Developments toward the Implementation of 44Sc Production at a Medical Cyclotron

44Sc has favorable properties for cancer diagnosis using Positron Emission Tomography (PET) making it a promising candidate for application in nuclear medicine. The implementation of its production with existing compact medical cyclotrons would mean the next essential milestone in the development of this radionuclide. While the production and application of 44Sc has been comprehensively investigated, the development of specific targetry and irradiation methods is of paramount importance. As a result, the target was optimized for the 44Ca(p,n)44Sc nuclear reaction using CaO instead of CaCO3, ensuring decrease in target radioactive degassing during irradiation and increased radionuclidic yield. Irradiations were performed at the research cyclotron at the Paul Scherrer Institute (~11 MeV, 50 µA, 90 min) and the medical cyclotron at the University of Bern (~13 MeV, 10 µA, 240 min), with yields varying from 200 MBq to 16 GBq. The development of targetry, chemical separation as well as the practical issues and implications of irradiations, are analyzed and discussed. As a proof-of-concept study, the 44Sc produced at the medical cyclotron was used for a preclinical study using a previously developed albumin-binding prostate-specific membrane antigen (PSMA) ligand. This work demonstrates the feasibility to produce 44Sc with high yields and radionuclidic purity using a medical cyclotron, equipped with a commercial solid target station.

Upon exposure to air, CaO undergoes hydration through adsorption of moisture, yielding 23 calcium hydroxide and subsequent carbonation (which involves CO2 fixation) to form calcium 24 carbonate. The net reaction of this process can be represented by: 25 26 (Eq. 1)

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The hydration process is fast, while the following carbonation process is slow, due to the 29 difference in water and CO2 concentration in air [1]. This indicates that, when using CaO as target 30 material, further measures have to be taken in order to prevent hydration and carbonation.

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The severity of such measures was investigated by exposing prepared CaO powder to air and 33 measuring X-ray Diffraction (XRD) spectra at different exposure times (0 h, 4 h, 16 h; Figure S1). Peaks 34 corresponding to the diffraction pattern of Ca(OH)2 can be identified in the spectra after 16 h 35 exposure. The data, however, indicates that the overall crystallinity of the material decreases over 36 time, as can be seen in the spectra recorded after 4 h. This could mean amorphisation of the present 37 phases (CaO and Ca(OH)2) and/or a formation of an amorphous CaCO3 phase. Such secondary phase 38 formations lead to changes of the crystal structure and result in volume changes (density ρCaO > ρCaCO3 39 > ρCa(OH)2). This has negative effects on prepared pellets from CaO powder, leading to cracking and 40 breaking when exposed to air. Taking this into consideration, directly after conversion of CaCO3 the 41 resultant CaO powder was pressed into target pellets, while the exposure time was kept to a 42 minimum (≤ 1 h). The resultant targets were encapsulated and stored under inert gas or in a desiccator 43 under vacuum.

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Molecules 2020, 25, x FOR PEER REVIEW 2 of 9 45 Figure S1. X-ray diffraction patterns of CaO powder exposed to air at different time points.

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Target holders used for irradiations at the Bern medical cyclotron only seal completely when 49 pressed together with 6 bars of pressure within the target station. Outside the target station, held 50 together only by the magnets of the "coin", the target holder exhibits a gap between front cover and 51 back part, as confirmed by three-dimensional measurements using a KEYENCE VR-3000 G2 52 profilometer ( Figure S2). It is important, therefore, to store such encapsulated targets under inert 53 conditions to prevent secondary phase formations as indicated by XRD ( Figure S3). Disastrous results

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were observed when such measures were not taken in the preparation of targets ( Figure S4).

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The first irradiation was performed at the Bern medical cyclotron using CaCO3 pellets. This 73 irradiation had to be stopped after a few minutes, at beam currents of about 1 µA, due to an air 74 contamination radiation safety alarm provoking the shutdown of the ventilation in the cyclotron 75 bunker. Figure S5 shows the measurement of air contamination at the main exhaust of the facility on 76 the day irradiation. The signals (shown as broad peaks), due to 41 Ar produced in air by neutrons via 77 the 40 Ar(n,γ) 41 Ar nuclear reaction during routine 18 F production, are clearly visible together with the 78 huge peak due to the 44 Sc production test. The peak can be explained by the fact that, due to the 79 increase of temperature, CaCO3 decomposed to CaO and CO2, leading to high pressure inside the

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The use of CaO pellets proved to be successful also from a radiation protection point-of-view.

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The radioactive gas released by the target was found to be far from the alarm values and had little 92 influence on the total amount of radioactivity released in the atmosphere by the whole facility, as 93 shown in Figure S6.

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The amount of released radioactivity in form of 13 N was found to be comparable to 41 Ar 101 produced during 18 F runs, thus, causing no problem from a radiation safety perspective. As shown 102 in Figure S6, a peak was observed after EOB due to the release of radioactive gas as soon as the piston 103 pressing the target in the solid target station was released. For this reason, a conservative waiting 104 period of about 30 minutes after EOB (corresponding to about three 13 N half-lives) was adopted 105 before releasing the piston to reduce the release of radioactive gas into the air of the bunker.

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It was found that the reading of the beam current on target and on the 12 mm diameter collimator 109 was not correct over the irradiation periods 2 to 5 (Table 2). This was due to a faulty connection 110 leading to a bad centering of the beam on the pellet. In some cases, only the tail of the beam was 111 hitting the target material, thus, producing low yields. In order to evaluate where the beam hit the

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It can be seen that the beam was not correctly centered for runs 3, 4 and 5. These experimental

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For some of the irradiations performed at the IBA cyclotron, problems were encountered in 124 removing the irradiated target from the target holder. In these cases (production runs 3 -5), the target 125 holder was placed directly into the 1.0 M HNO3 solution and the target dissolved. This led to a 126 leaching of Sm from the samarium-cobalt magnets (part of the target holder sealing mechanism) 127 ( Figure S8) [3]. This resulted in a poor radiolabeling capability of the final product (see Table 2

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DOTANOC peptide (2 µL, 1 mM) was added and the sample incubated at 95 °C for 15 min. A 137 reference sample consisting of HCl, sodium acetate and DOTANOC was prepared using the same 138 procedure. A 10-µL aliquot of the resultant sample, mixed with 20 µL EDTA solution, was used for 139 LC-ESI-TOF-MS analysis using the operating parameters specified in Table S1.

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In order to evaluate the amount of low-activity radionuclidic impurities, long-term γ-155 spectrometry of partially decayed 44 Sc eluate was measured (Fig. S10). Other than the final product 156 containing 44 Sc and 44m Sc (~2 %), trace amounts of 88 Y (~0.0013 %) were also discovered. This is likely 157 due to the Sr impurity in the target material ( Figure  The chemical structure of PSMA-ALB-02 ligand is shown in Figure S12. It comprises a DOTA-176 chelator for coordination of the 44 Sc. The p-iodophenyl-moiety was used an albumin binder in order 177 to increase the radioligand's blood circulation time and, hence, uptake in the tumor.

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A representative HPLC chromatogram of the quality control of 44

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A representative HPLC chromatogram of the 44 Sc-PSMA-ALB-01 is shown in Figure S13.