Theranostic Imaging Surrogates for Targeted Alpha Therapy: Progress in Production, Purification, and Applications

This article highlights recent developments of SPECT and PET diagnostic imaging surrogates for targeted alpha particle therapy (TAT) radiopharmaceuticals. It outlines the rationale for using imaging surrogates to improve diagnostic-scan accuracy and facilitate research, and the properties an imaging-surrogate candidate should possess. It evaluates the strengths and limitations of each potential imaging surrogate. Thirteen surrogates for TAT are explored: 133La, 132La, 134Ce/134La, and 226Ac for 225Ac TAT; 203Pb for 212Pb TAT; 131Ba for 223Ra and 224Ra TAT; 123I, 124I, 131I and 209At for 211At TAT; 134Ce/134La for 227Th TAT; and 155Tb and 152Tb for 149Tb TAT.


Introduction
Targeted alpha therapy (TAT) involves utilizing radiopharmaceuticals to precisely eliminate malignancies with alpha particle emissions, while sparing surrounding healthy tissues.These radiopharmaceuticals consist of alpha (α)-emitting radionuclides conjugated to a biological-targeting vector such as monoclonal antibodies, peptides, and nanocarriers [1].Key advantages of TAT include highly selective radiation delivery to the target, reduced patient side effects, and the ability to assess radiopharmaceutical uptake and, therefore, patient eligibility using a diagnostic radionuclide before therapy [2].
While beta minus (β − ) radiopharmaceuticals employing radionuclides such as 177 Lu have made significant advances in clinical care of advanced prostate and neuroendocrine tumors [3,4], alpha particle emissions are significantly more precise and cytotoxic than β − emissions.This is attributed to the much larger size of alpha particles (7300 times the mass of electrons), their 2+ charge resulting in a highly ionized emission path, and high linear energy transfer that deposits their energy over a path length of only several cell diameters.These properties make alpha emitters ideal for combatting metastatic cancers and other systemic malignancies where traditional treatment avenues have failed [2,[5][6][7].
While the potency of TAT offers significantly enhanced therapeutic efficacy, TAT must be treated as a double-edged sword with the possibility of severe off-target toxicity to nontarget organs and tissues.This mandates a comprehensive understanding of the stability, pharmacokinetics, and dosimetry of any TAT radiopharmaceutical.During preclinical tool for evaluating patient dosimetry, provided that the targeting vector exhibits rapid in vivo clearance, minimal off-target binding, and the radionuclide is stably incorporated within the radiopharmaceutical.Radiopharmaceutical pretargeting approaches may reduce the advantage of selecting diagnostic and therapeutic TAT radionuclides with similar half-lives; however, it is uncertain whether most theranostic targeting vectors will employ a pretargeting approach.
Regarding radioactive emissions, it is preferable that PET imaging surrogates possess a high positron branching ratio and low positron emission energy to facilitate high-resolution PET imaging and minimal co-emitted electrons and gamma/X-rays to reduce the radioactive dose.Radionuclides with lower positron branching ratios may require additional injected activity to resolve the same quality image.For SPECT imaging, radionuclides should possess lower energy gamma rays within the optimal energy window of scanners and minimal co-emitted electrons and gamma/X-rays.
To produce imaging surrogates, sufficient cyclotron or nuclear reactor facilities are required to synthesize the radionuclide.Target material (natural or isotopically enriched) should be available in adequate quantity and enrichment to support routine production, and a favorable nuclear cross-section must exist within the capabilities of production facilities.Radionuclide production should be performed safely, create few long-lived radionuclidic impurities, and be scalable to sufficient activities that allow distribution to clinical sites.Robust chemical-purification techniques must separate the imaging surrogate from potentially hazardous target material post-irradiation.Finally, the radionuclide progeny of the imaging surrogate should be considered since this can influence imaging quality and impact radioactive waste management.
Most radionuclides used in TAT are part of decay chains where each decay results in the recoil of the daughter nucleus with energy sufficient to liberate the daughter nucleus from the chelator into solution.Additionally, the alpha particle itself may induce radiolytic damage to the radiopharmaceutical, reducing the in vivo targeting and leading to further accumulation of radioactivity in nontarget tissue.These inherent physical properties are not easily covered by the surrogates in question, so they should be considered in experimental methods and conclusions.
In this article, a selection of 13 diagnostic imaging surrogates for promising alphaemitting radionuclides have been highlighted for their production, purification, applications, and overall strengths and limitations.

Theranostic Imaging Surrogates Proposed for Actinium-225
Actinium-225 (t 1/2 = 9.9 d) has been explored extensively for TAT.Its long half-life permits extended dose delivery and decay via a cascade of six short-lived radionuclide progeny with four alpha particle emissions to near-stable 209 Bi, making 225 Ac particularly attractive for TAT. 225Ac studies have demonstrated efficacy in metastatic prostate cancer and neuroendocrine tumors, and additional radiopharmaceuticals are under development for other cancers [11,[24][25][26][27][28][29][30] There are considerable efforts underway to significantly increase the 225 Ac supply to meet the significant anticipated clinical demand [31][32][33][34].
However, 225 Ac does not emit gamma rays of sufficient intensity for imaging.Although its 213 Bi and 221 Fr progeny possess gamma rays of suitable energy and intensity for SPECT imaging [9], the 225 Ac activities injected into patients (~50-200 kBq/kg [11]) would be insufficient to resolve a high-quality image within a reasonable scan duration.Additionally, the supply of high-purity 225 Ac from 225 Ra/ 225 Ac generators is limited, constraining AT development efforts [31].While other sources of 225 Ac from high-energy spallation reactions are available [32,35,36], these often contain a small activity of co-produced and inseparable 227 Ac (t 1/2 = 21 y), which complicates radioactive waste management.Therefore, the desire to enable 225 Ac imaging and enhance research throughput motivates the development of imaging surrogates.

