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Short communication [18F]FETO for adrenocortical PET imaging: a pilot study in healthy volunteers Wolfgang Wadsak1, 2, Markus Mitterhauser1, 3, 4, Gundula Rendl1, Matthias Schuetz1, Leonhard Key Mien1, 3, 5, Dagmar E. Ettlinger1, Robert Dudczak1, 6, Kurt Kletter1, Georgios Karanikas1 1

Department of Nuclear Medicine, Medical University of Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria Department of Inorganic Chemistry, University of Vienna, Vienna, Austria 3 Department of Pharmaceutical Technology and Biopharmaceutics, University of Vienna, Vienna, Austria 4 Hospital Pharmacy of the General Hospital of Vienna, Vienna, Austria 5 Department of Psychiatry, Medical University of Vienna, Vienna, Austria 6 Ludwig-Boltzmann-Institute for Nuclear Medicine, Vienna, Austria 2

Received: 14 September 2005 / Accepted: 12 December 2005 / Published online: 28 March 2006 © Springer-Verlag 2006

Abstract. Purpose: Functional imaging of the adrenal cortex by means of PET may play an important clinical role. Recently, we presented the synthesis and first evaluation of a novel 11β-hydroxylase inhibitor, [18F] FETO, in rats displaying high tracer accumulation in the adrenals. In this study, we aimed to investigate for the first time the potency of [18F]FETO as a PET tracer for the adrenal cortex in humans. Methods: An average preparation yielded 1–2 GBq of [18F]FETO ready to use. Ten healthy volunteers aged 24–57 years (five male and five female) were included in the study. After i.v. administration of 365 MBq [18F] FETO (246–391 MBq), dynamic images were acquired in 2D standard mode in 14 frames over 45 min. Afterwards, whole-body scanning was performed. In addition to visual interpretation, semi-quantitative analysis using standardised uptake values (SUVs) was conducted. Results: [18F]FETO distribution was similar in all scanned volunteers. Visually, pronounced accumulation of [18F] FETO was found in the adrenals, whereas moderate uptake was observed—at least in some of the subjects— for liver, renal calices, gallbladder, stomach walls and pancreas. Kidney and bowels showed only faint uptake. Median SUVs for the right and left adrenal glands were 15.6 (10.0–28.6) and 15.7 (10.3–35.9), respectively. The reference tissue (liver) displayed a median SUV of 2.5 (2.2–4.6). Conclusion: [18F]FETO is a valuable tracer for adrenocortical PET imaging, combining the longer half-life of 18F Wolfgang Wadsak ()) Department of Nuclear Medicine, Medical University of Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria e-mail: [email protected] Tel.: +43-1-404005255, Fax: +43-1-404005251

with a high 11β-hydroxylase selectivity. In accordance with our findings in rats, FETO PET revealed very high accumulation in the adrenal glands in healthy volunteers. Keywords: 11β-hydroxylase – Adrenal cortex – Fluorine-18 – Metomidate – Etomidate Eur J Nucl Med Mol Imaging (2006) 33:669–672 DOI 10.1007/s00259-005-0062-6

Introduction 11β-Hydroxylase (CYP11B1, P45011β), a key enzyme in the biosynthesis of adrenocortical steroid hormones, is a suitable target for the in vivo imaging of the adrenal cortex. [11C]metomidate (MTO) and [11C]etomidate (ETO), potent inhibitors of this enzyme, were presented for the first time in 1998 [1]. A first evaluation using positron emission tomography (PET) followed shortly thereafter [2]. Recently, the value of MTO PET in imaging adrenal masses and metastatic tumours of adrenocortical origin, as compared with FDG PET, was demonstrated [3–5]. However, the application of this promising tracer is still limited, mainly owing to its short half-life (20 min). To allow longer imaging protocols and also distribution of the tracer to PET centres without a cyclotron, we recently developed an analogue of MTO labelled with 18F, 2-[18F] fluoroethyl-desethyl-(R)-etomidate (FETO) [6]. In vitro binding studies revealed increased affinity for 11β-hydroxylase for FETO as compared with MTO (FETO IC50=3.0 nM, MTO IC50=3.69 nM) [7]. In rats, we recently demonstrated that FETO accumulates exclusively in the adrenals [7.52% injected dose per gram (ID/g)] whereas no significant uptake was observed in any of the other tissues

European Journal of Nuclear Medicine and Molecular Imaging Vol. 33, No. 6, June 2006

670 Fig. 1. The reaction scheme for the preparation of [18F]FETO starting from cyclotron produced [18F]fluoride in a two-step radiosynthesis

