Theragnostic Radionuclide Pairs for Prostate Cancer Management: 64Cu/67Cu, Can Be a Budding Hot Duo

Prostate cancer (PCa) is one of the preeminent causes of mortality in men worldwide. Theragnostic, a combination of therapy and diagnostic, using radionuclide pairs to diagnose and treat disease, has been shown to be a promising approach for combating PCa. In PCa patients, bone is one of the most common sites of metastases, and about 90% of patients develop bone metastases. This review focuses on (i) clinically translated theragnostic radionuclide pairs for the management of PCa, (ii) radionuclide therapy of bone metastases in PCa, and (iii) a special emphasis on emerging theragnostic radionuclide pair, Copper-64/Copper-67 (64Cu/67Cu) for managing the disease.


Introduction
Prostate cancer (PCa) is the most commonly diagnosed male malignancy worldwide [1]. Reportedly, PCa is more prevalent in the developed world, where it is a leading cause of cancer mortality among Swedish men and the second leading cause of cancer death in the United Kingdom [2]. In the rest of Europe, it is the third most commonly diagnosed cancer [3]. It is estimated that in 2022, in the United States, newly diagnosed PCa will account for 14% of all new cancer cases, and the estimated deaths will account for 5.7% of all cancer deaths [4].
A testosterone-reducing drug combined with androgen deprivation therapy is the first-line therapy for metastatic PCa. Most patients respond to these regimens initially. The disease, however, advances to metastatic castration-resistant prostate cancer (mCRPC) [5]. At this stage, patient survival may be improved by using theragnostic radionuclide pairs [6]. Theragnostic radionuclide pairs are a combination of diagnostic and therapeutic radionuclides sharing the same target inside the body. They can belong to the same or different elements and can have similar chemistry and pharmacokinetics. The use of theragnostic radionuclide pairs can be highly beneficial in the management of cancer as well as other life-threatening diseases. First, a diagnostic radiopharmaceutical is administered to the patient, and molecular imaging is performed. The findings allow to select a patient who is most likely to benefit from the therapy. Second, the biodistribution of the diagnostic radiopharmaceutical further helps to determine the possible adverse effects of therapeutic radiopharmaceutical. Third, the location of the primary lesion and its tumor volume determined from the pre-therapy scan, the estimated highest whole-body dose, and critical tissue radiation dose help to calculate a personalized therapeutic dose. Fourth, posttherapy diagnostic radiopharmaceutical help in predicting response to the treatment [7][8][9] ( Figure 1).

Figure 1.
The schematic shows the concept of theragnostic pairs. Targeting molecule conjugated with a bifunctional chelator (BFC) and radiolabeled with a diagnostic radionuclide (gamma/positron (γ/β + ) emitting) is injected into the patient (1). Molecular imaging is performed using positron emission tomography (PET) or single photon emission computed tomography (SPECT) to identify lesions (2). Patient selection is made based on scan findings, and pre−therapy dosimetry is performed to calculate personalized therapy dose. (3). For therapy, the same BFC−targeting molecule tagged with a therapeutic radionuclide is injected into the patient (4). Alpha/beta particles (α/β − ) cause the tumor cell killing, and response evaluation is done with molecular imaging (5). Created with BioRender.com, accessed on 25 September 2022.
A number of theragnostic radionuclide pairs have been translated clinically, such as Iodine-123/Iodine-131 ( 123 I/ 131 I), Gallium-68/Lutetium-177/Actinium-225 ( 68 Ga/ 177 Lu/ 225 Ac), Technetium-99m/Rhenium-188 ( 99m Tc/ 188 Re), 99m Tc/Yttrium-90 ( 99m Tc/ 90 Y) etc [10,11]. Radioiodine was the first theragnostic agent used in medicine to treat thyroid abnormalities. 123 I/ 131 I is an effective and true theragnostic radionuclide pair as both are chemically identical [12]. In thyroid conditions, 123 I-sodium iodide (NaI) is used to determine the percentage uptake in the thyroid. Following resection of differentiated thyroid carcinomas, 123 I-NaI identifies residual functional tissue in the thyroid bed and evaluates the amount of metastases and iodide avidity [13]. The therapeutic activity of 131 I-NaI is personalized based on the size of the patient's gland and the percentage of iodide uptake in order to deliver a 100-150 Gy radiation dose to the thyroid [14]. 68 Ga-prostate specific membrane antigen-11 ( 68 Ga-PSMA-11) has been successfully used in mCRPC patients for primary staging, lesion detection in biochemical recurrence cases, and assessing response to therapy with 177 Lu-/ 225 Ac-PSMA-617 [15]. Bone is one of the most common sites of metastases in PCa patients as approximately 90% of mCRPC patients develop bone metastases. Bone lesions can cause pain, disability, and skeletal events, as well as deteriorates quality of life [16]. This review discusses the theragnostic radionuclide pairs for PCa management and radionuclide therapy of bone metastases in patients (Table 1).

