrease in 11C-L-methionine uptake was observed in all tumors that responded positively to the treatment and thus foretold that the medical treatment, both concerning dopamine agonist and somatostatin analogue, was effective. In this respect, PET was valuable to monitor treatment. PET was also shown valuable in differential diagnosing between pituitary adenomas, meningiomas and skull base neuromas.

CONCLUSION

We have found PET to be highly valuable in the research and clinical handling of patients with a pituitary adenoma for in vivo tumor characterization.

Developments in PET for the detection of endocrine tumours

Positron emission tomography (PET) supplies a range of labelled compounds to be used for the characterization of tumour biochemistry. Some of these have proved to be of value for clinical diagnosis, treatment follow-up, and clinical research. (18)F-fluorodeoxyglucose PET scanning is now a widely accepted imaging approach in clinical oncology, reflecting increased expression of glucose transporters in cancerous tissue. This tracer, however, does not show sufficient uptake in well-differentiated tumours such as neuroendocrine tumours. Endocrine tumours have the unique characteristics of taking up and decarboxylating amine precursors. These so-called APUD characteristics offer highly specific targets for PET tracers. Using this approach, radiopharmaceuticals such as [(11)C]-5-hydroxytryptophan and [(11)C]-L-dihydroxyphenylalanine for localization of carcinoid and endocrine pancreatic tumours, 6-[(18)F]-fluorodopamine and [(11)C]-hydroxyephedrine for phaeochromocytomas, and [(11)C]-metomidate for adrenal cortical tumours have been developed. Functional imaging with PET using these compounds is now being employed to complement rather than replace other imaging modalities. Development of new PET radiopharmaceuticals may in the future allow in vivo detection of tumour biological properties, such as malignant potential and responsiveness to treatment.

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.

Usefulness of positron emission tomography in diagnosis and treatment follow-up of brain tumors

Clinical and experimental use of positron emission tomography (PET) is expanding and allows quantitative assessment of brain tumor's pathophysiology and biochemistry. PET therefore provides different biochemical and molecular information about primary brain tumors when compared to histological methods or neuroradiological studies. Common clinical indications for PET contain primary brain tumor diagnosis and identification of the metabolically most active brain tumor reactions (differentiation of viable tumor tissue from necrosis), prediction of treatment response by measurement of tumor perfusion, or ischemia. The interesting key question remains not only whether the magnitude of biochemical alterations demonstrated by PET reveals prognostic value with respect to survival, but also whether it identifies early disease and differentiates benign from malignant lesions. Moreover, an early identification of treatment success or failure by PET could significantly influence patient management by providing more objective decision criteria for evaluation of specific therapeutic strategies. Specially, as PET represents a novel technology for molecular imaging assays of metabolism and signal transduction to gene expression, reporter gene assays are used to trace the location and temporal level of expression of therapeutic and endogenous genes. PET probes and drugs are being developed together as molecular probes to image the function of targets without disturbing them and in mass amounts to modify the target's function as a drug. Molecular imaging by PET helps to close the gap between in vitro to in vivo integrative biology of disease.

Positron emission tomography (PET): expanding the horizons of oncology drug development

Positron emission tomography (PET) allows three-dimensional quantitative determination of the distribution of radioactivity permitting measurement of physiological, biochemical, and pharmacological functions at the molecular level. Until recently, no method existed to directly and noninvasively assess transport and metabolism of neoplastic agents as a function of time in various organs as well as in the tumor. Standard preclinical evaluation of potential anticancer agents entails radiolabeling the agent, usually with tritium or 14C, sacrifice experiments, and high-performance liquid chromatography (HPLC) analysis to determine the biodistribution and metabolism in animals. Radiolabeling agents with positron-emitting radionuclides allows the same information to be obtained as well as in vivo pharmacokinetic (PK) data by animal tissue and plasma sampling in combination with PET scanning. In phase I/II human studies, classic PK measurements can be coupled with imaging measurements to define an optimal dosing schedule and help formulate the design of phase III studies that are essential for drug licensure [1]. Many of the novel agents currently in development are cytostatic rather than cytotoxic and therefore, the traditional standard endpoints in phase I and II studies may no longer be relevant. The use of a specialized imaging modality that allows PK and pharmacodynamic (PD) evaluation of a drug of interest has been proposed to permit rapid and sensitive assessment of the biological effects of novel anticancer agents. The progress to date and the challenges of incorporating PET technology into oncology drug development from the preclinical to clinical setting are reviewed in this article.

