Ratiometric determination of etomidate based on an albumin-based indicator displacement assay (IDA)

The first fluorescent sensor based on the indicator displacement assay (IDA) for on-site determination of etomidate.

11C-Metomidate PET-CT versus adrenal vein sampling to subtype primary aldosteronism: a prospective clinical trial

OBJECTIVE

Adrenal vein sampling (AVS) is recommended to subtype primary aldosteronism, but it is technically challenging. We compared 11C-Metomidate-PET-computed tomography (PET-CT) and AVS for subtyping of primary aldosteronism.

METHODS

Patients with confirmed primary aldosteronism underwent both AVS and 11C-Metomidate PET-CT (post-dexamethasone). All results were reviewed at a multidisciplinary meeting to decide on final subtype diagnosis. Primary outcome was accuracy of PET versus AVS to diagnosis of unilateral primary aldosteronism based on post-surgical biochemical cure. Secondary outcome was accuracy of both tests to final subtype diagnosis.

RESULTS

All 25 patients recruited underwent PET and successful AVS (100%). Final diagnosis was unilateral in 22 patients, bilateral in two and indeterminate in one due to discordant lateralization. Twenty patients with unilateral primary aldosteronism underwent surgery, with 100% complete biochemical success, and 75% complete/partial clinical success. For the primary outcome, sensitivity of PET was 80% [95% confidence interval (95% CI): 56.3-94.3] and AVS was 75% (95% CI: 50.9-91.3). For the secondary outcome, sensitivity and specificity of PET was 81.9% (95% CI: 59.7-94.8) and 100% (95% CI: 15.8-100), and AVS was 68.2% (95% CI: 45.1-86.1) and 100% (95% CI: 15.8-100), respectively. Twelve out of 20 (60%) patients had both PET and AVS lateralization, four (20%) PET-only, three (15%) AVS-only, while one patient did not lateralize on PET or AVS. Post-surgery outcomes did not differ between patients identified by either test.

CONCLUSION

In our pilot study, 11C-Metomidate PET-CT performed comparably to AVS, and this should be validated in larger studies. PET identified patients with unilateral primary aldosteronism missed on AVS, and these tests could be used together to identify more patients with unilateral primary aldosteronism.

VIDEO ABSTRACT

http://links.lww.com/HJH/B918.

Preliminary Investigations into Use of Metomidate for Euthanasia of Ornamental Fishes

The goals of this study were to investigate the potential use of metomidate for one-step euthanasia of ornamental fish species representing commonly sold families in the ornamental fish trade and to determine a baseline euthanasia dose for most species tested. Metomidate hydrochloride, a rapid-acting, water-soluble, nonbarbiturate hypnotic related to etomidate, was tested at various concentrations and durations for euthanasia of species representing the following freshwater and marine aquarium fish families: Apogonidae, Ariidae, Blenniidae, Callichthyidae, Characidae, Cichlidae, Cyprinidae, Gobiidae, Gyrinocheilidae, Loricariidae, Melanotaeniidae, Osphronemidae, Pimelodidae, Poeciliidae, Pomacentridae, and Pseudochromidae. Fish in each trial were euthanized as a group (n = 1). Most groups contained 10-12 fish. Some higher doses required buffering. Euthanasia was considered successful if all fish in each group did not recover after 6 h in unmedicated water. All species immediately lost buoyancy and equilibrium, dropping to the tank bottom within 1 min and ceasing ventilation typically within minutes. However, reactivity to vibration, sound, or touch was noticeable for varying time periods afterward (8-66 min), so an additional 30 min of exposure after cessation of reactivity was included as part of the protocol. Although some species (Neon Tetras Paracheirodon innesi and Australian Rainbowfish Melanotaenia australis) were euthanized at a concentration of 40 mg/L metomidate for a total exposure time of 38 min, most species tested were successfully euthanized with metomidate at a concentration of 100 mg/L, with total exposure times ranging from 35-96 min. A few catfish species (Otocinclus sp. and Bronze Corydoras Corydoras aeneus) were refractory and recovered after 100 mg/L. However, Otocinclus sp. were successfully euthanized at 250 mg/L, and Bronze Corydoras were euthanized at 1,000 mg/L. Further studies are needed to provide additional data for these and other species and families, for different water chemistry conditions, and for various biological factors to fine-tune dosing regimens.

11C-Metomidate PET/CT Detected Multiple Ectopic Adrenal Rest Tumors in a Woman With Congenital Adrenal Hyperplasia

CONTEXT

Women with congenital adrenal hyperplasia (CAH) may present with androgen excess that is difficult to control with conventional suppressive doses of glucocorticoids. Clinical management is challenging, and the woman is at great risk of developing steroid-induced complications.

PATIENTS AND METHODS

A 32-year-old woman with salt-wasting CAH due to 21-hydroxylase deficiency underwent right-sided adrenalectomy because of a large myelolipoma. Over the years, androgens became increasingly difficult to suppress on prednisolone 5 + 0 + 2.5 mg daily, and at age 39 years the left adrenal with an enlarging myelolipoma was removed. A month later serum testosterone levels had increased from 4.1 preoperatively to 18.3 nmol/L (reference 0.2-1.8 nmol/L), and adrenocorticotropin levels from 32 to 283 pmol/L (reference < 14 pmol/L). No adrenal parenchyma was visualized on computed tomography (CT). In the further search for the source of the markedly elevated testosterone, positron emission tomography (PET) was performed with 2 different tracers, 18fluorodeoxyglucose (18FDG) reflecting glucose metabolism and 11C-metomidate, an inhibitor of 11-β-hydroxylase targeting adrenocortical tissue.

RESULTS

18FDG-PET/CT with cosyntropin stimulation showed ovarian/paraovarian hypermetabolism, suggestive of adrenal rest tumors. Further characterization with 11C-metomidate PET/CT showed uptakes localized to the ovaries/adnexa, behind the spleen, and between the right crus diaphragmaticus and inferior vena cava.

CONCLUSION

Adrenal rest tumors can give rise to high androgen levels in spite of suppressive supraphysiological glucocorticoid doses. This case illustrates, for the first time, the value of 11C-metomidate PET as a sensitive method in documenting adrenal rest tumors, currently considered rare in women with CAH.

11C-metomidate PET in the diagnosis of adrenal masses and primary aldosteronism: a review of the literature

BACKGROUND

Adrenal masses are commonly encountered in clinical practice, many of whom are incidental. Identifying malignancy, and excess hormone production is essential for appropriate management. Biochemical workup and imaging tests (dedicated adrenal CT and/or MRI) are used to determine the likelihood of excessive hormone function and malignancy, respectively. However, imaging cannot provide information about function and biochemical workup cannot localize the source. Furthermore, in primary aldosteronism, adrenal vein sampling, the gold standard for lateralization, has important limitations such as the technical expertise required, the elevated costs, and potential complications. Over the last decades, there has been a renewed interest in alternative noninvasive imaging techniques that provide information about adrenal function without the need for invasive procedures. In this review, we will evaluate the evidence and the potential role of ¹¹C-metomidate as a promising positron emission tomography (PET) tracer in clinical practice.

METHODS

A review of the English literature for articles describing the use of the tracer ¹¹C-metomidate in adrenal disorders.

RESULTS

A total of 12 studies were included in the systematic review, which altogether addressed the use of ¹¹C-metomidate in adrenal masses and the application of this tracer in primary aldosteronism.

CONCLUSIONS

¹¹C-metomidate, a selective inhibitor of 11-β-hydroxylase, demonstrated a high specificity for adrenocortical tissue. In addition, ¹¹C-metomidate is correlated with this enzyme activity making it a potentially useful PET tracer for the identification primary aldosteronism, in addition to detection of adrenocortical masses.

New insights for screening etomidate analogues in the human H295R cell model

Etomidate is a sedative-hypnotic with excellent pharmacological effects, including rapid onset and hemodynamic stability. However, etomidate causes adrenocortical toxicity via binding to 11β-hydroxylase. Therefore, developing an approach to screen new etomidate analogues without endocrine-disrupting effects is urgently warranted. In this study, we employed the adrenocortical tumour cell line, NCI-H295R, as an in vitro system for etomidate analogues screening and detected the hormone levels in these cells using a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method. After obtaining the concentration-response curves of hormone release, the "Adrenocortical Inhibitory Index" was used to evaluate the adrenocortical inhibitory potency of each compound. In summary, we demonstrate the benefits of our methods for screening of etomidate analogues that lack adrenocortical suppression, especially when this in vitro system is combined with in vivo testing.

