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There are presently no reliable ways to quantify human pancreatic beta cell mass (BCM) in vivo, which prevents an accurate understanding of the progressive beta cell loss in diabetes or following islet transplantation. Furthermore, the lack of beta cell imaging hampers the evaluation of the impact of new drugs aiming to prevent beta cell loss or to restore BCM in diabetes. We presently discuss the potential value of BCM determination as a cornerstone for individualized therapies in diabetes, describe the presently available probes for human BCM evaluation, and discuss our approach for the discovery of novel beta cell biomarkers, based on the determination of specific splice variants present in human beta cells. This has already led to the identification of DPP6 and FXYD2ga as two promising targets for human BCM imaging, and is followed by a discussion of potential safety issues, the role for radiochemistry in the improvement of BCM imaging, and concludes with an overview of the different steps from pre-clinical validation to a first-in-man trial for novel tracers.

Purpose: The aim of this study was to optimize the radiolabeling method of [ 18 F]fluoropropyl-(+)-dihydrotetrabenazine ([ 18 F]FP-(+)-DTBZ) to fulfill the demand of preclinical and clinical application., Procedures: Optimized labeling conditions were performed by altering the molar ratio of precursor to base (P/B), base species, solvents, reaction temperature, reaction time, and precursor concentration through manual radiosynthesis of [ 18 F]FP-(+)-DTBZ. The conditions with the highest radiochemical yield (RCY) were applied to automated radiosynthesis, and the crude product was purified with a Sep-Pak Plus C18 cartridge. Quality control and stability of [ 18 F]FP-(+)-DTBZ were carried out by HPLC. In vitro cellular uptake and blocking assays were conducted in human neuroblastoma cell line SH-SY5Y. In vivo imaging with small animal positron emission tomography (microPET) was performed with Sprague-Dawley rats., Results: Under the optimized conditions (P/K 2 CO 3  = 1:8, heating at 120 °C for 3 min in dimethyl sulfoxide), an RCY of 88.7 % was obtained with 1.0 mg precursor. The optimized reaction conditions were successfully applied to an automated module and gave a high activity yield (AY) of 30-55 % in about 40 min with a > 99.0 % radiochemical purity (RCP) and a > 44.4 GBq/μmol molar activity (A m ). Stability test displayed that the RCP retained > 98.0 % in 8 h in saline and in phosphate buffer saline (PBS, pH 7.4). In vitro cellular uptake assay showed accumulation of [ 18 F]FP-(+)-DTBZ in SH-SY5Y cells, which could be significantly inhibited by vesicular monoamine transporter 2 (VMAT2) inhibitor DTBZ. MicroPET images of rat brain displayed that the striatum showed the highest uptake with a standardized uptake value (SUV) of 3.91 ± 0.30 at ~ 70 min. Co-injection with DTBZ (1.0 mg/kg) resulted in a 75 % decrease of the striatal SUV, confirming the specificity of [ 18 F]FP-(+)-DTBZ to VMAT2., Conclusions: We obtained an optimized radiolabeling method of [ 18 F]FP-(+)-DTBZ and successfully applied it to a commercial available module. The automated synthesis gave a high AY and RCP of [ 18 F]FP-(+)-DTBZ with high and specific binding to VMAT2, facilitating its routine application for VMAT2 tracing.

