Eur J Nucl Med Mol Imaging (2014) 41:1947–1956 DOI 10.1007/s00259-014-2782-y
ORIGINAL ARTICLE
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C-ORM-13070, a novel PET ligand for brain α2C-adrenoceptors: radiometabolism, plasma pharmacokinetics, whole-body distribution and radiation dosimetry in healthy men Pauliina Luoto & Sami Suilamo & Vesa Oikonen & Eveliina Arponen & Semi Helin & Jukka Herttuainen & Johanna Hietamäki & Aila Holopainen & Marita Kailajärvi & Juha M. Peltonen & Juha Rouru & Jukka Sallinen & Mika Scheinin & Jere Virta & Kirsi Virtanen & Iina Volanen & Anne Roivainen & Juha O. Rinne
Received: 18 December 2013 / Accepted: 7 April 2014 / Published online: 17 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose 11C-labelled 1-[(S)-1-(2,3-dihydrobenzo[1,2]dioxin2-yl)methyl]-4-(3-methoxy-methylpyridin-2-yl)-piperazine (11C-ORM-13070) is a novel PET tracer for imaging of α2Cadrenoceptors in the human brain. Brain α2C-adrenoceptors may be therapeutic targets in several neuropsychiatric disorders, including depression, schizophrenia and Alzheimer’s disease. To validate the use of 11C-ORM-13070 in humans, we investigated its radiometabolism, pharmacokinetics, whole-body distribution and radiation dose. Electronic supplementary material The online version of this article (doi:10.1007/s00259-014-2782-y) contains supplementary material, which is available to authorized users. P. Luoto : S. Suilamo : V. Oikonen : E. Arponen : S. Helin : J. Virta : K. Virtanen : A. Roivainen : J. O. Rinne (*) Turku PET Centre, University of Turku and Turku University Hospital, FI-20521 Turku, Finland e-mail:
[email protected] S. Suilamo Department of Oncology and Radiotherapy, Turku University Hospital, Turku, Finland J. Herttuainen : J. Hietamäki : A. Holopainen : J. Rouru : J. Sallinen Orion Pharma, Espoo and Turku, Finland M. Kailajärvi Turku Imanet, GE Healthcare, Turku, Finland J. M. Peltonen : M. Scheinin : I. Volanen : J. O. Rinne CRST, University of Turku, Turku, Finland J. O. Rinne Unit of Clinical Pharmacology, TYKSLAB, Turku, Finland J. O. Rinne Division of Clinical Neurosciences, University of Turku and Turku University Hospital, Turku, Finland
Methods Radiometabolism was studied in a test–retest setting in six healthy men. After intravenous injection of 11C-ORM-13070, blood samples were drawn over 60 min. Plasma samples were analysed by radio-HPLC for intact tracer and its radioactive metabolites. Metabolite-corrected plasma time–activity curves were used for calculation of pharmacokinetics. In a separate group of 12 healthy men, the whole-body distribution of 11CORM-13070 and radiation exposure were investigated by dynamic PET/CT imaging without blood sampling. Results Two radioactive metabolites of 11C-ORM-13070 were detected in human arterial plasma. The proportion of unchanged 11C-ORM-13070 decreased from 81±4 % of total radioactivity at 4 min after tracer injection to 23±4 % at 60 min. At least one of the radioactive metabolites penetrated into red blood cells, while the parent tracer remained in plasma. The apparent elimination rate constant and corresponding half-life of unchanged 11C-ORM-13070 in arterial plasma were 0.0117±0.0056 min−1 and 73.6±35.8 min, respectively. The organs with the highest absorbed doses were the liver (12 μSv/MBq), gallbladder wall (12 μSv/MBq) and pancreas (9.1 μSv/MBq). The mean effective dose was 3.9 μSv/MBq, with a range of 3.6 – 4.2 μSv/MBq. Conclusion 11C-ORM-13070 was rapidly metabolized in human subjects after intravenous injection. The effective radiation dose of 11C-ORM-13070 was in the same range as that of other 11C-labelled brain receptor tracers. An injection of 500 MBq of 11C-ORM-13070 would expose a subject to 2.0 mSv of radiation. This supports the use of 11C-ORM13070 in repeated PET scans, for example, in receptor occupancy trials with novel drug candidates. Keywords α2C-Adrenoceptor . Antagonist . Carbon-11 . Dosimetry . PET . Radiometabolism . Whole-body distribution