11 C-Metomidate PET/CT is a useful adjunct for lateralization of primary aldosteronism in routine clinical practice

OBJECTIVE

To describe clinical practice experience of ¹¹ C-Metomidate PET/CT as an adjunct to adrenal vein sampling (AVS) in the lateralization of aldosterone-producing adenomas (APA) in primary aldosteronism (PA).

CONTEXT

Accurate lateralization of APA in the setting of PA offers the potential for surgical cure and improved long-term cardiovascular outcomes. Challenges associated with AVS, the current gold standard lateralization modality, mean that only a small proportion of potentially eligible patients currently make it through to surgery. This has prompted consideration of alternative strategies for lateralization, including the application of novel molecular PET tracers such as ¹¹ C-Metomidate.

DESIGN

Clinical Service Evaluation/Retrospective audit.

PATIENTS

Fifteen individuals with a confirmed diagnosis of PA, undergoing lateralization with ¹¹ C-Metomidate PET/CT prior to final clinical decision on surgical vs medical management.

MEASUREMENTS

All patients underwent screening aldosterone renin ratio (ARR), followed by confirmatory testing with the seated saline infusion test, according to Endocrine Society Clinical Practice Guidelines. Adrenal glands were imaged using dedicated adrenal CT. ¹¹ C-Metomidate PET/CT was undertaken due to equivocal or failed AVS. Management outcomes were assessed by longitudinal measurement of blood pressure, ARR, number of hypertensive medications following adrenalectomy or institution of medical therapy.

RESULTS

We describe the individual lateralization and clinical outcomes for 15 patients with PA.

CONCLUSION

¹¹ C-Metomidate PET/CT in conjunction with adrenal CT and AVS provided useful information which aided clinical decision-making for PA within a multidisciplinary hypertension clinic.

Development of [18F]FAMTO: A novel fluorine-18 labelled positron emission tomography (PET) radiotracer for imaging CYP11B1 and CYP11B2 enzymes in adrenal glands

INTRODUCTION

Primary aldosteronism accounts for 6-15% of hypertension cases, the single biggest contributor to global morbidity and mortality. Whilst ~50% of these patients have unilateral aldosterone-producing adenomas, only a minority of these have curative surgery as the current diagnosis of unilateral disease is poor. Carbon-11 radiolabelled metomidate ([11C]MTO) is a positron emission tomography (PET) radiotracer able to selectively identify CYP11B1/2 expressing adrenocortical lesions of the adrenal gland. However, the use of [11C]MTO is limited to PET centres equipped with on-site cyclotrons due to its short half-life of 20.4 min. Radiolabelling a fluorometomidate derivative with fluorine-18 (radioactive half life 109.8 min) in the para-aromatic position ([18F]FAMTO) has the potential to overcome this disadvantage and allow it to be transported to non-cyclotron-based imaging centres.

METHODS

Two strategies for the one-step radio-synthesis of [18F]FAMTO were developed. [18F]FAMTO was obtained via radiofluorination via use of sulfonium salt (1) and boronic ester (2) precursors. [18F]FAMTO was evaluated in vitro by autoradiography of pig adrenal tissues and in vivo by determining its biodistribution in rodents. Rat plasma and urine were analysed to determine [18F]FAMTO metabolites.

RESULTS

[18F]FAMTO is obtained from sulfonium salt (1) and boronic ester (2) precursors in 7% and 32% non-isolated radiochemical yield (RCY), respectively. Formulated [18F]FAMTO was obtained with >99% radiochemical and enantiomeric purity with a synthesis time of 140 min from the trapping of [18F]fluoride ion on an anion-exchange resin (QMA cartridge). In vitro autoradiography of [18F]FAMTO demonstrated exquisite specific binding in CYP11B-rich pig adrenal glands. In vivo [18F]FAMTO rapidly accumulates in adrenal glands. Liver uptake was about 34% of that in the adrenals and all other organs were <12% of the adrenal uptake at 60 min post-injection. Metabolite analysis showed 13% unchanged [18F]FAMTO in blood at 10 min post-administration and rapid urinary excretion. In vitro assays in human blood showed a free fraction of 37.5%.

CONCLUSIONS

[18F]FAMTO, a new 18F-labelled analogue of metomidate, was successfully synthesised. In vitro and in vivo characterization demonstrated high selectivity towards aldosterone-producing enzymes (CYP11B1 and CYP11B2), supporting the potential of this radiotracer for human investigation.

Effects of repeated anaesthesia on gill and general health of Atlantic salmon, Salmo salar

Fish are the second most widely utilized vertebrate group used for scientific procedures in the United Kingdom, but the development and application of 3Rs (the principles of replacement, reduction, and refinement) in aquaculture disease research lags behind methodologies in place for mammalian studies. With a need for individual monitoring and non-lethal sampling, the effect of repeat anaesthesia on experimental fish needs to be better understood. This study analyses the effect of repeat anaesthesia with MS-222, metomidate and AQUI-S upon the gill and general health of post-smolt Atlantic salmon Salmo salar. A single, lethal dose of anaesthetic was compared with seven anaesthetizing time points over 28 days, terminating in a lethal dose. No anaesthetic showed significant differences in accumulation in the muscle tissue, or changes in plasma glucose after repeated or single dosing. Fish repeatedly anaesthetized with MS-222 or AQUI-S exhibited upregulation of osmoregulatory genes in the gill and AQUI-S-treated individuals showed, histologically, epithelial lifting from the lamellae capillary irrespective of whether they had a single or repeated dose history. No significant changes were seen in inflammatory or stress genes in the head kidney of fish repeatedly anaesthetized with AQUI-S or metomidate, however MS-222 treatment resulted in upregulation of tnfα3. Repeated anaesthesia with MS-222 and metomidate gave a significant decrease and increase in peripheral blood neutrophils, respectively. This study concludes that no increase in cumulative stress or inflammation is induced by the repeated anaesthetization of S. salar with any of the tested anaesthetics, however gill osmotic regulation and blood parameters may be affected.

Anesthetic Drug Discovery and Development: A Case Study of Novel Etomidate Analogs

All currently available general anesthetic agents possess potentially lethal side effects requiring their administration by highly trained clinicians. Among these agents is etomidate, a highly potent imidazole-based intravenous sedative-hypnotic that deleteriously suppresses the synthesis of adrenocortical steroids in a manner that is both potent and persistent. We developed two distinct strategies to design etomidate analogs that retain etomidate's potent hypnotic activity, but produce less adrenocortical suppression than etomidate. One strategy seeks to reduce binding to 11β-hydroxylase, a critical enzyme in the steroid biosynthetic pathway, which is potently inhibited by etomidate. The other strategy seeks to reduce the duration of adrenocortical suppression after etomidate administration by modifying the drug's structure to render it susceptible to rapid metabolism by esterases. In this chapter, we describe the methods used to evaluate the hypnotic and adrenocortical inhibitory potencies of two lead compounds designed using the aforementioned strategies. Our purpose is to provide a case study for the development of novel analogs of existing drugs with reduced side effects.

Effect of repeated exposure to AQUI-S® on the viability and growth of Neoparamoeba perurans

There have been recent efforts amongst immunologists to develop approaches for following individual fish during challenges with viral and bacterial pathogens. This study contributes to assessing the feasibility of using such approaches to study amoebic gill disease (AGD). Neoparamoeba perurans, agent of AGD, has been responsible for widespread economic and fish loss in salmonid aquaculture. With the emergence of AGD in Europe, research into infection dynamics and host response has increased. This study investigated the effect of repeat exposure to anaesthesia, a necessary requirement when following disease progression in individual fish, on N. perurans. In vitro cultures of N. perurans were exposed every 4 days over a 28-day period to AQUI-S^® (isoeugenol), a popular anaesthetic choice for AGD challenges, at a concentration and duration required to sedate post-smolt salmonids. Population growth was measured by sequential counts of amoeba over the period, while viability of non-attached amoeba in the culture was assessed with a vital stain. AQUI-S^® was found to be a suitable choice for in vivo ectoparasitic challenges with N. perurans during which repetitive anaesthesia is required for analysis of disease progression.

The etomidate analog ET-26 HCl retains superior myocardial performance: Comparisons with etomidate in vivo and in vitro

OBJECTIVE

(R)-2-methoxyethyl1-(1-phenylethyl)-1H-imidazole-5-carboxylate hydrochloride (ET-26 HCl) is a novel etomidate analogue. The purpose of this study was to characterize whether ET-26 HCl could retain the superior myocardial performance of etomidate in vivo and in vitro.

