State of the art and future perspectives of new radionuclides in Nuclear Medicine

Abstract

This continuing education analyzes recent advances in Nuclear Medicine focused on the development of new radiopharmaceuticals that improve both the diagnosis and treatment of complex diseases. The focus is on teragnosis, which combines diagnosis and treatment by means of pairs of radiopharmaceuticals directed to the same molecular target, which allows the personalization of treatments.

This first part specifically reviews the teragnostic pairs copper-64/copper-67, lead-212/lead-203 and scandium-44/scandium-47, highlighting their physical characteristics, methods of production and potential clinical applications.

Despite the challenges in their production, their versatility and effectiveness are driving their clinical application in oncology and other diseases. The text also addresses the development of new radiopharmaceuticals and their impact on precision medicine, pointing out future directions and opportunities for research in this field.

Resumen

En esta formación continuada se analizan los recientes avances en Medicina Nuclear centrados en el desarrollo de nuevos radiofármacos que mejoran tanto el diagnóstico como el tratamiento de enfermedades complejas. Destaca la teragnosis, que combina diagnóstico y tratamiento mediante pares de radiofármacos dirigidos a una misma diana molecular, lo que permite personalizar los tratamientos.

En esta primera parte se revisan específicamente los pares teragnósticos cobre-64/cobre-67, plomo-212/plomo-203 y escandio-44/escandio-47, resaltando sus características físicas, métodos de obtención y potenciales aplicaciones clínicas.

A pesar de los desafíos en su producción, su versatilidad y efectividad están impulsando su aplicación clínica en oncología y otras enfermedades. El texto también aborda el desarrollo de nuevos radiofármacos y su impacto en la medicina de precisión, señalando las futuras direcciones y oportunidades de investigación en este campo.

Introduction

In the last years, Nuclear Medicine has made significant advances in the development of new radiopharmaceuticals that are not only improving the results of targeted therapies but are also opening new pathways of investigation in diseases which, to date, were difficult to treat.

Theragnosis (from the Greek word “therapeia” or treatment and “gnosis” or knowledge/diagnosis) is based on the use of radiopharmaceutical pairs, which have the capacity to join at the same target while using different radionuclides, and allow sequentially performing diagnostic images (positron emission tomography [PET] or single photon emission computerized tomography [SPECT]) and therapy. By only changing the radionuclide, treatments can be personalized and the selectivity for the therapeutic target(s) can be simultaneously monitored, raising precision medicine to a new level.¹

With the aim of staying at the forefront of these advances, we present this series of articles of continuing education. These articles will explore the characteristics of the new radionuclides that are being integrated into clinical practice and the radiopharmaceuticals under development as well as their benefits in comparison with traditional methods, the technical challenges they present, and the promising future that they represent for our patients.

This first article covers the theragnostic pairs that we consider of greatest relevance and their clinical applications: copper-64/copper-67, lead-212/lead-203 and scandium-44/scandium-47.

Table 1 summarizes their principal physical characteristics, energy, obtainment methods and radiopharmaceuticals of major clinical relevance developed.

Table 1.

Radionuclides and their main physical characteristics, energies, obtainment methods and radiopharmaceuticals with the greatest clinical relevance.

Radionuclide	Decay	Production	Radiopharmaceuticals	Application	Ref.
Copper-64	T1/2 = 12.7 hβ^+ = 17.5%EC = 43.5%β^− = 39.0%	Cyclotron^64Ni(p,n)^64Cu^64Ni(d,2n)^64CuReactor^64Zn(n,p)^64Cu^63Cu(n,γ)^64Cu	[^64Cu]CuCl2	Malignant urological diseases	^10–12
				Brain tumors	^13
				Non-small cell lung cancer	^14
			[^64Cu]Cu-DOTA-TATE	Neuroendocrine tumors	^18,19
			[^64Cu]Cu-PSMA	Prostate tumors	^3,21
			[^64Cu]Cu−NOTA−MX001	PDL1 positive lesions	^23
			[^64Cu]Cu-PCB-TE2A-PEG4-trastuzumab[^64Cu]CuMeCOSar- trastuzumab	HER-2 positive lesions	^24
					^25
Copper-67	T1/2 = 2.58 dβ^− = 100%	Accelerator ^68Zn(p,2p)^67Cu ^68Zn(γ,p)^67CuCyclotron ^70Zn(p,α)^67Cu	[^67Cu]CuSarTATE	Pediatric neuroblastoma	^3
			[^67Cu]Cu−NOTA-Pertuzumab	HER2 positive lesions	^6
Lead-212	T1/2 = 10.6 hβ^− = 100%; decay to α emitters ^212Bi and ^212Po	Generator^228Th/^212PbCyclotron^232Th	[^212Pb]-NG001	Prostate tumors	ClinicalTrial.govNCT05725070
			[^212Pb]TCMC-trastuzumab[^212Pb]TCMC-37A7	HER-2 positive lesions	^37,38
					^36
			[^212Pb]DOTA-ReCCMSH	Melanoma	^39
			[^212Pb]DOTA-GGNle-CycMSHhex	Melanoma, lung	^40
			[^212Pb]VMT01	Metastatic melanoma	^41
			[^212Pb]DOTAMTATE	SSTR2 positive neuroendocrine tumors	^42,43
			[^212Pb]RM2	Tumors with overexpression of GRPR	^44
			[^212Pb]376.96	Tumors with overexpression of the B7-H3 protein (pancreas, ovary and prostate)	^45
Lead-203	T1/2 = 51.9 hEC a ^203Tl; γ emission: 81%	Cyclotron^203Tl	[^203Pb]Pb-TCMC-PSMA	Prostate cancer	^30,31
			[^203Pb]Pb-TCMC-CA012		^32^a
			[^203Pb]DOTA-GGNle-CycMSHhex	Melanoma, lung	^40
			[^203Pb]VMT01	Melanoma, lung	^41
			[^203Pb]RM2	Tumors with overexpression of GRPR	^44
Scandium-44	T1/2 = 4.04 h β^+ = 94.2% EC = 5.8%	Cyclotron ^44Ca(d,2n)^44ScGenerator ^44Ti/^44Sc ^natCa(p,n)^44Sc	[^44Sc]Sc-DOTATOC	Tumores neuroendocrinos metastásicos	^52–54
			[^44Sc]Sc-DOTA-c(RGD)2[^44Sc]Sc-NODAGA-c(RGD)2	Overexpression of integrin receptors (Glioblastoma, pancreatic tumor)	^57,58
			[^44Sc]Sc-DOTA-BN[2-14]NH2	Overexpression of GRPR (prostate tumors)	^51,60
			[^44Sc]Sc-PSMA-617[^44Sc]Sc-picaga-DUPA	Prostate tumors	^61
					^62,63
			[^44Sc]Sc-DO3AM-NI	Hypoxia	^65
			[^44Sc]Sc-DOTA-Ahx-A7R, [^44Sc]Sc-DR7A-DLys(DOTA-Sc)[^44Sc]Sc-DOTA-Ahx-K4R	Overexpression of neuropilin	^67
Scandium-47	T1/2 = 3.35 d β^− = 100%	^46Ca(n,γ)^47Ca → ^47Sc ^47Ti(n,p)^47Sc	[^47Sc]Sc-DOTATOC	Metastatic neuroendocrine tumors	^52–54