Objectives: Recently, a deuterated tracer, D6-[ 18 F]FP-(+)-DTBZ, 9-O-hexadeutero-3-[ 18 F]fluoropropoxyl-(+)-dihydrotetrabenazine ([ 18 F]9), targeting vesicular monoamine transporter 2 (VMAT2) in the central nervous system, was reported as a useful imaging agent for the diagnosis of Parkinson's disease (PD). The production of [ 18 F]9 was optimized and simplified by using solid-phase extraction (SPE) purification., Methods: Three major nonradioactive impurities were synthesized and characterized. The preparation of [ 18 F]9 was optimized by using different labeling conditions, and an SPE purification method was evaluated. The influence of chemical impurities in the final dose of [ 18 F]9 was assessed by an in vitro binding assay, an assay of the in vivo biodistribution in mice, and ex vivo and in vitro autoradiography of brain sections., Results: Optimized fluorination conditions for [ 18 F]9 were found - heating at 130 °C for 10 min in DMSO, and a high radiochemical yield and three major chemical impurities were observed. An SPE method involving a Sep-Pak® tC18 Plus Light cartridge with a two-step elution process was successfully implemented. This process gave a good radiochemical yield (38.7 ± 10.5%, decay corrected; radiochemical purity >99%) and low chemical impurities. An in vivo biodistribution study and autoradiography of brain sections showed that there was no significant difference between HPLC-purified and SPE-purified [ 18 F]9., Conclusion: A VMAT2 targeting imaging agent, D6-[ 18 F]FP-(+)-DTBZ, [ 18 F]9, was prepared by optimized labeling conditions and an easy SPE purification. This method offers a short preparation time and operational simplicity. In conjunction with PET imaging, this new VMAT2 agent might be a useful clinical tool for diagnosing PD., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Cassette dosing is also known as N-in-One dosing: several compounds are simultaneously administrated to a single animal and then the samples are rapidly detected by LC-MS/MS. This approach is a successful strategy to enhance the efficiency of drug discovery and reduce animal usage. However, no report on the utility of the cassette approach in radiotracer discovery has appeared in the literature. This study designed a cassette microdose with LC-MS/MS method to enhance the throughput for screening radiopharmaceutical biodistribution in the rat brain directly. Three unradiolabeled compounds (FPBM FPBM2 and AV-133) were chosen as model drugs administrated intravenously to the rats as a cassette as opposed to discrete study. The rat brain biodistribution data, target localization, the differential uptake ratio (%ID/g) and the brain tissue-specific binding ratio were obtained by the LC-MS/MS analysis. These data matched very well with the values obtained by the standard radioactivity measurements. Moreover, no significant differences between discrete dosing and cassette dosing were observed. By circumventing the need for radiolabeled molecules, this method may be high-throughput and safe for the research and development of new PET imaging agents. The combination of cassette microdosing and LC-MS/MS would be a medium throughput screening tool at an early stage in the discovery/development process of PET imaging agents., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Background: The feasibility of beta cell mass (BCM) imaging and quantification with positron emission tomography (PET) in the pancreas is controversial. In an effort to shed some light on this topic, we have used a xenograft model of rat insulinoma (RIN) in mice, mimicking an intramuscular islet transplantation situation., Methods: A total of 105 RIN cells were subcutaneously implanted in nude mice (N.=8). Tumor size and glycaemia levels were determined daily. Rat C-peptide was measured to demonstrate rat insulin production. PET imaging with 11C-(+)-α-dihydrotetrabenazine (11C-DTBZ) was done at 3 and 4 weeks and compared with 18F-FDG and 18F-DOPA studies in the same mice. Ex-vivo autoradiography with 11C-DTBZ was carried out in frozen sections of tumors. VMAT2 expression was measured by Western-blot and immunohistochemistry in tumors and RIN cells., Results: Functional rat insulin production in mice was demonstrated by substantial decrease in glycaemia (<50 mg/dL by week 4) and rat C-peptide levels (7.2±2.6 ng/mL) similar to those measured in control rats. PET studies showed that tumor imaging with 11C-DTBZ at four (N.=8) and five (N.=5) weeks was negative; only bigger tumors could be seen with 18F-DOPA. In explanted tumors 11C-DTBZ autoradiography was negative, albeit VMAT2 expression measured by Western-blot and immunohistochemistry was lower than in cultured RIN cells., Conclusions: Although insulinomas are fully functional it does not seem feasible to use 11C-DTBZ for in-vivo measuring of BCM. This might either be due to inherent technical limitations of PET, decrease in VMAT2 expression in the tumors due to unknown reasons, or other biological limiting facts.

In the early 1990s, 9-(+)-11C-dihydrotetrabenazine (9-(+)-11C-DTBZ) was shown to be a useful positron emission tomography (PET) imaging agent for various neurodegenerative disorders. Here, we described the radiosynthesis and evaluation of the 9-(+)-11C-DTBZ analog, 10-(+)-11C-DTBZ, as a vesicular monoamine transporter 2 (VMAT2) imaging agent and compare it with 9-(+)-11C-DTBZ. 10-(+)-11C-DTBZ was obtained by 11C-MeI methylation with its 10 hydroxy precursor in the presence of 5 M NaOH. It had a slightly better average radiochemical yield of 35.3 ± 3.6% (decay-corrected to end of synthesis (EOS)) than did 9-(+)-11C-DTBZ (30.5 ± 2.3%). MicroPET studies showed that 10-(+)-11C-DTBZ had a striatum-to-cerebellum ratio of 3.74 ± 0.21 at 40 min post-injection, while the ratio of 9-(+)-11C-DTBZ was 2.50 ± 0.33. This indicated that 10-(+)-11C-DTBZ has a higher specific uptake in VMAT2-rich brain regions, and 10-(+)-11C-DTBZ may be a potential VMAT2 radioligand. Our experiment is the first study of 10-(+)-11C-DTBZ to include dynamic brain distribution in rat brains., Competing Interests: The authors have declared that no competing interests exist.

Dihydrotetrabenazine (DTBZ) derivatized from (+) Tetrabenazine (TBZ) has been used for imaging the expression of VMAT2 when labeled with (11)C (t1/2=20.3 min) or (18)F (t1/2=110 min) in neurodegenerative diseases or pancreatic beta-cell. Because (11)C or (18)F radiolabels are only available in the proximity of a biomedical cyclotron facility, here we report our work of derivatizing (+) and (-) DTBZ using a (64)Cu-specific bifunctional chelator scaffold ((64)Cu: t1/2=12.7 h) for the preparation of long-lived VMAT2 targeted radiotracers, (64)Cu-CB-TE2A-(+)-DTBZ and (64)Cu-CB-TE2A-(-)-DTBZ. The specific VMAT2 binding affinity of (64)Cu-CB-TE2A-(+)-DTBZ measured using rat brain homogenate or porcine islets was not compromised by our chemical modifications while that of its (-) counterpart remained low as in (11)C or (18)F labeled (±) DTBZ., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

A novel series of analogs of 2-amino-dihydrotetrabenazine derivatives, 4-6, targeting the vesicular monoamine transporter have been prepared. In vitro binding was carried out in tissue homogenates prepared from rat striatal tissue homogenates with both [(125)I]-iodovinyl-TBZ and [(3)H]DTBZ. There was a good correlation (r(2)=0.925) between the affinities of the different compounds for [(125)I]-iodovinyl-TBZ and [(3)H]-DTBZ binding. Compound 5 exhibited a better affinity for the vesicular monoamine transporter (K(i)=8.68+/-1.26 nM and 7.01+/-0.07 nM, respectively), which may be a good lead compound for further structural modification to develop useful probes for VMAT2.