Lanthanum-133 (PET)
Lanthanum-133 (t 1/2 = 3.9 h) has been synthesized via the 135 Ba(p,3n) 133 La and 135 Ba(p,2n) 133 La nuclear reactions on medical cyclotrons [45].Natural Ba metal can be used as a target material, with one study producing 231 MBq 133 La and 166 MBq 135 La for 500 µA•min cyclotron irradiations at 22 MeV.Subsequent chemical processing using a diglycolamide (DGA) resin produced a highly pure [ 133 La]LaCl 3 product that, when used to radiolabel DOTA and macropa chelators, achieved molar activities sufficient for preclinical and clinical application [40].Co-production of 135 La (t 1/2 = 18.9 h (44)) is unavoidable using natural barium target material.While 135 La has potential applications for Auger-Meitner electron therapy, it would add additional patient radioactive dose and is undesirable for 133 La PET imaging applications.
Alternatively, natural or isotopically enriched BaCO 3 can be employed to simplify target preparation to boost 133 La yields and selectivity from co-produced 135 La.Another study irradiated [ 135 Ba]BaCO 3 at a 23.3 MeV proton energy, significantly improving 133   [41].Another approach involved irradiating isotopically enriched [ 134 Ba]BaCO 3 at a proton energy of 22 MeV, with subsequent purification yielding up to 1.2-1.8GBq [ 133 La]LaCl 3 with 0.4% co-produced 135 La and a radionuclidic purity of >99.5%.The decay of 133 La into its long-lived daughter 133 Ba (t 1/2 = 10.6 y) resulted in 4 kBq 133 Ba per 100 MBq 133 La, which was deemed uncritical concerning dose and waste management [42].
As shown in Figure 1, 133 La PET imaging analysis was performed in Derenzo phantoms and compared with other common PET radionuclides, with 133 La found to have superior spatial resolution compared to 44 Sc, 68 Ga, and another radiolanthanum positron emitter, 132 La [41].For SPECT imaging, 226 Ac is an elementally matched surrogate for 225 Ac.Radiolanthanum isotopes 133 La, 132 La, and 134 La are particularly attractive for PET imaging of 225 Ac due to the similar ionic radii of La 3+ and Ac 3+ (~1.03 and ~1.12 Å, respectively [37,38]) and their resulting similar chemistries.Both lanthanum and actinium possess similar chelation chemistry with chelators such as DOTA, macropa, and crown ethers, and exhibit similar in vivo biodistributions [39][40][41][42][43][44].The subsequent sections will outline the properties, strengths, and limitations of 133 La, 132 La, 134 Ce/ 134 La, and 226 Ac.

Lanthanum-133 (PET)
Lanthanum-133 (t1/2 = 3.9 h) has been synthesized via the 135 Ba(p,3n) 133 La and 135 Ba(p,2n) 133 La nuclear reactions on medical cyclotrons [45].Natural Ba metal can be used as a target material, with one study producing 231 MBq 133 La and 166 MBq 135 La for 500 µA•min cyclotron irradiations at 22 MeV.Subsequent chemical processing using a diglycolamide (DGA) resin produced a highly pure [ 133 La]LaCl3 product that, when used to radiolabel DOTA and macropa chelators, achieved molar activities sufficient for preclinical and clinical application [40].Co-production of 135 La (t1/2 = 18.9 h (44)) is unavoidable using natural barium target material.While 135 La has potential applications for Auger-Meitner electron therapy, it would add additional patient radioactive dose and is undesirable for 133 La PET imaging applications.
As shown in Figure 1, 133 La PET imaging analysis was performed in Derenzo phantoms and compared with other common PET radionuclides, with 133 La found to have superior spatial resolution compared to 44 Sc, 68 Ga, and another radiolanthanum positron emitter, 132 La [41].[41], with 18 F, 64 Cu, 44 Sc, and 68 Ga data from Ferguson et al. [46].
As depicted in Figure 2, PET imaging was performed with [ 133 La]La-PSMA I&T in a prostate cancer mouse model.The LNCaP prostate cancer tumors were delineated with  [41], with 18 F, 64 Cu, 44 Sc, and 68 Ga data from Ferguson et al. [46].
As depicted in Figure 2, PET imaging was performed with [ 133 La]La-PSMA I&T in a prostate cancer mouse model.The LNCaP prostate cancer tumors were delineated with high spatial resolution and minimal off-target uptake, demonstrating the potential for further 133 La PET imaging applications [41].high spatial resolution and minimal off-target uptake, demonstrating the potential for further 133 La PET imaging applications [41].Strengths of 133 La include its 3.9 h half-life that allows sufficient time for separation and distribution to external clinics; a lower positron emission energy compared to 68 Ga, 44 Sc, and 132 La that results in a higher PET imaging spatial resolution [47]; and low energy and intensity co-emitted gamma rays that reduce the radioactive dose.Limitations include the production requirement of medium-energy cyclotron facilities; its lower positron branching ratio of 7.2% that may require additional injected activity relative to other PET radionuclides such as 18 F; and its decay into relatively long-lived 133 Ba.
Strengths of 132 La include its 4.6 h half-life, which allows ease of radiopharmaceutical preparation and distribution compared to shorter-lived PET emitters such as 68 Ga; its stable 132 Ba decay daughter; and its significant 41.2% positron branching ratio [9].Limitations include severe cyclotron production constraints owing to the 0.1% natural isotopic abundance of 132 Ba target material; high energy and intensity co-emitted gamma rays that contribute to excess radioactive dose; and the high maximum positron emission energy of 3.67 MeV, which leads to a low PET spatial resolution and image blurring as shown in Figure 1.Strengths of 133 La include its 3.9 h half-life that allows sufficient time for separation and distribution to external clinics; a lower positron emission energy compared to 68 Ga, 44 Sc, and 132 La that results in a higher PET imaging spatial resolution [47]; and low energy and intensity co-emitted gamma rays that reduce the radioactive dose.Limitations include the production requirement of medium-energy cyclotron facilities; its lower positron branching ratio of 7.2% that may require additional injected activity relative to other PET radionuclides such as 18 F; and its decay into relatively long-lived 133 Ba.
Strengths of 132 La include its 4.6 h half-life, which allows ease of radiopharmaceutical preparation and distribution compared to shorter-lived PET emitters such as 68 Ga; its stable 132 Ba decay daughter; and its significant 41.2% positron branching ratio [9].Limitations include severe cyclotron production constraints owing to the 0.1% natural isotopic abundance of 132 Ba target material; high energy and intensity co-emitted gamma rays that contribute to excess radioactive dose; and the high maximum positron emission energy of 3.67 MeV, which leads to a low PET spatial resolution and image blurring as shown in Figure 1.

