2.1 Radium-223

Radium is an alkaline earth metal that originates from the decay of ²³⁵U or one of its daughter nuclides, such as ²³¹Th (25.5 h), ²³¹Pa (3.28 × 10⁴ y), ²²⁷Ac (21.7 years), or ²²⁷Th (18.7 days) [5]. The production of ²²³Ra for clinical applications uses ²²⁷Ac/²²⁷Th generators in which the parent isotopes are on actinide chromatographic resins. A ²²³Ra-chloride solution is obtained after elution and purification on a cation exchange column. The disintegration sequence of ²²³Ra generates four high energy α particles, two β particles, and γ rays leading to total energy emitted of approximately 28 MeV, which includes ²¹⁹Rn (3.96 s), ²¹⁵Po (1.78 ms), ²¹¹Pb (36.1 min), ²¹¹Bi (2.14 min), ²⁰⁷Tl (4.77 min), or ²¹¹Po (0.52 s) and stable ²⁰⁷Pb. The cascade of α-particles produced (approximately 96% of the total energy) allows increasing the radiation dose received by the targeted tissues, while low energy γ-components (154 keV, 269 keV) are of interest for monitoring [6].

2.2 Astatine-211

The nuclear reaction ²⁰⁹Bi(α,2n) ²¹¹At is produced by bombarding solid ²⁰⁹Bi targets with α-beams at energies below 28.4 MeV [7]. When the energy is <28.4 MeV, ²¹¹At is created, which is isotopically pure. An undesired byproduct, ²¹⁰At, is coproduced at energies >30 MeV. One stable ²⁰⁷Bi α-emission with a half-life of 7.21 h is produced by the radioactive decay of ²¹¹At, making it appropriate for use in medical procedures.

2.3 Actinium-225

With a half-life of approximately 10 days, ²²⁵Ac decays to ²⁰⁹Bi; the decay path generates a net four α-particles and two β-particles with maximal energies of 1.6 and 0.6 MeV. In preclinical research and clinical administration, ²²⁵Ac is primarily radiochemically isolated from thorium-229 (²²⁹Th) generated due to the decay of aged uranium-233 (²³³U). As an alternative, ²²⁵Ac can be produced by bombarding radium-226 (²²⁶Ra) in a medical cyclotron with <20 MeV beam energy, causing it to undergo ²²⁶Ra(p,2n) ²²⁵Ac reaction. ²²⁵Ac is also produced in substantial quantities by irradiating thorium-232 (²³²Th) with high-energy protons, where ²²⁶Ac and ²²⁷Ac are produced [8-10].

2.4 Bismuth-213

²¹³Bi is a promising α-emitting radionuclide that has been successfully used in clinical trials for various blood and solid tumors [11]. It is a decay product of ²²⁵Ac with a relatively short half-life of 45.6 min and commonly used as an in vivo radioisotope generator in the form of a ²²⁵Ac/²¹³Bi generator [12]. The ²²⁵Ac/²¹³Bi generators use cation and anion exchange or extraction chromatography [13, 14]. The double generator using AG MP-50 cation exchange resin is the most mature, the most widely used, and the preferred system in clinical research.

2.5 Lead-212

Thorium nuclide ²¹²Pb is part of the ²³²Th and ²³²U decay chains: ²³²Th (T_1/2 = 1.4 × 10¹⁰ y) undergoes an α decay to produce ²²⁸Ra (T_1/2 = 5.75 years); ²²⁸Ra undergoes a β⁻ decay to produce daughter ²²⁸Ac (T_1/2 = 6.15 h); ²²⁸Ac undergoes β⁻ decay again with ²²⁸Th (T_1/2 = 1.91 years), which then forms ²²⁴Ra (T_1/2 = 3.6 days); and ²¹²Pb can be separated from ²²⁸Th and ²²⁴Ra, the natural decay products of ²³²Th. Therefore, ²²⁸Th and ²²⁴Ra are parent radionuclides suitable for the ²¹²Pb generator [12].

Friedman et al. deposited ²²⁸Th on a column containing sodium titanate and used the water flow to carry out ²²⁰Rn [15], and a D50 organic cation exchange resin column was used to elute ²¹²Pb. Another approach is to use ²²⁴Ra, which has a shorter half-life than ²²⁸Th, as the parent nuclide. ²²⁴Ra is the preferred source of radionuclides because of its ability to reduce radiation hazards. Radium and thorium are separated by dissolving thorium in concentrated nitric acid to form the thorium nitrate anion; the solution is added to an anion exchange resin to elute ²²⁴Ra. Eluted ²²⁴Ra was loaded onto the cation exchange resin and used as a ²¹²Pb/²¹²Bi generator.