Theragnostic pairs

Copper-64 and copper-67

Physical characteristics and processes for obtaining copper radionuclides with clinical application

Copper-based radiopharmaceuticals are not new, having been investigated for almost 30 years. However, their currently increasing popularity is due to the versatility and the notable physicochemical characteristics of the radioisotopes of copper as well as the logistical advantages they provide in the obtainment of radiopharmaceuticals.

Copper-64

Copper-64 has a very particular decay scheme in which it combines β⁺ and β⁻ emitters with electron capture. It is considered to be an ideal radionuclide for theragnosis since it can generate PET images with good spatial resolution (low β⁺ energy and the absence of other γ emitters) and, at high activities, it has therapeutic potential. Of note, β⁺ and β⁻ particles provide doses at the cellular level, while electron capture produces Auger electrons, which, despite their short range, can be highly cytotoxic if the radionuclide penetrates the cell (Fig. 1).

Figure 1.

Diagram of the range of cell irradiation of the DNA according to the type of radionuclide emitter. AE: Auger electron. Image adapted from Bolcaen, J. et al.⁶

The half-life of copper-64 is 12.7 hours, which is adequate for both drugs with rapid kinetics (small molecules and peptides) as well as for transport molecules of slower kinetics (i.e., antibodies). In addition, this period allows their transfer from a production cyclotron to satellite installations.

The production of copper-64 in medical cyclotrons is simple and well established, generally through the nuclear reaction ⁶⁴Ni(p,n)⁶⁴Cu, by the bombardment of a target of nickel-64 with a proton beam of 11–15 MeV. This is precisely the bombardment energy necessary that can be produced in medical cyclotrons that are available in healthcare centers in Spain. However, obtainment of the radionuclide precursor can present certain challenges. Firstly, the natural abundance of nickel-64 is low (0.9%), leading to limited availability and a high cost. Secondly, although there is the possibility of irradiating nickel-64 in a liquid target, this technique is not yet completely developed, and thus, the irradiation of solid targets continues to be the most common method, elevating the cost in economic, personnel and time terms, since it involves the extraction of copper-64 and the recovery of nickel-64 for reuse.^2,3

From a dosimetric point of view, radiopharmaceuticals labeled with copper-64 are within the range of other radionuclides used for diagnostic purposes, providing between 0.01–0.06 mSv/MBq administered.⁴ The irradiation of critical organs, such as the kidneys, bladder, lungs or bone marrow, varies based on different factors including the kinetics of the transport molecule.⁵

Copper-67

Copper-67 emits β⁻ particles with energies that are comparable to those of other commonly used therapeutic radionuclides, such as lutetium-177 and iodine-131. In addition to emitting β⁻ particles in 100% of the decays, copper-67 also emits a photon of 185 KeV in 49% of the decays, making it adequate for obtaining SPECT images. It has a half-life of 2.6 days. Although the half-life and the energy of the emissions of copper-67 make it a very interesting radionuclide, its use is quite limited due to its low availability.

The production of copper-67 requires intermediate energy protons, that cannot be achieved in medical cyclotrons. The main production method is the ⁶⁸Zn(p,2p)⁶⁷Cu reaction, which requires protons with energies greater than 100 MeV and zinc targets enriched in zinc-68. The ⁷⁰Zn(p,α)⁶⁷Cu reaction has also been investigated, being feasible with energies less than 30 MeV, but its efficiency is very low.⁷

Some companies in the United States have recently been able to produce quantities of gigabecquerels of copper-67 by a reaction induced by ⁶⁸Zn(γ,p)⁶⁷Cu photons and it is now commercially available.⁸

Theragnostic pair copper-64/copper-67

The literature has described copper-64 and copper-67 as a theragnostic pair. However, as reported previously, both radioisotopes can be used for diagnosis and therapy (Fig. 2). If so, why are they proposed as a theragnostic pair and are not used separately? This association is due to several key factors:

• •

  The half-life of copper-64 is less than that of copper-67, implying a shorter time of irradiation of the target. This aspect is especially critical when the transport molecule of the radiopharmaceutical has slow kinetics, since it may reduce its efficacy.

• •

  During its decay, copper-64 emits β⁻ particles in 38% of the decays, while copper-67 does so in 100% of the decays. Although copper-64 emits Auger electrons, favoring the cytotoxic effect, the effectiveness of this process depends on the internalization of the radiopharmaceutical since it has a range of less than 500 μm.

• •

  The γ emissions of copper-67 allow monitoring the biodistribution of the radiopharmaceutical, but the quality of the images is less than that obtained with the PET image of copper-64.

• •

  The main advantage of this theragnostic pair with respect to others, such as gallium-68/lutetium-177 or gallium-68/actinium-225, is that they are the same chemical element. This is not a minor detail, since it allows the use of the same chelator for both radioisotopes; that is, the same molecule can be used for not only diagnosis but also for treatment, simplifying the development and production processes.⁵

Figure 2.