Figure 1.
The schematic shows the concept of theragnostic pairs. Targeting molecule conjugated with a bifunctional chelator (BFC) and radiolabeled with a diagnostic radionuclide (gamma/positron (γ/β + ) emitting) is injected into the patient (1). Molecular imaging is performed using positron emission tomography (PET) or single photon emission computed tomography (SPECT) to identify lesions (2). Patient selection is made based on scan findings, and pre−therapy dosimetry is performed to calculate personalized therapy dose. (3). For therapy, the same BFC−targeting molecule tagged with a therapeutic radionuclide is injected into the patient (4). Alpha/beta particles (α/β − ) cause the tumor cell killing, and response evaluation is done with molecular imaging (5). Created with BioRender.com, accessed on 25 September 2022.
A number of theragnostic radionuclide pairs have been translated clinically, such as Iodine-123/Iodine-131 ( 123 I/ 131 I), Gallium-68/Lutetium-177/Actinium-225 ( 68 Ga/ 177 Lu/ 225 Ac), Technetium-99m/Rhenium-188 ( 99m Tc/ 188 Re), 99m Tc/Yttrium-90 ( 99m Tc/ 90 Y) etc. [10,11]. Radioiodine was the first theragnostic agent used in medicine to treat thyroid abnormalities. 123 I/ 131 I is an effective and true theragnostic radionuclide pair as both are chemically identical [12]. In thyroid conditions, 123 I-sodium iodide (NaI) is used to determine the percentage uptake in the thyroid. Following resection of differentiated thyroid carcinomas, 123 I-NaI identifies residual functional tissue in the thyroid bed and evaluates the amount of metastases and iodide avidity [13]. The therapeutic activity of 131 I-NaI is personalized based on the size of the patient's gland and the percentage of iodide uptake in order to deliver a 100-150 Gy radiation dose to the thyroid [14]. 68 Ga-prostate specific membrane antigen-11 ( 68 Ga-PSMA-11) has been successfully used in mCRPC patients for primary staging, lesion detection in biochemical recurrence cases, and assessing response to therapy with 177 Lu-/ 225 Ac-PSMA-617 [15]. Bone is one of the most common sites of metastases in PCa patients as approximately 90% of mCRPC patients develop bone metastases. Bone lesions can cause pain, disability, and skeletal events, as well as deteriorates quality of life [16]. This review discusses the theragnostic radionuclide pairs for PCa management and radionuclide therapy of bone metastases in patients (Table 1).
Amongst the PCa theragnostics, Copper-64 and Copper-67 ( 64 Cu/ 67 Cu) is an emerging theragnostic radionuclide pair. Their nuclear properties are highly suitable for diagnosis and therapy. The half-life (t 1 2 ) of 64 Cu is 12.7 h, long enough for uptake and biodistribution studies of a radiopharmaceutical, including antibodies that may require a longer time for localization. Copper-64 is a positron-emitting radionuclide (E β + = 0.653 MeV) that permits positron emission tomography (PET) imaging and has beta emissions (E β − max = 0.579 MeV), which may be useful for theragnostic applications. Copper-67, on the other hand, decays with beta emissions of (E β − max = 0.561 MeV) with t 1 2 of 2.57 d, long enough to cause tumor cell killing but short enough not to induce excessive radiation burden to normal tissues. The coordination chemistry of copper is such that the radionuclides can be linked with several chelators, peptides, antibodies, and small molecules [17]. Since both belong to the same element and share similar chemistry, 67 Cu can be used in lieu of 64 Cu for radionuclide therapy. This paper briefly discusses the production, and clinical applications of 64 Cu/ 67 Cu in PCa patients.

