High specific activity 61Cu via 64Zn(p,α)61Cu reaction at low proton energies

Production of the PET radionuclide 61Cu via the 62Ni(p,2n)61Cu nuclear reaction

BACKGROUND

There are only a handful of true theranostic matched pairs, and in particular the theranostic radiocopper trio 61Cu, 64Cu and 67Cu, for diagnosis and therapy respectively, is a very attractive candidate. In fact, the alternative of two imaging radionuclides with different half-lives is a clear advantage over other theranostic pairs, since it offers a better matching for the tracer biological and radionuclide physical half-lives. Due to the high availability of 64Cu, its translation into the clinic is being successfully carried out, giving the example of the FDA approved radiopharmaceutical Detectnet (copper Cu 64 dotatate injection). However, a shorter-lived PET radionuclide such as 61Cu may as well be beneficial.

RESULTS

Proton irradiation of enriched 62Ni electrodeposited targets with a compact cyclotron produced the desired radionuclide via the 62Ni(p,2n)61Cu nuclear reaction, leading to 61Cu activities of up to 20 GBq at end of bombardment and 8 GBq at end of purification. Furthermore, two purification methods are compared leading to comparable results regarding separation yield and product purity. Following the radiochemical separation, quality assessment of this product [61Cu]CuCl2 solution proved radionuclidic purities (RNP) over 99.6% and apparent molar activities (AMA) of 260 GBq/µmol with the 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA) chelator, end of purification corrected.

CONCLUSIONS

In the current article a comprehensive novel production method for the PET radionuclide 61Cu is presented, providing an alternative to the most popular production routes. Characterization of the [61Cu]CuCl2 product showed both high RNP as well as high AMA, proving that the produced activity presented high quality regarding radiolabeling up to 9 h after end of purification. Furthermore, production scalability could be easily achieved by increasing the irradiation time.

64Cu production via the 68Zn(p,nα)64Cu nuclear reaction: An untapped, cost-effective and high energy production route

INTRODUCTION

Copper-64 (64Cu, t1/2 = 12.7 h) is a positron emitter well suited for theranostic applications with beta-emitting 67Cu for targeted molecular imaging and radionuclide therapy. The present work aims to evaluate the radionuclidic purity and radiochemistry of 64Cu produced via the 68Zn(p,nα)64Cu nuclear reaction. Macrocyclic chelators DOTA, NOTA, TETA, and prostate-specific membrane antigen ligand PSMA I&T were radiolabeled with purified 64Cu and tested for in vitro stability. [64Cu]Cu-PSMA I&T was used to demonstrate in vivo PET imaging using 64Cu synthesized via the 68Zn(p,nα)64Cu production route and its suitability as a theranostic imaging partner alongside 67Cu therapy.

METHODS

64Cu was produced on a 24 MeV TR-24 cyclotron at a beam energy of 23.5 MeV and currents up to 70 μA using 200 mg 68Zn encapsulated within an aluminum‑indium-graphite sealed solid target assembly. 64Cu semi-automated purification was performed using a NEPTIS Mosaic-LC synthesis unit employing CU, TBP, and TK201 (TrisKem) resins. Radionuclidic purity was measured by HPGe gamma spectroscopy, while ICP-OES assessed elemental purity. Radiolabeling was performed with NOTA at room temperature and DOTA, TETA, and PSMA I&T at 95 °C. 64Cu incorporation was studied by radio-TLC. 64Cu in vitro stability of [64Cu]Cu-NOTA, [64Cu]Cu-DOTA, [64Cu]Cu-TETA, and [64Cu]Cu-PSMA I&T was assessed at 37 °C from 0 to 72 h in human blood serum. Preclinical PET imaging was performed at 1, 24, and 48 h post-injection with [64Cu]Cu-PSMA I&T in LNCaP tumor-bearing mice and compared with [68Ga]Ga-PSMA I&T.

RESULTS

Maximum purified activity of 4.9 GBq [64Cu]CuCl2 was obtained in 5 mL of pH 2-3 solution, with 2.9 GBq 64Cu concentrated in 0.5 mL. HPGe gamma spectroscopy of purified 64Cu detected <0.3 % co-produced 67Cu at EOB with no other radionuclidic impurities. ICP-OES elemental analysis determined <1 ppm Al, Zn, In, Fe, and Cu in the [64Cu]CuCl2 product. NOTA, DOTA, TETA, and PSMA I&T were radiolabeled with 64Cu, resulting in maximum molar activities of 164 ± 6 GBq/μmol, 155 ± 31 GBq/μmol, 266 ± 34 GBq/μmol, and 117 ± 2 GBq/μmol, respectively. PET imaging in PSMA-expressing LNCaP xenografts resulted in high tumor uptake (SUVmean = 1.65 ± 0.1) using [64Cu]Cu-PSMA I&T, while [68Ga]Ga-PSMA I&T yielded an SUVmean of 0.76 ± 0.14 after 60 min post-injection.