Decay schemes of copper-64 and copper-67 and the main scheme of the use of the theragnostic pair ⁶⁴Cu/⁶⁷Cu for preimaging, dosimetry, and the consequent radiotherapy. Adapted from Krasnovskaya et al.³

Radiopharmaceuticals with copper-64 and copper-67 and applications

[⁶⁴Cu]CuCl₂

Copper-64 chloride ([⁶⁴Cu]CuCl₂) may be a useful radiopharmaceutical for diagnosis in oncology since copper is a necessary element in angiogenesis and cellular proliferation, both of which are increased in tumoral cells.⁹ Copper is incorporated into the cell through the high-affinity transporter hCtr1, which is overexpressed in the tumoral tissue of different neoplasias, converting it into an adequate target for both diagnosis and therapy.¹⁰

Among the diseases in which [⁶⁴Cu]CuCl₂ may be used for diagnosis by molecular imaging, the following are of note:

• •

  Malignant urological diseases, such as cancer of the prostate, bladder or penis. In 2018, Piccardo et al.¹¹ described the possibility of detecting metastasis of prostate cancer in patients with biochemical recurrence by PET/CT with [⁶⁴Cu]CuCl₂. A prospective study of 50 patients with biochemical recurrence following surgery or external radiotherapy compared the findings of the PET/CT with [⁶⁴Cu]CuCl₂ with those obtained by PET/CT with [¹⁸F]-Fluoromethylcholine and multiparametric magnetic resonance imaging (mpMRI). PET/CT with [⁶⁴Cu]CuCl₂ showed a higher global rate of detection (84%) versus PET/CT with [¹⁸F]Fluoromethylcholine (56%) and mpMRI (74%).

In 2021, Mascia et al. performed a phase 2 clinical trial in 23 patients with cancer of the prostate, penis and bladder. The results showed significantly greater maximum standard uptake values (SUVmax) in the primary tumor compared to healthy tissue, with no adverse effects related to the radiopharmaceutical being observed.¹²

• •

  Brain tumors: The use of [⁶⁴Cu]CuCl₂ has recently been described in pediatric patients with diffuse high-grade gliomas (n = 10), in which it was especially interesting that the uptake of the radiopharmaceutical in the tumor was concordant with the MRI findings and even greater in the presence of radiological signs of necrosis. Likewise, the SUVmax values of the tumor background ratio (TBR) progressively increased in the images at 24 and 72 hours, supporting the hypothesis of its role as a therapeutic agent in future studies.¹³

• •

  Non-small cell lung cancer: Although there is still limited evidence, in comparison with 2-[¹⁸F]FDG PET/CT in patients before initiating chemotherapy with platinum, PET/CT with [⁶⁴Cu]CuCl₂ showed good capacity for detecting primary lesions and lymph node metastases, although with lower detection rates than PET/CT with 2-[¹⁸F]FDG. Nonetheless, taking into account that the uptake of [⁶⁴Cu]CuCl₂ is based on the expression of the Ctr1 transporter, its use could help in identifying patients who would respond better to platinum-based chemotherapies. However, further studies are needed to confirm these results.¹⁴

• •

  Wilson’s disease: In Wilson’s disease (WD) the mutations in the ATP7B copper transported generate deficient biliary excretion of copper and low levels of ceruloplasmin in the circulation. In view of the limitations of previously developed therapies, gene therapy based on adeno-associated vectors (AAV) has shown to be a promising solution to non-surgically correct the metabolism of copper in mice with WD. VTX-801, an AAV that has a miniaturized version of the ATP7B gene, has shown to reduce copper concentrations in the liver and urine and restore fecal excretion of copper.^15,16

Preclinical trials with [⁶⁴Cu]CuCl₂ have allowed the measurement and monitoring of the metabolism of copper in real time and it could be a valuable tool for sensitive, non-invasive evaluation of the effects of VTX-801 in patients with WD.¹⁷

Radiopharmaceuticals targeting somatostatin receptors (SSTR)

[⁶⁴Cu][Cu(DOTATATE)] is a radiopharmaceutical agent for PET imaging that was approved by the FDA in 2020 and is indicated for the localization of neuroendocrine tumors (NETs) positive for SSTR. Most NETs overexpress SSTRs, having a density that reflects the grade of tumor differentiation and proliferation, which is useful for the diagnosis, treatment and follow-up of these patients. Octreotide and Tyr₃-octreotate (TOC) are among the somatostatin analogs most used in routine clinical practice in patients with NETs. They are peptides which are of note for their capacity to bind to the SSTRs with diagnostic radionuclides, such as gallium-68, and therapeutic radionuclides, such as lutetium-177.^3,18,19 Both radiopharmaceuticals are based on tyrosine-octreotate, conjugated with DOTA (tetraxetan) as a metal chelator. In a comparative study between [⁶⁴Cu][Cu(DOTATATE)] and [⁶⁸Ga][Ga(DOTATOC)], the sensitivity of both radiopharmaceuticals was similar in the analysis based on patients; however, [⁶⁴Cu][Cu(DOTATATE)] had the capacity to localize more truly positive lesions compared to [⁶⁸Ga][Ga(DOTATOC)] (33 vs. 7, respectively). One of the possible explanations for this better performance is the lower range of emission compared with gallium-68. It remains to be demonstrated whether the detection of more lesions translates into better therapeutic management for patients.²⁰ With respect to therapy with copper-67, one of the first ongoing trials is evaluating the efficacy and safety of [⁶⁷Cu][Cu(SarTATE)] (analog of Tyr3-octreotate) in pediatric patients with neuroblastoma (NCT04023331).³

Radiopharmaceuticals targeting the prostate-specific membrane antigen (PSMA) receptor

The prostate-specific membrane antigen (PSMA) is a key glycoprotein in tumors tissues of prostate cancer, showing significant overexpression in comparison with normal tissues.²¹