2.6 Thorium-227

²²⁷Thorium is the progenitor nuclide of ²²³Ra [16]. Its half-life is 18.7 days, and it primarily decays as α particle emission with five high-energy α particles and two β particles in the decay chain, which subsequently decays to stable ²⁰⁷Pb [17, 18]. ²²⁷Th is isolated from the ²²⁷Ac-based generator. Furthermore, high-purity ²²⁷Th is obtained through repeated extraction and separation processes of the short-lived decay product ²²³Ra.

3.1 Radium-223

Radium belongs to group 2 of the periodic table and therefore has similar chemical properties to magnesium, calcium, and barium. With an electronic configuration of [Rn]7s², the corresponding divalent cation Ra²⁺ is the only species formed. Because ²²³Ra²⁺ is the largest divalent cation in the periodic table (8-coordinate ionic radius, 148 p.m.), the identification of suitable chelators for ²²³Ra²⁺ remains challenging [19]. Studies on the complexation of radium cations by 1-,4-,7-,10-tetraazcyclododecane-1,4-,7-,10-tetra acetic acid (DOTA), amino carboxylic acids, cryptands, and calixarenes were unsuccessful. The coordination chemistry of radium remains rather limited. Thus, the lack of an efficient chelator has prevented the conjugation of ²³Ra to biomolecules as with other alpha emitters. Furthermore, even if retention in structures such as nanoparticles is possible, ²²³Ra is mainly used in its chlorine salt form [²²³Ra]RaCl₂ [20].

We reasoned that macropa would be an effective chelator for this promising therapeutic radiometal. Macropa quantitatively formed the [²²³Ra][Ra(macropa)] complex within 5 min at RT and pH 6. The concentration of ligand required to achieve 50% radiolabeling efficiency was 13 mM. Importantly, [²²³Ra][Ra(macropa)] remained ∼90% intact in human serum at 37°C after 12 days.

3.2 Astatine-211

While At has a metallic quality, no chelating ligand studied thus far has been able to create a ²¹¹At complex stable enough for TAT applications [21]. Nonactivated aryl-At bonding is used in several At labeling techniques, with electrophilic substitution processes using organometallic compounds, such as compound 2, N-succinimidyl 3-(tri-alkylstannyl)benzoate, as the most common technique.

Nonactivated arylboronic acid derivatives react rapidly with electrophilic At species. Recent research demonstrated that nucleophilic substitution processes using aryl boronic acids/esters can produce extremely effective ²¹¹At-labeling. Additionally, mAb conjugate labeling by nucleophilic substitution is possible with aryliodonium salts [22]. In preclinical and clinical research, the boron cage compound isothiocyanatophenyl-closo-decaborate(2-) (B10) was used as a reagent for ²¹¹At-labeling. The aromatic B10 moiety (75%–90% radiochemical yield, 1 min) contributes to excellent ²¹¹At tagging efficiency and in vivo stability. New boron cage labeling reagents are being investigated for more favorable pharmacokinetic properties and better tissue distribution to potentially stabilize ²¹¹At in a higher oxidation state.

3.3 Actinium-225

The free daughters of ²²⁵Ac such as ²¹³Bi and ²²¹Fr might spread out or be carried to different target organs, where they accumulate and induce radiotoxicity, particularly in the kidney. The kidney is a prime location for the accumulation of free radioactive bismuth because renal proximal tubular cells have proteins that resemble metallothionein and bind bismuth. Additionally, renal tubular cells may be able to collect ²²¹Fr [23]. Chelators were used to improve the stability of ²²⁵Ac, and DOTA is the most widely used chelator. Previous studies reported that macrocyclic ligand macropa has better labeling characteristics for ²²⁵Ac radiometal compared with ligands [24]. However, ²²⁵Ac has yet to be incorporated into radiopharmaceuticals used in clinical settings. In vivo kinetic and thermodynamic instability prevents the use of linear bifunctional chelators such as ethylenediamine tetraacetic acid (EDTA) and diethylene triamine pentaacetic acid (DTPA) for tagging ²²⁵Ac-based radiopharmaceuticals.