CONCLUSIONS

64Cu was purified in a small volume amenable for radiolabeling, with yields suitable for preclinical and clinical application. The 64Cu production and purification process and the favourable PET imaging properties confirm the 68Zn(p,nα)64Cu nuclear reaction as a viable 64Cu production route for facilities with access to a higher energy proton cyclotron, compared to using expensive 64Ni target material and the 64Ni(p,n)64Cu nuclear reaction.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Our 64Cu production technique provides an alternative production route with the potential to improve 64Cu availability for preclinical and clinical studies alongside 67Cu therapy.

Copper radionuclides for theranostic applications: towards standardisation of their nuclear data. A mini-review

Copper has several clinically relevant radioisotopes and versatile coordination chemistry, allowing attachment of its radionuclides to biological molecules. This characteristic makes it suitable for applications in molecular imaging or radionuclide targeted therapy. Of particular interest in nuclear medicine today is the theranostic approach. This brief review considers five radionuclides of copper. These are Cu-60, Cu-61, Cu-62, Cu-64, and Cu-67. The first four are positron emitters for imaging, and the last one Cu-67 is a β--emitting radionuclide suitable for targeted therapy. The emphasis here is on theory-aided evaluation of available experimental data with a view to establishing standardised cross-section database for production of the relevant radionuclide in high purity. Evaluated cross section data of the positron emitters have been already extensively reported; so here they are only briefly reviewed. More attention is given to the data of the 68Zn(p,2p)67Cu intermediate energy reaction which is rather commonly used for production of 67Cu.

Cyclotron-Based Production of 67Cu for Radionuclide Theranostics via the 70Zn(p,α)67Cu Reaction

Theranostic matched pairs of radionuclides have aroused interest during the last couple of years, and in that sense, copper is one element that has a lot to offer, and although 61Cu and 64Cu are slowly being established as diagnostic radionuclides for PET, the availability of the therapeutic counterpart 67Cu plays a key role for further radiopharmaceutical development in the future. Until now, the 67Cu shortage has not been solved; however, different production routes are being explored. This project aims at the production of no-carrier-added 67Cu with high radionuclidic purity with a medical 30MeV compact cyclotron via the 70Zn(p,α)67Cu reaction. With this purpose, proton irradiation of electrodeposited 70Zn targets was performed followed by two-step radiochemical separation based on solid-phase extraction. Activities of up to 600MBq 67Cu at end of bombardment, with radionuclidic purities over 99.5% and apparent molar activities of up to 80MBq/nmol, were quantified.

Efficient Production of the PET Radionuclide 133La for Theranostic Purposes in Targeted Alpha Therapy Using the 134Ba(p,2n)133La Reaction

Targeted Alpha Therapy is a research field of highest interest in specialized radionuclide therapy. Over the last decades, several alpha-emitting radionuclides have entered and left research topics towards their clinical translation. Especially, 225Ac provides all necessary physical and chemical properties for a successful clinical application, which has already been shown by [225Ac]Ac-PSMA-617. While PSMA-617 carries the DOTA moiety as the complexing agent, the chelator macropa as a macrocyclic alternative provides even more beneficial properties regarding labeling and complex stability in vivo. Lanthanum-133 is an excellent positron-emitting diagnostic lanthanide to radiolabel macropa-functionalized therapeutics since 133La forms a perfectly matched theranostic pair of radionuclides with the therapeutic radionuclide 225Ac, which itself can optimally be complexed by macropa as well. 133La was thus produced by cyclotron-based proton irradiation of an enriched 134Ba target. The target (30 mg of [134Ba]BaCO3) was irradiated for 60 min at 22 MeV and 10−15 µA beam current. Irradiation side products in the raw target solution were identified and quantified: 135La (0.4%), 135mBa (0.03%), 133mBa (0.01%), and 133Ba (0.0004%). The subsequent workup and anion-exchange-based product purification process took approx. 30 min and led to a total amount of (1.2−1.8) GBq (decay-corrected to end of bombardment) of 133La, formulated as [133La]LaCl3. After the complete decay of 133La, a remainder of ca. 4 kBq of long-lived 133Ba per 100 MBq of 133La was detected and rated as uncritical regarding personal dose and waste management. Subsequent radiolabeling was successfully performed with previously published macropa-derived PSMA inhibitors at a micromolar range (quantitative labeling at 1 µM) and evaluated by radio-TLC and radio-HPLC analyses. The scale-up to radioactivity amounts that are needed for clinical application purposes would be easy to achieve by increasing target mass, beam current, and irradiation time to produce 133La of high radionuclide purity (>99.5%) regarding labeling properties and side products.