In Spain, [⁶⁸Ga][Ga(PSMA-11)], [¹⁸F]FDCPyL and [¹⁸F]F-PSMA-1007 are available. The sum of the interests of the radiopharmaceuticals against PSMA and of copper-64 has led to greater investment in the development of radiopharmaceuticals, offering potential advantages with respect to sensitivity on comparison with [⁶⁸Ga][Ga(PSMA-11)], and image quality and a similar detection range with respect to fluorinated radiopharmaceuticals.²²

In regard to radiopharmaceuticals with copper-67, at present a phase I/IIa clinical trial with step-up dosing and expansion of cohorts is ongoing with the aim of evaluating the safety and antitumoral efficacy of [⁶⁷Cu][Cu(SAR-bisPSMA)] (NCT04868604). The patients treated with this radiopharmaceutical are those with castration-resistant metastatic prostate cancer in progression and with previous exposure to one or more androgen receptor pathway inhibitors who present positivity PET with [⁶⁴Cu][Cu(SAR-bisPSMA)]. The key primary and secondary objectives include the evaluation of the safety and dosimetry of the two radiopharmaceutical, the determination of the maximum dose tolerated or maximum feasible dose and the antitumoral efficacy of [⁶⁷Cu][Cu(SAR-bisPSMA)].

Radiopharmaceuticals targeting anti PD-1 and PD-L1 antibodies

The radiolabeling of antibodies against PD-1 or PD-L1 is especially interesting for evaluating their expression in vivo, given their capacity to improve immune response versus tumoral cells. In a preclinical model, Xu et al. demonstrated a good correlation between the anti-PD-L1 antibody (MX001) labeled with copper [⁶⁴Cu][Cu(NOTAMX001)] and PD-L1 expression by immunohistochemistry in several types of cancer, being not only a potential diagnostic but also therapeutic agent.²³

Radiopharmaceuticals targeting human epidermal growth factor receptor 2 (HER-2)

HER-2 is overexpressed in 15–20% of breast tumors and its positivity has been associated with greater disease aggressiveness.⁶

Trastuzumab and pertuzumab are among the first therapeutic lines against breast cancer. These two drugs are widely used and target HER-2. Lee et al. demonstrated excellent in vivo stability and high affinity for positive HER-2 tumoral cells using the [⁶⁴Cu][Cu(PCB-TE2A-PEG4-trastuzumab)] conjugate, by an innovative focus of conjugation based on the click chemistry reaction. In addition, [⁶⁴Cu][Cu(PCB-TE2A-PEG4-trastuzumab)], showed excellent in vivo stability and elevated affinity for HER-2 tumoral cells, achieving an adequate background signal ratio.²⁴

The efficacy of radioimmunotherapy with [⁶⁷Cu][Cu(NOTA-pertuzumab) has recently been evaluated, which involves the conjugation of p-SCN-Bn-NOTA with pertuzumab and its posterior radiolabeling in murine models with positive HER-2 tumors. A dose-dependent inhibition of tumoral growth was observed, even with low doses and the activity in the tumoral cells could be visualized by SPECT up to 5 days after administration.⁶

Keinänen et al. reported an in vivo pre-targeting strategy using the theragnostic pair copper-64/copper-67 to perform PET images and carry out sequential radioimmunotherapy in human SW1222 colorectal carcinoma.²⁵

Key points

• •

  Radiopharmaceuticals labeled with copper-64 and copper-67 are very promising for the diagnosis and treatment of several types of cancer.

• •

  Copper-64 is an ideal radionuclide for theragnosis due to its combination of β⁺ and β⁻ emissions.

• •

  The main advantage that this theragnostic pair presents with respect to others, such as gallium-68/lutetium-177 or gallium-68/actinium-225, is that the two radionuclides are the same chemical element.

Lead-212 and lead-203

Physical characteristics and processes to obtain lead radionuclides with clinical application

The lead isotopes lead-212 and lead-203 are a theragnostic pair of great interest.

Lead-203 is generated in cyclotrons with proton beams of energies less than 30 MeV from a thallium-203 target. Its half-life is 51.9 hours and decays through electron capture to thallium-203, followed by the emission of a γ photon of 279 keV (81%). Thanks to the γ photon and that it decays to a stable nucleus, it allows visualizing the distribution of the radionuclide using conventional gamma cameras and due to the absence of radioactive products, the radiation dose is reduced, simplifying the dosimetry. Therefore, this radionuclide has a diagnostic role.

Lead-212 is produced with a thorium-228/lead-212 generator. The generator is obtained from spallation reactions produced in a cyclotron by protons of 500 MeV on a thorium-232 target. Its half-life is 10.6 hours, and 100% of the decays are via β⁻ particles which decay the α emitters bismuth-212 and polonium-212. Due to these α emitters, the therapeutic component of the theragnostic pair is provided by lead-212.

When the estimations used are based on lead-203 for the dosimetry of lead-212, ambiguity potentially appears due to the nature of the decay chain of lead-212, which must be studied in depth. The decay of lead-212 to bismuth-212 can generate the separation of the bismuth-212 radionuclide from the molecule where its parent was bound. The different biodistribution of this free radionuclide, especially in the case of α particle emitters, adds uncertainty to the dosimetry.

From simulations modeling particle transportation (particle and heavy ion transport code system – PHITS), it has been estimated that α particles contribute to more than 90% of the deposition of the dose in cells of 10 μm in diameter and in micro-metastasis of 1 cm in diameter, while the electrons from β decay contribute more significantly when the tumor size is greater. The half-life of lead-212 is very short (0.3 μs), avoiding notable relocalization with respect to the progenitor. However, in the case of bismuth-“!” (1 hour), redistribution is possible if it is separated from the molecule where the parent was found.

Independently of the production of lead-203 (via cyclotron) or lead-212 (via generator), the isotopes of the lead must be purified and isolated for use with radiopharmaceuticals (Fig. 3). Li et al. have published a review of the purification procedures of these radioisotopes prior to the processes of radiochemical labeling for the obtainment of the corresponding radiopharmaceuticals.²⁶

Figure 3.

