Potential production of Ac-225 at Egyptian second research reactor (ETRR-2) through neutron induced transmutation of Ra-226

A considerable focus has been paid to the production of 225Ac due to its effective therapeutic action in alpha-targeted radiotherapy. Considering the future global clinical demand, it is necessary to increase the production capacity of 225Ac. A feasibility study was conducted to investigate the production of 225Ac through neutron induced transmutation of 226Ra at the Egyptian Second Research Reactor (ETRR-2) using the MCNPX code. The calculations were carried out for 1 g of 226Ra target exposed to the highest neutron flux in the irradiation grid surrounding the reactor core. The 227Ra, 225Ra, 227Ac, and 225Ac generated activities as a function of irradiation and decay times were estimated. Our study revealed that in this non-linear production process, 39.22 MBq of pure 225Ac could be obtained after three days of irradiation, while 148.74 MBq could be obtained after fifteen days of continuous irradiation.

Primary standardization and half-life determination of 225Ac at NRC

A solution of 225Ac was standardized by NRC using the triple-to-double coincidence ratio (TDCR) method. The counting efficiencies were calculated assuming a counting efficiency of 100% for alpha decays and those calculated using the MICELLE2 Monte Carlo code for beta decays and was approximately 500% for the NRC TDCR system. The relative uncertainty for the activity concentration was determined to be 0.25%. This agreed with measurements performed using gamma spectroscopy and a predicted calibration factor for the Vinten 671 ionization chamber as calculated using an EGSnrc model, implementing radioactive decay. Finally, the half-life of 225Ac was determined from long-term measurements using ionization chambers and liquid scintillation counting. The NRC measured half-life for 225Ac was found to be 9.914(4) days and is consistent within an expanded uncertainty coverage of k = 2 with the most recent (Kossert et al., 2020; Pommé et al., 2012) measurements of this decay parameter.

α(PS)-γ(Ge) digital anti-coincidence spectroscopy and its application to activity measurement of 225Ac

Activity of 225Ac was measured by the digital anti-coincidence spectroscopy technique using a 4πα-γ detector configuration, composed of a sandwich type 4π plastic scintillator and Ge detectors. Ultrathin plastic scintillators were used for selective detection of α-particles emitted from 225Ac and its progenies, and the α-counting efficiencies of a 4π plastic scintillation detector for individual nuclides in the decay chain were determined as well. A list-mode multichannel analyzer was employed to record coincidence/anti-coincidence events for off-line analyses. The time difference distribution spectra revealed α-particle emission following 213Po decay without β-particle interference from 213Bi.

Measurement of the activity and determination of the half-life of 225Ac at POLATOM

A method for absolute measurements of the 225Ac activity in equilibrium with its progeny was developed. Measurements were performed using the triple-to-double coincidence ratio (TDCR) method in two different TDCR counters. The activity concentration of an 225Ac solution was determined and the solution was sent to the SIR system for a comparison. The half-life of 225Ac was determined by one of the TDCR counters and found to be 9.9150(63) days.

Actinium-225 Targeted Agents: Where Are We Now?

α-particle targeted radionuclide therapy has shown promise for optimal cancer management, an exciting new era for brachytherapy. Alpha-emitting nuclides can have significant advantages over gamma- and beta-emitters due to their high linear energy transfer (LET). While their limited path length results in more specific tumor 0kill with less damage to surrounding normal tissues, their high LET can produce substantially more lethal double strand DNA breaks per radiation track than beta particles. Over the last decade, the physical and chemical attributes of Actinium-225 (225Ac) including its half-life, decay schemes, path length, and straightforward chelation ability has peaked interest for brachytherapy agent development. However, this has been met with challenges including source availability, accurate modeling for standardized dosimetry for brachytherapy treatment planning, and laboratory space allocation in the hospital setting for on-demand radiopharmaceuticals production. Current evidence suggests that a simple empirical approach based on 225Ac administered radioactivity may lead to inconsistent outcomes and toxicity. In this review article, we highlight the recent advances in 225Ac source production, dosimetry modeling, and current clinical studies.