Molecular imaging of cancer with positron emission tomography

The imaging of specific molecular targets that are associated with cancer should allow earlier diagnosis and better management of oncology patients. Positron emission tomography (PET) is a highly sensitive non-invasive technology that is ideally suited for pre-clinical and clinical imaging of cancer biology, in contrast to anatomical approaches. By using radiolabelled tracers, which are injected in non-pharmacological doses, three-dimensional images can be reconstructed by a computer to show the concentration and location(s) of the tracer of interest. PET should become increasingly important in cancer imaging in the next decade.

PET radiopharmaceuticals: state-of-the-art and future prospects

In this review we provide a conceptual overview of radiopharmaceuticals containing positron-emitting isotopes, not a catalog of radiopharmaceuticals or details of syntheses. We hope to provide an integrated framework for understanding the radiopharmaceuticals that are available at this time, describing both their strengths and weaknesses, and to look forward to some of the improvements that might be anticipated in the next decade. The range of biology that can be studied with positron emission tomography (PET) radiopharmaceuticals has greatly expanded, involving more sophisticated tracers and more sophisticated data analysis. PET measurements now encompass increasingly more specific aspects of human biochemistry and physiology as described in this review. As the biology being studied becomes more complex, the demands on the radiopharmaceutical and the methods of data analysis also become more complex. New synthetic chemistry and data analysis must develop in tandem. Radiopharmaceuticals must be designed to ensure that the rate determining step that is of interest is the one reflected in the data from the radiopharmaceutical. The challenge to the PET community of chemists, biologists, and physicians is to apply new knowledge of human biochemistry for developing and validating useful PET radiopharmaceuticals that will, in turn, produce useful nuclear medicine procedures. Initially the synthesis of a compound containing a short-lived radionuclide was a triumph in itself. However as the science advances the radiochemical synthesis becomes just the first step in a long trail that terminates in the compound being used to provide data on biological processes via a well-designed PET experiment. The resulting list of compounds and experiments should be as diverse as all of human biology and pathophysiology.

Differentiation of clinically non-functioning pituitary adenomas from meningiomas and craniopharyngiomas by positron emission tomography with [18F]fluoro-ethyl-spiperone

The differential diagnosis among various types of non-functioning sellar and parasellar tumours is sometimes difficult using currently available methods of morphological imaging. The aim of this study was to define whether assessment of the uptake of [18F]fluoro-ethyl-spiperone (FESP) with positron emission tomography (PET) could be helpful for the differential diagnosis of pituitary adenomas and other parasellar lesions, and for establishing the appropriate therapeutic approach. The population examined comprised 16 patients with the diagnosis of primary tumour of the sellar/parasellar region who were waiting to undergo surgical treatment. The results demonstrated that PET with [18F]FESP is a very specific method for differentiating adenomas from craniopharyngiomas and meningiomas. The visual interpretation of images allows such differentiation at approximately 70 min after tracer injection. Semiquantitative analysis of the dynamic PET data confirmed the results of visual interpretation, demonstrating that the uptake of [18F]FESP was consistently (i.e. throughout the series) at least two- to threefold higher in non-functioning adenomas than in other parasellar tumours as early as 70 min after tracer injection, and that it increased still further thereafter. It is concluded that PET with [18F]FESP might be of clinical value in those cases in which the differential diagnosis among various histological types of sellar tumour is uncertain with conventional methods.

The history of Swedish neuroradiology

The history of Swedish neuroradiology is reviewed.