Production and purification of lead-203 (left) and decay chain of thorium-228 as a generator of lead -212 (right). Adapted from Inghman et al.²⁷

Radiopharmaceuticals labeled with lead-212 and lead-203 and applications

The challenge of the use of lead radioisotopes for producing radiopharmaceuticals is the change of chemical element produced in these radioisotopes during decay. The chemical nature of a radioisotope determines its chemical-physical characteristics, among which the constant of stability is found when a complex with a chelator is formed. Consequently, one of the main problems of the use of these radionuclides is bone marrow radiotoxicity provoked by premature release of the metal by the chelating agent. Specifically, bismuth-212 presents lower affinity for the chelators commonly used for other radiometals of routine clinical use, which suggests that up to 30% of this radioisotope of the chelator is released and the patient receives internal radiation with no therapeutic benefits. In addition, it presents physiological uptake in the kidney, and thus, the dosimetry of this organ is the main limitation of the dose of radiopharmaceuticals labeled with lead-212 that can be administered.²⁸

Investigations in this setting have been focused on the search for a bifunctional chelator that is kinetically stable for both radionuclides. In 2000, Chappell et al. described a new bifunctional chelator: [4-NCS-Bz-TCMC], hereafter referred to as TCMC.²⁹ With lead this chelator forms a kinetically more stable complex ([Pb(TCMC)]) that is formed with the DOTA chelator, and in addition, allows wider pH ranges.

TCMC presents additional advantages over DOTA as a chelator agent of lead-212, such as a more efficient conjugation reaction with antibodies, with preservation of the immunoreactivity as well as more efficient radiolabeling reactions and with greater radiochemical performance.

Another chelator used for coordinating the radionuclides of lead is DOTAM. This chelator is derived from DOTA and allows generating a more thermodynamically stable complex.

Although important advances have been made in recent years in the development of new agents that maintain lead-212 and bismuth-212 chelated to the transporter molecule, TCMC continues to be one the agents most used for lead-212 and lead-203.²⁶

Radiopharmaceuticals targeting the PSMA receptor

Most of the research carried out for obtaining a PSMA molecule radiolabeled with lead-212 has been focused on the development of a molecule with the same affinity, selectivity and efficacy as [¹⁷⁷Lu]PSMA-617, attempting to introduce a chelator that generates a final compound that is more stable (TCMC instead of DOTA). In this sense, Banerjee et al. proposed several peptidomimetic molecules of PSMA inhibitors based on Glu-Urea-Lys (similar structure to PSMA-617). All of these molecules were for radiolabeling with lead-203, using TCMC or DOTA (as a control) as the chelator. All of the molecules were used in studies of in vivo biodistribution after intravenous injection of the compounds in a mouse model (n = 4) of prostate carcinoma. The results obtained indicated that, in addition to the stability of the chelator-radionuclide bond, the use of the TCMC chelator improved the uptake of the radiolabeled compounds in the target and, consequently, the therapeutic index. Moreover, the uptake was significantly lower in the kidney (main organ limiting the dose that can be administered for being the main route of excretion) compared with the compounds using DOTA as the chelator. The dosimetry received by the kidney is considered a critical parameter in radiopharmaceuticals with PSMA due to the elevated renal excretion, as in the case of lutetium-177 and actinium-225. However, in the case of radiopharmaceuticals with shorter half-lives, such as lead-212 or astate-211, they may represent a potential cause of toxicity due to the greater doses of radiation in the kidney during the first hours.^30,31

Along the same line, in two patients with metastatic prostate cancer Do Santo et al. performed dosimetry studies with [²⁰³Pb][Pb(CA012)], a PSMA ligand conjugated with TCMC which shared the same binding site as PSMA-617. In this first study in humans, an activity of 250–300 MBq de [²⁰³Pb][Pb(CA012)] was chosen using planar scintigraphy at 0.4, 4, 18, 28 and 42 hours post-injection, achieving an effective dose of 2.5 mSv per 100 MBq. The preliminary dosimetry study suggested that the injection activity could be safety increased to 750 MBq for future studies, with an effective dose of 18 mSv. The dosimetry was made for lead-212 using data extrapolated from the kinetics and biodistribution of lead-203 with the use of planar images.³²

There are numerous studies with promising results that have proposed different molecules similar to PSMA-617 with the TCMC chelator for labeling with lead radioisotopes, including even some studies in the clinical trial phase.^31,33 Recently, the phase 0/1 trial in the United States (NCT05725070) was completed. The aim of this trial was mainly to evaluate the biodistribution of the compound and the quality of the images, as well as the safety and tolerability of the NG001 molecule radiolabeled with lead-212 in patients with castration-resistant metastatic prostate cancer who had received all the therapeutic lines available. The results are still pending publication.

Radiopharmaceuticals targeting HER-2

Several articles have been published on the conjugation of lead-212 with trastuzumab by chelation with TCMC for directing the radiation of α particles to the tumoral microenvironment, involved in mechanisms of resistance and immune evasion in HER-2 positive tumors (Fig. 4). The overexpression of HER-2 in 20–30% of prostate cancers gained interest in directing the treatment against HER-2 in this type of tumor.³⁴ Tan et al. found that 20 μCi of [²¹²Pb][Pb(TCMC-trastuzumab)] injected intravenously suppressed tumoral growth and prolonged the median survival by 50% in a xenograft model in orthotopic human prostate cancer in comparison with the control arm.³⁵ Likewise, Boudousq et al. compared internalized [²¹²Pb][Pb(TCMC-trastuzumab)] and non-internalized [²¹²Pb][Pb(TCMC-37A7)] targeting the carcinoembryonic antigen (CEA) in squamous HER+/CEA+A-431 carcinoma tumors in mice, demonstrating that internalization and dosimetry at a cellular level were crucial for accurately evaluating the results of α radiotherapy.³⁶ Westrom et al. carried out an initial study with three patients with ovarian cancer with metastatic disease who were administered a single intraperitoneal injection of 7.4 MBq for evaluating the distribution, pharmacokinetics and safety. The injection of up to 274 MBq of [²¹²Pb][Pb(TCMC-trastuzumab)] was well tolerated and studies with a greater dose step-up seem feasible for better tumoral response.³⁷

Figure 4.