Radiation dosimetry and efficacy of an 89Zr/225Ac-labeled humanized anti-MUC5AC antibody

INTRODUCTION

Theranostic applications are currently difficult to achieve owing to the limited evaluation of suitable chelators for therapeutic nuclides, such as 225Ac and 227Th. With a focus on targeted α therapy and theranostics using human IgG as a drug-delivery system (i.e., combining highly cytotoxic α-particle emitter radiation with efficient tumor targeting), we developed a recombinant humanized Nd2 (hNd2) as an anti-MUC5AC antibody since MUC5AC is highly expressed in patients with pancreatic cancer. Therefore, we aimed to evaluate the performance of 89Zr- (for diagnosis) and 225Ac- (for therapy) labeling of these antibodies using well-controlled radioisotope (RI)-labeling technology in pancreatic cancer mouse models.

METHODS

89Zr-labeled hNd2 (NMK89) and 225Ac-labeled hNd2 (NMT25) were manufactured by chemical conjugation using affinity peptides. A binding assay and the evaluation of plasma stability were performed in vitro to confirm the properties of NMK89 and NMT25. In vivo, we evaluated biodistribution, positron emission tomography (PET)/computed tomography (CT) imaging, antitumor effects, and toxicity. Moreover, the exposure dose in humans was estimated based on the biodistribution evaluation in normal mice.

RESULTS

NMK89 and NMT25 showed binding specificity to MUC5AC and stability with radiochemical purity ≥90% in mice and human plasma following incubation for 168 h. NMK89 showed high accumulation in tumors and low non-specific accumulation in normal tissues. The antitumor effect of NMT25 was dose-dependent and significantly suppressed tumor growth in the NMT25 treatment groups compared with the control group (p < 0.05). NMK89 and NMT25 showed similar pharmacokinetics and biodistribution characteristics. Additionally, the human estimated exposure dose of NMK89 and NMT25 was confirmed, and the effective dose of NMK89 and NMT25 was 0.33 mSv/MBq and 177.5 mSv/MBq, respectively.

CONCLUSION

NMK89 showed specific accumulation in the MUC5AC-expressing tumors, while NMT25 showed strong antitumor effects. These results suggest NMK89 and NMT25 as promising theranostic agents for pancreatic cancer.

Radiolabeling of a polypeptide polymer for intratumoral delivery of alpha-particle emitter, 225Ac, and beta-particle emitter, 177Lu

INTRODUCTION

Radiotherapy of cancer requires both alpha- and beta-particle emitting radionuclides, as these radionuclide types are efficient at destroying different types of tumors. Both classes of radionuclides require a vehicle, such as an antibody or a polymer, to be delivered and retained within the tumor. Polyglutamic acid (pGlu) is a polymer that has proven itself effective as a basis of drug-polymer conjugates in the clinic, while its derivatives have been used for pretargeted tumor imaging in a research setup. trans-Cyclooctene (TCO) modified pGlu is suitable for pretargeted imaging or therapy, as well as for intratumoral radionuclide therapy. In all cases, it becomes indirectly radiolabeled via the bioorthogonal click reaction with the tetrazine (Tz) molecule carrying the radionuclide. In this study, we report the radiolabeling of TCO-modified pGlu with either lutetium-177 (¹⁷⁷Lu), a beta-particle emitter, or actinium-225 (²²⁵Ac), an alpha-particle emitter, using the click reaction between TCO and Tz.

METHODS

A panel of Tz derivatives containing a metal ion binding chelator (DOTA or macropa) connected to the Tz moiety directly or through a polyethylene glycol (PEG) linker was synthesized and tested for their ability to chelate ¹⁷⁷Lu and ²²⁵Ac, and click to pGlu-TCO. Radiolabeled ¹⁷⁷Lu-pGlu and ²²⁵Ac-pGlu were isolated by size exclusion chromatography. The retention of ¹⁷⁷Lu or ²²⁵Ac by the obtained conjugates was investigated in vitro in human serum.

RESULTS

All DOTA-modified Tzs efficiently chelated ¹⁷⁷Lu resulting in average radiochemical conversions (RCC) of >75%. Isolated radiochemical yields (RCY) for ¹⁷⁷Lu-pGlu prepared from ¹⁷⁷Lu-Tzs ranged from 31% to 55%. TLC analyses detected <5% unchelated ¹⁷⁷Lu for all ¹⁷⁷Lu-pGlu preparations over six days in human serum. For ²²⁵Ac chelation, optimized RCCs ranged from 61 ± 34% to quantitative for DOTA-Tzs and were quantitative for the macropa-modified Tz (>98%). Isolated radiochemical yields (RCY) for ²²⁵Ac-pGlu prepared from ²²⁵Ac-Tzs ranged from 28% to 51%. For 3 out of 5 ²²⁵Ac-pGlu conjugates prepared from DOTA-Tzs, the amount of unchelated ²²⁵Ac stayed below 10% over six days in human serum, while ²²⁵Ac-pGlu prepared from macropa-Tz showed a steady release of up to 37% ²²⁵Ac.