METHODS

In vivo, the influence of ET-26 HCl and etomidate on the cardiac function of dogs was confirmed using echocardiography and electrocardiogram. In vitro, a Langendorff preparation was used to examine direct myocardial performance in isolated rat hearts, and a whole-cell patch-clamp technique was used to study effects on the human ether-a-go-go-related gene (hERG) channel.

RESULTS

In vivo, after a single bolus administration of ET-26 HCl or etomidate, no significant difference in echocardiography and electrocardiogram parameters was observed. No arrhythmia occurred and no QT interval prolongation happened during the study period. In the in vitro Langendorff preparation, none of the cardiac parameters were abnormal, and the hERG recordings showed that ET-26 HCl and etomidate inhibited the tail current of the hERG in a concentration-dependent manner with an IC50 of 742.51 μM and 263.60 μM, respectively.

CONCLUSIONS

In conclusion, through an in vivo experiment and a whole organ preparation, the current study found that ET-26 HCl can maintain a myocardial performance that is similar to that of etomidate. In addition, the electrophysiology study indicated that ET-26 HCl and etomidate inhibited the hERG at a supra-therapeutic concentration.

Effects of Using Tricaine Methanesulfonate and Metomidate before Euthanasia on the Contractile Properties of Rainbow Trout (Oncorhynchus mykiss) Myocardium

Because many anesthetics work through depressing cell excitability, unanesthetized euthanasia has become common for research involving excitable tissues (for example muscle and nerve) to avoid these depressive effects. However, anesthetic use during euthanasia may be indicated for studies involving isolated tissues if the potential depressive effects of brief anesthetic exposure dissipate after subsequent tissue isolation, washout, and saline perfusion. We explore this here by measuring whether, when applied prior to euthanasia, standard immersion doses of 2 fish anesthetics, tricaine methanesulfonate (TMS; 100 mg/L, n = 6) and methyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate (metomidate, 10 mg/L, n = 6), have residual effects on the contractile properties (force and work output) of isolated and saline-perfused ventricular compact myocardium from rainbow trout (Oncorhynchus mykiss). Results suggest that direct exposure of muscle to immersion doses of TMS-but not metomidate-impairs muscle contractile performance. However, brief exposure (2 to 3 min) to either anesthetic during euthanasia only-providing that the agent is washed out prior to tissue experimentation-does not have an effect on the contractile properties of the myocardium. Therefore, the use of TMS, metomidate, and perhaps other anesthetics that depress cell excitability during euthanasia may be indicated when conducting research on isolated and rinsed tissues.

Nuclear imaging in the diagnosis of primary aldosteronism

PURPOSE OF REVIEW

Primary aldosteronism is increasingly recognized as a common secondary cause of hypertension. Successful demonstration of a unilateral cause (e.g. a classical 'Conn's adenoma') offers the potential for curative adrenalectomy. Adrenal vein sampling (AVS), in conjunction with cross-sectional imaging, remains the 'gold standard' for distinguishing unilateral and bilateral disease, but is technically demanding and frequently unsuccessful or inconclusive. As such, alternative strategies for lateralization, including nuclear medicine techniques, are being developed and brought into clinical practice.

RECENT FINDINGS

Metomidate, a potent ligand of CYP11B1 and CYP11B2, can be C11H3-labelled as a PET tracer and has been shown to offer a rapid noninvasive alternative to AVS for localizing unilateral aldosterone-producing adenomas.

SUMMARY

Increasing experience with 11C-metomidate PET-CT supports its use as an adjunct to AVS when this has failed, is ambiguous, or cannot be undertaken.

Conditioned place avoidance of zebrafish (Danio rerio) to three chemicals used for euthanasia and anaesthesia

Zebrafish are becoming one of the most used vertebrates in developmental and biomedical research. Fish are commonly killed at the end of an experiment with an overdose of tricaine methanesulfonate (TMS, also known as MS-222), but to date little research has assessed if exposure to this or other agents qualifies as euthanasia (i.e. a "good death"). Alternative agents include metomidate hydrochloride and clove oil. We use a conditioned place avoidance paradigm to compare aversion to TMS, clove oil, and metomidate hydrochloride. Zebrafish (n = 51) were exposed to the different anaesthetics in the initially preferred side of a light/dark box. After exposure to TMS zebrafish spent less time in their previously preferred side; aversion was less pronounced following exposure to metomidate hydrochloride and clove oil. Nine of 17 fish exposed to TMS chose not to re-enter the previously preferred side, versus 2 of 18 and 3 of 16 refusals for metomidate hydrochloride and clove oil, respectively. We conclude that metomidate hydrochloride and clove oil are less aversive than TMS and that these agents be used as humane alternatives to TMS for killing zebrafish.

[¹²³I]Iodometomidate imaging in adrenocortical carcinoma

CONTEXT

Imaging with [¹²³I]iodometomidate ([¹²³I]IMTO) has been shown to diagnose adrenocortical lesions with high sensitivity and specificity.

OBJECTIVE

Our objective was to evaluate the clinical utility of [¹²³I]IMTO imaging in adrenocortical carcinoma (ACC).

DESIGN

We conducted a prospective monocentric diagnostic study and a prospective case series at a single tertiary referral center.

PATIENTS AND INTERVENTIONS

Fifty-eight patients with histologically confirmed ACC, all European Network for the Study of Adrenal Tumors stage IV (with distant metastases), received 185 MBq [¹²³I]IMTO. Sequential planar whole-body scans until 24 hours post injection and single photon emission computed tomography/computed tomography (SPECT/CT) hybrid imaging 4 to 6 hours post injection were performed.

MAIN OUTCOME MEASURES

Outcome measures included uptake of [¹²³I]IMTO in ACC lesions, sensitivity and specificity of [¹²³I]IMTO imaging compared with conventional imaging, and number of patients eligible for [¹³¹I]IMTO therapy.

RESULTS

Of 430 lesions detected by conventional imaging, 30% showed strong, 8% moderate, and 62% no tracer accumulation. [¹²³I]IMTO detected both primary and metastatic lesions of ACC. However, a substantial percentage of lesions failed to show [¹²³I]IMTO uptake. The overall sensitivity and specificity values were 38% and 100%, respectively. Thirty-four patients (59%) had at least 1 [¹²³I]IMTO-positive lesion. Cortisol and aldosterone secretion by ACC was positively correlated to [¹²³I]IMTO uptake (P = .01); cytotoxic chemotherapy and mitotane treatment presumably did not influence tracer uptake. Twenty-one patients (36.2%) had radiotracer uptake in all lesions ≥ 2 cm and therefore were potential candidates for targeted systemic radiotherapy with [¹³¹I]IMTO.

CONCLUSION

About one-third of patients with ACC show specific retention of [¹²³I]IMTO in metastatic lesions. This study provides support for the conduct of a prospective trial to determine whether the first molecular informed therapy using [¹³¹I]IMTO will be of value to patients with metastatic ACC.

Functional characterization of adrenal lesions using [123I]IMTO-SPECT/CT

CONTEXT

Adrenal tumors are highly prevalent and represent a wide range of different pathological entities. Conventional imaging often provides only limited information on the origin of these lesions. Novel specific imaging methods are, therefore, of great clinical interest.

OBJECTIVE

We evaluated [(123)I]iodometomidate ([(123)I]IMTO) imaging for noninvasive characterization of adrenal masses.

DESIGN/SETTING

This was a prospective monocentric diagnostic study in a tertiary care center.

PATIENTS AND INTERVENTION

A total of 51 patients with an adrenal lesion underwent [(123)I]IMTO imaging after injection of 185 MBq of [(123)I]IMTO. Sequential planar whole-body scans until 24 hours postinjection and single photon emission computed tomography (SPECT)/computed tomography imaging 4 to 6 hours postinjection were performed.

MAIN OUTCOME MEASURE

Sensitivity and specificity of [(123)I]IMTO imaging for the noninvasive characterization of adrenal lesions were measured.

RESULTS

Adrenocortical tissue showed high and specific tracer uptake with a short investigation time and low radiation exposure. Qualitative analysis of SPECT/computed tomography data resulted in a sensitivity of 89% and a specificity of 85% for differentiating adrenocortical tumors from lesions of nonadrenocortical origin. Receiver-operating characteristic analysis of semiquantitative data revealed a sensitivity of 83% and a specificity of 86% for identification of adrenocortical lesions at a cutoff value of tumor to liver ratio of 1.3.