Peptide Receptor Radionuclide Therapy (PRRT)
Increased prostate-specific antigen (PSA) levels are one of the hallmarks of PCa. Expression of prostate-specific membrane antigen (PSMA) is positively linked with PSA levels. PSMA is a type 2 integral membrane glycoprotein with intracellular, transmembrane, and extracellular domains. It consists of 750 amino acids and two monomers [18]. PSMA is expressed at low levels in the cytoplasm of normal prostate epithelial cells. In salivary glands, lacrimal glands, kidneys, and gastrointestinal tract it is overexpressed physiologically [19]. However, in 90-95% of PCa cells, it dwells in the luminal epithelium of prostatic ducts where it is up to one thousand times more abundant [20]. Moreover, PSMA expression levels are highest in metastatic and castrate-resistant diseases. This, differential expression in normal tissues and cancer cells, as well as its ability to be internalized after ligand binding, make PSMA an appealing target for theragnostics [20,21]. For targeting PCa, PSMA has been radiolabeled with theragnostic radionuclide pair 68 Ga and 177 Lu. 68 Ga is a generator-produced positron (1.92 MeV) emitting radionuclide having a t 1/2 of 68 min. Like 68 Ga, 177 Lu is also a trivalent cation with a t 1/2 of 6.7 d. Having E β − max = 0.496 MeV for therapy and E γ = 0.208 MeV (10.4%) for imaging 177 Lu renders itself as an attractive theragnostic radionuclide [22].
2.1. Gallium-68-Prostate Specific Membrane Antigen-11 ( 68 Ga-PSMA-11) (Locametz) 68 Ga-PSMA-11 belongs to a class of urea-based, peptidomimetic PSMA inhibitors. It is indicated for imaging PSMA-positive lesions in men with PCa. Diagnostic imaging is performed with PET in those patients who have (i) suspected metastases, (ii) are candidates for definitive therapy, and (iii) with suspected recurrence based on elevated serum PSA levels [23]. The sensitivity of 68 Ga-PSMA-11 correlates positively with serum PSA levels and performs relatively well at low PSA levels. A clinical trial in PCa patients reported a sensitivity of 47% and specificity of 90% [24].

Lutetium-177-Prostate Specific Membrane Antigen-617 ( 177 Lu-PSMA-617) (Pluvicto)
The PSMA-binding therapeutic radioligand 177 Lu-PSMA-617 ( 177 Lu-vipivotide tetraxetan) is indicated for the treatment of PSMA-positive adult mCRPC patients, who have previously been treated for androgen receptor pathway inhibition and with taxane-based chemotherapy ( Table 2). A 68 Ga-PSMA-11 pre-therapy scan is performed to determine PSMA expression in PCa. Those who express PSMA, qualify for 6 cycles of intravenous administration of 7.4 GBq 177 Lu-PSMA-617 every 6 weeks. The radiopharmaceutical is distributed in various organs within 2.5 h of injection and 60-70% of which binds to human plasma proteins [9]. 177 Lu-PSMA-617 beta emissions cause DNA damage, resulting in cytotoxicity in PSMA-expressing and nearby cells [25]. An international, open-label, phase 3 trial of 177 Lu-PSMA-617 reported imaging-based progression-free survival (PFS) and overall survival of 8.7 and 15.3 months. Patients reported fatigue, dry mouth, nausea, anemia, back pain, arthralgia, decreased appetite, constipation, and diarrhea [26]. The highest mean calculated absorbed dose for lacrimal gland was 92 Gy for the cumulative administered activity of 44.4 GBq (6 × 7.4 GBq). For salivary gland the dose was 28 Gy, for the large intestine it was 92 Gy, for kidneys 19 Gy, for urinary bladder 14 Gy and for blood marrow 1.5 Gy [27]. The primary route of excretion is the kidneys, with a mean terminal elimination half-life of 41.6 h. Patients with kidney impairment may be more susceptible to the renal toxicity. A follow-up patient study found a link between cumulative radiation doses and worsening kidney function after 13 ± 9 months of therapy [28]. However, in another radioligand therapy study, did not induce deterioration in kidney function as observed in a subgroup of mCRPC patients having kidney impairment [29].