Schematic representation of radioimmunoconjugation for obtaining 212Pb-TCMC-trastuzumab targeting the HER-2 receptor. Adapted from Yong et al.³⁸

Radiopharmaceuticals targeting the melanocortin type 1 receptor (MC1R)

An alternative pathway for the introduction of therapy targeting MC1R is the use of radiolabeled analogs of the native peptide of MCIR, that is, α melanocyte stimulating hormone (α-MSH). Theragnosis with α-MSH analogs labeled with lead-203/lead-212 has been reported for the first time using α-MSH analogs through reaction of rhenium cyclization conjugated with DOTA (DOTA-ReCCMSH). In this study, 40% of the murine melanoma B16F0 tumors were completely eradicated following the administration of 7.4 MBq of [²¹²Pb][Pb(DOTA-ReCCMSH)] receiving a dose of 112.4 Gy in the tumor, 72.2 Gy in the kidney and 91 Gy in blood. The B16F0 xenograft was visualized by SPECT/CT at 2 hours post-injection, with the pharmacokinetic profile of [²⁰³Pb][Pb(DOTA-ReCCMSH)] being comparable to [²¹²Pb][Pb(DOTA-ReCCMSH)].³⁹ Along the same line, Yang et al. demonstrated that [²⁰³Pb][Pb(DOTA-GGNle-CycMSHhex)] (an analog of α-MSH) specifically bound to not only the melanoma tumoral cells but also to the metastatic lesions in the lung, and given these properties, there is the possibility of studying this analog as a new therapeutic agent in this type of tumor by α therapy with [²¹²Pb][Pb(DOTA-GGNle-CycMSHhex)].⁴⁰

For this indication, there is currently an ongoing early phase trial (NCT05655312) of step-up doses of the compound [²¹²Pb][Pb(VMT01)] in 52 patients with non-resectable metastatic melanoma.⁴¹ The trial is considering a dosimetric sub-study by imaging of the compound, but radiolabeled with lead-203, with the aims of estimating the dosimetry of the patient and correlate the efficacy and possible toxicities with the uptake of the radiopharmaceutical in the tumoral lesion.

Radiopharmaceuticals targeting somatostatin receptor 2 (SSTR2)

Stallons et al. evaluated [²¹²Pb][Pb(DOTAMTATE)] in an AR42J xenograft model positive for SSTR2, demonstrating that a single dose of 1.5 MBq [²¹²Pb][Pb(DOTAMTATE)] caused acute toxicity and death within one week with 3 cycles of 0.5 MBq [²¹²Pb][Pb(DOTAMTATE)] were well tolerated. These studies demonstrated that the administration of 3 cycles of [²¹²Pb][Pb(DOTAMTATE)] at 2-week intervals with an activity of 3.7 MBq/cycle prolonged the mean survival of 3 weeks in the controls to 12 weeks in treated mice.⁴² In 2018, this compound completed the phase 1 clinical trial in both the evaluation of safety and in dose step-up in patients with neuroendocrine tumors positive for SSTR2 (NCT03466216). The preliminary results in 20 subjects have recently been published and are very promising.⁴³ At present, a phase 2 clinical trial is ongoing (NCT05153772) to evaluate the effectiveness of the treatment. This active trial is an open multicenter study aimed at adult patients with a NET confirmed (by imaging and histology) with and without a previous history of therapy with radioligands. Although this study is the most advanced, there are currently five other early phase clinical trials that are active or planned to open in the near future to evaluate different peptides radiolabeled with lead-212 (NCT05636618, NCT06148636, NCT06479811, NCT06427798, NCT05153772).

Radiopharmaceuticals targeting gastrin-releasing peptide receptors (GRPR)

Okoye et al. evaluated the use of the bombesin peptide RM2, radiolabeled with lead-203 in mice with xenografts of prostate cancer with overexpression of GRPR. The study showed rapid uptake and prolonged retention of the compound in the tumors, allowing SPECT/CT images with excellent contrast to be obtained. However, discrepancies were observed in the maximum uptake values between the compounds labeled with lead-212 and lead-203, attributed to differences in molar activity.⁴⁴ At present, studies are underway to determine the therapeutic efficacy and the maximum dose tolerated of the compound labeled with lead -212.

Radiopharmaceuticals targeting the B7-H3 (CD276) protein

Antibodies targeting the B7-H3 protein labeled with lead-212 versus B7-H3 have been evaluated in preclinical studies. This co-stimulator protein is overexpressed in different types of tumors, such as cancer of the pancreas, ovary and prostate. Kasten et al. used the antibody 376.96 radiolabeled with lead-212 in xenografts of human ovarian cancer and found significant uptake of the antibody in the tumors at 24 hours post-injection. However, no notable difference was observed in the uptake compared with a non-specific antibody due to the high uptake of the latter in the tumors.⁴⁵ Despite the A2870cp20 cells having more binding sites than ES-2 cells, the mean survival did not improve with [²¹²Pb][Pb(376.96)], thereby highlighting the need to better understand the differences between the in vitro and in vivo studies.

Key points

• •

  One of the major challenges in the use of lead-212 is bone marrow radiotoxicity produced by premature release of the metal of the chelating agent.

• •

  With the obtainment of radiopharmaceuticals with lead-212 and lead-203 it has been demonstrated that the TCMC chelator is more efficient than DOTA, because it increases the kinetic stability of the complex.

• •

  The decay of lead-212 to bismuth-212 can produce a release of the radionuclide and alter the biodistribution of the same, affecting treatment accuracy.

Scandium-43, scandium-44 and scandium-47

Physical characteristics and processes for obtaining radionuclides of scandium with clinical application

There are more than 20 scandium radioisotopes with half-lives that range from a few minutes up to several weeks. Among these, scandium-43, scandium-44 and scandium-47 have generated the greatest interest for applications in nuclear medicine because of the favorable emission characteristics they present. Scandium-43 and scandium-44 are positron emitters (can be used for diagnosis) while scandium-47 is an emitter of β⁻ particles and low energy (can be used in therapy) (Fig. 5).⁴⁶

Figure 5.