CONCLUSIONS

[(123)I]IMTO is a highly specific radiotracer for imaging of adrenocortical tissue with a short investigation time and low radiation exposure. Because of the general availability of SPECT technology, [(123)I]IMTO scintigraphy has the potential to become a widely used tool to noninvasively characterize the biology of adrenal lesions.

Effects of metomidate hydrochloride sedation on blood glucose and marketability of transported threespot gourami Trichogaster trichopterus

Our objectives were to determine whether sedation with metomidate hydrochloride (hereafter, "metomidate") during transportation of threespot gourami Trichogaster trichopterus would prevent an increase in blood glucose levels and improve fish marketability (i.e., based on appearance and behavior) in comparison with unsedated controls. Threespot gourami are obligate air-breathers that possess a labyrinth organ, enabling the fish to respire air above the water surface; these fish should be lightly sedated during transport. Fish were transported for approximately 24 h via truck and domestic airline. Blood was sampled at 0, 2, 6, and 12 h posttransport ation, and appearance and behavior were observed at 0, 2, 4, 6, and 12 h and 7 d posttransportation. Metomidate concentrations tested were 0.0 (control), 0.1, 0.2, 0.3, and 0.4 mg/L. At the concentrations tested, metomidate neither inhibited elevations in blood glucose nor improved marketability. Fish that were transported with 0.3-mg/L metomidate were less marketable based on behavioral indices, and fish that were transported with 0.4-mg/L metomidate had higher glucose levels than control fish. Use of metomidate as a transport sedative for threespot gourami should be considered with caution and may be problematic at the concentrations tested; however, further research examining additional indices of stress may clarify metomidate use for this species.

Pre-anaesthetic metomidate sedation delays the stress response after caudal artery cannulation in Atlantic cod (Gadus morhua)

Recovery from caudal artery cannulation with and without pre-anaesthesia metomidate sedation was assessed in Atlantic cod (Gadus morhua). The levels of plasma cortisol, glucose, electrolytes and acid-base parameters were compared between sedated and unsedated cod and to those in uncannulated individuals, where the samples were obtained by sacrificial sampling (reference level). Metomidate sedation delayed the stress response, causing sedated cod plasma cortisol to return to the reference level more slowly [day 4 post surgery (PS)] than in unsedated cod (day 2 PS). Plasma glucose was elevated in both sedated and unsedated cod up to and including day 5 PS. Plasma K(+) was lower and pH was higher in cannulated cod than in the reference from 24 h PS until the end of experimentation, indicating a stress effect of sacrificial sampling on plasma K(+) and pH that was likely caused by an acute stress response. Metomidate sedation delayed the stress response following CA cannulation and should therefore not be used as a pre-anaesthetic sedation in Atlantic cod. The caudal artery cannulation can be a useful tool in obtaining repeated blood samples from Atlantic cod given an adequate recovery time, which was determined to be 6 days irrespective of pre-anaesthesia sedation status.

[131I]iodometomidate for targeted radionuclide therapy of advanced adrenocortical carcinoma

CONTEXT

In advanced adrenocortical carcinoma (ACC), many patients have progressive disease despite standard treatment, indicating a need for new treatment options. We have shown high and specific retention of [123I]metomidate ([123I]IMTO) in ACC lesions, suggesting that labeling of metomidate with 131I offers targeted radionuclide therapy for advanced ACC.

OBJECTIVE

Safety and efficacy of radionuclide therapy with [131I]IMTO in advanced ACC.

DESIGN/SETTING

This monocentric case series comprised 19 treatments in 11 patients with nonresectable ACC.

PATIENTS AND INTERVENTION

Between 2007 and 2010, patients with advanced ACC not amenable to radical surgery and exhibiting high uptake of [123I]IMTO in their tumor lesions were offered treatment with [131I]IMTO (1.6-20 GBq in one to three cycles of [131I]IMTO).

MAIN OUTCOME MEASURE

Tumor response was assessed according to response evaluation criteria in solid tumors (RECIST version 1.1) criteria, and side effects were assessed by Common Toxicity Criteria (version 4.0).

RESULTS

Best response was classified as partial response in one case with a change in target lesions of -51% from baseline, as stable disease in five patients, and as progressive disease in four patients. One patient died 11 d after treatment with [131I]IMTO unrelated to radionuclide therapy. In patients responding to treatment, median progression-free survival was 14 months (range, 5-33) with ongoing disease stabilization in three patients at last follow-up. Treatment was well tolerated, but transient bone marrow depression was observed. Adrenal insufficiency developed in two patients.

CONCLUSIONS

Radionuclide therapy with [131I]IMTO is a promising treatment option for selected patients with ACC, deserving evaluation in prospective clinical trials.

Evaluation of the sensitivity and specificity of (11)C-metomidate positron emission tomography (PET)-CT for lateralizing aldosterone secretion by Conn's adenomas

CONTEXT

Identification of unilateral aldosterone-producing (Conn's) adenomas has traditionally required lateralization by the invasive and technically difficult procedure of adrenal vein sampling (AVS). (11)C-metomidate, a potent inhibitor of adrenal steroidogenic enzymes, is a positron emission tomography (PET) radiotracer that is selectively accumulated by Conn's adenomas.

OBJECTIVE

The objective of the study was to compare the sensitivity and specificity of (11)C-metomidate PET-computed tomography (CT) against the current gold standard of AVS.

DESIGN

The design of the study was within-patient comparison of diagnostic techniques.

SETTING

The study was conducted at a single center-university teaching hospital.

PATIENTS

Thirty-nine patients with primary hyperaldosteronism (PHA) and five with nonfunctioning adenomas (incidentalomas) participated in the study.

INTERVENTION(S)

The first six PHA patients were studied on three occasions to determine whether steroid pretreatment reduced (11)C-metomidate uptake by normal adrenal. Subsequent patients received dexamethasone for 3 d prior to injection of (11)C-metomidate 150-500 MBq.

MAIN OUTCOME MEASURE(S)

Maximum standardized uptake values (SUV(max)) over regions of interest determined from 35-45 min after injection were measured.

RESULTS

Dexamethasone increased tumor to normal adrenal SUV(max) ratio by 25.6 ± 5.0% (P < 0.01). PET-CT visualized subcentimeter adenomas and distinguished hot from cold adenomas within a gland. In 25 patients with PHA and AVS lateralization to the side of an adenoma, SUV(max) over tumor (mean ± sem) of 21.7 ± 1.6 was greater than over normal adrenal, 13.8 ± 0.6 (P = 0.00003); this difference was absent in 10 patients without lateralization on AVS (P = 0.28) and in four of five incidentalomas. On receiver-operator characteristics analysis, an SUV(max) ratio of 1.25:1 provided a specificity of 87% [95% confidence interval (69, 104)] and sensitivity of 76% (59, 93); in tumors with SUV(max) greater than 17, the specificity rose to 100%.

CONCLUSIONS

(11)C-metomidate PET-CT is a sensitive and specific noninvasive alternative to AVS in the management of PHA.

Functional imaging of neuroendocrine tumors

Neuroendocrine tumors (NET) have several distinct pathophysiological features that can be addressed by specific radiolabeled probes. An overview on the different radiopharmaceuticals that have been developed for positron emission tomography (PET) of NET are presented. The focus is on fluordeoxyglucose (F-18 FDG), biogenic amine precursors, somatostatin analogs, and hormone syntheses markers. Due to the highly specific tracers lacking any clear anatomical landmarking, the advantages of integrated functional and morphological imaging systems such as PET-CT are obvious. Based on the up to now published literature and one's own experience, it is concluded that amine precursors (e.g. fluor-dihydroxyphenylalanin and hydroxytryptophane) should be employed in most gastroenteropancreatic NET, whereas F-18 FDG should be preserved for more aggressive less-differentiated NETs. Hormone syntheses markers have up to now only been used in few centers and their broad clinical value remains uncertain. The different available somatostatin analogs are the most promising tracers, since they can improve dosimetry in cases where peptide receptor radiotherapies are planned. Of specific interest are the somatostatin analogs addressing several subtypes of the somatostatin receptor (e.g. DOTANOC) that allow detecting also subtypes not expressing the "classically" addressed subtype 2 and 5. Since NET have a high variety of different features, the individual diagnostic approach using PET or integrated PET-CT should be tailored, depending on the histological classification and the differentiation of the tumor.