Lutetium-177-J591 ( 177 Lu-J591)
Anti-PSMA monoclonal antibody J591 has been used for targeting mCRPC. A dose of 1665 MBq/m 2 given at a 2-week interval showed a median survival of 42.3 months and a reduction in PSA levels. 79.6% of the patients had positive 68 Ga-PSMA-11 imaging. Patients with low PSMA expression had poor responses. At high doses, PSA levels decreased with an increase in overall survival as well as increased toxicity [30]. The authors added that the cumulative radiation dose was higher when 177 Lu-J591 was administered in fractions.

Actinium-225-Prostate Specific Membrane Antigen-617 ( 225 Ac-PSMA-617)
Alpha emitting radionuclides are evaluated for PRRT due to their short penetration range (50-80 µm) and high linear energy transfer (80-100 keV/µm). The alpha radiations induce double-strand DNA breaks, and DNA cluster breaks causing highly effective cell killing ( Figure 2). 225 Ac is an alpha-emitting radionuclide (6 MeV) with a t 1/2 of 9.9 d. A study with 225 Ac-PSMA-617 treatment (100 KBq/kg body weight) showed a ≥90% decline in PSA levels in 82.3% of PCa patients. Furthermore, 88.2% of patients had a more than 50% decline in lesions avidity, as showed in 68 Ga-PSMA-PET/CT scan ( Table 2). In 64.7% of patients, all the metastatic lesions were completely resolved. There were side effects in the form of xerostomia, bone marrow toxicity, and renal impairment [31,32].

Radionuclide Therapy for Bone Metastases
Metastatic PCa cells interact with bone microenvironments and release various growth factors. These growth factors disturb the tightly regulated intercellular communication between osteoblasts and osteoclasts and dysregulate their activity. It results in a profusion of new, disordered bone. In turn, osteoblasts produce growth factors that stimulate the growth and survival of PCa cells. This bidirectional positive-feedback loop gives rise to osteoblastic bone metastases, characteristic of PCa, accounting for significant morbidity, Biomedicines 2022, 10, 2787 6 of 14 bone fracture, pain, and even death [36]. Bone-seeking therapeutic radiopharmaceuticals, having an affinity for hydroxyapatite, have been used for the treatment of bone metastases.

Radium-223-Chloride ( 223 RaCl 2 ) (Xofigo)
223 RaCl 2 (t 1/2 -11.4 d) is an alpha-emitting radiopharmaceutical indicated for treating CRPC patients with symptomatic bone metastases but no known visceral metastatic disease. A dose of 50 kBq/kg body weight is administered every 4-week intervals for 6 cycles. Six injections of 223 RaCl 2 per patient have shown overall median survival of 14.9 months, improved quality of life, favorable safety profiles, and low rates of myelosuppression. Moreover, the development of skeletal complications was delayed, and the risks of spinal cord compression were significantly reduced. A 3-year follow-up analysis suggested long-term safety of 223 RaCl 2 as no association was found between 223 RaCl 2 treatment and secondary malignancies [33,38].

Strontium-89-Chloride ( 89 SrCl 2 ) (Metastron)
Strontium-89 is a beta-emitting radionuclide (E β − max = 1.463 MeV) with a t 1/2 of 50.5 d. Strontium-89 chloride acts as a calcium analog and is retained in metastatic bone lesions for much longer period than in normal bone. The recommended dose, which clears rapidly from the blood, is 148 MBq. Clinical trials have witnessed relief of pain which usually begins 10-20 d after its administration and lasts up to 6 months, improving the quality of life [34,39].