General view of the peptide radiopharmaceuticals labeled with scandium-44 and scandium-47 for their use in diagnosis and therapy and decay diagrams of both radionuclides. GRPR: gastrin-releasing peptide receptor Adapted from Trencsényi et al.⁵¹

In comparison with gallium-68, the radioisotopes of scandium-43 and scandium-44 have longer half-lives, less positron energy and greater intensity of positron emission.

Scandium-44 has the particularity of emitting a γ photon in coincidence (actually with a few picoseconds of delay) with the positron. Although the possibility of performing PET images using the three photons has been studied to improve spatial resolution, at present, the third photon is considered an inconvenience more than an advantage.⁴⁷ Overall, this is because by being a high energy photon (1157 keV) it complicates the procedures from the point of view of radiological protection. Therefore, scandium-43 is gaining interest as a nuclide with better properties. However, it is difficult and costly to produce, especially with an elevated purity. Nonetheless, it is true that the use of mixtures of scandium-43/scandium-44 has been tried and it does not seem to be a problem from the point of view of PET image quality.⁴⁸

The production of the scandium radioisotopes using accelerators, cyclotrons or reactors has been exhaustively investigated. A recent review has described the nuclear reactions reported to date in the literature, describing variable results in terms of performance and purity.⁴⁶

An alternative that is also being investigated is production in a generator for decay of a parent radionuclide. Scandium-44 is a decay product of titanium-44, while scandium-47 is a product of the decay of calcium-47.^49,50 This solution with both isotopes, one of diagnosis and one of therapy, obtained with generators, would be ideal in clinical practice, but does not yet seem to be viable since the generators developed up to now do not allow obtaining sufficient activity for clinical applications.

Radiopharmaceuticals with scandium-44 and scandium-47 and applications

Radiopharmaceuticals targeting somatostatin receptor 2 (SSTR2)

Peptide analogs of somatostatin, such as [⁴⁴Sc][Sc(DOTATOC)], are used. Similarly to gallium-68, scandium-44 can form thermodynamically stable complexes with the DOTA chelator with the advantage of it longer half-life (68 min vs. 2.97 h) allowing remote distribution of the radiopharmaceutical and the performance of longer studies.^52,53

The labeling of [⁴⁴Sc][Sc(DOTATOC)] is carried out by incubation of scandium-44 with the DOTATOC peptide for 25 minutes at a temperature of 95% in an oil bath and posterior purification of the radiopharmaceutical by reverse phase chromatography, obtaining labeling yields greater than 98%. The incubation time may be reduced to 3 minutes with the use of a microwave oven.⁵⁴ The radiopharmaceutical presents a stability greater than 98% in physiological saline 0.9% and phosphate buffer solution.

In 2017, Singh et al. published the first experience in humans (two patients with metastatic NETs) with the radiopharmaceutical [⁴⁴Sc][Sc(DOTATOC)] obtained in a cyclotron.⁵⁵ Multiple whole body PET/CT images were acquired between 10 minutes and 24 hours after administration of the radiopharmaceutical and these were compared with those previously obtained with [⁶⁸Ga][Ga(DOTATOC)]. [⁴⁴Sc][Sc(DOTATOC)] demonstrated excellent uptake of the radiopeptide in the lesions of both patients, achieving the best image quality at 4 hours after administration as well as a greater number of hepatic lesions. In addition, no visually significant uptake of [⁴⁴Sc][Sc(DOTATOC)] was identified in the pituitary or salivary glands or in the intestine. Neither were any clinical adverse effects observed and the hematological, renal and hepatic profiles remained unaltered, and thus, this radiopharmaceutical was considered highly sensitive and safe for use in humans. The authors concluded that this scandium-44/scandium-47 conjugate is ideal due to its theragnostic potential.

Radiopharmaceuticals targeting integrin receptors

The radiopharmaceuticals [⁴⁴Sc][Sc(DOTA-c(RGD)₂)] and [⁴⁴Sc][Sc(NODAGA-c(RGD)₂)] are based on the arginylglycylaspartic acid (RGD) peptide, which is a tripeptide that acts as an inhibitor of the integrin receptors, a protein that plays an important role in angiogenesis.⁵⁶

The difference between the two lies in the chelator used during labeling, which is performed by incubation of the RGD peptide with the chelator at a temperature of 90 °C for 10 minutes, obtaining radiochemical purities of greater than 95%. The radiopharmaceutical labeled with DOTA presents greater stability than that labeled with NODAGA (98% vs. 77 % after 4 half-lives) and greater blood clearance after intravenous administration.^57,58

It is important to know that the presence of cations may produce displacement and release of the radioactive isotope, with this phenomenon being more frequent in the case of labeling with the NODAGA chelator.⁵⁴

The biodistribution and the in vivo PET/CT images in mice with human glioblastoma cells and murine exocrine pancreatic tumor were studied. [⁴⁴Sc]]Sc(DOTA-c(RGD)₂)] and [⁴⁴Sc][Sc(NODAGA-c(RGD)₂)] presented the same tumoral distribution, although curiously, hepatic uptake of the first was greater at 30 minutes post-injection, leveling out in later acquisitions. On comparing the biodistribution of these same compounds but with gallium-68, those labeled with DOTA presented greater hepatic uptake compared to those of scandium-44, in agreement with the in vivo PET data. Since the pattern of distribution of [⁴⁴Sc][Sc(NODAGA-c(RGD)₂)] was similar to that of [⁴⁴Sc]]Sc(DOTA-c(RGD)₂)], NODAGA seems to be an attractive alternative as a chelator for PET radiopharmaceuticals labeled with scandium-44.⁵⁸

Radiopharmaceuticals targeting bombesin receptors

The radiopharmaceutical [⁴⁴Sc][Sc(DOTA-BN[2-14]NH₂)] is a radiolabeled analog of bombesin (peptide that binds with great affinity to the gastrin-releasing peptide receptors [GRPR]), which is overexpressed in prostate cancer.⁵⁹