Anesthesia induces stress in Atlantic salmon (Salmo salar), Atlantic cod (Gadus morhua) and Atlantic halibut (Hippoglossus hippoglossus)

Stress in response to anesthesia with benzocaine, MS-222, metomidate and isoeugenol was studied in Atlantic salmon (Salmo salar), Atlantic halibut (Hippoglossus hippoglossus), and Atlantic cod (Gadus morhua) with no concomitant stress from handling or confinement in association with anesthesia or sampling. All of the anesthetics tested induced a stress response in all species, displayed by a release of cortisol to the water. MS-222 anesthesia elicited the highest cortisol release rates, reaching maximum levels 0.5 h post-exposure and returning to basal levels after 3-4 h. Benzocaine anesthesia caused a bimodal response where the initial peak in cortisol release rate was followed by a second increase lasting towards the end of the trial (6 h). This bimodality was more profound in Atlantic salmon than in Atlantic halibut and Atlantic cod. Metomidate anesthesia induced the lowest release of cortisol of the agents tested in both Atlantic halibut and Atlantic cod, but resulted in a bimodal response in Atlantic salmon where the initial increase in cortisol release was followed by a larger increase peaking at 2-2.5 h post exposure before returning to basal levels after 5 h. The stress induced in Atlantic salmon by isoeugenol anesthesia resembled that of MS-222, but did not reach the same elevated level. Overall, the cortisol release was most profound in Atlantic salmon followed by Atlantic halibut and Atlantic cod.

Masked volume wise Principal Component Analysis of small adrenocortical tumours in dynamic [11C]-metomidate Positron Emission Tomography

BACKGROUND

In previous clinical Positron Emission Tomography (PET) studies novel approaches for application of Principal Component Analysis (PCA) on dynamic PET images such as Masked Volume Wise PCA (MVW-PCA) have been introduced. MVW-PCA was shown to be a feasible multivariate analysis technique, which, without modeling assumptions, could extract and separate organs and tissues with different kinetic behaviors into different principal components (MVW-PCs) and improve the image quality.

METHODS

In this study, MVW-PCA was applied to 14 dynamic 11C-metomidate-PET (MTO-PET) examinations of 7 patients with small adrenocortical tumours. MTO-PET was performed before and 3 days after starting per oral cortisone treatment. The whole dataset, reconstructed by filtered back projection (FBP) 0-45 minutes after the tracer injection, was used to study the tracer pharmacokinetics.

RESULTS

Early, intermediate and late pharmacokinetic phases could be isolated in this manner. The MVW-PC1 images correlated well to the conventionally summed image data (15-45 minutes) but the image noise in the former was considerably lower. PET measurements performed by defining "hot spot" regions of interest (ROIs) comprising 4 contiguous pixels with the highest radioactivity concentration showed a trend towards higher SUVs when the ROIs were outlined in the MVW-PC1 component than in the summed images. Time activity curves derived from "50% cut-off" ROIs based on an isocontour function whereby the pixels with SUVs between 50 to 100% of the highest radioactivity concentration were delineated, showed a significant decrease of the SUVs in normal adrenal glands and in adrenocortical adenomas after cortisone treatment.

CONCLUSION

In addition to the clear decrease in image noise and the improved contrast between different structures with MVW-PCA, the results indicate that the definition of ROIs may be more accurate and precise in MVW-PC1 images than in conventional summed images. This might improve the precision of PET measurements, for instance in therapy monitoring as well as for delineation of the tumour in radiation therapy planning.

Identification of binding sites in the nicotinic acetylcholine receptor for TDBzl-etomidate, a photoreactive positive allosteric effector

Etomidate, one of the most potent general anesthetics used clinically, acts at micromolar concentrations as an anesthetic and positive allosteric modulator of gamma-aminobutyric acid responses, whereas it inhibits muscle-type nicotinic acetylcholine receptors (nAChRs) at concentrations above 10 microm. We report here that TDBzl-etomidate, a photoreactive etomidate analog, acts as a positive allosteric nAChR modulator rather than an inhibitor, and we identify its binding sites by photoaffinity labeling. TDBzl-etomidate (>10 microm) increased the submaximal response to acetylcholine (10 microm) with a 2.5-fold increase at 60 microm. At higher concentrations, it inhibited the binding of the noncompetitive antagonists [(3)H]tetracaine and [(3)H]phencyclidine to Torpedo nAChR-rich membranes (IC(50) values of 0. 8 mm). nAChR-rich membranes were photolabeled with [(3)H]TDBzl-etomidate, and labeled amino acids were identified by Edman degradation. For nAChRs photolabeled in the absence of agonist (resting state), there was tetracaine-inhibitable photolabeling of amino acids in the ion channel at positions M2-9 (deltaLeu-265) and M2-13 (alphaVal-255 and deltaVal-269), whereas labeling of alphaM2-10 (alphaSer-252) was not inhibited by tetracaine but was enhanced 10-fold by proadifen or phencyclidine. In addition, there was labeling in gammaM3 (gammaMet-299), a residue that contributes to the same pocket in the nAChR structure as alphaM2-10. The pharmacological specificity of labeling of residues, together with their locations in the nAChR structure, indicate that TDBzl-etomidate binds at two distinct sites: one within the lumen of the ion channel (labeling of M2-9 and -13), an inhibitory site, and another at the interface between the alpha and gamma subunits (labeling of alphaM2-10 and gammaMet-299) likely to be a site for positive allosteric modulation.

[123 I]Iodometomidate for molecular imaging of adrenocortical cytochrome P450 family 11B enzymes

BACKGROUND

Due to advances in conventional imaging, adrenal tumors are detected with increasing frequency. However, conventional imaging provides only limited information on the origin of these lesions, which represent a wide range of different pathological entities. New specific imaging methods would therefore be of great clinical value. We, therefore, studied the potential of iodometomidate (IMTO) as tracer for molecular imaging of cytochrome P450 family 11B (Cyp11B) enzymes.

METHODS

Inhibition of Cyp11B1 and Cyp11B2 by IMTO, etomidate, metomidate, and fluoroetomidate was investigated in NCI-h295 cells and in Y1 cells stably expressing hsCyp11B1 or hsCyp11B2. Pharmacokinetics and biodistribution after iv injection of [(123/125)I]IMTO were analyzed in mice in biodistribution experiments and by small-animal single-photon emission computed tomography (SPECT). Furthermore, four patients with known adrenal tumors (two metastatic adrenal adenocarcinomas, one bilateral adrenocortical adenoma, and one melanoma metastasis) were investigated with [(123)I]iodometomidate-SPECT.

RESULTS

In cell culture experiments, all compounds potently inhibited both Cyp11B1 and Cyp11B2. Adrenals showed high and specific uptake of [(123/125)I]IMTO and were excellently visualized in mice. In patients, adrenocortical tissue showed high and specific tracer uptake in both primary tumor and metastases with short investigation time and low radiation exposure, whereas the non-adrenocortical tumor did not exhibit any tracer uptake.

CONCLUSION

We have successfully completed the development of an in vivo detection system of adrenal Cyp11B enzymes by [(123)I]IMTO scintigraphy in both experimental animals and humans. Our findings suggest that [(123)I]IMTO is a highly specific radiotracer for imaging of adrenocortical tissue. Due to the general availability of SPECT technology, we anticipate that [(123)I]IMTO scintigraphy may become a widely used tool to characterize adrenal lesions.

Identification of anesthetic binding sites on human serum albumin using a novel etomidate photolabel

We have synthesized a novel analog of the general anesthetic etomidate in which the ethoxy group has been replaced by an azide group, and which can be used as a photolabel to identify etomidate binding sites. This acyl azide analog is a potent general anesthetic in both rats and tadpoles and, as with etomidate, is stereoselective in its actions, with the R(+) enantiomer being significantly more potent than the S(-) enantiomer. Its effects on alpha1beta2gamma2s GABA(A) receptors expressed in HEK-293 cells are virtually indistinguishable from the parent compound etomidate, showing stereoselective potentiation of GABA-induced currents, as well as direct mimetic effects at higher concentrations. In addition, a point mutation (beta2 N265M), which is known to attenuate the potentiating actions of etomidate, also blocks the effects of the acyl azide analog. We have investigated the utility of the analog to identify etomidate binding sites by using it to photolabel human serum albumin, a protein that binds approximately 75% of etomidate in human plasma and which is thought to play a major role in its pharmacokinetics. Using HPLC/mass spectrometry we have identified two anesthetic binding sites on HSA. One site is the well-characterized drug binding site I, located in HSA subdomain IIA, and the second site is also an established drug binding site located in subdomain IIIB, which also binds propofol. The acyl azide etomidate may prove to be a useful new photolabel to identify anesthetic binding sites on the GABA(A) receptor or other putative targets.