Initial clinical experiences with dopamine D2 receptor imaging by means of 2'-iodospiperone and single-photon emission computed tomography

Dopamine D2 receptor imaging was performed with 123I labeled 2'-iodospiperone (2'-ISP) and single-photon emission computed tomography (SPECT) in 9 patients: 4 with idiopathic Parkinson's disease, 2 with parkinsonism, 1 with Wilson's disease and 2 with pituitary tumor, and the results were compared with the data for 9 normal subjects. Following an intravenous injection of 123I-2'-ISP, early (within 30 min) and late (between 2 and 4 hr) SPECT images were obtained by means of a multi-detector SPECT scanner or a rotating gamma camera. In normal subjects, early SPECT images demonstrated uniform distribution of radioactivity in the cerebral gray matter and cerebellum reflecting regional cerebral blood flow, whereas late SPECT images showed high radioactivity only in the basal ganglia. All the patients with Parkinson's disease also demonstrated symmetrical basal ganglia uptake in the late SPECT images, but it was diminished in parkinsonism and Wilson's disease. One patient with a growth hormone-producing pituitary tumor had a positive uptake in the tumor. These preliminary clinical data demonstrated that 2'-ISP can be used for SPECT imaging of D2 dopamine receptors and may be of clinical value for the diagnosis and planning of the treatment of neurological diseases.

Imaging of pituitary tumours

In the neuroradiological study of pituitary tumours, second generation CT, dynamic CT and MRI provide information about the extent of the tumour and its anatomical relations with the surrounding tissues. Sometimes these techniques can distinguish primary anterior pituitary lesions from primary parasellar lesions with presentations in the sellar region. In general, contrast-enhanced MRI and dynamic CT are more sensitive than conventional CT for the diagnosis of pituitary microadenomas, as well as for the precise delineation of the parasellar invasion of macroadenomas. Radiological techniques usually cannot distinguish clinically non-functioning from functioning pituitary adenomas. BSIPSS is used for confirmation of the diagnosis of Cushing's disease as well as for the lateralization of pituitary microadenomas in Cushing's disease and some other anterior pituitary hyperfunctional states. Neurotransmitter-receptor ligand imaging by SPECT with 123I-IBZM and/or 111In-DTPA-octreotide characterizes the dopamine D2 and somatostatin receptor status of pituitary adenomas, respectively. In selected cases, these techniques may be used for the differential diagnosis of pituitary tumours as well as for the differential diagnosis of primary anterior pituitary lesions and primary parasellar lesions with presentations in the sellar region. If medical therapy of these tumours with receptor agonists is being considered, these techniques can help in selecting the first-line treatment. Furthermore, the effects of medical therapy on the tumour can be evaluated. The introduction of newer and more receptor-specific radioligands may expand the clinical use of these techniques in the future. The availability of PET for the clinical diagnosis of pituitary tumours is still limited, but promising results have been described.

Radiopharmaceuticals for brain imaging

Brain imaging is performed using radiopharmaceuticals by single photon emission computed tomography (SPECT) and positron emission tomography (PET). SPECT and PET radiopharmaceuticals are classified according to blood-brain-barrier permeability, cerebral perfusion and metabolism receptor-binding, and antigen-antibody binding. The blood-brain-barrier (BBB) SPECT agents, such as 99mTcO4-, [99mTc]DTPA, 201TI and [67Ga]citrate are excluded by normal brain cells, but enter into tumor cells because of altered BBB. These agents were used in the earlier period for the detection of brain tumors. SPECT perfusion agents such as [123I]IMP, [99mTc]HMPAO, [99mTc]ECD are lipophilic agents and therefore, diffuse into the normal brain. These tracers have been successfully used to detect various cerebrovascular diseases such as stroke, Parkinson disease, Huntington's disease, epilepsy, dementia, and psychiatric disorders. Xenon-133 and radiolabeled microspheres have been used for the measurement of cerebral blood flow (CBF). Important receptor-binding SPECT radiopharmaceuticals include [123I]QNE, [123I]IBZM, and [123I]iomazenil. These tracers bind to specific receptors in the brain, thus displaying their distribution in various receptor-related cerebral diseases. Radioiodinated monoclonal antibodies were used for the detection of brain tumors. PET radiopharmaceuticals for brain imaging are commonly labeled with positron-emitters such as 11C, 13N, 15O, and 18F, although other radionuclides such as 82Rb, 62Cu and 68Ga also were used. The brain uptake of [13N]glutamate, [68Ga]EDTA and [82Rb]RbCl depends on the BBB permeability, but these are rarely used for brain imaging. Several cerebral perfusion agents have been introduced, of which [15O]water, [13N]ammonia, and [15O]butanol have been used more frequently. Regional CBF has been quantitated by using these tracers in normal and different cerebral disease states. Other perfusion agents include [15O]O2, [11C]CO, [11C]CO2, [18F]fluoromethane, [15O]O2, [11C]butanol, and [62Cu]PTSM. Among the PET cerebral metabolic agents, [18F]fluorodeoxyglucose (FDG) is most commonly used to detect metabolic abnormalities in the brain. Various brain tumors have been graded by [18F]FDG PET. This technique was used to detect epileptic foci by showing increased uptake in the foci during the ictal period and decreased uptake in the interictal period. Differentiation between recurrent tumors and radiation necrosis and the detection of Alzheimer's disease have been made successfully by [18F]FDG PET. Other PET metabolic agents such as [11C]deoxyglucose, and [11C]methylmethionine have drawn attention in the detection of brain tumors. [18F]fluorodopa is a cerebral neurotransmitter agent, which has been found very useful in the detection of Parkinson disease that shows reduced uptake of the tracer in the striatum of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of cutaneous contact hypersensitivity in the mouse with systemic or topical spiperone: topical application of spiperone produces local immunosuppression without inducing systemic neuroleptic effects