1.

cyclotron:

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O (p,n) 18 F K 2CO3 / Kryptofix (K2.2.2 ) Acetonitrile 100ºC, 10min

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Br TfO

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+ O + TBA

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Synthesis of FETO FETO was synthesised as previously reported [6]. Briefly, cyclotronproduced [18F]fluoride was treated with the aminopolyether kryptofix 2.2.2 and potassium carbonate, azeotropically dried in the presence of acetonitrile and finally reacted with 2-bromoethyltriflate for 10 min at 100°C to give 2-bromo-1-[18F]fluoroethane ([18F] BFE). Purification of the intermediate substrate was achieved via distillation. Subsequently, purified [18F]BFE was reacted with activated (R)-1-(1-phenylethyl)-1H-imidazole-5-carboxylic acid in DMF for 20 min at 135°C. The crude reaction mixture was purified using Waters SepPak C18plus solid phase extraction cartridges in an automated synthesis module. A reaction scheme is presented in Fig. 1.

O N

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[ F]BFE

Materials and methods

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[8]. Metabolic stability of FETO was inferred from absence of uptake in bone (defluorination) and kidney (ester cleavage with renal excretion), and the tracer was found to undergo hepatobiliary clearance. Additionally, FETO was proven to exert a modulatory effect on the GABA (γaminobutyric acid) receptor in the rat brain. Based on the previous findings regarding (1) the binding affinities towards 11β-hydroxylase and (2) our biodistribution experiments in rats, we aimed to investigate whether the expected exclusive uptake in the adrenals could also be observed in humans.

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FETO PET imaging All study patients underwent FETO PET imaging on a dedicated full ring PET scanner (Advance, General Electric Medical Systems, Milwaukee, WI, USA). FETO imaging was performed in all patients following the same protocol. A 10-min transmission scan was performed for attenuation correction of subsequent emission scans, and thereafter a median activity of 365 MBq FETO was administered intravenously (range 246–391 MBq). A 1-min scout scan one field of view (FOV: 14.5 cm) downwards from the xiphoid was used to locate the adrenal glands. Subsequently, the adrenals were positioned centrally within the FOV for the dynamic study. All patients were injected already positioned within the PET scanner. Dynamic images were acquired in two-dimensional standard mode with a matrix size of 128×128 according to a previously published protocol; acquisition was performed in 14 frames over 45 min (frames 1–5, 60 s; frames 6– 10, 180 s; frames 11–13, 300 s; frame 14, 600 s). Following the dynamic study, an attenuation whole-body scan from the symphysis to the head was performed (equal to a [18F]FDG whole-body scan). Scan time was 5 min per step, and acquisition was performed in twodimensional standard mode with a matrix size of 128×128. The images were reconstructed using an iterative algorithm and a 6-mm Hanning filter. In addition to visual interpretation of both the dynamic and the summed images, semi-quantitative analysis was performed. Standardised uptake values (SUVs) were calculated by drawing regions of interest over the right and left adrenal glands. In addition, an upper region in the liver was used as the reference region. Maximal SUVs of each region of interest are given. Since the aorta could not be visualised satisfactorily, the kinetic study was performed by calculating adrenal gland-to-liver ratios (ALRs) for all subjects over 35 min.

Results Healthy subjects

Synthesis of FETO Ten healthy volunteers were investigated using FETO PET imaging (median age 28 years, range 24–57 years, five males and five females). The study was approved by the Ethical Committee of the Medical University of Vienna, and the study participants gave written informed consent to their participation. Inclusion criteria were: (1) normal laboratory findings with regard to adrenocortical pathologies and (2) no irregularities sonographically detectable in the abdomen. Exclusion criteria were: (1) pregnancy, (2) breast-feeding, (3) inability to lie motionless for 1 h and (4) participation in other studies including a radiation burden during the last 12 months. Pregnancy was excluded by a pregnancy test performed before tracer application.

The synthesis and quality control were finished in 100 min from EOB using standard PET equipment. It is noteworthy that no semi-preparative high-performance liquid chromatography (HPLC) purification of the final product is required—in contrast to the preparation of MTO [9]. The radiochemical purity of FETO assessed by radioHPLC and radio-thin-layer chromatography was 99.5±0.5%, and total amounts of radioactivity up to 3.3 GBq (1.55±0.71%) with a specific activity of 42–214 GBq/μmol were achieved. Typical activity concentrations ranged from 50 to 200 MBq/ml.