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Eur J Nucl Med Mol Imaging (2014) 41:1947–1956
Introduction
Subjects
α2C-Adrenoceptors in the brain are potential therapeutic targets in several neuropsychiatric disorders, including depression, schizophrenia and Alzheimer’s disease [1]. Their physiological functions and pharmacological effects in the brain are still incompletely understood, but as they are involved in the regulation of the neuronal release of noradrenaline (autoinhibition) and many other neurotransmitters such as γ-aminobutyric acid, acetylcholine, dopamine and serotonin (as presynaptic heteroreceptors), they may play an important ‘fine-tuning’ role in the modulation of brain neurotransmission [1, 3, 4]. 11 C-Labelled 1-[(S)-1-(2,3-dihydrobenzo[1,2]dioxin-2yl)methyl]-4-(3-methoxy-methylpyridin-2-yl)-piperazine (11CORM-13070) is a novel PET ligand for the in vivo imaging of α2C-adrenoceptors in the human brain. Preclinical studies of 11CORM-13070 in rats and mice have revealed high and specific uptake in the striatum and olfactory tubercle, i.e. brain regions known to contain relatively high densities of α2C-adrenoceptors [2, 5–9]. Selectivity for α2C-adrenoceptors, as opposed to α2Aadrenoceptors that are more abundant in the mouse brain than α2C-adrenoceptors and have a different anatomical distribution, has been demonstrated in mouse lines with targeted inactivation of the genes encoding these receptor subtypes [9]. The objectives of the present study were: to map the wholebody biodistribution of intravenously (i.v.) administered 11CORM-13070 in humans, for use in the assessment of its safety as a PET tracer; to determine the effective radiation dose and the radiation exposure of critical target organs; and to evaluate the plasma pharmacokinetics and radiometabolism of 11CORM-13070. This information is essential for the validation of 11C-ORM-13070 for human use.
The whole-body distribution and radiation dosimetry of 11CORM-13070A were investigated by PET/CT imaging in 12 healthy male volunteers (mean±SD age 27±6 years, range 21 – 41 years; weight 75±5 kg). The radiometabolism and plasma pharmacokinetics of i.v. administered 11C-ORM13070 were investigated in another six healthy male volunteers (age 26±7 years, range 21 – 41 years; weight 78±6 kg) subjected to two PET imaging experiments within a single day. In this test–retest setting, the subjects underwent two brain PET imaging sessions with 11C-ORM-13070 with at least a 3-h interval between the tracer injections (i.e. 20 min, nine times the physical half-life of 11C). The study protocol was reviewed and approved by the Ethics Committee of Southwest Finland Hospital District and by the National Agency for Medicines, Finland. All subjects gave their written informed consent before participation. The study was registered as a clinical trial (NCT00735774).
Materials and methods Chemicals and reagents The desmethyl precursor, (2-{4-[(S)-1-(2,3dihydrobenzo[1,2]dioxin-2-yl)methyl]piperazin-1-yl}pyridin-3-yl) methanol, ORM-13333, and the reference compound, ORM-13070, were obtained from Orion Pharma (Espoo, Finland). All other reagents were purchased from commercial suppliers and were either synthesis grade or analytical grade and were used without further purification.
Preparation of 11C-ORM-13070 11
C-ORM-13070 was synthesized by 11C-methylation of Odesmethyl-ORM-13070 with 11C-methyl triflate prepared from 11C-methane via 11C-methyl iodide [2].