Samarium-153-Ethylenediaminetetramethylenephosphonic Acid ( 153 Sm-EDTMP) (Quadramet)
153 Sm has a t 1/2 of 46.3 h and emits medium-energy beta-particles and gamma-photons. A dose of 37 MBq/kg body weight is administered intravenously [35]. 153 Sm-EDTMP is effective in treating bone metastases due to its fast blood clearance, high bone uptake, and low nonosseous uptake. Patients often decrease their analgesics. Turner et al. reported pain alleviation in 65% of patients over 4 to 35 weeks, and in some patients, transitory myelosuppression and delayed thrombocytopenia were observed. Sartor et al. reported that repeat dosing in patients (37 MBq/kg body weight) having painful bone metastases was safe and effective. The treatment is reasonable in patients who still have bone pain after an initial dose and have adequate hematologic function [40][41][42][43][44].

Copper-64 and Copper-67
Copper is the third most abundant transition trace metal in humans, after iron and zinc (Zn) [50]. Copper plays a vital role in the metabolic pathways of cells, such as in neurotransmitter synthesis and pigment formation. Copper is a transition metal with a mass number 63.55 and an atomic number 29. With electronic configuration [Ar] 3d 10 4s 1 , Copper belongs to group 11 of the periodic table. Copper has multiple oxidation states; Cu(II) is the most favorable for radiopharmaceutical preparation. Cu(II) is less labile to ligand exchange because of crystal field stabilizing energy. It can exist in chloride, nitrate, bromide, etc., forms and is generally stable and soluble in water. Copper has two naturally occurring and stable isotopes, of which 69.15% is 63 Cu, and 30.85% is 65 Cu. It has 27 radioactive isotopes and out of these 60 Cu (E β + = 2.9 MeV, t 1/2 = 23.7 min), 61 Cu (E β + = 1.21 MeV, t 1/2 = 3.33 h), 62 Cu (E β + = 2.9 MeV, t 1/2 = 9.7 min), 64 Cu (E β + = 0.653 MeV, E β − max = 0.57 MeV, t 1/2 = 12.7 h) and 67 Cu (E β − max = 0.561 MeV, t 1/2 = 2.57 d) are used for imaging and radiotherapy [51,52].

Copper-64
Amongst all the radioactive isotopes of copper, 64 Cu is the most studied owing to its ideal nuclear properties. Copper-64 decays to Nickel-64 ( 64 Ni) with E β + = 0.653 MeV (17.8%) and electron capture (43.6%). It also decays to Zinc-64 ( 64 Zn) with E β − max = 0.579 MeV (38.48%) (Figure 3a). It has a t 1/2 of 12.7 h, which is suitable for imaging using small as well as large molecules such as antibodies and peptides. The relatively short t 1/2 of 64 Cu does not add unnecessary radiation burden to the patient after imaging studies have been performed, and yet, 64 Cu radiopharmaceuticals can be shipped long distances without excessive radioactivity decay [53].

Copper-64-Chloride ( 64 CuCl2)
Ionic 64 CuCl2 has also been used to screen patients with PCa. 185-370 MBq 64 CuCl2 administered had given good tumor to background ratio. It was shown that the tracer delivered a few nanograms of copper to cells and had no known cytotoxic effect. The radiotracer had a rapid blood clearance. Dosimetry studies in healthy volunteers suggested the liver, intestine, and pancreas as the critical organs [56] In most tumor microenvironments, increased cell proliferation and decreased neoangiogenesis lead to low oxygen levels. In many cases, the hypoxic conditions render tumors

Production of Copper-64
64 Cu can be produced by neutron bombardment in a reactor or by proton bombardment in a cyclotron. To produce 64 Cu in a reactor, 63 Cu(n,γ) 64 Cu or 64 Zn(n,p) 64 Cu nuclear reactions are used. The use of 63 Cu as the target, produce 64 Cu with low specific activity. 64 Zn target requires fast neutron flux and produces 65 Zn (t 1/2 = 245 d) as an impurity. These factors limit the availability of 64 Cu from the reactor. Cyclotron production involves 64 Ni(p,n) 64 Cu reaction, which is widely used in the US to meet the need for 64 Cu. In this production method, enriched nickel (99.6%) is electroplated onto a gold disk or on a copper substrate having a gold layer and bombarded with protons to obtain 64 Cu. It has been reported that bombarding 40 mg Ni with 15.5 MeV protons for 4 h at 60 µA gives 18.5 GBq 64 Cu. After production, copper is carefully separated from nickel by an ion-exchange column chromatography [54,55].