Identification of a GABAA receptor anesthetic binding site at subunit interfaces by photolabeling with an etomidate analog

General anesthetics, including etomidate, act by binding to and enhancing the function of GABA type A receptors (GABA(A)Rs), which mediate inhibitory neurotransmission in the brain. Here, we used a radiolabeled, photoreactive etomidate analog ([(3)H]azietomidate), which retains anesthetic potency in vivo and enhances GABA(A)R function in vitro, to identify directly, for the first time, amino acids that contribute to a GABA(A)R anesthetic binding site. For GABA(A)Rs purified by affinity chromatography from detergent extracts of bovine cortex, [(3)H]azietomidate photoincorporation was increased by GABA and inhibited by etomidate in a concentration-dependent manner (IC(50) = 30 microm). Protein microsequencing of fragments isolated from proteolytic digests established photolabeling of two residues: one within the alphaM1 transmembrane helix at alpha1Met-236 (and/or the homologous methionines in alpha2,3,5), not previously implicated in etomidate function, and one within the betaM3 transmembrane helix at beta3Met-286 (and/or the homologous methionines in beta1,2), an etomidate sensitivity determinant. The pharmacological specificity of labeling indicates that these methionines contribute to a single binding pocket for etomidate located in the transmembrane domain at the interface between beta and alpha subunits, in what is predicted by structural models based on homology with the nicotinic acetylcholine receptor to be a water-filled pocket approximately 50 A below the GABA binding site. The localization of the etomidate binding site to an intersubunit, not an intrasubunit, binding pocket is a novel conclusion that suggests more generally that the localization of drug binding sites to subunit interfaces may be a feature not only for GABA and benzodiazepines but also for etomidate and other intravenous and volatile anesthetics.

Synthesis of trifluoromethylaryl diazirine and benzophenone derivatives of etomidate that are potent general anesthetics and effective photolabels for probing sites on ligand-gated ion channels

To locate the binding sites of general anesthetics on ligand-gated ion channels, two derivatives of the intravenous general anesthetic etomidate (2-ethyl 1-(phenylethyl)-1H-imidazole-5-carboxylate), in which the 2-ethyl group has been replaced by photoactivable groups based on either aryl diazirine or benzophenone chemistry, have been synthesized and characterized pharmacologically. TDBzl-etomidate (4-[3-(trifluoromethyl)-3H-diazirin-3-yl]benzyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate) and BzBzl-etomidate (4-benzoylbenzyl-1-(1-phenylethyl)-1H-imidazole-5-carboxylate are both potent general anesthetics with half-effective anesthetic concentrations of 700 and 220 nM, respectively. Both agents resembled etomidate in enhancing currents elicited by low concentrations of GABA on heterologously expressed GABAA receptors and in shifting the GABA concentration-response curve to lower concentrations. They also allosterically enhanced the binding of flunitrazepam to mammalian brain GABAA receptors. Both agents were also effective and selective photolabels, photoincorporating into some, but not all, subunits of the Torpedo nicotinic acetylcholine receptor to a degree that was allosterically regulated by an agonist or a noncompetitive inhibitor. Thus, they have the necessary pharmacological and photochemical properties to be useful in identifying the site of etomidate-induced anesthesia.

Rates of cortisol increase and decrease in channel catfish and sunshine bass exposed to an acute confinement stressor

Channel catfish and sunshine bass were exposed to a low-water stress event and allowed to recover in fresh water or a solution of metomidate (dl-1-(1-phenylethyl)-5-(metoxycarbonyl) imidazole hydrochloride), which inhibits the synthesis of cortisol. Change in time of plasma cortisol was used as an index of cortisol secretion and clearance. Plasma cortisol and glucose increased during the exposure to low-water stress in both fish, but the changes of both plasma components were more dramatic in sunshine bass. Exposure to metomidate during recovery resulted in a short-term increase in plasma glucose but differences between controls and metomidate-exposed fish were relatively minor thereafter. Cortisol began to decrease in catfish immediately after the removal of the stress but continued to increase for 15 min in sunshine bass recovering in fresh water and for 5 min in bass recovering in metomidate. Catfish recovering in fresh water had a cortisol elimination rate of -1.28 ng/mL/min compared with -2.45 ng/mL/min for fish recovering in metomidate (P>0.05) while sunshine bass recovering in fresh water had an elimination rate of -6.96 ng/mL/min compared with -4.50 ng/mL/min for fish recovering in metomidate (P>0.05). These data indicate that the rapid decrease of plasma cortisol after removal of the stressor is due to an almost immediate decrease of secretion, tissue uptake and a rapid renal loss due to the absence of a plasma binding protein.

[11C]metomidate positron emission tomography of adrenocortical tumors in correlation with histopathological findings

CONTEXT

Adrenal incidentalomas are common findings necessitating extensive laboratory work-up and repetitive radiological examinations. Positron emission tomography (PET) using (11)C-labeled metomidate (MTO) has previously been described as a tool for specific adrenocortical imaging.

OBJECTIVE

We evaluated 212 MTO-PET examinations in 173 patients to identify its role in the management of adrenal tumors.

DESIGN

Seventy-five histopathological examinations from 73 patients were retrospectively analyzed.

SETTING

All examinations were performed at a referral center.

PATIENTS

Patients who were operated or biopsied due to adrenal tumors had histopathological diagnoses of adrenocortical adenoma (n = 26), adrenocortical cancer (ACC; n = 13), adrenocortical hyperplasia (n = 8), pheochromocytoma (n = 6), metastasis (n = 3), and tumors of nonadrenal origin (n = 19).

MAIN OUTCOME MEASURES

The main outcome measures were statistical analyses and findings while scrutinizing images. The hypothesis that MTO-PET is of value in the management of adrenal tumors, especially incidentaloma, was stated before data collection.

RESULTS

Sensitivity was 0.89 and specificity was 0.96 for MTO-PET in proving adrenocortical origin of the lesions. Pheochromocytomas, metastases to the adrenal gland, and nonadrenal masses were all MTO negative. PET measurements using standardized uptake values (SUV) in pathological adrenocortical tissue could differentiate lesions larger than 1-1.5 cm from normal adrenocortical tissue. SUV was higher in aldosterone-hypersecreting adenomas, and the SUV ratio between the tumor and the contralateral gland was significantly higher in all hormonally hypersecreting adenomas as well as in ACC.

CONCLUSION

MTO-PET is a specific and sensitive method for diagnosing adrenocortical tumors. MTO-PET is useful in the imaging work-up of adrenal incidentalomas and may be beneficial for the examination of patients with primary aldosteronism or ACC.

R (+) etomidate and the photoactivable R (+) azietomidate have comparable anesthetic activity in wild-type mice and comparably decreased activity in mice with a N265M point mutation in the gamma-aminobutyric acid receptor beta3 subunit

A photoactivable diazirine derivative of etomidate, azietomidate, shares many actions of etomidate, including a capacity to abolish the righting reflexes in tadpoles and enhance gamma-aminobutyric acid (GABA)-induced currents. Azietomidate's usefulness in studies of mechanisms of anesthesia depends on the assumption that it shares a site of action with etomidate. Mice bearing an N265M beta3 subunit point mutation in GABA(A) receptors have a markedly decreased sensitivity to loss of righting reflexes induced by etomidate over a range of doses. Accordingly, in the present study we measured the time to recovery of righting reflexes of wild type and mutant mice as a function of dose given as an IV bolus. Analysis of the data for azietomidate yielded mean times to recovery of righting reflexes at a dose of 7.5 mg/kg of 10.0 +/- 0.9 min and 3.0 +/- 1.6 min for wild type and mutant mice, respectively (mean +/- sd). A similar analysis for etomidate yielded mean times to recovery of righting reflexes at a dose of 7.5 mg/kg of 12.0 +/- 1.3 min and 4.0 +/- 0.7 min for wild type and mutant mice respectively. Thus, at this dose a single mutation, N265M on the beta3 subunit of the GABA(A) receptor, approximately halved the time to recovery of righting reflexes for both etomidate and azietomidate (by 7.6 +/- 1.5 min and 7.2 +/- 1.8 min, respectively), emphasizing the contribution of this residue as a determinant of a behavioral response of azietomidate in mice.