The labeling of this transport molecule is performed by the incubation of scandium-44 with the peptide [⁴⁴Sc][Sc(DOTA-BN[2-14]NH₂)] at a temperature of 95 °C for 25 minutes and posterior purification of the radiopharmaceutical by extraction in solid phase, obtaining yields greater than 80%.⁶⁰ The radiopharmaceutical presents elevated stability in vitro, with 89.6% remaining intact after two hours of incubation in human serum at 37 °C. Following intravenous administration, it presents rapid blood clearance with elevated renal excretion.⁵¹

Koumarianou et al. directly compared [⁴⁴Sc][Sc(DOTA-BN[2-14]NH₂)] with [⁶⁸Ga][Ga(DOTA-BN[2-14]NH₂)].⁶⁰ In this study, both radiopharmaceuticals showed compared tumoral uptake both in terms of global uptake and the dynamics of distribution. Similar patterns of distribution were observed as well as equivalent time constants and elimination. The uptake was slightly superior in the peripheral regions of the tumor, possibly due to differences in the expression of the GRPR in the tissue or to variations in the tumoral microenvironment. These findings suggest that the utility of the preparations with gallium-68 or scandium-44 for the detection of tumors with GRPR is equivalent.⁶⁰

Radiopharmaceuticals targeting the prostate-specific membrane antigen (PSMA)

[⁴⁴Sc][Sc(PSMA-617)] is analogous to [¹⁷⁷Lu][Lu(PSMA-617)] but is radiolabeled with scandium-44.

Eppard et al. performed labeling by incubation of the scandium-44 with PSMA for 20 minutes at a temperature of 95 °C and posterior purification of the radiopharmaceutical with a extraction cartridge in solid phase, obtaining yields greater than 98%, presenting an elevated stability in vitro in physiological saline 0.9% and plasma, and with more than 95% remaining intact 24 hours after incubation. This same group, performed the first PET study in humans with [⁴⁴Sc][Sc(PSMA-617)] in 4 men with prostate cancer. They compared the dynamic images at 30 min and 2 h post-injection with those obtained with [⁶⁸Ga][Ga(PSMA-11)], with the images being of similar quality and with no differences in the quantitative analysis of uptake in most organs, although less uptake of [⁴⁴Sc][Sc(PSMA-617)] was observed in the kidneys compared to [⁶⁸Ga][Ga(PSMA-11)]. The pathological lesions were detected despite using a dose with less activity and with the possibility of performing acquisitions at up to 19 h post-injection, thereby avoiding activity in the urinary tract and kidneys. Among the advantages of their half-life, they noted the possibility of performing pretherapeutic dosimetry.⁶¹

The radiopharmaceutical [⁴⁴Sc][Sc(picaga-DUPA)] also binds to PSMA and is obtained by incubation of scandium-44 with the picaga-DUPA ligand for 30 minutes at a temperature of 80 °C, followed by purification by extraction in solid phase.^62,63

In relation to the possibility of using this chelator with scandium-47 for performing treatment, Vaugh et al. demonstrated that the response of [⁴⁴Sc][Sc(picaga-DUPA)] in mice was comparable to that of [¹⁷⁷Lu][Lu(PSMA-617)] and [¹⁷⁷Lu][Lu(picaga-DUPA)].⁶⁴

Radiopharmaceuticals as markers of hypoxia

The radiopharmaceutical [⁴⁴Sc][Sc(DO3AM-NI)] is a derivative of 2-nitroimidazol, which penetrates into the cell by passive diffusion and is reduced by the nitroreductase enzyme. In the presence of oxygen, the radiopharmaceutical is reoxidized and released from the cell, while in conditions of hypoxia it is again reduced and remains retained within the interior of the hypoxic cells.⁶⁵ Hypoxic cells are known for promoting angiogenesis, the growth cancerous cells and resistance to treatments.

The synthesis is carried out by incubation of scandium-44 with DO3AM-NI for 15 minutes at a temperature of 95 °C and posterior purification of the radiopharmaceutical with an extraction cartridge in solid phase, obtaining elevated labeling yields (>93%) and radiochemical purity greater than 95%. Following it administration the radiopharmaceutical presents greater renal clearance than its analogs labeled with gallium-68.

In in vivo and ex vivo biodistribution studies [⁴⁴Sc][Sc(DO3AM-NI)] was compared with its analog labeled with gallium-68 in healthy and KB tumor (human epidermal carcinoma) carrier severe combined immunodeficient (SCID) mice. PET/MR studies were performed at 90 and 240 minutes after intravenous administration. Both radiopharmaceuticals showed similar uptake in the KB tumors; however, greater uptake of [⁶⁸Ga][Ga(DO3AM-NI)] was observed in non-target tissues and organs (liver, spleen, kidney, intestine, lung, heart and brain), resulting in greater background activity. This finding suggests that scandium-44 could have an advantage by providing images with less background activity.⁶⁶

Radiopharmaceuticals targeting neuropilin receptors

The radiopharmaceuticals [⁴⁴Sc][Sc(DOTA-Ahx-A7R)], [⁴⁴Sc][Sc(DR7A-DLys(DOTA-Sc))] and [⁴⁴Sc][Sc(DOTA-Ahx-K4R)] inhibit neuropilin 1, which is a co-receptor of vascular endothelial growth factor, that is overexpressed in different type of tumors.⁶⁷

Labeling is performed by incubation of scandium-44 with the different peptides (DOTA-Ahx-A7R, DOTA-DLys-DR7A and DOTA-Ahx-K4R) for 20 minutes at a temperature of 95 °C, obtaining labeling yields greater than 95% and a radiochemical purity also greater than 95%. All of these peptides showed elevated stability in phosphate buffer solution. Unfortunately, all the radiopharmaceuticals presented low stability in plasma, making the search for other inhibitors of angiogenesis necessary.⁶⁸

Key points

• •

  Scandium radionuclides present advantages over gallium-68 by having a longer half-life thereby allowing more prolonged studies.

• •

  The production of scandium-43 and scandium-47 is complex due to difficulties in their production methods and high cost.

• •

  Preclinical and clinical studies with these Radiopharmaceuticals have demonstrated high tumoral uptake, and low toxicity, highlighting their theragnostic potential.