Production of GMP-Compliant Clinical Amounts of Copper-61 Radiopharmaceuticals from Liquid Targets

PET imaging has gained significant momentum in the last few years, especially in the area of oncology, with an increasing focus on metal radioisotopes owing to their versatile chemistry and favourable physical properties. Copper-61 (t_1/2 = 3.33 h, 61% β⁺, E_max = 1.216 MeV) provides unique advantages versus the current clinical standard (i.e., gallium-68) even though, until now, no clinical amounts of ⁶¹Cu-based radiopharmaceuticals, other than thiosemicarbazone-based molecules, have been produced. This study aimed to establish a routine production, using a standard medical cyclotron, for a series of widely used somatostatin analogues, currently labelled with gallium-68, that could benefit from the improved characteristics of copper-61. We describe two possible routes to produce the radiopharmaceutical precursor, either from natural zinc or enriched zinc-64 liquid targets and further synthesis of [⁶¹Cu]Cu-DOTA-NOC, [⁶¹Cu]Cu-DOTA-TOC and [⁶¹Cu]Cu-DOTA-TATE with a fully automated GMP-compliant process. The production from enriched targets leads to twice the amount of activity (3.28 ± 0.41 GBq vs. 1.84 ± 0.24 GBq at EOB) and higher radionuclidic purity (99.97% vs. 98.49% at EOB). Our results demonstrate, for the first time, that clinical doses of ⁶¹Cu-based radiopharmaceuticals can easily be obtained in centres with a typical biomedical cyclotron optimised to produce ¹⁸F-based radiopharmaceuticals.

Expanding PET-applications in life sciences with positron-emitters beyond fluorine-18

Positron-emission-tomography (PET) has become an indispensable diagnostic tool in modern nuclear medicine. Its outstanding molecular imaging features allow repetitive studies on one individual and with high sensitivity, though no interference. Rather few positron-emitters with near favourable physical properties, i.e. carbon-11 and fluorine-18, furnished most studies in the beginning, preferably if covalently bound as isotopic label of small molecules. With the advancement of PET-devices the scope of in vivo research in life sciences and especially that of medical applications expanded, and other than "standard" PET-nuclides received increasing significance, like the radiometals copper-64 and gallium-68. Especially during the last decades, positron-emitters of other chemical elements have gotten into the focus of interest, concomitant with the technical advancements in imaging and radionuclide production. With known nuclear imaging properties and main production methods of emerging positron-emitters their usefulness for medical application is promising and even proven for several ones already. Unfortunate decay properties could be corrected for, and β+-emitters, especially with a longer half-life, provided new possibilities for application where slower processes are of importance. Further on, (bio)chemical features of positron-emitters of other elements, among there many metals, not only expanded the field of classical clinical investigations, but also opened up new fields of application. Appropriately labelled peptides, proteins and nanoparticles lend itself as newer probes for PET-imaging, e.g. in theragnostic or PET/MR hybrid imaging. Furthermore, the potential of non-destructive in-vivo imaging with positron-emission-tomography directs the view on further areas of life sciences. Thus, exploiting the excellent methodology for basic research on molecular biochemical functions and processes is increasingly encouraged as well in areas outside of health, such as plant and environmental sciences.

Uses of alpha particles, especially in nuclear reaction studies and medical radionuclide production

Alpha particles exhibit three important characteristics: scattering, ionisation and activation. This article briefly discusses those properties and outlines their major applications. Among others, α-particles are used in elemental analysis, investigation and improvement of materials properties, nuclear reaction studies and medical radionuclide production. The latter two topics, dealing with activation of target materials, are treated in some detail in this paper. Measurements of excitation functions of α-particle induced reactions shed some light on their reaction mechanisms, and studies of isomeric cross sections reveal the probability of population of high-spin nuclear levels. Regarding medical radionuclides, an overview is presented of the isotopes commonly produced using α-particle beams. Consideration is also given to some routes which could be potentially useful for production of a few other radionuclides. The significance of α-particle induced reactions to produce a few high-spin isomeric states, decaying by emission of low-energy conversion or Auger electrons, which are of interest in localized internal radiotherapy, is outlined. The α-particle beam, thus broadens the scope of nuclear chemistry research related to development of non-standard positron emitters and therapeutic radionuclides.