Whole-body PET/CT imaging The biodistribution of 11C-ORM-13070 (496±20 MBq, range 463 – 536 MBq; 0.50±0.16 μg, range 0.30 – 0.72 μg) was investigated using a whole-body PET/CT scanner (Discovery VCT; GE Medical Systems, Milwaukee, WI) [10]. The scanner is a combined 64-slice CT and PET camera containing 24 rings of bismuth germanate detectors, and acquires 47 imaging planes with an axial field of view (FOV) of 15.7 cm. The scanner was calibrated using a body phantom prior to the study. In three subjects who underwent whole-body scanning (from head to thigh), a whole-body PET scan was started at the same time as tracer injection. Seven bed positions (covering 100 cm) were used to scan the body from mid-brain to the thighs. Fourteen time frames were collected within 2 h with increasing bed position times (15 s in frames 1 – 4, 30 s in frames 5 – 7, 60 s in frames 8 – 10 and 120 s in frames 11 – 14). The camera was used in two-dimensional (2-D) mode for the first 25 min (the first seven frames) and then in 3-D mode. The basic performance characteristic tests on this camera indicated full-width at half-maximum of 5 – 6 mm. Prior to injection of 11C-ORM-13070, a low-dose CT scan was performed for attenuation correction. PET images were reconstructed with an iterative, ordered-subsets expectation maximization method with two iterations and 20 subsets for the 2-D scans and two iterations and 28 subsets for the 3-D scans. Scatter correction, random counts and dead time corrections were incorporated into the reconstruction algorithms. The reconstruction matrices used were 256×256 with a transverse FOV of 70 cm. Dynamic PET scans were performed in another nine subjects over limited anatomical regions with only one bed position. Three subjects were scanned over each of the following
Eur J Nucl Med Mol Imaging (2014) 41:1947–1956
regions: the neck including salivary glands, the abdomen including the liver, gallbladder and parts of the bowel, and the lower abdomen including the pelvic area and urinary bladder. 3-D dynamic scans were performed with the following time frames: 10×1 min and 16×5 min. The total scan time was 90 min. A low-dose CT scan was used for attenuation correction, and the PET images were reconstructed in the same way as the whole-body 3-D images. In the first three subjects with whole-body scans, quantitative analysis was performed by delineating volumes of interest (VOIs) on transverse planes with iPlan RT Image software version 4.1 (BrainLab AG, Feldkirchen, Germany). For the dynamic regional PET scans, Xeleris software (GE Healthcare, Milwaukee, WI) was used to define VOIs. The total radioactivity of the source organ at each time point was calculated from the radioactivity concentration (becquerels per millilitre) of the VOI by multiplying it with the volume of the source organ. The volumes of the source organs were either defined from the CT images or the volumes were values of the Reference Male’s organs [11]. Estimation of human radiation dose The residence times of the source organs were calculated from the areas under the time–activity curves. The curves were fitted with two-exponential or three-exponential functions in a least-squares manner and plotted as percent of the injected radioactivity. To calculate the residence time for the remainder of the body, the sum of the residence times of all source organs was subtracted from the theoretical maximum value; t½/ ln(2)=0.49 h, where t½ is the physical half-life of the 11C radionuclide (0.33 h). No model for excretion was used because of the short half-life of the radionuclide. Doses were estimated in specified organs and in the whole body using OLINDA/EXM 1.0 software (Vanderbilt University, Nashville, TN) which applies the MIRD scheme [12,13]. The model for a 70-kg adult male was used. In the whole-body scans, residence times and dose estimates were defined for all organs listed in Table 1. In the dynamic regional PET scans, values were defined for those organs that were completely or partly within the scanned region. Because of limited information on the radioactivity distribution in the dynamic regional PET scans, effective total body doses were calculated only in those subjects who underwent scanning of the abdomen. Many critical organs were located in the abdominal scanning region, and calculation of the effective dose was therefore achievable with high accuracy.