Copper-64-Chloride ( 64 CuCl 2 )
Ionic 64 CuCl 2 has also been used to screen patients with PCa. 185-370 MBq 64 CuCl 2 administered had given good tumor to background ratio. It was shown that the tracer delivered a few nanograms of copper to cells and had no known cytotoxic effect. The radiotracer had a rapid blood clearance. Dosimetry studies in healthy volunteers suggested the liver, intestine, and pancreas as the critical organs [56].

Copper-64-Diacetyl-bis-N-4-Methylthiosemicarbazone ( 64 Cu-ATSM)
In most tumor microenvironments, increased cell proliferation and decreased neoangiogenesis lead to low oxygen levels. In many cases, the hypoxic conditions render tumors resistant to chemotherapy and radiation therapy. 64 Cu-ATSM has been employed in these cases as a diagnostic marker for hypoxia imaging to assess the prognosis. In 64 Cu-ATSM, copper is present in a 64 Cu(II) oxidation state. It is considered that inside the tumor, it gets reduced to 64 Cu(I), the complex becomes unstable, and 64 Cu is trapped within the tumor. 64 Cu-ATSM has no urinary excretion and can also image PCa [57][58][59][60] Clinical trials are ongoing with 64 Cu-SAR-bisPSMA to identify PSMA-expressing mCRPC. Patient's recruitment started in 2021 and is currently recruiting [63]. In the phase I study one administration of 200 MBq 64 Cu-SAR-bisPSMA will be given to the patients for dosimetric determination, two administrations for dose escalation studies, and three for cohort expansion study. The purpose of the research is to determine the safety and efficacy of 67 Cu-SAR-bisPSMA in patients.

Copper-64-TP3805 ( 64 Cu-TP3805)
Thakur et al. developed and evaluated TP3805, a peptide analog of pituitary adenylate cyclase-activating peptide (PACAP) having a high affinity for VPAC1 receptors. VPAC1 receptors (a combined for vasoactive intestinal peptide (VIP) and PACAP) are minimally expressed in normal cells and benign tumors but expressed in high density on many types of malignant cells. TP3805 was conjugated with N 2 S 2 (diaminedithiol(N 2 S 2 -Benzoyl) 2 ) chelating agent at the C-terminus and radiolabeled with 64 Cu. A kit was also formulated to make it convenient for reliable and routine radiolabeling [64].
In a clinical study of 25 PCa patients, PET imaging with 64 Cu-TP3805 identified lesions in the prostate gland as confirmed by post-surgical histology. Digital autoradiography (DAR) with 64 Cu-TP3805 identified 98% PCa foci, 100% high-grade intraepithelial neo-plasia, and other malignant lesions. For benign lesions, DAR was negative. The study demonstrated that 64 Cu-TP3805 is highly specific for PCa and merits further investigation [65]. Another study with 64 Cu-TP3805 in urothelial bladder cancer patients (n = 19) clearly visualized the lesions due to negligible urinary excretion [66]. 64 Cu-TP3805 can also identify other malignant lesions and bone metastases. In a clinical study of 19 breast cancer patients [67], positron emission mammography uptake value/background value ratios of the 15 min, post-injection images did not alter significantly for up to 5 h of imaging indicating high in vivo stability of 64 Cu-TP3805.
Hence, it was proposed that 67 Cu-TP3805, with excellent theragnostic qualities (Tables 3 and 4 However, 67 Cu-TP3805 has no uptake in salivary glands. Hence, can treat patients without undergoing multiple pretreatment procedures. (iii) 67 Cu-TP3805 has no urinary excretion and can treat bladder cancer, primary PCa their metastatic lesions as well as involved lymph nodes. (iv) Cancer stem cells express VPAC receptors [68,69] and can be targeted with 67 Cu-TP3805, perhaps preventing recurrence of the disease.