Lanthanum-134/Cerium-134 (PET)
Lanthanum-134 (t 1/2 = 6.5 min) can be produced via irradiation of natural barium target material; however, its short half-life precludes its direct use for PET imaging.Cerium-134 (t 1/2 = 3.2 d) decays into 134 La, permitting an in vivo generator configuration where 134 Ce can be labelled to a targeting vector, with 134 La progeny used for PET imaging.Production involves irradiating nat La metal, with yields of 59 MBq•µA −1 •h −1 at proton energies of 62.1-72.1 MeV [52].A subsequent production route utilized 100 MeV protons to irradiate nat La metal, producing over 3 Ci of 134 Ce with a 100 µA irradiation for 30 h.Chemical purification can be performed with Bio-Rad AGMP-1 resin, where 134 Ce is eluted with 0.05 M HNO 3 . 134Ce can then be used to label DTPA in its 3+ oxidation state, allowing 134 Ce to act as a 225 Ac imaging surrogate, while 134 Ce can label 3,4,3-LI(1,2-HOPO) in its 4+ oxidation state and act as a 227 Th imaging surrogate [53,54].A PET imaging phantom study investigating the spatial resolution and recovery coefficient of 134 La was found to be inferior and similar to 18 F, respectively [52].
Strengths of 134 Ce/ 134 La include the 3.2 d half-life of 134 Ce, which permits PET imaging at extended time points after injection to track 225 Ac and 227 Th radiopharmaceuticals; the significant 63.6% positron branching ratio of 134 La [9]; the stable 134 Ba decay daughter of 134 La; and the ability for 134 Ce to act as a surrogate for both 225 Ac and 227 Th.Limitations include a scarcity of production facilities capable of achieving a ~100 MeV proton beam energy; the high positron emission energy of 134 La, which would result in lower PET spatial resolution; unavoidable co-produced radionuclidic impurities ( 139 Ce, t 1/2 = 137.6);and the potential for in vivo 134 La daughter redistribution following decay from 134 Ce that could blur PET imaging [9,39].
A phantom assembly with rods between 0.85 and 1.7 mm in diameter and a mi-croSPECT/CT system was used to assess resolution using a high-energy ultra-high resolution (HEUHR) collimator and an extra ultra-high sensitivity (UHS) collimator.The primary 158 keV and 230 keV gamma photopeaks were reconstructed, with the 158 keV photopeak images demonstrating slightly better contrast recovery.For resolution, as depicted in Figure 3, the HEUHR collimator resolved all rods, while the UHS collimator could only resolve rods >1.3 mm and >1.5 mm for the 158 keV and 230 keV photopeaks, respectively [55].This demonstrated the feasibility of using 226 Ac as a SPECT imaging surrogate for 225 Ac.

Theranostic Imaging Surrogates Proposed for Lead-212
Lead-212 (t1/2 = 10.6 h) has cultivated a significant interest for TAT due to its payload of one alpha and two β -particles in its decay chain and the rapid decay of its progeny to stable 208 Pb.A recent study using a 212 Pb somatostatin analogue demonstrated a significant antitumor effect in patients with metastatic neuroendocrine tumors, and additional radiopharmaceuticals are under development to treat other cancers [1,[58][59][60][61][62]. Production of 212 Pb involves synthesizing its parent radionuclide, 228 Th (t1/2 = 1.9 y), via 226 Ra irradiation in a nuclear reactor or high-energy proton spallation of 232 Th target material. 212Pb can then be extracted in a convenient generator setup from 228 Th or one of its intermediate progeny, 224 Ra (t1/2 = 3.6 d) [12,[63][64][65][66][67].
Previous clinical trials have employed imaging techniques with conventional radiometals such as 68 Ga [58].While direct SPECT imaging of 212 Pb can be performed using its 239 keV (44%) gamma emissions [9], it is desirable to have an imaging surrogate that can be used for research owing to the limited supply of 212 Pb and to provide the most accurate pre-therapy scans to assess patient eligibility for 212 Pb TAT radiopharmaceuticals.While Advantages of 226 Ac include its relatively long 29.4 h half-life compared to 132 La and 133 La, permitting imaging at extended time points, and its identical chemical properties to 225 Ac.Limitations include challenges associated with routine irradiation of hazardous 226 Ra target material, significant β − co-emissions that would increase patient dose, and its decay to β − emitting 226 Th (t 1/2 = 30 min), which further decays via multiple alpha and β − emitting progeny before stabilizing at 206 Pb [9].