3.4 Bismuth-213

The key consideration for ²¹³Bi labeling is the appropriate rate of complex formation to accommodate its short half-life. The complexation kinetics of acyclic ligand DTPA is very fast at room temperature and has a high thermodynamic stability constant for many metal ions [25]. The stability constant of 10 for the complex of this ligand with Bi(III), the most common and stable oxidation state of Bi, is sufficient for the formation of a kinetically inert complex in vivo. However, despite the very high initial stability constant of the Bi(III)DTPA complex, the expected good in vivo stability was not observed with bifunctional DTPA derivatives conjugated with monoclonal antibodies. The division of the base of the cyclohexyl DTPA in the ligand of the DTPA derivative trans-cyclohexyl DTPA (CHX-A″-DTPA) into chelators increases rigidity, provides preorganization at metal ion binding sites, and increases kinetic inertness [25]. The complexation of CHX-A″-DTPA ligand with Bi(III) not only maintains relatively rapid complex formation kinetics but also provides very high in vivo stability [26]. The current ²¹³Bi TAT studies mainly use CHX-A″-DTPA, which is the most suitable ligand for bismuth chelation [27].

3.5 Lead-212

²¹²Pb has a strong affinity to nitrogen and oxygen, and the coordination properties of bifunctional chelators contribute to the formation of radioactive metal complexes that can be attached to carrier molecules. Ligands that stabilize the chelation of ²¹²Pb and its decay to form ²¹²Bi are more attractive because off-target free bismuth from the labeling complex tends to accumulate and lead to toxicity in kidneys [28].

A chelating agent of 1, 4, 7, 10-tetramethylcarboformyl-1, 4, 7, 10-tetrazazecyclododecane (TCMC) was developed; it was more stable to Pb(II) and more resistant to complex dissociation at lower pH in acidic cells compared with Pb(II)DOTA complexes. TCMC ligands also show other benefits, including more efficient conjugation with monoclonal antibodies and higher labeling yields [29-31]. These advantages support the use of TCMC as a ligand for the in vivo generator of ²¹²Pb/²¹²Bi.

3.6 Thorium-227

The therapeutic radionuclide ²²⁷Th is primarily used for labeling of numerous different vectors such as monoclonal antibodies and nanocarriers. Monoclonal antibodies with tumor-targeting properties can be efficiently labeled using ²²⁷Th via the covalent attachment of the ε-amino group of the lysine residue to the octadentate 3,2-HOPO chelator, resulting in a product with high yield, purity, and stability under physiological conditions [32].

A summary of the chelators of α-emitters is shown in Figure 3.

5.1 Astatine-211

Patients with metastatic differentiated thyroid carcinoma are commonly treated with ¹³¹I. However, some tumors are resistant to this treatment. ²¹¹At is a radiohalogen with iodine-like chemical characteristics. In studies of in vitro treatments with thyroid cancer cells, [²¹¹At]NaAt significantly increased DNA double-strand damage compared with [¹³¹I]NaI, and a higher therapeutic efficacy was seen for [²¹¹At]NaAt in an animal model of preclinical thyroid cancer [39]. Therefore, the use of ²¹¹At in TAT to treat advanced differentiated thyroid carcinoma is quite promising.

Our group synthesized ²¹¹At-SPC-octreotide, which could target somatostatin receptor subtype 2. It exhibits rapid organ clearance 24 h after injection and increased uptake in the spleen, lung, stomach, and intestines in the early stages of the post-injection period. A radiation dose–dependent induction of apoptosis in cells by ²¹¹At-SPC-octreotide was observed. Therefore, ²¹¹At-labeled octreotide may be a potential cancer treatment [40].

In addition to peptides, monoclonal antibodies also serve as targets. Trastuzumab is a monoclonal antibody that targets human epidermal growth factor type 2 (HER2). By binding to HER2, it prevents human epidermal growth factors from adhering to HER2 and thereby limits the development of cancer cells. The capacity of ²¹¹At-trastuzumab to bind to breast cancer cells was found to be extremely selective. The biological effect of ²¹¹At-trastuzumab was twice as great as that of external radiation therapy [41], and ²¹¹At-trastuzumab inhibited the growth of HER2-positive tumor cells. Another study showed that ²¹¹At-trastuzumab extended median survival without posing a severe risk of harm in the treatment of liver metastases from primary gastric cancer [62].

Trastuzumab has shown some promising benefits, but the size of intact antibodies causes sluggish and uneven transport to tumors and prolonged residence time in healthy tissues [63].

