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Copper, a cofactor for many enzymes, is a bioelement that is involved in many main biochemical processes; although high levels of copper promote the proliferation of cancer cells. Further development of radiopharmaceuticals based on copper radioisotopes depend on understanding and taking advantage of its biochemical pathways in oncogenesis. As with other radiometals used in molecular imaging and/or targeted therapy, biological vectors are employed to transport copper radioisotopes to a target, aiming for high specific uptake at tumor sites and precise delivery of ionizing radiation. Evidence of the clinical utility of copper radioisotopes in the ionic form CuCl2 were also proven in an
The natural occurrence of copper (69.17% 63Cu, 30.83% 65Cu [
Copper is an essential microelement involved in important biochemical processes, such as: homeostasis, iron transport, respiration, and metabolism, as a result of its redox abilities in the biological environment: reversible translation between oxidized form (cupric ion, Cu2+) and the reduced form (cuprous ion, Cu+). It is a transition metal with 29 isotopes, out of which 27 are radioactive [
Along with the progress of nuclear medicine practices and technology, approaching molecular imaging and personalized treatment, five of the copper radioisotopes have gained interest for medical applications, considering their emissions, energies, production route, and availability, with half-lives ranging from 9.7 min (62Cu) to 2.6 days (67Cu) [
Recent studies demonstrated the usefulness of 64/67Cu agents, containing biological vectors to carry radioisotopes to target, aiming for high specific uptake at tumor sites, and precise delivery of ionizing radiation, such as peptides, antibodies, or other biologically active small molecules [
As many of these findings are evidence-based and sourced directly from clinical practice (e.g., the significantly higher copper levels measured in serum and tumor cells of patients with cancer compared to normal subjects [
Humans are exposed to environmental copper from water, food, and tools or household goods, therefore the World Health Organization (WHO) defined a safe range for copper intake and acknowledged its effects, either positive or negative, on human health [
Copper bioavailability is fairly affected by dietary factors, such as carbohydrate, iron, zinc, molybdenum, and ascorbic acid co-ingestion. Large quantities of dietary zinc can decrease copper absorption and induce the symptoms of systemic copper deficiency. Also, an increased molybdenum intake drives the organism toward secondary copper deficiency, which can be rapidly corrected by copper supplementation. On the other hand, iron-copper interactions in the intestines conduct the regulation of copper transport modulation by the iron levels. Reduced levels of copper lead to a series of physiological changes, inducing pathological conditions, while high intake of copper, found as chronic or acute exposure, can result in liver damage [
The intestines are the main absorption site, the process being conducted by the enterocytes, with the participation of copper permease and human copper transporter-1 (hCTR1) [
Copper is further transferred to the cytoplasm, in inter-mitochondrial and intra-mitochondrial spaces, where it becomes a constituent of cytochrome c oxidase (CcO) and superoxide dismutase-1 (SOD1) [
Reduced or minimal activity of copper-dependent enzymes results in symptoms that may include hypochromic anemia, neutropenia, thrombocytopenia, and hypopigmentation, bone, cardiovascular, and neurological abnormalities, as well as immune system depression [
Children can develop potentially fatal idiopathic copper toxicosis when drinking contaminated water or food [
The proliferation of cancer cells is promoted by high levels of copper [
Molecular imaging allows for the quantification of functional parameters of an organ or process; moreover the interactions of a drug with its desired target can be analyzed, side effects can be determined, and the delivery, absorption, distribution, metabolism, and elimination in a living system can be precisely evaluated [
The positron-emitting radionuclide is customarily selected taking into account several factors, such as: the half-life of the radionuclide (this should match with the vector pharmacokinetics to allow optimal uptake), the energy of the positron emission (which determines the precision and image resolution), and the availability and cost of the production. Moreover, the specific/molar activity and carrier-free specifications, as quality parameters, become tremendously important when associated with molecular term (either imaging or therapy), together with radiobiological parameters, mainly the affinity, uptake, and retention profiles (radio)toxicity, blood clearance, and elimination route.
Chelators used for binding radio‐copper to biomolecules.
Five radioisotopes of copper (
Radioisotopes of copper produced in medium energy cyclotrons.