Theranostic Imaging Surrogates Proposed for Lead-212
Lead-212 (t 1/2 = 10.6 h) has cultivated a significant interest for TAT due to its payload of one alpha and two β − particles in its decay chain and the rapid decay of its progeny to stable 208 Pb.A recent study using a 212 Pb somatostatin analogue demonstrated a significant antitumor effect in patients with metastatic neuroendocrine tumors, and additional radiopharmaceuticals are under development to treat other cancers [1,[58][59][60][61][62]. Production of 212 Pb involves synthesizing its parent radionuclide, 228 Th (t 1/2 = 1.9 y), via 226 Ra irradiation in a nuclear reactor or high-energy proton spallation of 232 Th target material. 212Pb can then be extracted in a convenient generator setup from 228 Th or one of its intermediate progeny, 224 Ra (t 1/2 = 3.6 d) [12,[63][64][65][66][67].
Previous clinical trials have employed imaging techniques with conventional radiometals such as 68 Ga [58].While direct SPECT imaging of 212 Pb can be performed using its 239 keV (44%) gamma emissions [9], it is desirable to have an imaging surrogate that can be used for research owing to the limited supply of 212 Pb and to provide the most accurate pre-therapy scans to assess patient eligibility for 212 Pb TAT radiopharmaceuticals.While no positron-emitting Pb isotopes are suitable for use as 212 Pb imaging surrogates, multiple gamma-ray emitters exist, with 203 Pb being a prime candidate for SPECT imaging.

Lead-203 (SPECT)
Lead-203 (t 1/2 = 51.9h) emits X-rays and a primary 279 keV (81%) gamma photon that can be used for SPECT imaging. 203Pb has been synthesized via 203 Tl(p,n) 203 Pb, 203 Tl(d,2n) 203 Pb, and 205 Tl(p,3n) 203 Pb nuclear reactions on cyclotrons [21, 45,63,64,[68][69][70][71]. Natural thallium metal can be used as a target material; however, significant precautions must be taken owing to the high toxicity of Tl, and its low thermal conductivity and melting point (304 • C) that makes it prone to melt or sublime under intense heat of a cyclotron beam.Natural Tl metal has been used as a target material, with one technique bombarding nat Tl at 25-26 MeV, producing up to 21 GBq 203 Pb five days after end of bombardment [61].However, irradiating nat Tl produces significant activities of 201 Pb (t 1/2 = 9.3 h), which must be permitted to decay significantly to achieve a 203 Pb product with high radionuclidic impurity. 203Pb can be produced at lower proton energies using natural or isotopically enriched 203 Tl and the 203 Tl(p,n) 203 Pb nuclear reaction 63,71 , with one process yielding up to 138.7 ± 5.1 MBq 203 Pb [64].However, yields are limited due to the low nuclear reaction cross-section in this energy window [45].Alternatively, isotopically enriched 205 Tl can be irradiated at 23-24 MeV proton energies to produce 203 Pb via the 205 Tl(p,3n) 203 Pb reaction.This produces significant activities of 203 Pb (>12 GBq at the end of purification) with a high radionuclidic purity (>99.9%) made possible by the near absence of 203 Tl and its resulting 201 Pb co-production 21,63 .Enriched 203 Tl can also be bombarded with deuterons to produce 203 Pb via the 203 Tl(d,2n) 203 Pb reaction; however, this production route has a lower maximum cross-section compared to the 205 Tl(p,3n) 203 Pb reaction, and 203 Tl (29.5% natural isotopic abundance) is more expensive to enrich than 205 Tl (70.5% natural isotopic abundance). 203Pb can be separated using ion exchange resins such as Pb resin, carboxymethyl resin, and Dowex-1X8 anion exchange resin.This can yield a concentrated 203  Phantom imaging of 203 Pb has been performed, with imaging spatial-resolution results comparable to 99m Tc for 1.6-4.8mm diameter fillable rod regions [72].In vivo preclinical and clinical SPECT imaging of uncomplexed and chelated 203 Pb has been performed [71,73].Studies have included 203/212 Pb-labeled PSMA and gastrin-releasing peptide receptortargeting agents for imaging and radiotherapy of prostate-cancer-bearing mice [60,61,74,75], and 203/212 Pb-labeled anti-melanin antibodies and melanocortin subtype 1 receptor targeting ligands for imaging and therapy of melanoma-bearing mice [59,72,73,[76][77][78][79].As shown in Figure 4, a PSMA-targeting 203 Pb agent, [ 203 Pb]Pb-CA012, exhibited a comparable biodistribution to [ 177 Lu]Lu-PSMA 617 with high tumor uptake relative to other tissues [74].
photon emission spectrum that enables SPECT imaging using a low or high-energy collimator; its ability to rapidly and stably radiolabel targeting vectors under mild chemical conditions at room temperature (similar to 212 Pb); and established production processes that provide 203 Pb with high radionuclidic purity in yields suitable for multiple patients per production run.Limitations include risks associated with preparing and irradiating highly toxic thallium targets and potential uncertainties with using 203 Pb pharmacokinetic data for 212 Pb therapy planning due to the release of 212 Bi progeny during 212 Pb decay [80].