Preclinical research showed that HER2-single-domain antibodies successfully transport to HER2-positive brain tumors and penetrate tumors much more quickly than whole antibodies [63, 64]. To investigate the effectiveness of the HER2 single-domain antibody, Feng et al. designed iso-²¹¹At-SAGMB-5F7 and iso-²¹¹At-SAGMB-VHH_1028. In most treated animals, full tumor regression was seen 200 days following therapy, and the treatments showed good therapeutic efficacy against HER2-positive breast cancer xenografts at a single dose [42]. However, radiolabeling prosthetic groups such as iso-[²¹¹At]SAGMB react with multiple lysine residues in the single-domain antibody (sdAb) fragment, resulting in a combination of radioconjugates with a range of properties. Because of the variability and irreproducibility inherent in random labeling, particularly at higher radioactivity levels required for therapeutic TAT, there are significant barriers to the efficient use of radioimmunoconjugates. Feng et al. thus described a method for radiohalogenating sdAbs at a particular location using a prosthetic compound that possesses both a residualized moiety and a thiol-reactive PODS activity. The in vitro stability was higher for iso-[²¹¹At]AGMB-PODS-5F7GGC than for iso-[²¹¹At]MEAGMB-5F7GGC, and tumor targeting was excellent [43].

In addition to directly killing tumor cells, the ²¹¹At-conjugates improved CD4+ T cell and macrophage recruitment [65]. More data is required to support the pro-inflammatory effects because CD4+ T cells also contain immunosuppressive T-regulatory cells. Our group constructed ²¹¹At-ATE-MnO₂-BSA, which showed enhanced anti-cancer immune activity by increasing the proportion of dendritic cells. In combination with anti-PD-L1, distant tumor growth was inhibited from the abscopal effect [44].

5.2 Actinium-225

Many ²²⁵Ac labeled ligands are undergoing preclinical or early-stage clinical trials. Because of its long half-life, ²²⁵Ac is a suitable candidate for monoclonal labeling. Several targeted receptors overexpressed in cancer, such as HER2, fibroblast activating protein (FAP), and CD45, are in the development and preclinical stages [45-47]. HER2 is an important target for breast cancer. HER2-targeting mAb labeled with ²²⁵Ac α-particle emitters provides highly lethal and localized radiation to targeted cancer cells with minimal exposure to surrounding healthy tissues. FAP is highly expressed in cancer-associated fibroblasts in epithelial cancer stroma and its high expression is associated with poor prognosis. FAP inhibitors (FAPI) are used for theranostics in oncology. A study investigated the therapeutic effects of [²²⁵Ac]FAPI-04 in FAP-expressing human pancreatic cancer [66]. Another study suggested the possible application of FAPI radioligand therapy (RLT) in FAP-expressing pancreatic cancer [45]. Moreover, radio-labeled CD45 antibodies have shown clinical promise as a targeted modulator prior to bone marrow transplantation as a more effective alternative to chemotherapy and/or whole-body irradiation of a medullary conditioning regimen. In addition to monoclonal antibodies, peptides and small molecule compounds labeled with ²²⁵Ac are promising new radioconjugates for targeted α-particle therapy.

Quantitative imaging plays a significant role in improving therapeutic outcomes and guiding clinical trials. Without an intense emission from ²²⁵Ac decay, measuring ²²⁵Ac typically relies on detecting the 218 keV γ-ray from the ²²¹Fr decay and the 440 keV γ-ray from the ²¹³Bi decay; single-photon emission computed tomography (SPECT) is used to produce quantifiable dosage maps [67]. However, the required doses were two orders of magnitude over the mouse toxicity limit [68]. The requirement for adequate techniques to quantitatively assess ²²⁵Ac dosimetry in the clinic setting remains unfulfilled.

5.3 Bismuth-213

Many preclinical studies have been carried out on ²¹³Bi in leukemia, non-Hodgkin/s lymphoma, malignant melanoma, brain tumors, neuroendocrine tumors, and other cancer types using multiple targeting strategies, antibodies, antibody fragments, peptides, liposomes, and other carriers.

In a highly aggressive B16F10 mouse melanoma model, ²¹³Bi-labeled humanized 8C3 (h8C3) monoclonal antibody showed high uptake rates in tumors and virtually no uptake in natural melanotic tissues. The ²¹³Bi-h8C3 treatment group exhibited a dose-dependent reduction in tumor volume without significant hematological or systemic side effects. Compared with ²¹³Bi-h8C3, ¹⁷⁷Lu-h8C3 had a worse effect on tumor growth at the same dose. No significant dose response was observed between ¹⁷⁷Lu-h8C3 and ¹⁷⁷Lu-H8C3 at a high dose (14.8 MBq) and low dose (7.4 MBq) [48].