CONCLUSION

We labeled TCO-modified pGlu polymers with alpha- and beta-emitting radionuclides in acceptable RCYs. All ¹⁷⁷Lu-pGlu preparations and some ²²⁵Ac-pGlu preparations showed excellent stability in human plasma. Our work shows the potential of pGlu as a vehicle for alpha- and beta-radiotherapy of tumors and demonstrated the usefulness of Tz ligation for indirect radiolabeling.

Transport model predictions of 225Ac production cross sections via energetic p, d and α irradiation of 232Th targets

Monte Carlo transport codes PHITS and MCNP6 were used to calculate the production cross sections of ^225,227Ac, ^227,229Th, ^223,225Ra, and ^229,230,231Pa via the bombardment of a²³²Th target with energetic protons, deuterons, and α-particles. The incident projectile energies ranged between 10 and 800 MeV/nucleon. When possible, the predicted production cross sections were compared with the available experimental data and other predictions. The degree of the codes' abilities to match the measured data provides a qualitative assessment of the codes' abilities to predict data from similar, but unmeasured, projectile/target systems. In addition, a comparison between calculated cross sections and data may provide insight into possible improvements in the physics models employed by those transport codes.

Pairing of thorium with selected primary target materials in tandem configurations: Co-production of 225Ac/213Bi and 230U/226Th generators with a 70 MeV H- cyclotron

Several radionuclide production facilities based on a new generation of high-current, 70 MeV H^- cyclotrons have been established in recent years and a number of new facilities are either under construction or being planned. In this study, the feasibility to produce ²²⁵Ac/²¹³Bi and ²³⁰U/²²⁶Th generators via Th + p reactions at such a facility has been investigated. From the perspective of production yields, it has been found useful to compare the relevant reactions on thorium with those on other targets regularly employed in this energy region, in particular the ^natMg(p,x)²²Na, ^natGa(p,x)⁶⁸Ge, and ^natRb(p,x)⁸²Sr reactions that have been exploited with 66 MeV proton beams at iThemba LABS for many years. Therefore, various tandem configurations of thorium with magnesium, gallium and rubidium are discussed based on the relevant nuclear data. In a few cases, the available data were judged to be insufficient in this energy region. New experimental cross sections are presented for ²²⁵Ac, ²²⁵Ra, and ²³⁰Pa in Th + p reactions as well as ^22,24Na in ^natMg + p reactions. Integral yields have been derived for those radionuclides of main interest. Production yields expected from extended 70 MeV proton bombardments, on selected tandem-target configurations, are presented for ²²Na, ⁶⁸Ge, ⁸²Sr, ²²⁵Ac, and ²³⁰Pa. It is concluded that ²²⁵Ac/²¹³Bi generators can, in principle, be added to a production programme based at a 70 MeV H^- cyclotron facility as the production rate is of similar magnitude to those of other well-established radionuclides in this energy region. Also, radio-contaminant levels for ²²⁵Ac are similar to those found in higher proton-energy windows. The scope for ²³⁰U/²²⁶Th generators, via bulk production of the precursor ²³⁰Pa, seems to be more limited due to a lower yield but can nevertheless be produced in clinically relevant quantities.

Construction of a thorium/actinium generator at the Canadian Nuclear Laboratories

Targeted Alpha Therapy (TAT) has demonstrated considerable promise in the treatment of a range of cancers in both preclinical and, more recently clinical research. In particular, work with the alpha-emitting radionuclide ²²⁵Ac has been effectively employed due to the relatively rapid decay cascade that leads to 4 alpha and 2 beta emissions. One limitation for TAT has been caused by access to the vital radionuclide. Traditionally, ²²⁵Ac has been sourced from thorium/actinium generators based on the alpha decay of stockpiles of ²²⁹Th. ²²⁹Th is itself the alpha-decay product from ²³³U. Due to proliferation issues associated with ²³³U, only three thorium/actinium generators have been reported in the literature, capable of supporting clinical research. This paper describes the construction and operation of a thorium/actinium radionuclide generator at the Canadian Nuclear Laboratories, capable of supporting preclinical and limited clinical research in the area of TAT. Thorium was recovered and purified by a combination of anion exchange and extraction chromatography from aged ²³³U stockpiles. A separation scheme for ²²⁵Ra and ²²⁵Ac has been developed, based upon the chemical composition of the thorium material to allow for regular, routine milkings capable of supplying up to 3.7 GBq (100 mCi) of radiochemically pure ²²⁵Ac annually. This routine separation is accomplished using a combination of anion exchange chromatography to separate Ac and Ra isotopes from Th and extraction chromatography employing TEVA and DGA-N resins to separate actinium from radium and breakthrough thorium.