We tested the ability of the neuroleptic agent spiperone (8-[3-(p-fluorobenzoyl)propyl]-1-phenyl-1,3,8-triazaspiro-[4.5] decan-4- one) to influence the tissue swelling and leukocyte infiltration associated with T-cell--dependent immune responses, i.e., contact hypersensitivity reactions, in mice. Contact hypersensitivity reactions were elicited by applying the haptens oxazolone or dinitrofluorobenzene topically to one or both ears 5-8 d after epicutaneous sensitization. When spiperone was given subcutaneously at a dose of 30 or 150 mg/kg, 1 h after challenge with oxazolone, cutaneous contact hypersensitivity to this hapten was significantly diminished. When applied topically in concentrations as low as 0.08% (w/w), preparations of spiperone significantly suppressed both the tissue swelling and the leukocyte infiltration associated with the elicitation phase of contact hypersensitivity. Topical treatment with spiperone also suppressed the sensitization phase of contact sensitivity. However, mice treated topically with spiperone, unlike those treated systemically, exhibited no drowsiness or other evidence of central nervous system effects. Spiperone expresses both serotonin and dopamine receptor antagonist activity. However, unlike spiperone, the chemically unrelated serotonin antagonists, trazadone and mianserin, and the dopamine receptor antagonist, haloperidol, were not effective in suppressing contact hypersensitivity. Our results indicate that spiperone can have immunosuppressive effects on contact hypersensitivity reactions in the mouse, even when applied topically in doses that lack neuroleptic effects, and that the mechanism of action of spiperone on the immune response may be independent of its serotonin or dopamine receptor blocking properties.

Differentiation of pituitary adenoma and meningioma: visualization with positron emission tomography and [11C]-L-deprenyl

Seven patients with clinically nonsecreting pituitary adenoma and 5 patients with meningioma were examined with positron emission tomography using [11C]-LL-deprenyl and [11C]-LL-methionine. The dynamics of the uptake of [11C]-L-deprenyl in the pituitary adenomas demonstrated a rapid and high uptake immediately after the injection, and, later, an almost constant level was observed that was equal to or higher than that observed in normal brain tissue. In the meningiomas, however, the initially high uptake was followed by a marked decrease with time, reaching a level that was approximately half that observed in brain tissue. The study demonstrated high binding of [11C]-L-deprenyl to monoamine oxidase B in pituitary adenomas, whereas the binding in meningiomas was very low. This fact can be used in the differential diagnosis of pituitary adenoma and parasellar meningioma. Operative samples from 10 patients with meningioma and from 5 patients with pituitary adenoma were analyzed biochemically for activity of monoamine oxidase B, using [14C]-phenyl-ethylamine as substrate. The nonsecreting pituitary adenomas demonstrated high enzyme activity, the secreting adenomas about one-tenth of that of the nonsecreting, and the meningiomas one-thirtieth of that of nonsecreting adenomas.