Theranostic Imaging Surrogates Proposed for Radium-223/224
Radium-223 (t1/2 = 11.4 d) is used as an alpha therapy for men with bone-metastatic castration-resistant prostate cancer.It works as a calcium-mimetic by accumulating in and irradiating osteoblastic lesions, while sparing most surrounding healthy tissue [81].It is the only FDA-proved alpha-particle-emitting radiopharmaceutical (Xofigo ® ) and has been used to treat over 18,000 patients since 2013 [82].However, unlike targeted alpha therapy, 223 Ra is currently administered as a [ 223 Ra]RaCl2 salt in an aqueous buffer without a chelator or biological-targeting agent.Therefore, the established clinical efficacy and safety of 223 Ra makes it an attractive TAT candidate [82].Similarly, 224 Ra (t1/2 = 3.6 d) has been employed in a dual targeting strategy with 212 Pb, where 224 Ra accumulates at primary bone cancer sites or bone metastases, while extra-skeletal metastases can be targeted with a 212 Pb-labeled cancer-specific vector [83,84].[ 224 Ra]RaCl2 (marketed as 224 SpondylAT ® (Eckert & Ziegler, Berlin, Germany) has also been used to treat bone and joint disease, ankylosing spondylitis [85], while 224 Ra is also under investigation for a novel brachytherapy Strengths of 203 Pb include its relatively long 51.9 h half-life, which permits imaging at extended time points to inform 212 Pb TAT dosimetry; its relatively clean X-ray and gamma photon emission spectrum that enables SPECT imaging using a low or high-energy collimator; its ability to rapidly and stably radiolabel targeting vectors under mild chemical conditions at room temperature (similar to 212 Pb); and established production processes that provide 203 Pb with high radionuclidic purity in yields suitable for multiple patients per production run.Limitations include risks associated with preparing and irradiating highly toxic thallium targets and potential uncertainties with using 203 Pb pharmacokinetic data for 212 Pb therapy planning due to the release of 212 Bi progeny during 212 Pb decay [80].

Theranostic Imaging Surrogates Proposed for Radium-223/224
Radium-223 (t 1/2 = 11.4 d) is used as an alpha therapy for men with bone-metastatic castration-resistant prostate cancer.It works as a calcium-mimetic by accumulating in and irradiating osteoblastic lesions, while sparing most surrounding healthy tissue [81].It is the only FDA-proved alpha-particle-emitting radiopharmaceutical (Xofigo ® ) and has been used to treat over 18,000 patients since 2013 [82].However, unlike targeted alpha therapy, 223 Ra is currently administered as a [ 223 Ra]RaCl 2 salt in an aqueous buffer without a chelator or biological-targeting agent.Therefore, the established clinical efficacy and safety of 223 Ra makes it an attractive TAT candidate [82].Similarly, 224 Ra (t 1/2 = 3.6 d) has been employed in a dual targeting strategy with 212 Pb, where 224 Ra accumulates at primary bone cancer sites or bone metastases, while extra-skeletal metastases can be targeted with a 212 Pblabeled cancer-specific vector [83,84].[ 224 Ra]RaCl 2 (marketed as 224 SpondylAT ® (Eckert & Ziegler, Berlin, Germany) has also been used to treat bone and joint disease, ankylosing spondylitis [85], while 224 Ra is also under investigation for a novel brachytherapy called diffusing alpha-emitter radiation therapy (DaRT).In DaRT, 224 Ra-infused seeds are inserted into solid tumors, which are then irradiated with alpha emissions released during the diffusion and subsequent decay cascade of its 220 Rn progeny [86][87][88][89][90][91][92][93][94][95].Both 223 Ra and 224 Ra are currently produced in significant activities as by-products and decay daughters of neutron irradiation of 226 Ra in a nuclear reactor.With proven purification techniques, this positions these radionuclides well for TAT [67,96,97]. 223Ra has recently been stably complexed with the chelator macropa, where a [ 223 Ra]Ramacropa complex exhibited rapid clearance and low 223 Ra bone absorption, suggesting in vivo stability.This has opened the possibility of using 223 Ra complexed using functionalized chelators to target metastases beyond the bone, similar to other radionuclides used in targeted alpha therapy [82,98].
While 223 Ra possesses several gamma emissions within an energy window suitable for SPECT imaging ( 223 Ra: 269 keV, (13%); 154 keV (6%); 224 Ra: 241 keV (4.1%)), the low intensity of these gamma photons would likely be insufficient to generate a high-quality SPECT image when considering the relatively low injected therapeutic activity (~50 kBq/kg) injected [9,81].Similarly, a relatively low injection activity of 224 Ra due to its 3.6 d half-life could complicate direct SPECT imaging.Therefore, an imaging surrogate is desirable to assess the viability of 223/224 Ra radiopharmaceuticals, with 131 Ba emerging as a candidate.