Positron emission tomography imaging of adrenal masses: (18)F-fluorodeoxyglucose and the 11beta-hydroxylase tracer (11)C-metomidate

PURPOSE

(11)C-metomidate (MTO), a marker of 11beta-hydroxylase, has been suggested as a novel positron emission tomography (PET) tracer for adrenocortical imaging. Up to now, experience with this very new tracer is limited. The aims of this study were (1) to evaluate this novel tracer, (2) to point out possible advantages in comparison with( 18)F-fluorodeoxyglucose (FDG) and (3) to investigate in vivo the expression of 11beta-hydroxylase in patients with primary aldosteronism.

METHODS

Sixteen patients with adrenal masses were investigated using both MTO and FDG PET imaging. All patients except one were operated on. Five patients had non-functioning adrenal masses, while 11 had functioning tumours(Cushing's syndrome, n=4; Conn's syndrome, n=5; phaeochromocytoma, n=2). Thirteen patients had benign disease, whereas in three cases the adrenal mass was malignant (adrenocortical cancer, n=1; malignant phaeochromocytoma, n=1; adrenal metastasis of renal cancer, n=1).

RESULTS

MTO imaging clearly distinguished cortical from non-cortical adrenal masses (median standardised uptake values of 18.6 and 1.9, respectively, p<0.01). MTO uptake was slightly lower in patients with Cushing's syndrome than in those with Conn's syndrome, but the difference did not reach statistical significance. The expression of 11beta-hydroxylase was not suppressed in the contralateral gland of patients with Conn's syndrome, whereas in Cushing's syndrome this was clearly the case. The single patient with adrenocortical carcinoma had MTO uptake in the lower range.

CONCLUSION

MTO could not definitely distinguish between benign and malignant disease. FDG PET, however, identified clearly all three study patients with malignant adrenal lesions. We conclude: (1) MTO is an excellent imaging tool to distinguish adrenocortical and non-cortical lesions; (2) the in vivo expression of 11beta-hydroxylase is lower in Cushing's syndrome than in Conn's syndrome, and there is no suppression of the contralateral gland in primary aldosteronism; (3) for the purpose of discriminating between benign and malignant lesions, FDG is the tracer of choice.

Imaging of adrenal incidentalomas with PET using (11)C-metomidate and (18)F-FDG

Our aim was to evaluate the use of PET with (11)C-metomidate and (18)F-FDG for the diagnosis of adrenal incidentalomas.

METHODS

Twenty-one patients underwent hormonal screening before dynamic imaging of the upper abdomen with (11)C-metomidate, and for 19 of these 21 patients, static (18)F-FDG imaging followed. Uptake of (11)C-metomidate and (18)F-FDG in incidentalomas was quantified and correlated with the hormonal work-up and the mass size on CT (median, 2.5 cm; range, 2-10 cm).

RESULTS

The final diagnoses were hormonally active adenoma (n = 7), nonsecretory adenoma (n = 5), adrenocortical carcinoma (n = 1), pheochromocytoma (n = 2), benign noncortical tumor (n = 2), normal adrenal (n = 1), and malignant noncortical tumor (n = 3). Diagnosis was established at surgery (n = 9), percutaneous biopsy (n = 4), or follow-up (n = 8). The highest uptake of (11)C-metomidate, expressed as standardized uptake value (SUV), was found in adrenocortical carcinoma (SUV = 28.0), followed by active adenomas (median SUV = 12.7), nonsecretory adenomas (median SUV = 12.2), and noncortical tumors (median SUV = 5.7). Patients with adenomas had significantly higher tumor-to-normal-adrenal (11)C-metomidate SUV ratios than did patients with noncortical tumors. (18)F-FDG detected 2 of 3 noncortical malignancies but failed to detect adrenal metastases from renal cell carcinoma. All inactive and most active adenomas were difficult to detect with (18)F-FDG against background activity, whereas both pheochromocytomas and adrenocortical carcinoma showed slightly increased uptake of (18)F-FDG. There was no correlation between uptake of (11)C-metomidate or (18)F-FDG and mass size.

CONCLUSION

(11)C-Metomidate is a promising PET tracer to identify incidentalomas of adrenocortical origin. (18)F-FDG should be reserved for patients with a moderate to high likelihood of neoplastic disease.

Identification of binding sites in the nicotinic acetylcholine receptor for [3H]azietomidate, a photoactivatable general anesthetic

To identify binding domains in a ligand-gated ion channel for etomidate, an intravenous general anesthetic, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with a photoactivatable analog, [(3)H]azietomidate. Based upon the inhibition of binding of the noncompetitive antagonist [(3)H]phencyclidine, azietomidate and etomidate bind with 10-fold higher affinity to nAChRs in the desensitized state (IC(50) = 70 microm) than in the closed channel state. In addition, both drugs between 0.1 and 1 mm produced a concentration-dependent enhancement of [(3)H]ACh equilibrium binding affinity, but they inhibited binding at higher concentrations. UV irradiation resulted in preferential [(3)H]azietomidate photoincorporation into the nAChR alpha and delta subunits. Photolabeled amino acids in both subunits were identified in the ion channel domain and in the ACh binding sites by Edman degradation. Within the nAChR ion channel in the desensitized state, there was labeling of alphaGlu-262 and deltaGln-276 at the extracellular end and deltaSer-258 and deltaSer-262 toward the cytoplasmic end. Within the acetylcholine binding sites, [(3)H]azietomidate photolabeled alphaTyr-93, alphaTyr-190, and alphaTyr-198 in the site at the alpha-gamma interface and deltaAsp-59 (but not the homologous position, gammaGlu-57). Increasing [(3)H]azietomidate concentration from 1.8 to 150 microm increased the efficiency of incorporation into amino acids within the ion channel by 10-fold and in the ACh sites by 100-fold, consistent with higher affinity binding within the ion channel. The state dependence and subunit selectivity of [(3)H]azietomidate photolabeling are discussed in terms of the structures of the nAChR transmembrane and extracellular domains.

Comparison of three different purification methods for the routine preparation of [11C] Metomidate

PET with (R)-[O-methyl-11C] metomidate ([11C] MTO) is an attractive method for the characterisation of adrenal masses discriminating lesions of adrenal cortical origin from noncortical lesions. [11C] MTO was prepared by the reaction of [11C] methyliodide with the corresponding free acid. Three purification methods have been compared. The method of choice uses preparative HPLC with a ready-to-use weak acidic solvent.

Pharmacokinetic and pharmacodynamic properties of metomidate in turbot (Scophthalmus maximus) and halibut (Hippoglossus hippoglossus)

Metomidate was administered to halibut (Hippoglossus hippoglossus) and turbot (Scophthalmus maximus) intravenously at a dose of 3 mg/kg bodyweight, as a bath treatment at a dose of 9 mg/L water for 5 min to study the disposition of metomidate, and as bath treatment (9 mg/L) for 10 min to study the absorption and effect of metomidate on respiration and balance/motor control. Additionally, turbot were given metomidate orally at a dose of 7 mg/kg. The studies were performed in seawater at a temperature of 10.3 +/- 0.4 degrees C (halibut) and 18.0 +/- 0.3 degrees C (turbot). Pharmacokinetic modeling of the data showed that metomidate had shorter elimination half-life and higher plasma concentrations in turbot compared with halibut, both species displaying a rapid uptake, distribution and excretion. Following intravenous administration, the volumes of distribution at steady state (Vd(ss)) were 0.21 L/kg (halibut) and 0.44 L/kg (turbot). Plasma clearances (Cl) were 0.099 L/h.kg in halibut and 0.26 L/h.kg in turbot and the elimination half-lives (t(1/2)lambdaz) were calculated to be 5.8 h and 2.2 h in halibut and turbot, respectively. Mean residence times (MRT) were 2.2 h in halibut and 1.7 h in turbot. Following oral administration, the t(1/2)lambdaz was 3.5 h in turbot. The maximum plasma concentration (Cmax) was 7.8 mg/L in turbot 1 h after administration. The oral bioavailability (F) was calculated to 100% in turbot. Following 5 min bath the maximum plasma concentrations (Cmax), which were observed immediately after end of the bath, were 9.5 mg/L and 13.3 mg/L in halibut and turbot, respectively. Metomidate rapidly immobilized the fish, with respiratory depression, reduced heart rate, and loss of balance/motor control within 1 min (mean). Recovery was slow, with resumed balance/motor control after 26.4 min. Opercular respiration movements were resumed more rapidly with a recorded mean of 1.7 min. Oral administration was demonstrated to be a way of immobilizing fish, for example in large aquariums, without exposing them to unwanted stress.