Functional imaging of the brain using single photon emission computerized tomography (SPECT)

The use of tracers is an important technique available for studying cerebral function. Changes in 'signal' are large, but as a result of its photon limited nature, the measurement of this signal is limited: spatially, temporally and in terms of accuracy. The most commonly used single photon (SPECT) system (as apposed to positron) is that with a rotating gamma camera, although multi-headed devices and special purpose rings are now also commonly available. The problems of obtaining good functional information are however identical. Firstly the devices need to be optimised in terms of resolution and sensitivity. Secondly several sources of error, notably those associated with scatter, attenuation and limited spatial resolution, need to be corrected, with the aim of obtaining quantitative estimates of radioactivity concentration. Finally such quantitative estimates need to be converted into meaningful estimates of physiological variables by use of an appropriate model. The general aim of many SPECT measurements is to estimate blood flow for example using Tc-99m labelled HMPAO as a tracer. Good results have been obtained in many clinical conditions: stroke, dementia, tumour and epilepsy, for example. Many other tracers are also available, for example to measure density of receptor sites. The use of SPECT in conjunction with other techniques after image registration is suggested as being an essential tool in extracting maximal clinical information.

Preclinical evaluation of a positron emitting progestin ([18F]fluoro-16 alpha-methyl-19-norprogesterone) for imaging progesterone receptor positive tumours with positron emission tomography

Three 21-fluoro-progestins were investigated as potential imaging agents for the in vivo assessment of human progesterone receptor positive neoplasms with positron emission tomography. In competitive binding assays these compounds demonstrated high specificity, competing only for progesterone receptors. Binding to other steroid receptor types was negligible. Based on its high affinity binding, 21-fluoro-16 alpha-methyl-19-norprogesterone was selected for further evaluation in vivo. Tissue distribution studies in immature estrogen primed female rats revealed high uterine uptake of 21-[18F]fluoro-16 alpha-methyl-19-norprogesterone ([18F]FMNP). At 60 min after injection the ratio of uptake of radioactivity by uterine tissue to that of blood was 7. This ratio increased to 24 at 180 min. A selective decrease in uterine uptake was observed after administration of [18F]FMNP with excess unlabelled progestin. Rats bearing hormone responsive MT-W9A mammary adenocarcinomas were used to examine [18F]FMNP for tumour uptake. Animals were used irrespective of the phase of the estrous cycle. At 180 min the uterus to blood ratio and the tumour to blood ratio ranged from 3 to 20 and 3 to 17, respectively. Uterine and tumour tissue was assayed for cytosolic estrogen and progesterone receptors using a dextran-coated charcoal method and Scatchard plot analysis. The results indicate that the in vivo uptake of [18F]FMNP by uterine and mammary tumour tissue correlates well with the progesterone receptor concentration (rs = 0.98 and rs = 0.88, respectively). It is concluded that the uptake of [18F]FMNP by progesterone receptor positive tissue in vivo is primarily receptor related and that this uptake is attributable to the progesterone receptor. The study demonstrates the potential applicability of [18F]FMNP and positron emission tomography for imaging progesterone receptor positive neoplasms.

Advances in medical imaging for cancer diagnosis and treatment

Over the last several decades, significant "new eyes" have been developed that improve the diagnosis, treatment, planning, and monitoring of human cancer: computer tomography (CT), magnetic resonance imaging (MRI) and spectroscopy (MRS), single photon emission computed tomography (SPECT), and positron emission tomography (PET). Innovative advances in both morphologic and functional imaging have led to a dramatic improvement in our ability to diagnose and monitor human cancer. Frequently, anatomic detail can be demonstrated in ways that exceed views at surgery, and functional biochemical imaging is being used to show the metabolic activity and receptor status of normal and pathologic states. In vivo functional and biochemical studies differentiate normal from neoplastic or nonviable tissue, and make it possible to measure progression or regression of the disease. Because physiologic changes often precede morphologic findings in many disease processes, the use of in vivo biochemical probes can demonstrate disease before anatomic abnormalities become evident. Gross changes in anatomy are no longer adequate endpoints for therapy protocols. Today, using physiologic imaging, we can evaluate the response to treatment within hours of administration of therapy. Adjuvant metabolic tumor imaging studies provide complimentary information to morphologic evaluation of human cancers that will ultimately lead to better patient care.