Radioisotope and half-life | Decay mode and energy | Most intense γ emissions | Nuclear reaction and cross-section data [ |
---|---|---|---|
67Cu 61.8 h | β− (100%) |
91.2 keV (7%) |
|
64Cu 12.7 h | β− (38.5%) |
- | |
EC and β+ (61.5%) |
1,345.77 keV (0.475%) | ||
62Cu 9.7 min | EC and β+ (100%) |
875.7 keV (0.15%) |
|
61Cu 3.32 h | EC and β+ (100%) |
282.9 keV (12.2%) |
|
60Cu 23.7 min | EC and β+ (100%) |
1,332.4 keV (88%) |
Researchers are investigating different routes to produce carrier-free and high specific activity copper radioisotopes [
The most common way to produce 64Cu is by using a small/medium energy cyclotron [
The 64Ni(p,n)64Cu reaction is used at large scale for the production of 64Cu, although bearing the disadvantage of costly target material, this route is preferred for the high yields that can be achieved, even at small medical cyclotrons [
64CuCl2 is used either as a radiopharmaceutical or as a precursor for radiolabeling specific carriers, such as monoclonal antibodies, peptides, amino acids, hormones, nanoparticles, or small molecules, using chelating agents [
After IV administration, 64CuCl2 accumulates in the liver (uptake fraction 0.65), brain (uptake fraction 0.1), kidney (uptake fraction 0.01), and pancreas (uptake fraction 0.0002). Based on preclinical studies, the calculated effective dose (ED) is 70 mSv for the whole body of a 70 kg adult, after the intravenous injection of 925 MBq of 64CuCl2 [
The chelators used for binding radio-copper to biomolecules (
Comparing the biodistribution, at 24 h p.i., of 64Cu-CB-DO2A, 64Cu-CB-TE2A, 64Cu-DOTA, and 64Cu-TETA, a larger amount of 64Cu-labeled cross-bridged chelates was cleared form the blood, liver, and kidney than the non cross-bridged analogues; moreover, 64Cu-CB-TE2A was the most resistant to
64CuCl2 shows an increased and specific uptake in melanoma expressing high hCTR1: 12.7% ± 0.26 in B16F10 cells and 4.6% ± 0.04 in A375M cells, the tumor-to-muscle ratio was 4.11 ± 0.07 for B16F10 and 3.46 ± 1.25 for A375M. During 64CuCl2 treatment, tumor growth in both melanoma models was slower than without treatment, suggesting that 64CuCl2 radiotherapy is effective for hCTR1 high-expressing tumors [
In a xenograft model of glioblastoma multiforme (GBM) U87MG, the biodistribution of 64CuCl2 indicated no brain uptake, while PET images showed an uptake in glioma cells; a decrease of the tumor volume with more than 68% was noticed, raising the survival rate of the treated mice [
In a study using the hypoxia-selective agent 64Cu-ATSM on hamsters implanted with GW39 (human colorectal carcinoma), the inhibition of tumor growth was observed for a 220 MBq injected dose; the animals presented an increased rate of survival with no acute toxicity. After administration, PET scans revealed that 64Cu-ATSM was localized in the GW39 tumor and PET imaging could be performed regularly [
Administration of 555 MBq of 64Cu-TETA-Y3-TATE in a single dose to CA20948 rats, a model of somatostatin receptor-positive pancreatic cancer, decreased the tumor volume (29–73%) and inhibited its growth. The multiple dose radiotherapy study (3 × 370 MBq) decreased the tumor volume (36–81%) and provided a tolerable radiation exposure level over an extended period [
64Cu-ATSM (64Cu-diacetyl-bis(N-4-methylthiosemicarbazone) showed a high cytotoxic effect, decreasing the clonogenic survival of LL/2 cells (Mouse Lewis Lung carcinoma cells) in a dose dependent manner; the uptake of 1.50 Bq/cell of 64Cu killed 99% of the cells. Under hypoxic conditions, 64Cu was accumulated in the cells and produced DNA damage, detected by comet assay and Annexin V-FITC and propidium iodide staining methods [
DU-145 human prostate cancer xenografts were visualized by PET using 64CuCl2, the cellular uptake was mediated by hCTR1, demonstrated by negative control PC-3 prostate cancer cells. Knockdown of hCTRl reflected the decreased cellular uptake and inhibition of tumor growth [
The biochemical pathways show copper metabolism in normal cells and highlight its increased activity in human cancer cells, at a higher metabolic rate. Its involvement in tumor progression and angiogenesis and its pivotal role in preserving the intracellular homeostasis are particular indicators used in functional imaging. Thus, specific processes are targeted by radio-copper chloride, but also specific vectors radiolabeled with copper radioisotopes are used. Moreover, the copper presence in intermitochondrial and intramitochondrial spaces, as constituents of cytochrome
The uptake mechanism, kinetics, and metabolic parameters are very important findings for PET imaging using 60Cu, 61Cu, 62Cu, or 64Cu which are decisive when designing an individualized targeted therapy and, also, for dose calculations of high LET Auger electrons and β− emissions of 64Cu and 67Cu. The concept of theranostic applications applies perfectly to copper radioisotopes, by matching pairs for diagnostics and therapy (e.g., 61Cu and 67Cu) or by taking advantage of the dual emissions of 64Cu for both purposes. In this latter case, a real-time therapy follow-up brings important benefits for patients.
DN, RD, and RL reviewed the data regarding copper radioisotopes production and radiochemistry and edited the article. LC, RS, and DAN reviewed the data regarding biological assessment of 64Cu-labelled biomolecules, CD, AN, ID, and DD reviewed pharmaceutical and pharmacological data.
The work has been funded by the UEFISCDI, Romanian Ministry of Education and Research, under contract 64PCCDI/2017. The work of Ramona Dusman, PhD student, has been funded by the Operational Program Human Capital of the Ministry of European Funds through the Financial Agreement 51668/09.07.2019, SMIS code 124705.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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