Adrenal imaging agents: rationale, synthesis, formulation and, metabolism

Ring opening of epoxides with [18F]FeF species to produce [18F]fluorohydrin PET imaging agents

A simple technique for the preparation of [18F]HF has been developed and applied to the generation of an [18F]FeF species for opening sterically hindered epoxides. This method has been successfully employed to prepare four drug-like molecules, including 5-[18F]fluoro-6-hydroxy-cholesterol, a potential adrenal/endocrine PET imaging agent. This easily automated one-pot procedure produces sterically hindered fluorohydrin PET imaging agents in good yields and high molar activities.

Current and future treatments for malignant pheochromocytoma and sympathetic paraganglioma

Pheochromocytomas (PHs) and sympathetic paragangliomas (SPGs) are rare neuroendocrine tumors. Approximately 17 % of these tumors are malignant, but because no molecular or histologic markers for malignancy exist, patients are often diagnosed with malignant PHs or SPGs after unresectable disease has formed. Patients with progressive metastatic tumors and overwhelming symptoms are currently treated with systemic chemotherapy and radiopharmaceutical agents such as metaiodobenzylguanidine. These therapies lead to partial radiographic response, disease stabilization, and symptomatic improvement in approximately 40 % of patients, and systemic chemotherapy is associated with a modest improvement in overall survival duration. However, over the past decade, substantial progress has been made in clinical, biochemical, and radiographic diagnosis of PHs and SPGs. Approximately 50 % of patients with malignant PHs and SPGs have been found to carry hereditary germline mutations in the succinate dehydrogenase subunit B gene (SDHB), and anti-angiogenic agents such as sunitinib have been found to potentially play a role in the treatment of malignant disease, especially in patients with SDHB mutations. In some patients, treatment with sunitinib has been associated with partial radiographic response, disease stabilization, decreased fluorodeoxyglucose uptake on positron emission tomography, and improved blood pressure control. These findings have led to the development of prospective clinical trials of new targeted therapies for metastatic disease. Here, we provide an updated review of the clinical and genetic predictors of malignant disease, radiographic diagnosis of malignant disease, and information from the most relevant studies of systemic therapies, as well as proposed treatment guidelines for patients with metastatic or potentially malignant PHs and SPGs.

Radioiodinated metaiodobenzylguanidine (MIBG): radiochemistry, biology, and pharmacology

As an analogue of adrenergic neurotransmitter norepinephrine (NE), metaiodobenzylguanidine (MIBG) demonstrates high uptake both in normal sympathetically innervated tissues, such as the heart and salivary glands, and in tumors that express the NE transporter (NET), specifically those of neural crest and neuroendocrine origin. In 1994, (131)I-MIBG, also known as iobenguane I-131 intravenous, received Food and Drug Administration (FDA) approval as an imaging agent. In 2008, (123)I-MIBG was also approved by FDA as a tumor imaging agent. Commercial formulations of radioiodinated MIBG are prepared on the basis of radioiodide exchange reaction with unlabeled MIBG as a precursor and contain large mass amounts of unlabeled MIBG, or "cold carrier," molecules. Because the cold MIBG molecules competitively inhibit the uptake of radiolabeled MIBG molecules by adrenergic and neuroendocrine cells expressing NET, no-carrier-added (n.c.a.), high specific activity (SA) radioiodinated MIBG preparations have been developed on the basis of electrophilic radioiodination reaction and solid-phase technology by using dibutylstanyl benzylguanidine precursor linked to polymers. On the basis of n.c.a. synthetic procedures, therapeutic doses of [(131)I]MIBG can be administered with very high SA (1600 mCi/μmol or 5734 mCi/mg). The very high SA of n.c.a. [(131)I]MIBG drug would increase the specific cellular uptake of adrenergic neurons and neuroendocrine tumor cells expressing NET.

[123/131I]Iodometomidate as a radioligand for functional diagnosis of adrenal disease: synthesis, structural requirements and biodistribution

Metomidate [(R)-1-(1-phenylethyl)-1H-imidazole-5-carboxylic acid methyl ester] (MTO, 1, Fig. 1) is a potent and selective inhibitor of the cytochrome P-450 enzyme system in the adrenal cortex. Labelled in the 4-position with radioiodine, (R)-4-[131I]iodometomidate, 2, [131I]IMTO has been evaluated by in-vitro studies and also ex-vivo in rats. [131I]IMTO was synthesized by oxidative radioiododestannylation using a suitable precursor which was prepared by a new stereoselective synthesis. Optimization of the labelling reaction was performed by systematic variation of the most important reaction parameters. Under optimum reaction conditions, a labelling yield of 95% was obtained. In-vitro-stability of the tracer was studied over 8 days, indicating slow deiodination (0.27%/h). Displacement studies using [131I]IMTO and rat adrenal membranes revealed the structural requirements for high affinity binding, namely an intact ester group and (R)-configuration of the radioligand. Pharmacokinetic studies in rats showed fast accumulation of [131I]IMTO in the adrenals (approx. 10% ID/g tissue) with an activity plateau for 2 hours. Metabolic degradation was indicated by a steady increase of renal activity up to 4 hours post injection. Based on target to non-target ratios the highest contrast for imaging of the adrenals was observed between 30 and 60 min post injection of [131I]IMTO. We conclude that SPECT using [123I]IMTO will be a promising method for the characterization of adrenal incidentalomas.