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47 MBq of 11C-ORM-13070 (range 362 – 537 MBq; 2.5± 1.5 μg, range 0.7 – 5.3 μg of ORM-13070). Before injection, a blood sample was taken for determination of haematocrit (HCT). To obtain an input function, arterial radioactivity concentrations were measured using an online detector (ABSS; Allogg AB, Mariefred, Sweden) for the first 5 min after injection covering the time of peak radioactivity in the blood. Thereafter, arterial blood samples (2 mL) were collected manually at 6, 8, 10, 15, 20, 30, 45, 60, 75 and 90 min after injection and radioactivity was measured with an automated gamma counter (1480 Wizard 3; Perkin Elmer, Turku, Finland). In addition, arterial blood samples were obtained at 4, 10, 20, 30, 45 and 60 min after injection and processed for high-performance liquid chromatography (radio-HPLC) analysis as described below. The online detector and the automated gamma counter were cross-calibrated with the PET camera. Blood samples (2 mL) were collected into heparinized tubes and plasma was obtained by centrifugation at 2,100×g for 5 min at 4 °C. Plasma proteins were precipitated with acetonitrile (500 μL plasma+700 μL acetonitrile) containing 10 μg of “cold” ORM-13070 as an internal standard. An aliquot of the supernatant (1 mL) was analysed with radioHPLC as described below. The radio-HPLC system consisted of a LaChrom L-7100 pump (Merck Hitachi, Darmstadt, Germany), L-7400 UV detector and D-7000 interface, an online radioactivity detector (Radiomatic 150TR flow scintillation analyser; Packard, Meriden, CT) and a computerized data acquisition system. RadioHPLC was performed using a μ-Bondapak® C-18 column (125 Å, 10 μm, 7.8×300 mm; Waters, Milford, MA) at a flow rate of 6.0 mL/min and a gradient of acetonitrile (A) and phosphoric acid, 50 mmol/L (B) as follows: 15 % A and 85 % B at 0 – 3 min, 60 % A and 40 % B at 3 – 8 min, and 15 % A and 85 % B at 8 – 10 min. The peaks of radioactivity were integrated, and intact 11C-ORM-13070 was identified by the retention time of non-radioactive ORM-13070 with UV detection at 246 nm. A Hill-type function was fitted simultaneously to the fractions of parent tracer (f parent ) and the two observed radiometabolites (fmetab1 and fmetab2): 8 > > > > > > > > >
> > > > > > > > =
f metab1 ðt Þ ¼ E > > > C þ ðt − DÞB > > > > B > > > > > ð t − D Þ > > A > > > > : f metab2 ðt Þ ¼ ð1 − E Þ B; C þ ðt − DÞ
Blood analyses in the context of brain PET imaging For dynamic PET imaging of the brain (procedures and results to be reported separately; Virta J et al., manuscript in preparation), the subjects received an i.v. bolus injection of 482±
These functions enabled calculation of the plasma concentration curves for the parent tracer and both radiometabolites. These were used to model the impact of a possible brainpenetrating radiometabolite. D is the delay time, which was
1950 Table 1 Residence times and radiation dose estimates for 11 C-ORM-13070
a b
SD/mean×100
Organs were delineated in subjects who underwent dynamic scanning of the abdomen. In these subjects, effective doses were estimated using the values of the delineated organs and the remainder of the body
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Source organ
No. of subjects
Adrenals Brain Breasts Cortical bone Gallbladderb Gallbladder wall Heart (contents) Heart wallb Kidneysb Liverb Lower large intestine (contents)
3 6 3 3 6 6 3 5 6 6 6
Lower large intestine wall Lungs Muscle Osteogenic cells Ovaries Pancreasb Red marrowb Skin Small intestine (contents)b Small intestine wall Spleenb Stomach (contents)b Stomach wall Testes Thymus Thyroid Trabecular bone Upper large intestine (contents)b
6 3 6 3 3 6 9 3 6 6 6 6 6 3 3 3 3 6
Upper large intestine wall Urinary bladder (contents) Urinary bladder wall Uterus Remainder (whole-body scans) Remainder (neck scans) Remainder (abdomen scans)b Remainder in body (lower abdomen scans) Total body Effective dose equivalent Effective dose
6 6 6 3 3 3 3 3
first fitted as a free parameter, but for the final analysis it was constrained to the median of all experiments. When t