The ²¹³Bi-labeled HER2-targeted sdAb ²¹³Bi-DTPA-2Rs15d sdAb demonstrated excellent in vivo stability and rapid tumor-specific uptake in HER2-positive tumor models with peritoneal metastasis. Efficacy evaluation showed significantly improved median survival and lower normal tissue toxicity in mice compared with 0.9% NaCl group [49]. Thus, ²¹³Bi-DTPA-2Rs15d sdAb is a promising new radioconjugate for the treatment of HER2-positive metastatic cancers.

Peptides with low molecular weight and strong diffusivity enable rapid targeting, making them a promising combination with ²¹³Bi with a short half-life. A ²¹³Bi-labeled somatostatin analog [DOTA⁰, Tyr³] octreopeptide (DOTATOC) was evaluated for antitumor efficacy in a rat pancreatic cancer model and showed good therapeutic efficacy in a linear relationship with an absorbed dose [50]. In in vitro studies, ²¹³Bi-labeled DOTA-DPhe1-Tyr3-octreotate (DOTATATE) showed better tumor killing effects than ¹⁷⁷Lu-DOTATATE. Tumor cell viability was only 10% at a 3 Gy dose of ²¹³Bi-DOTATATE, while the cytotoxicity of ¹⁷⁷Lu-DOTATATE was much lower than that of ²¹³Bi-DOTATATE at the same absorbed dose. ¹⁷⁷Lu-DOTATATE requires six times the dose of ²¹³Bi-DOTATATE to achieve a 10% cell survival rate [51].

5.4 Lead-212

Preclinical studies have reported the antitumor effects of untargeted ²¹²Pb without a targeting vector. Intraperitoneal injection of ²¹²Pb-labeled sulfur colloids in an ovarian cancer mouse model showed effective tumor killing with ascites reduction and longer survival. However, when the dosage of ²¹²Pb reached 70 mCi, severe gastrointestinal toxicity occurred, resulting in mouse death [52]. Based on the fact that the intraperitoneal residence time of ferric hydroxide is longer than that of ferric sulfide or ferric hydroxide, [²¹²Pb]Fe(OH)₂ was further prepared as a colloidal carrier for ²¹²Pb [53]. In animal studies, dose-dependent survival was observed after intraperitoneal infusion of [²¹²Pb]Fe(OH)₂. ²¹²Pb significantly increased the radiosensitivity and chromosome aberration of tumor cells. [²¹²Pb]Fe(OH)₂ up to 2.6 mCi was used in dogs without any significant adverse effects or toxicity.

Targeted therapy studies were carried out for the labeling of ²¹²Pb with monoclonal antibodies, peptides, and other targeting molecules. Ruble et al. demonstrated the effectiveness of ²¹²Pb-labeled monoclonal antibody 103A targeting Rauscher leukemia virus in the treatment of leukemia. All leukemia model mice treated with ²¹²Pb-DOTA-103A were cured histologically. However, at a dose of only 20 mCi, there was severe myelotoxicity, and all animals died of leukopenia and secondary infection. TAT was also conducted for non-Hodgkin's lymphoma with the ²¹²Pb-labeled rituximab and anti-CD37 antibody NNV003 [54, 55]. Median survival was significantly prolonged after intravenous administration of ²¹²Pb-rituximab or ²¹²Pb-NNV003, demonstrating the efficacy of α nuclide in the treatment of hematologic tumors.

The combination of ²¹²Pb with small molecules such as PSMA inhibitors is also being investigated. PSMA ligands L1-L5 were synthesized for chelating ²⁰³Pb/²¹²Pb. ²¹²Pb-L2 showed high tumor-specific uptake in PSMA-positive tumor-bearing mice and achieved a good antitumor effect. When combined with the ²⁰³Pb labeled PSMA ligand, it enables real-time adjustment of treatment [56]. ²⁰³Pb, which has the same element as the therapeutic ²¹²Pb, has a half-life of 52 h and emits 80.1% γ rays at 279 keV, so it is capable of SPECT imaging. Therefore, ²⁰³Pb is highly expected to be used as a radionuclide tracer matching ²¹²Pb for accurate in vivo biodistribution imaging and targeted determination of ²¹²Pb radionuclide molecules, thus achieving integration of diagnosis and treatment.