Barium-131 (SPECT)
Barium-131 (t 1/2 = 11.5 d) decays via electron capture to 131 Cs (t 1/2 = 9.7 d) and subsequently to stable 131 Xe, emitting gamma rays suitable for SPECT imaging (496 keV (48%); 216 keV (20%); 124 keV (30%); 371 keV (14%)) [9].Additionally, approaches designed to sequester Ra (nanoparticles, chelation via macropa or ligands based on the arene scaffold) [99,100] should be transferrable owing to the proven use of Ba as a non-radioactive surrogate for Ra [101].Therefore, the favorable imaging emissions of 131 Ba compared to other Ba radionuclides ( 135m Ba, 133m Ba), and the similar half-life and chemistry of 131 Ba to 223/224 Ra positions 131 Ba as a promising surrogate to track in vivo 223/224 Ra biodistribution. 131Ba can be produced via neutron irradiation of isotopically enriched 130 Ba (natural abundance = 0.1%) in a nuclear reactor, which would co-produce significant activities of 133 Ba [45,102].Alternatively, 131 Ba can be produced via proton irradiation of natural cesium target material in a cyclotron via the 133 Cs(p,3n) 133 Ba nuclear reaction with a small 133 Ba contamination (0.01%) at beam energies of 27.5 MeV [45,101].A 4 h irradiation yielded 190 ± 26 MBq 131 Ba, and an SR resin was used to separate 131 Ba from the Cs target material. 131Ba was subsequently successfully radiolabeled to macropa, and exhibited stability in human serum [101].
SPECT imaging was performed in a cylindrical syringe, which enabled visualization of the radionuclide distribution.However, image quality was limited due to artifacts caused by the higher energy gamma photon emissions.As highlighted in Figure 5, small animal SPECT/CT was performed with [ 131 Ba]Ba(NO 3 ) 2 , showing 131 Ba accumulation within the entire skeleton 1 h post-injection, which was still present 24 h after injection.Additional SPECT imaging was performed with [ 131 Ba]Ba-macropa, with rapid clearance observed through the intestines and gallbladder [101].This demonstrated the feasibility of using 131 Ba as a SPECT imaging surrogate for 223/224 Ra.

Theranostic Imaging Surrogates Proposed for Astatine-211
Astatine-211 (t1/2 = 7.2 h) has garnered interest for TAT owing to its decay to either 207 Bi (t1/2 = 31.6y) via alpha emission or to 211 Pb via electron capture followed by alpha Advantages of 131 Ba include its relatively long half-life, which is similar to 223 Ra, permitting imaging at extended time points; the ability to sequester 131 Ba in the macropa chelator similar to 223 Ra; and established 131 Ba production routes.Limitations include higher energy gamma photon emissions, which increase unintended patient dose and can cause image artifacts.The presence of co-produced 133 Ba may also require additional dosimetric analysis.Additionally, the decay of 131 Ba to 131 Cs with X-ray emissions adds a suboptimal patient radioactive dose compared to an imaging radionuclide with direct decay to stable progeny.Finally, further improvements in the cyclotron production route would be required to synthesize enough activity for multiple patients in a single batch.

Theranostic Imaging Surrogates Proposed for Astatine-211
Astatine-211 (t 1/2 = 7.2 h) has garnered interest for TAT owing to its decay to either 207 Bi (t 1/2 = 31.6y) via alpha emission or to 211 Pb via electron capture followed by alpha decay to stable 207 Pb [9].Therefore, each 211 At decay yields one alpha particle.The 211 At decay chain also emits few high-energy gamma photons, which avoids excess radiation dose [8]. 211At can be produced in medium-energy alpha cyclotrons using bismuth target material and the 209 Bi(α,2n) 211 At nuclear reaction or via heavy ion irradiation and the 209 Bi( 7 Li,5n) 211 Rn reaction, where 211 At is obtained via decay of its longer-lived parent 211 Rn (t 1/2 = 14.6 h) in a generator configuration [8,103,104].Production yields of up to 6.6 GBq have been reported, which would be sufficient for clinical radiopharmaceutical production for several patients and distribution several hours from the production site [8,105]. 211At was initially investigated for treating thyroid disorders and is currently being evaluated in clinical trials for multiple myeloma, leukemia, myelodysplastic syndromes, thyroid cancer, and malignant pheochromocytoma [106].While direct SPECT imaging of 211 At is possible using the X-rays emitted during 211 At decay to 211 Po, it is desirable to have an imaging surrogate to perform pre-therapy assessment scans and research, owing to the limited supply and short half-life of 211 At that generally precludes its use at facilities located more than several hours from a production site.Several candidates exist for use as 211 At diagnostic imaging surrogates: chemically identical 209 At, or chemically similar 123 I, 124 I and 131 I.

Iodine-123 (SPECT)
Iodine-123 (t 1/2 = 13.2 h) decays via electron capture to near-stable 123 Te, and is commonly used in nuclear medicine and research of various malignancies and biological processes, including thyroid diseases and tumor imaging [107].Its X-ray emissions and primary gamma photopeak of 159 keV (83.6%) are well suited for SPECT imaging [9].
123 I is primarily produced via the 124 Xe(p,2n) 123 I nuclear reaction using a highly enriched 124 Xe gas target, which enables 123 I production with a high yield and radionuclidic purity.The subsequent 123 I product is commercially available in dilute NaOH solutions [108,109].
Strengths of 123 I include its favorable emission spectrum for SPECT imaging, similar half-life relative to 211 At, and commercial availability.Limitations include hazards associated with volatile radioactive products, the lower image quality of SPECT images to PET imaging, and the low natural abundance (0.095%) of 124 Xe target material.