The role of PET in the surgical approach to adrenal disease

BACKGROUND

Appropriate surgical approach to diseases of the adrenal requires a diagnosis sufficient to determine the biochemical status of adrenal dysfunction and anatomic evaluation sufficient to differentiate unilateral from bilateral disease, intra-adrenal from extra-adrenal neoplasm, adrenal tumor recurrence or adrenal metastases. High resolution computed tomography (CT) and magnetic resonance have been the primary imaging modalities for the evaluation of anatomy, while scintigraphic studies have played a secondary role in diagnosis. The recent availability of functional imaging provided by positron emission tomography (PET) with radiopharmaceuticals designed to depict substrate precursor uptake, cellular metabolism or receptor binding in neoplasms and CT as a single modality, hybrid PET/CT, to directly correlate function and anatomy has had a significant impact upon the diagnostic and therapeutic approach to many cancers and has been applied to adrenal disease with some early success that we describe in this review.

METHODS

In addition to the authors' experience, a search of Medline and PubMed databases was performed using search terms: 'adrenal scintigraphy', 'positron tomography', 'computed tomography', 'adrenal surgery', 'adrenal mass', '(18)F-fluorodeoxyglucose', 'adrenal carcinoma', 'adrenal medulla' and 'pheochromocytoma'.

CONCLUSIONS

Present PET radiopharmaceuticals and their use in hybrid PET/CT have demonstrated efficacy in the preoperative and follow-up evaluation of neoplasms of the adrenal cortex and medulla that hopefully will continue to improve with the development of newer tracers that continue to exploit unusual characteristics of the adrenals.

Adrenocortical carcinoma in children. Review and recent innovations

Adrenocortical carcinoma in childhood is a rare potentially fatal disease. Despite its often dramatic presentation, there typically has been a distressingly long delay between the onset of symptoms and the time of diagnosis. This delay undoubtedly has contributed to the historically poor prognosis in these children by permitting the disease to reach an advanced stage before treatment is started. It is imperative that the physician recognizes the endocrine manifestations of these tumors early and has a high index of suspicion. Although biochemical and histologic evaluations are helpful, they often cannot differentiate benign lesions from malignant neoplasms and should not unduly delay intervention. Aggressive complete surgical resection continues to be the mainstay of treatment and is the best prognosticator of overall survival. The role of adjuvant therapy and chemotherapy continues to evolve. Molecular studies have increased understanding of cancer biology and may provide possible novel therapeutic approaches in the future. It is hoped that increased familiarity with this unusual tumor will result in earlier detection, prompt intervention, and improved survival for children with adrenocortical carcinoma.

Synthesis, biological evaluation, and baboon PET imaging of the potential adrenal imaging agent cholesteryl-p-[18F]fluorobenzoate

Cholesteryl-p-[18F]fluorobenzoate ([18F]CFB) was investigated as a potential adrenal positron emission tomography (PET) imaging agent for the diagnostic imaging of adrenal disorders. We describe the synthesis, biodistribution, adrenal autoradiography, and baboon PET imaging of [18F]CFB. The synthesis of [18F]CFB was facilitated by the use of a specially designed microwave cavity that was instrumental in effecting 70-83% incorporation of fluorine-18 in 60 s via [18F]fluoro-for-nitro exchange. Tissue distribution studies in mature female Sprague-Dawley rats showed good accumulation of [18F]CFB in the steroid-secreting tissues, adrenals and ovaries, at 1 h postinjection. The effectiveness of [18F]CFB to accumulate in diseased adrenals was shown through biodistribution studies in hypolipidemic rats, which showed a greater than threefold increase in adrenal uptake at 1 h and increased adrenal/liver and adrenal/kidney ratios. Analysis of the metabolites at 1 h in the blood, adrenals, spleen, and ovaries of hypolipidemic and control rats showed the intact tracer representing greater than 86%, 93%, 92%, and 82% of the accumulated activity, respectively. [18F]CFB was confirmed to selectively accumulate in the adrenal cortex versus the adrenal medulla by autoradiography. Normal baboon PET imaging with [18F]CFB effectively showed adrenal localization as early as 15 min after injection of the tracer, with enhanced adrenal contrast seen at 60-70 min. These results suggest that [18F]CFB may be useful as an adrenal PET imaging agent for assessing adrenal disorders.