5.5 Thorium-227

The therapeutic radionuclide ²²⁷Th is primarily used for labeling targeting vectors to enhance treatment efficacy. Targeted thorium-227 conjugate (TTC) therapy is a promising potent tool for TAT. Several studies have shown potential clinical applications of ²²⁷Th.

The 3,2-HOPO chelator and PSMA-targeting antibody are covalently linked to form the alpha particle emitter PSMA-targeted thorium-227 conjugate (PSMA-TTC). PSMA-TTC is preferentially internalized into cells with positive PSMA and demonstrated uptake by and efficacy against PSMA-positive tumors [57].

Mesothelin is a GPI-anchored membrane glycoprotein that is overexpressed in mesothelioma, ovarian, pancreatic, lung, and breast cancers, while showing limited expression in healthy tissues. A mesothelin-targeted ²²⁷Th conjugate (MSLN-TTC) has strong antitumor efficacy against disseminated lung cancer, resulting in a significant survival benefit [58].

Initial studies showed ²²⁷Th conjugated to antibodies such as trastuzumab and anti-CD20 monoclonal antibodies had significant antitumor effects in breast, ovarian, and lymphoma models [59, 60]. Later reports demonstrated ²²⁷Th-labeled CD22-targeted antibodies were safe and tolerated in patients with CD22-positive relapsed/refractory B-cell non-Hodgkin's lymphoma (R/R-NHL), promising clinical translation [61].

6.1 Radium-223

Because its chemical properties are similar to those of calcium, Ra²⁺ naturally targets this mineral component and is incorporated into the bone matrix via substitution for calcium ions. Thus, radium could be incorporated into hydroxyapatite. Consequently, ²²³Ra irradiation may be a promising option for the treatment of skeletal metastases. Studies in mice have shown that <1% of ²²³Ra decay products migrate away from the bone, thereby limiting the toxic effects and maximizing the exposure of bone metastases to alpha particle radiation [69].

²²³RaCl₂ was approved for clinical use by the FDA and the European Medicines Agency in 2013 and has been widely used in men with symptomatic mCRPC; a major milestone in the ALSYMPCA trial showed a survival benefit with ²²³Ra uptake and favorable safety profile. With no subsequent redistribution, peak skeletal uptake occurs within 1 h of injection. Blood clearance is rapid after intravenous administration and skeletal uptake is estimated to be 40%–60% of administered activity. Additionally, non-negligible levels of ²²³RaCl₂ are found in the spleen, stomach, and intestines and are rapidly excreted in the gastrointestinal tract [70]. In clinical reports, ²²³RaCl₂ produced a pain response in approximately 71% of patients and had significant positive effects on bone alkaline phosphatase and prostate-specific antigen levels and overall survival. These studies show no evidence of long-term toxicity during the 2 years of follow-up; the most common adverse events were transient diarrhea, fatigue, nausea, and vomiting [71].

Treatment with ²²³RaCl₂ may potentially have abscopal effects. Kwee et al. reported observations in two patients with castrate-resistant prostate cancer that supported a ²²³RaCl₂ abscopal effect and good response. Plasma interleukin 6 levels increased, indicating that immunological activation may have played a role in the response [72].

6.2 Astatine-211

Seven clinical investigations using ²¹¹At are ongoing. As reported in ClinicalTrials.gov, there are five early-phase clinical trials at the Fred Hutchinson Cancer Center using ²¹¹At-based radionuclide therapy to enhance outcome following hematopoietic cell transplantation. Investigations are now being conducted on the ²¹¹At-BC8-B10 and ²¹¹At-OKT10-B10 authorized constructions.

The NCT03128034 study is testing increasing doses of ²¹¹At-BC8-B10 for high-risk acute myeloid leukemia (AML) and acute lymphocytic leukemia. The patient population, transplantation technique, and conditioning regimen are different from those of the NCT03670966 phase I/II trial on ²¹¹At-BC8-B10 followed by donor stem cell transplantation for the treatment of patients with relapsed or refractory high-risk acute leukemia.

Two clinical trials are being conducted in Japan. At Fukushima Medical University, one trial is exploring ²¹¹At-MABG for patients with malignant pheochromocytoma. The study will follow the 3 + 3 study design and may increase to doses of 1.3 and 2.6 MBq kg⁻¹ depending on toxicity. Another trial at Osaka University Hospital is investigating [²¹¹At]NaAt for differentiated thyroid carcinoma. This dose-escalation phase I study employing a single intravenous dose of TAH-1005 includes patients with differentiated thyroid carcinoma who have not responded to conventional therapy. The starting dose is 1.25 and 10 MBq kg⁻¹ is the maximum dose.