Production of Copper-67
Copper-67 decays to 67 Zn with E β − max = 0.561 MeV, E γ = 0.184 MeV (48.7%) and a t 1/2 of 2.57 d (Figure 3b) [17]. Although 67 Cu can be produced from the reactor, the low yield and expensive target material ( 67 Zn) limit the availability of reactor-produced 67 Cu. In a cyclotron, 67 Cu can be produced using Zn, Ni, Cu, and Ga as target material ( Table 5). The main route of production using Zn as the target material is a 68 Zn(p,2p) 67 Cu reaction with proton-beam energy 38-50 MeV. Enriched 68 Zn targets not only enhance 67 Cu yield but also reduce co-production of other Cu-radionuclides. At the proton beams of up to 30 MeV, the 70 Zn(p,α) 67 Cu reaction is feasible without the co-production of 64 Cu. The reaction requires enriched 70 Zn (95.47%), which is expensive and makes recovery and re-use of the irradiated target material a vital task. Using deuteron beams of 7-25 MeV 70 Zn(d,x) 67 Cu reaction seems promising also [70]. Alpha beams are used to irradiate nickel in a 64 Ni(α,p) 67 Cu reaction, but the reaction has a low yield. Production of 67 Cu using 65 Cu(α,2p) 67 Cu reaction yields 67 Cu with a very low specific activity. Production of 67 Cu from Ga using 71 Ga(p,x) 67 Cu nuclear reaction requires 20-40 MeV proton beam energy and 24 h irradiation time. The disadvantages of the reaction are low yield and high 64 Cu contamination. Furthermore, the low melting point of gallium makes target preparation difficult for nuclear bombardment [71][72][73][74][75][76][77]. Photonuclear production using 68 Zn(γ,p) 67 Cu reaction is another method for 67 Cu production. It is reported that bombarding 55.5 g enriched 68 Zn with 40 MeV bremsstrahlung photons in electron linear accelerator for 53.5 h gives 62.9 GBq activity and >1850 Bq/mg 67 Cu at the end of bombardment (EOB) without detecting 64 Cu as a contaminant. The enriched target material is mandatory to avoid the co-production of zinc and copper radionuclides as impurities [78].

Copper-67 Radiopharmaceutical for Prostate Cancer Therapy
Copper-67-Sarcophagine-bis-Prostate Specific Membrane Antigen ( 67 Cu-SAR-bisPSMA) Clinical trials are being conducted to investigate the safety and efficacy of 67 Cu-SAR-bisPSMA in PSMA-expressing mCRPC patients [63]. Patients will receive two doses at the recommended dose level determined by dose escalation during the cohort expansion phase.

Conclusions
PCa theragnostics involve various radiopharmaceuticals that target both primary lesions as well as bone metastases. FDA approved, 177 Lu-PSMA-617 increases OS and imaging-based PFS in PCa patients. In mCRPC patients, targeted alpha therapy (TAT) with 225 Ac-PSMA-617 causes relatively low toxicity; hence, it is an effective and safe treatment option. However, clinical trials are required to compare the therapeutic effects and survival benefits with existing clinical treatments. TAT of bone metastases in PCa with 223 RaCl 2 remains an important component of the treatment paradigm as it has proven survival benefits. Tumor-specific TAT seems promising due to its high therapeutic efficiency, minimal damage to normal tissue, and ability to target small volume disease. 89 SrCl 2 and 153 Sm-EDTMP enable pain relief and improve the quality of life of PCa patients having bone metastases. 64 Cu and 67 Cu are chemically identical radionuclides. Both have similar in vivo behavior which facilitates the use of the former as a predictor of the biodistribution and toxicity of the latter. Moreover, a 64 Cu pre-therapy dosimetry scan helps calculate a personalized 67 Cu patient therapy dose. 67 Cu-bisPSMA and 67 Cu-TP3805 are emerging radiopharmaceuticals for PCa theragnostics. The efficient and optimized production methods yielding large amounts of 64 Cu and 67 Cu with high specific activity can enhance routine availability and enable its more widespread use.