Update on basic research and clinical experience with metaiodobenzylguanidine

I 131-metaiodobenzylguanidine (MIBG) is an aralkylguanidine with certain structural similarities to norepinephrine (NE). It is concentrated, stored, and released from chromaffin granules in a manner almost identical with that of NE. It will image the enlarged adrenal medullae of adrenal medullary hyperplasia when the CAT and NMR scans are normal. It is more sensitive in detecting extra-adrenal pheochromocytomas than CAT and NMR imaging. Because 46% of our 176 patients with histopathologically proved "benign" pheochromocytomas (pheos) have developed demonstrable metastases, with or without elevated plasma and urinary catecholamines, we now image all patients with "benign" pheos yearly. As of January 22, 1986 we had treated 28 patients with malignant pheos 71 times with MIBG. As of July 24, 1986, we had given 34 neuroblastoma patients 55 tracer doses. In some cases MIBG demonstrates more neuroblastoma than all other imaging modalities and this is helpful in staging. We have had 30-50% objective regressions in neuroblastoma tumor mass in 3 out of the first 12 patients treated. These three patients had slower-growing tumors and a lower body burden than the nonresponders. We also record the sensitivity of MIBG imaging of neuroendocrine tumors other than pheos and neuroblastomas.

Adrenal imaging with technetium-99m-labelled low density lipoproteins

Evaluation of adrenal cortical function by external imaging is currently accomplished by injection of radiolabelled analogs of cholesterol. Although the adrenals do utilized exogenous cholesterol for steroid hormone synthesis, the cholesterol is delivered to the glands not as free cholesterol but through the uptake of low density lipoproteins (LDL), which are subsequently degraded within the adrenal cortical cells to provide cholesterol. Thus, we sought to assess the use of 99mTc-labelled LDL injected into rabbits to obtain external images of the adrenal glands. Adrenal images of all nine rabbits tested were obtained within 18 to 21 hours after injection of 99mTc-LDL. Seven of the rabbits were subjected to adrenal cortical suppression with dexamethasone and then all nine rabbits were imaged a second time. In the untreated animals, visualization of the adrenal glands was accompanied by normal serum cortisol concentrations and accumulation of radiolabel in the adrenals, whereas in the dexamethasone-treated animals, lack of visualization of the adrenal glands was correlated with low serum cortisols, and greatly decreased accumulation of the radionuclide in the adrenals. These findings demonstrate for the first time that LDL, when labelled with 99mTc, can be used to evaluate adrenal cortical function by external imaging.

Synthesis of radioiodinated metyrapone--a potential adrenal imaging agent

Metyrapone has been labelled with radioiodine selectively in the 4'-position of ring B. The synthesis of 123I-(131)-metyrapone involves four intermediate compounds. 4'-Bromo-metyrapone serves as a stable precursor that is labelled before use. Studies of the biodistribution of [131I]metyrapone in rats indicate the highest concentration in the adrenal gland 10 min after the injection with a fast elimination of the radioactivity. However, the absolute uptake in the normal adrenal is low. When [123I]metyrapone was used in a patient with bilateral hyperplasia, faint adrenal images were obtained.

I-labeled 19-iodinated and 6 beta-iodomethyl-19-nor steroids: effect of structural modification on the adrenal accumulation

19-Iodinated and 6 beta-iodomethyl-19-nor derivatives of cholesterol and 17-ketosteroid labeled with 131I were tested in rats to determine the critical structural features required for maximal adrenal uptake. The introduction of the 17-keto group in place of the 17 beta-side chain of cholesterol caused most of the radioactivity to be taken up by the thyroids. Fluorination at the C-3 position had deleterious effects on the adrenal concentration and led to the loss of adrenal specificity. A beta-hydroxy group at the C-3 position is substantially required for adrenal uptake.

Selenium-sulfur analogs III: synthesis and biodistribution of two 75Se-labeled 4-substituted-1,2,3-selenadiazoles

The 75Se-labeled 4-substituted-1,2,3-selenadiazole analogs of two drugs that inhibit the adrenocorticosteroid 11 beta- and 17 alpha-hydroxylase enzymes were prepared by the [75Se]selenious acid oxidation of the appropriate methyl ketone semicarbazones. The concentration of the radiolabeled compounds in the adrenal glands of rats over a 0.25--24-hr period was determined and compared with that in the blood, liver, and kidneys. The concentration in the adrenal glands and the target to nontarget ratios were much lower than those reported for other adrenocorticosteroid inhibitors. Therefore, these 1,2,3-selenadiazole agents do not have potential as adrenocorticosteroid imaging agents.

Selenium 75 selenomethyl cholesterol: a new agent for quantitative functional scintigraphy of the adrenals: physical aspects

Fifty-two patients with suspected or known adrenal disorders have been studied with 75Se selenomethyl cholesterol using a rectilinear scanner or gamma camera computer system. Radiation dosimetry was estimated from the blood clearance and whole body retention of 75Se selenomethyl cholesterol. A method of quantitation of adrenal uptake is described, use of which is essential for the separation of normal from hyperactive adrenal glands. The adrenal depth was found to vary between 4.4 and 12 cm. The lack of need for thyroid suppression and the availability and ease of handling and storage of 75Se selenomethyl cholesterol make it preferable to 131I iodo-cholesterol derivatives and the current agent of choice for adrenal imaging.