2-(3-Methyl-3H-diaziren-3-yl)ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate: a derivative of the stereoselective general anesthetic etomidate for photolabeling ligand-gated ion channels

To locate general anesthetic binding sites on ligand-gated ion channels, a diazirine derivative of the potent intravenous anesthetic, R-(+)-etomidate (2-ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate), has been synthesized and characterized. R-(+)-Azietomidate [2-(3-methyl-3H-diaziren-3-yl)ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate] anesthetizes tadpoles with an EC(50) of 2.2 microM, identical to that of R-(+)-etomidate. At this concentration both agents equally enhanced GABA-induced currents and decreased binding of the caged-convulsant [(35)S]TBPS to GABA(A) receptors. In all of the above actions R-(+)-azietomidate is about an order of magnitude more potent than S-(-)-azietomidate, an enantioselectivity comparable to etomidate's. R-(+)-Azietomidate also inhibits acetylcholine-induced currents in nicotinic acetylcholine receptors, with about twice the potency of the parent compound. [(3)H]Azietomidate photoincorporated into Torpedo nicotinic acetylcholine receptor-rich membranes. Desensitization decreased photoincorporation into the delta-subunit and increased that into the alpha-subunit. The latter increase was confined to a proteolytic fragment containing the first three transmembrane segments. Thus, R-(+)-azietomidate is a potent stereoselective general anesthetic and an effective photolabel.

11C-metomidate PET imaging of adrenocortical cancer

The diagnostic potential of positron emission tomography (PET) with carbon-11 metomidate in patients with adrenocortical cancer (ACC) was evaluated. Thirteen PET examinations were performed in 11 patients with CT-detected primary tumours or recurrence and/or metastases from a previously histopathologically proven ACC. The findings at PET were compared with those at CT and verified by histopathology. Six studies (group A) were performed in patients who at the time of imaging were free of medication that could interfere with 11beta-hydroxylase activity and thereby tracer uptake at PET, such as adrenal steroid inhibitors or chemotherapy. The remaining seven studies (group B) were carried out in patients who were monitored during treatment with one or a combination of these drugs. PET visualised all viable tumours with high tracer uptake, and revealed two more lesions than were seen on CT. Three necrotic tumours were detected as false negative observations, as confirmed at surgery and histopathological examination. A true negative observation was obtained at PET in the case of a suspected liver metastasis on CT that was found to have fat vacuolation at histopathological examination of an ultrasonically guided core biopsy specimen. Group A showed apparently higher uptake in normal tissues than group B (adrenal, P=0.03; liver, P=0.01). The metomidate uptake was increased in tumour lesions as compared with normal tissues (adrenal, P=0.02; liver, P=0.005). ACC could be clearly visualised with (11)C-metomidate PET except when the tumour was necrotic. Medication with adrenal steroid inhibitors and chemotherapy decreased the tracer uptake.

The role of PET in localization of neuroendocrine and adrenocortical tumors

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

PET imaging of adrenal cortical tumors with the 11beta-hydroxylase tracer 11C-metomidate

The purpose of the study was to evaluate PET with the tracer 11C-metomidate as a method to identify adrenal cortical lesions.

METHODS

PET with 11C-metomidate was performed in 15 patients with unilateral adrenal mass confirmed by CT. All patients subsequently underwent surgery, except 2 who underwent biopsy only. The lesions were histopathologically examined and diagnosed as adrenal cortical adenoma (n = 6; 3 nonfunctioning), adrenocortical carcinoma (n = 2), and nodular hyperplasia (n = 1). The remaining were noncortical lesions, including 1 pheochromocytoma, 1 myelolipoma, 2 adrenal cysts, and 2 metastases.

RESULTS

All cortical lesions were easily identified because of exceedingly high uptake of 11C-metomidate, whereas the noncortical lesions showed very low uptake. High uptake was also seen in normal adrenal glands and in the stomach. The uptake was intermediate in the liver and low in other abdominal organs. Images obtained immediately after tracer injection displayed high uptake in the renal cortex and spleen. The tracer uptake in the cortical lesions increased throughout the examination. For quantitative evaluation of tracer binding in individual lesions, a model with the splenic radioactivity concentration assigned to represent nonspecific uptake was applied. Values derived with this method, however, did show the same specificity as the simpler standardized uptake value concept, with similar difference observed for cortical versus noncortical lesions.

CONCLUSION

PET with 11C-metomidate has the potential to be an attractive method for the characterization of adrenal masses with the ability to discriminate lesions of adrenal cortical origin from noncortical lesions.

[Differential diagnosis in adrenal gland tumors using PET and 11C-metomidate]

We have investigated 13 patients with incidentaloma using PET with 11C-metomidate. The patients included six adrenocortical adenomas, one adrenal cancer, two pheochromocytomas, one myelolipoma, one benign cyst and two metastases. A very high tracer uptake was observed in all tumors originating from the adrenal cortex, allowing excellent visualisation, whereas all other processes were negative. PET with 11C-metomidate is a very promising method for the characterisation of incidentalomas and is suggested to be included very early in the evaluation of these patients.

[Ban of the use of metomidate (Hypnodil) in swine. Background, consequences and alternatives]

Since the beginning of 1997 the use of the hypnotic drug metomidate (Hypnodil) in swine is not longer allowed. This ban caused a substantial therapeutic deficit for anesthesia in swine. The ban is based on provisions of the European Council Directive 2377/90 for fixing maximum residue limits (MRL) of veterinary drugs in animal-derived food and due to the fact that the pharmaceutical industry has not submitted any application for determining an MRL for metomidate. According to the regulations of the German drug law only those substances can be used as alternatives which are authorized for use in swine or another food-producing animal species. Registrations for use in swine exist for the barbiturates thiamylal and narcobarbital, for ketamine and for the neuroleptics azaperone, acepromazine and propionylpromazine. In the exceptional case of therapeutic emergency levomethadone or xylazine can be used. The administration of propofol, thiopentone, opioids (except levomethadone) or benzodiazepines (except brotizolam) to swine is not allowed since these drugs are authorized only for use in humans or companion animals. At present the most appropriate alternative for anesthesia in swine is thiamylal after premedication of azaperone and ketamine inducing general anesthesia with sufficient tolerance at low risks. In current studies general anesthesia with tolerance could further be produced by combinations of levomethadone, ketamine and azaperone or xylazine. Combined use of levomethadone with benzodiazepines appeared to be not suitable in swine.

Anaesthetics for general anaesthesia in growing pigs

A comparison was made between different anaesthetics for general anaesthesia in growing pigs, with focus on minor surgery under field conditions and for experiments in clinical research. Healthy cross-breed pigs (Hampshire x Yorkshire x Swedish Landrace) weighing 20-45 kg were used. The anaesthetics combinations compared were 1) azaperone plus metomidate (AM), 2) Zoletil (zolazepam + tiletamine) plus xylazine (ZX), and 3) Zoletil plus xylazine plus ketamine (ZXK). Parameters measured were: heart rate, respiratory rate, blood pressure, body temperature, and depth of analgesia (pin-prick). Minor surgery was performed to test the reliability of the "pin-prick" tests. It was clearly shown that AM produces anaesthesia with good cardiovascular stability and is a drug combination that is suitable for minor surgery. ZX also produces a good anaesthesia characterized by reliable and rapid induction. Good cardiovascular function is maintained, and the laryngeal relaxation makes intubation possible. These characteristics are very useful in a laboratory environment, as easy handling to avoid stress is necessary for research. Although it is difficult to evaluate the quality of analgesia from this study, it is concluded that ZX did not provide a superior anaesthesia and analgesia compared to AM in crossbreed pigs. However, these drugs are too expensive for regular use in ambulatory practice. The effects of ZXK resemble those of ZX, but the ZXK-drug combination has no anaesthetic advantages and is more laborious to work with.

[The action of amphiphilic drugs on human fibroblasts in vitro. Their possible use in antiviral and antitumor chemotherapy]

The LDH cytotoxicity test is able to emphasize the stabilisation of human embryo fibroblast membranes, in vitro, by two amphiphilic drugs: the metomidate and the thymolol. The membrane stabilisation is emphasized by the reduced LDH externalization as compared with the untreated cells. The use of the LDH test for the selection of some natural complexes or synthetic drugs with membrane stabilising and potential antiviral activity is proposed. The inhibition of LDH by metomidate was recorded, as decreasing of the enzyme activity with increasing metomidate concentration. This fact explains the known lactic acid accumulation, under metomidate treatment of human subjects. The use of metomidate in classical cancer treatment potentiation is proposed instead of lactate externalization inhibitors, already used for this aim.