671 Fig. 2. FETO PET scintigraphic findings. Images show the distribution of FETO in a 31-year-old male (74 kg) after administration of 365 MBq [18F] FETO (a, b) and in a 25-year-old female (58 kg) after administration of 333 MBq [18F]FETO (c, d). a and c represent OSEM reconstructed whole-body images (50–95 min p.i.). b and d show summed images of the dynamic scans (frames 1–14, 0–45 min p.i.)

FETO distribution was similar in all scanned volunteers. Visually, pronounced accumulation of FETO in all subjects was only found in the adrenals. In some scans the renal calices (9/10), stomach wall (7/10), gallbladder (6/10) and pancreas (3/10) were clearly outlined. Furthermore, the liver, which was used as the reference tissue, showed moderate uptake whereas kidney and bowels showed only faint uptake in all subjects. Typical images are presented in Fig. 2. As early as 3 min post tracer administration, the adrenal glands were clearly visible in all subjects, and accumulation of FETO occurred until the acquisition was terminated. The liver had a median SUV of 2.5 (range 2.2–4.6), and the median SUV of the adrenal glands was 15.7 (range 10.0– 35.9). All adrenal glands could be easily delineated visually. The kinetic study (Fig. 3) revealed a decreasing ALR in the initial phase (3 min p.i.) followed by a steady increase of the imaging contrast for the rest of the study. A satisfactory ratio for the visualisation of the adrenals (ALR>2) is reached 12–15 min post tracer administration. Discussion Since functional imaging of the 11β-hydroxylase expression and its alterations in the adrenal cortex may play an

important role in clinical routine, the demand for a [18F] fluorinated tracer recently resulted in the synthesis of [18F] FETO [6]. First evaluations in rats demonstrated a high selectivity for the adrenals and therefore suggested that FETO might be safely used in human PET imaging [8]. The data of the present first human PET study are in accordance with the nanomolar affinity previously found in binding assays [7] and with our biodistribution experiments in rats [8]. The investigations in the animal model

6.0 5.0 4.0 ALR

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Fig. 3. Adrenal gland-to-liver ratios (ALRs). The ALR is a measure for the visual contrast between adrenals and the reference region (liver). All values represent arithmetic means; error bars indicate standard deviation (n=10)


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were a good predictor for the human distribution, the two studies revealing almost exclusive uptake in the adrenals. Since the structural differences between FETO and MTO are minuscule, we expected a similar behaviour in vivo. Nevertheless, FETO PET visually showed a tissue distribution different from that observed in our prior MTO PET study [3]: After just 3 min the adrenal glands were clearly visible. Furthermore, interfering uptake in liver, stomach, bowels, kidneys and spleen was reduced using FETO instead of MTO. Uptake in the bladder was low and did not interfere with the delineation of the adrenals. Especially, the significantly reduced uptake in the liver and stomach of FETO compared with MTO is advantageous for routine PET imaging of the adrenal cortex. 6β-[131I]Iodomethyl-norcholesterol (NCL-6-I) has been widely used in nuclear medicine [10] but FETO is clearly advantageous in terms of its practicability: (1) the radiation burden for the patient and the staff is significantly reduced; (2) imaging can be performed a lot earlier (20 min vs 5 days), simplifying the patient management and (3) the PET technique is far superior to planar scintigraphy with a 131Ilabelled radiopharmaceutical. Since PET is evidently preferable to conventional imaging and FETO might differentiate cortical from non-cortical lesions in a similar manner to MTO, it could become an interesting tracer for scintigraphy of the adrenal gland when tested in a clinical setting. MTO was shown to be suitable for differentiating cortical from non-cortical lesions [2, 3] but unable to evaluate the status (benign vs malignant) of incidentalomas [3]. These results pointed to the possibility that longer imaging protocols—as are possible with FETO—could enhance small differences in various entities to allow further clinical applications. Conclusion From the present investigation we conclude that [18F] FETO is a valuable tracer for adrenocortical PET imaging. It combines the long half-life of 18F with high 11βhydroxylase selectivity. As expected from the preclinical evaluation, very high accumulation in the adrenal glands was also observed in healthy volunteers. Hence, FETO PET may become an additional tool in the clinical evaluation of adrenal pathologies.

Acknowledgements. This work was partly funded by the Dr. PfeifferStipendium 2004 of the Austrian Society of Nuclear Medicine (OGN). The authors thank the staff of the Viennese PET centre—I. Leitinger, B. Reiterits, G. Wagner and R. Bartosch—for their commitment to this study. This project was partially funded by the ‘Jubilaeumsfonds der Oesterreichischen Nationalbank’ (project number: 8263).”

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