Iodine-124 (PET)
Iodine-124 (t 1/2 = 4.2 d) undergoes positron decay to stable 124 Te and is employed for PET imaging studies.Its relatively long half-life allows extended radiosynthesis, quantitative imaging over several days, and distribution to sites far from production facilities [9]. 124I is typically produced using isotopically enriched 124 Te and the 124 Te(d,2n) 124 I or 124 Te(p,n) 124 I nuclear reactions [110,111].Applications in nuclear medicine and research have been extensive, including thyroid and parathyroid imaging, studies of neurotransmitter receptors, and monoclonal antibody imaging in cancer [110].
Strengths of 124 I include its long half-life that eases logistics and allows imaging at extended time points.Limitations include hazards associated with volatile radioactive products; a relatively low positron branching ratio (22.7%); relatively high average positron emission energy (E mean = 820 keV) that results in a lower spatial resolution compared to other PET radionuclides; and co-emitted gamma rays (603 keV (63%), 1691 keV (11%)) that increase dose and shielding requirements [9].
Strengths of 131 I include its 8 d half-life that permits imaging at extended time points, commercial availability, and primary 364 keV (81.5%) gamma emission that is well suited for SPECT imaging.However, limitations include hazards associated with volatile radioactive products and significant β − emissions that would increase patient dose [9].

Astatine-209 (SPECT)
Astatine-209 (t 1/2 = 5.4 h) decays via alpha emissions (4%) to 205 Bi (t 1/2 = 14.9 d) followed by decay to stable 205 Pb, or via electron capture (96%) to 209 Po (t 1/2 = 124 y).During decay to 209 Po, X-rays and gamma emissions (545 keV (91.0%), 195 keV (22.6%), and 239 keV (12.4%) enable SPECT imaging. 209At can be produced via high-energy proton spallation of a uranium carbide target, followed by online surface ionization and A = 213 isobars separation.This can yield 209 At in activities on the order of 10 2 MBq [113].Subsequent chemical purification employs a Te column to obtain purified 209 At [113,114].As shown in Figure 6, subsequent studies using 209 At for phantom imaging demonstrated that image reconstruction with 209 At X-ray emissions was superior to using its gamma emissions [114].Additionally, in vivo imaging measurements of 209 At uptake in mice matched ex vivo measurements within 10%.This demonstrated the potential of using 209 At to accurately determine astatine biodistributions [114].

Theranostic Imaging Surrogates Proposed for Thorium-227
Thorium-227 (t1/2 = 18.7 d) decays via alpha emission to 223 Ra and can be harvested from a generator containing 227 Ac (t1/2 = 21.8 y) that is produced via nuclear reactor irradiation of 226 Ra [115].Thorium can be complexed with octadentate 3,2-hydroxypyridinone (3,2-HOPO) chelators attached to biological-targeting vectors 115 .Ongoing clinical studies involving 227 Th TAT include targeting tumors expressing human epidermal growth factor Strengths include identical chemistry to 211 At, which would give more certainty to 209 At pharmacokinetic data.Limitations include alpha emissions in 209 At decay that would require dosimetric evaluation; numerous high-energy gamma rays that complicate shielding and increase patient dose; the need to consider longer-lived 205 Bi in dosimetry evaluations; and production/logistical challenges associated with distributing relatively short-lived 209 At from a limited number of facilities capable of high-energy proton spallation and separation of 211 At from actinide targets [8].

Theranostic Imaging Surrogates Proposed for Thorium-227
Thorium-227 (t 1/2 = 18.7 d) decays via alpha emission to 223 Ra and can be harvested from a generator containing 227 Ac (t 1/2 = 21.8 y) that is produced via nuclear reactor irradiation of 226 Ra [115].Thorium can be complexed with octadentate 3,2-hydroxypyridinone (3,2-HOPO) chelators attached to biological-targeting vectors 115 .Ongoing clinical studies involving 227 Th TAT include targeting tumors expressing human epidermal growth factor receptor 2 (HER2), PSMA, mesothelin (MSLN), and CD22 [116]. 227Th does emit a 236 keV (12.9%) gamma photon that would be suitable for SPECT imaging.However, the long half-life of 227 Th relative to other TAT radionuclides would likely result in a low injected therapeutic activity, which could be insufficient for direct imaging 9 .Therefore, an imaging surrogate to assess 227 Th radiopharmaceutical pharmacokinetics is desirable, with the 134 Ce/ 134 La PET imaging pair showing promise (see Section 3.3).A significant uncertainty of using any theranostic imaging pair with 227 Th involves its long-lived 223 Ra progeny, which has the potential for substantial redistribution and alpha irradiation of healthy tissue after decay from 227 Th.This would significantly complicate direct comparisons between imaging and inferred therapeutic dosimetry and require further study.

Theranostic Imaging Surrogates Proposed for Terbium-149
Terbium-149 (t 1/2 = 4.1 h) is a unique radionuclide for TAT.It emits low-energy alpha particles with a short tissue range and decays via several daughter radionuclides to stable 145 Nd and 141 Pr, without any subsequent alpha emissions [9].This absence of alphaemitting progeny is regarded as a potential strength for 149 Tb TAT. 149Tb is produced via high-energy proton spallation of a tantalum target followed by online isotope separation or 3 He bombardment of a 151 Eu target [19,20,117,118].100 MBq of 149 Tb was obtained in a solution suitable for preclinical applications and successfully labeled to a DOTANOC targeting vector [118].While PET images were successfully obtained using [ 149 Tb]Tb-DOTANOC in a mouse model, 149 Tb possesses a relatively low positron branching ratio (21%) and relatively high positron emission energy (E mean = 805 keV).These physical factors could present challenges to obtaining high-quality clinical PET images.Additionally, due to limited production and the resulting extreme scarcity of 149 Tb, imaging surrogates would be helpful research tools to evaluate its potential for TAT.Two surrogate candidates are 155 Tb and 152 Tb.
Advantages of 155 Tb include its accessible production routes that can synthesize multi-patient activities per run, decay to stable 155 Gd, and its long half-life that enables long-duration imaging.Limitations include relatively low imaging performance compared to other diagnostic radionuclides, such as PET emitters.
Advantages of 152 Tb include a relatively long half-life permitting imaging at extended time points and its decay to near-stable 152 Gd.Limitations include the scarcity of facilities capable of achieving proton energies for production, the higher average positron emission energy, and significant co-emitted gamma rays that increase the radioactive dose.