Positron emission tomography studies of brain receptors

Probing the regional distribution and affinity of receptors in the brain, in vivo, in human and non human primates has become possible with the use of selective ligands labelled with positron emitting radionuclides and positron emission tomography (PET). After describing the techniques used in positron emission tomography to characterize a ligand receptor binding and discussing the choice of the label and the limitations and complexities of the in vivo approach, the results obtained in the PET studies of various neurotransmission systems: dopaminergic, opiate, benzodiazepine, serotonin and cholinergic systems are reviewed.

Striatal binding of 11C-NMSP studied with positron emission tomography in patients with persistent tardive dyskinesia: no evidence for altered dopamine D2 receptor binding

Dopamine D2 receptor binding characteristics were studied by positron emission tomography (PET) using N-11C-methyl spiperone as receptor ligand in patients on longterm treatment with neuroleptic drugs and in control subjects. Eight of the patients had symptoms of tardive dyskinesia whereas three patients did not have any symptoms. Control subjects comprised 5 healthy volunteers and 7 patients with pituitary tumors. All patients had been free of neuroleptic drugs for at least 4 weeks. The time dependent regional radioactivity in the striatum was measured and the receptor binding rate, k3, proportional to receptor number, Bmax and association rate for the receptor was calculated in relation to the cerebellum. The lack in difference in k3 values between TD patients, neuroleptic treated patients without TD and control subjects throws doubt on the hypothesis that changes in striatal D2 dopamine receptor number or binding affinity is an etiological mechanism for persistent TD.

Where have we got to with neuroreceptor mapping of the human brain?

In the past two decades, tritiated radioligand receptor binding, a tool commonly used to investigate the site of action of drugs in laboratory animals, has provided a vast body of information on neuropharmacology and neurobiology. Several neurological and psychiatric diseases have been related to neurotransmitter and receptor disorders. In order to study ligand interactions with receptors in vivo in humans, new tracers capable of carrying a gamma-emitting radionuclide to the receptor have been designed. Emission computerized tomography (ECT) techniques such as positron (PET) or single photon emission tomography (SPET) allow monitoring of the time-course of regional tissue concentration of these radiolabelled ligands. PET and SPET each have their inherent advantages and drawbacks. The cyclotron-based technology of PET is a demanding and expensive technique that, to date, is still mainly reserved for research purposes. It is hoped that once the scientific basis of a physiopathological study is established using PET, diagnostic information might be provided by the more readily available SPET technology. The purpose of this article is to review the current state of receptor-binding gamma-emitting radioligands and to present the clinical potential of these new kinds of radiopharmaceuticals in clinical investigation.

PET in clinical oncology

Positron Emission Tomography (PET) is an imaging technique that produces cross sectional images based on tissue biochemical and physiological processes. PET complements other anatomic imaging techniques such as x-ray CT and magnetic resonance imaging (MRI). Fundamental processes such as glucose metabolism, oxygen metabolism, and blood flow can be imaged and quantified with PET, in addition to many other processes of both clinical and investigative interest. PET is now emerging as a clinical tool in oncology and is useful in noninvasively grading tumors, in determining tumor activity and recurrence, and in monitoring the effects of a variety of therapeutic interventions with tumors. While most of the applications of PET in oncology to date have been in brain tumors, the technique is now being applied in tumor evaluations outside of the central nervous system.

[Ferrocene, ruthenocene and rhodocene analogs in haloperidol synthesis and organ distribution after labeling with 103Ru and 103mRh]

Ferrocene-Haloperidol was synthesized by N-alkylation of 4-(4'-chlorophenyl)- 4-hydroxypiperidine with 1-ferrocenyl-4-chlor-butan-1-on. By heating the ferrocene-haloperidol with 103RuCl3 the 103Ru-labelled ruthenocene-haloperidol was obtained. This compound showed a high affinity for lung but not for brain in rats and mice. The decay of the 103Ru labelled compound results in the formation of the 103mRh labelled rhodocene-haloperidol, which is rapidly oxidized by air to the corresponding rhodocinium-haloperidol. This compound can be separated by extraction and TLC.