Synthesis and characterization of natural lanthanum labelled DOTA-Peptides for simulating radioactive Ac-225 labeling

This research aimed to assess the labeling procedure of Lanthanum to DOTATATE and PSMA-617 peptide for simulating Actinium-225 (²²⁵Ac) labeling. For this purpose labeling procedure, purification and characterization conditions were optimized. Results showed that for the high reaction yield labeling, microwave heating should be preferred and Sephadex G10 column was found to be more suitable as a purification system compared to Toyopearl column. For the characterization of complex, more precise and reliable results were obtained by the chromatographic system.

Radiometric evaluation of diglycolamide resins for the chromatographic separation of actinium from fission product lanthanides

Actinium-225 is a potential Targeted Alpha Therapy (TAT) isotope. It can be generated with high energy (≥ 100MeV) proton irradiation of thorium targets. The main challenge in the chemical recovery of ²²⁵Ac lies in the separation from thorium and many fission by-products most importantly radiolanthanides. We recently developed a separation strategy based on a combination of cation exchange and extraction chromatography to isolate and purify ²²⁵Ac. In this study, actinium and lanthanide equilibrium distribution coefficients and column elution behavior for both TODGA (N,N,N',N'-tetra-n-octyldiglycolamide) and TEHDGA (N,N,N',N'-tetrakis-2-ethylhexyldiglycolamide) were determined. Density functional theory (DFT) calculations were performed and were in agreement with experimental observations providing the foundation for understanding of the selectivity for Ac and lanthanides on different DGA (diglycolamide) based resins. The results of Gibbs energy (ΔG_aq) calculations confirm significantly higher selectivity of DGA based resins for Ln^III over Ac^III in the presence of nitrate. DFT calculations and experimental results reveal that Ac chemistry cannot be predicted from lanthanide behavior under comparable circumstances.

Thorium-229 quantified in historical Thorium-228 capsules

Thorium-229 is a valuable, but scarce, radionuclide for nuclear clock applications or targeted alpha therapy. While it is mostly produced by the decay of ²³³U, ²²⁹Th can also be produced by neutron irradiation of ²²⁶Ra. At SCK•CEN, capsules containing mainly ²²⁸Th (by-product of ²²⁶Ra irradiation) were characterized to quantify the present amounts of ²²⁹Th, ²²⁸Th, ²²⁷Ac, ²²⁶Ra with high resolution gamma spectroscopy, after a decay period of 40 years in which ²²⁸Th has decayed. High purity ²²⁹Th was quantified, and after recovery using radiochemical separation processes, it can be used to support ongoing research.

Large scale accelerator production of 225Ac: Effective cross sections for 78-192MeV protons incident on 232Th targets

Actinium-225 and ²¹³Bi have been used successfully in targeted alpha therapy (TAT) in preclinical and clinical research. This paper is a continuation of research activities aiming to expand the availability of ²²⁵Ac. The high-energy proton spallation reaction on natural thorium metal targets has been utilized to produce millicurie quantities of ²²⁵Ac. The results of sixteen irradiation experiments of thorium metal at beam energies between 78 and 192MeV are summarized in this work. Irradiations have been conducted at Brookhaven National Laboratory (BNL) and Los Alamos National Laboratory (LANL), while target dissolution and processing was carried out at Oak Ridge National Laboratory (ORNL). Excitation functions for actinium and thorium isotopes, as well as for some of the fission products, are presented. The cross sections for production of ²²⁵Ac range from 3.6 to 16.7mb in the incident proton energy range of 78-192MeV. Based on these data, production of curie quantities of ²²⁵Ac is possible by irradiating a 5.0gcm^{-2 232}Th target for 10 days in either BNL or LANL proton irradiation facilities.