6.3 Actinium-225

Some ²²⁵Ac-labeled ligand/antibody portions have also reached routine clinical practice for the treatment of neuroendocrine tumors, prostate cancer, and acute myelocytic leukemia. Somatostatin receptors are highly expressed in most neuroendocrine tumors. In 2011, the first human clinical study of a ²²⁵Ac-labeled small molecule for the treatment of patients with neuroendocrine tumors was conducted. Advanced and metastatic neuroendocrine tumors were treated with ²²⁵Ac-DOTA-TOC, and the results showed that most patients were stable and their symptoms were relieved, without side effects such as hematoxicity and liver and kidney function injury. Recently, a retrospective analysis of 39 patients who received ²²⁵Ac-DOTA-TOC was conducted to determine the safe level of ²²⁵Ac-DOTA-TOC. The analysis found that approximately 20 MBq per cycle (treatment every 4 months) and cumulative doses of 60–80 MBq of ²²⁵Ac-DOTATOC were reasonable for patients with advanced stage malignancies. Chronic renal toxicity was observed at these doses but pre-existing renal risk factors were important cofactors. In a long-term study of patients with metastatic gastro-pancreatic neuroendocrine tumors receiving both ²²⁵Ac-DOTATATE-TAT and capecitabine, ²²⁵Ac-DOTATATE TAT showed satisfactory outcomes and improved overall survival. In patients previously refractory to ¹⁷⁷Lu-DOTATATE, there were brief and acceptable adverse reactions [73, 74].

Several small-molecule PSMA ligands that can be conjugated to radioisotopes, such as ¹⁸F, ⁶⁸Ga, ¹⁷⁷Lu, and ²²⁵Ac, have been developed. The value of ⁶⁸Ga-PSMA (FDA approved in 2020) and [¹⁸F]DCFPyL for the treatment of prostate cancer patients has been widely recognized. PSMA-based radioactive tracers, such as ¹⁷⁷Lutetium-PSMA (¹⁷⁷Lu-PSMA) and ²²⁵Ac-PSMA (²²⁵Ac-PSMA), have shown unique diagnostic and therapeutic value [75]. ²²⁵Ac-PSMA, which emits a very narrow range of radiation equivalent to a few cell diameters, is more effective for treatment than ¹⁷⁷Lu-PSMA, which emits β rays. A recent meta-analysis by Lee et al. showed that prostate specific antigen decreased in approximately 84% of patients and prostate specific antigen decreased more than 50% in approximately 61% of patients after receiving ²²⁵Ac-PSMA RLT, which is far better than the 46% and 57% for ¹⁷⁷Lu-PSMA RLT [76]. Clinical data on PSMA TAT was available only for ²²⁵Ac-PSMA-617. TAT for prostate cancer using ²²⁵Ac-PSMA-617 showed a significant therapeutic effect and has the potential to overcome resistance to beta-transmitter therapy. However, TAT of ²²⁵Ac PSMA-617 can cause side effects including xerostomia and hematotoxicity. To reduce toxic side effects, new approaches are needed. PSMA-I&T was introduced in 2014 as a theranostic PSMA-targeting small molecule. Mathias and colleagues reported the first clinical experience of patients receiving ²²⁵Ac-PSMA-I&T; the results showed a promising antitumor effect in advanced metastatic castration-resistant prostate cancer, and grade 3/4 hematological side effect were reduced, indicating that this may be an additional therapeutic option in end-stage mCRPC patients [77].

In addition to prostate cancer and neuroendocrine tumors, hematological diseases can also be treated by ²²⁵Ac radiopharmaceuticals. Lintuzumab targeting HL60 leukemia shows strong specificity [78]. The first-in-human, phase I dose-escalation trial of 18 patients with relapsed or refractory AML showed that targeted therapy with ²²⁵Ac-lintuzumab is feasible, has an acceptable safety profile, and exhibits activity against advanced AML. Clinical trials for RIT for glioblastoma are also underway. While therapeutic efficacy of ²²⁵Ac radiopharmaceuticals was observed, further studies should focus on the improvement of the biological and chemical properties of ²⁵Ac radiopharmaceuticals, patient selection, and fractionation regimes.