Summary and Outlook for Alpha-Emitter Imaging Surrogates
As highlighted in this article, multiple SPECT and PET imaging surrogates have demonstrated the potential to enhance clinical TAT applications and research.Table 1 presents a summary of proposed theranostic imaging surrogates for alpha emitters, along with their properties and production status.Production capabilities must be augmented to enable more patients and research efforts to benefit from TAT imaging surrogates.Existing medium-energy cyclotron facilities are well positioned to improve the supply chain of imaging surrogates such as 133  La, 203 Pb, and 155 Tb by adapting and optimizing established production techniques to the unique capabilities of each facility.A stable supply of isotopically enriched accelerator target material will be required to support growing production efforts for many of these radionuclides.Other imaging surrogates such as 226 Ra, 152 Tb, 209 At, and 134 Ce/ 134 La require high-energy accelerators, bombarding hazardous target material, and techniques such as mass sep-aration to enable their production.While these surrogates have demonstrated research potential, their widespread deployment for radiopharmaceutical development and clinical application may be limited owing to the scarcity of facilities capable of their production.
Except for 149 Tb, which possesses a single alpha emission in its decay chain, most TAT radionuclides, including 225 Ac, 212 Pb, 223 Ra, 224 Ra, 227 Th, and 211 At, possess a cascade of decay progeny that are released from the original target site due to recoil energy and deposit additional alpha radiation in surrounding healthy tissues.While the highlighted imaging surrogates are well positioned to provide more accurate dosimetry data for the TAT parent radionuclide decay, there will be a degree of uncertainty regarding the dose from alpha-emitting decay progeny.This uncertainty will depend on the type of malignancy, internalization within targeted cells, and other factors within the disease microenvironment that influence the radiopharmaceutical pharmacokinetics.However, this limitation does not negate the improved accuracy of biodistribution dosimetry data conferred by using imaging surrogates matched to the TAT parent radionuclide, particularly when radionuclides are stably bound to their targeting vector.Therefore, TAT imaging surrogates have the potential to assist the preclinical development and clinical deployment of TAT radiopharmaceuticals and represent a significant improvement over conventional PET and SPECT imaging radionuclides currently paired with TAT.

Conclusions
Recent preclinical and clinical advances in targeted alpha therapy have spurred significant interest in utilizing alpha-emitting radiopharmaceuticals to treat metastatic cancers and other malignancies.Despite their strong potential, TAT radiopharmaceuticals suffer from an acute supply shortage of alpha-emitting radionuclides due to production constraints.This severely restricts the availability for patient therapy and slows the development of new TAT radiopharmaceuticals. Additionally, many alpha-emitting radionuclides do not possess radioactive emissions suitable for diagnostic imaging.This often leads to diagnostic radiopharmaceuticals being employed with suboptimally paired imaging radionuclides that possess different chemistries from their therapeutic counterpart, which can potentially result in different radiopharmaceutical biodistributions.Therefore, increasing the availability of SPECT and PET imaging TAT surrogates has strong potential to improve the accuracy of dosimetry and treatment tracking, and enhance TAT research output by using more economical and less potent diagnostic radionuclides for preclinical radiopharmaceutical development.Therefore, TAT imaging surrogates hold potential to improve the accuracy of diagnostic scans, equipping clinicians and researchers with more accurate biodistribution and dosimetry data that they can use to expedite the development and deployment of novel TAT radiopharmaceuticals.

Figure 1 .
Figure 1.Derenzo phantom PET images reconstructed with MAP for different PET radionuclides, listed in order of increasing positron emission energy.Figure from Nelson et al.[41], with 18 F,64 Cu,44 Sc, and68 Ga data from Ferguson et al.[46].

Figure 1 .
Figure 1.Derenzo phantom PET images reconstructed with MAP for different PET radionuclides, listed in order of increasing positron emission energy.Figure from Nelson et al.[41], with 18 F,64 Cu,44 Sc, and68 Ga data from Ferguson et al.[46].

Figure 3 .
Figure 3. Inter-rod contrast measurements were used to assess image resolution from 226 Ac SPECT images acquired using two collimators.Figure from Koniar et al. [55].

Figure 3 .
Figure 3. Inter-rod contrast measurements were used to assess image resolution from 226 Ac SPECT images acquired using two collimators.Figure from Koniar et al. [55].

Figure 6 .
Figure 6.SPECT images and inter-rod contrast data for a phantom containing 209 At [114].

Author
Contributions: B.J.B.N. prepared the manuscript draft; B.J.B.N., J.W., J.D.A. and F.W. performed review and editing; and J.D.A. and F.W. provided supervision.All authors have read and agreed to the published version of the manuscript.Funding: The authors would like to thank the Dianne and Irving Kipnes Foundation for supporting this work.

Table 1 .
Summary of prominent TAT radionuclides and their proposed theranostic SPECT and PET imaging surrogates.