6.4 Bismuth-213

²¹³Bi is the first α particle emitter to enter clinical studies. The labeled lintuzumab, HuM195, was tested in a phase I clinical trial in 18 patients with relapsed or refractory AML. Studies showed that ²¹³Bi-HuM195 was rapidly ingested in the bone marrow, liver, and kidney, where leukemia cells were aggregated, and 78% of patients exhibited a reduced proportion of myeloblasts. ²¹³Bi-HuM195 was well tolerated, with no evidence of obvious extramedullary toxicity [79]. The effectiveness and safety of ²¹³Bi-HuM195 against leukemia were demonstrated in this study, which was the first validation for targeted α-emitting radionuclide immunotherapy in humans.

Peptide receptor alpha therapy with ²¹³Bi combined with low molecular weight, rapidly diffusing peptides has shown promise for glioma treatment. Substance P labeled with ²¹³Bi targeting G protein-coupled receptor neurokinin Type 1 receptor (NK-1R) was used for the treatment of recurrent glioblastoma multiforme (GBM). 20 GBM patients treated with ²¹³Bi-DOTA-SP showed significant improvements in progression-free survival and overall survival compared with patients treated with other therapies such as surgery or chemotherapy [80].

²¹³Bi represents a new treatment option for the targeted treatment of prostate cancer with PSMA-617. Prostate-specific antigen levels decreased nearly four-fold (from 237 to 43 μg/L) [81]. However, the dosimetric estimation of ²¹³Bi from ⁶⁸Ga-PSMA-617 imaging showed that ²¹³Bi-PSMA-617 was less effective than ²²⁵Ac-PSMA-617 [82].

6.5 Lead-212

The first clinical trials of the ²¹²Pb/²¹²Bi in vivo generator were conducted from 2011 to 2015. Peritoneal treatment was administered to 18 individuals with HER2-positive tumors (ovarian and colon cancers). In the first human study, conducted by Meredith et al., the biological distribution, pharmacokinetics, and safety of ²¹²Pb-TCMC-trastuzumab were evaluated. Six doses of 0.2–0.74 mCi/m² (7.4–27.4 MBq/m²) were tested using a single intraperitoneal injection of ²¹²Pb-TCMC-trastuzumab (7.4 MBq/m²), followed by a dose escalation study. Minimal extraperitoneal radiopharmaceutical redistribution and no significant myelosuppression were observed; the maximum dosage did not induce major hematologic, renal, or hepatic damage, suggesting the dose might be increased in conjunction with other medications. However, none of the patients showed a partial response, and all patients had disease progression within <8 months [83].

Another study, based on successful preclinical studies of ²¹²Pb-DOTAMTATE for somatostatin receptor–positive metastatic neuroendocrine tumors [84], is a phase I clinical trial initiated in 2018. The optimal dose was found, and the objective radiological response was as high as 83%, much higher than the 13% of ¹⁷⁷Lu-DOTATATE. The clinical study for dosage escalation of ²¹²Pb-DOTAMTATE in humans in 2022 demonstrated good treatment tolerability, no drug-related serious adverse events, and encouraging therapeutic effects in 10 subjects with neuroendocrine tumors [85].

6.6 Thorium-227

Four clinical development initiatives based on TTC are in progress. ²²⁷Th conjugates targeting PSMA are currently being evaluated in patients with metastatic castration-resistant prostate cancer. The study is determining the safety and tolerability profile of PSMA-TTC alone or in combination with darolutamide, the maximum tolerated dose, the recommended dose for further clinical development, and how the drug is distributed and cleared in vivo.

Additionally, ²²⁷Th labeled antibody-chelator conjugate is being evaluated for the treatment of patients with advanced recurrent epithelioid mesothelioma or serous ovarian cancer in the NCT03507452 trial. The patients received a single intravenous dose on day 1 of each cycle for a total of 6 weeks (42 days), starting at 1.5 MBq and increasing by increments of 1.0 or 1.5 MBq.

A ²²⁷Th-labeled antibody-chelator conjugate that targets HER2 on cancer cells is being studied in an open-label study (NCT04147819). The study included patients with HER2-overexpressing breast cancer, HER2 low expressing breast cancer, and other HER2-expressing cancers. Each patient will receive the drug once every 6 weeks for up to 6 cycles and will be monitored for safety and efficacy for up to 3 years.

In the phase I study by Lindén et al., 21 patients with CD22-positive relapsed/refractory R/R-NHL were treated with ²²⁷Th-labeled CD22-targeting antibodies. While the drug was safe and tolerable with an objective response rate of 25% (5/21 patients), including one complete response and four partial responses, adverse events such as neutropenia, thrombocytopenia, and leukopenia were observed in 10 patients.

The current clinical trials of α-emitters are summarized in Table 3.