Objective. High-energy proton irradiation of 232 Th produces both 225 Ac and its precursor 225 Ra, the latter being underexplored despite its potential applicability at relevant facilities. This study aims to provide a theoretical basis for 232 Th-based 225 Ra production by modeling the associated yields and radioisotopic impurity levels. Approach. Production yields of 225 Ra and levels of 228 Ra impurity were calculated using Monte Carlo simulations for proton energies of 100–800 MeV, total 232 Th target thicknesses of 0.125–8 mm, and an irradiation time of 15 d. A theoretical model was developed for predicting the cumulative yield of 225 Ac eluates from a 225 Ra yield. The levels of 228 Ra-derived 228 Ac impurity in 225 Ac eluates and the postelution decay times required to suppress 228 Ac to acceptable thresholds were modeled and computed. Main results. The calculated 225 Ra yields at the end of bombardment for a 15 d irradiation ranged from 2.3 ± 0.3 MBq µA −1 at 100 MeV to 563.1 ± 12.3 MBq µA −1 at 700 MeV. Increases in 225 Ra yield were most pronounced when the proton energy was raised from 100 to 200 MeV, with diminishing returns at higher energies. The cumulative yield of 225 Ac eluates was about 29.1% of a 225 Ra yield under combined conditions of hypothetical and empirical settings. Postelution times of at least 25.1–30.1 h should be allowed to reduce 228 Ac impurity levels to below 0.1%. Significance. The developed models provide a quantitative basis for evaluating and optimizing 225 Ra production via proton irradiation of 232 Th. This study quantitatively predicted the presence of 228 Ac in 225 Ac eluates under various conditions and showed how 228 Ac can be suppressed by introducing minimal postelution decay times.

Objective. High-energy proton irradiation of Th-232 produces both Ac-225 and its precursor Ra-225, the latter being underexplored despite its potential applicability at relevant facilities. This study aims to provide a theoretical basis for Th-232-based Ra-225 production by modeling the associated yields and radioisotopic impurity levels. Approach. Production yields of Ra-225 and levels of Ra-228 impurity were calculated using Monte Carlo simulations for proton energies of 100-800 MeV, total Th-232 target thicknesses of 0.125-8 mm, and an irradiation time of 15 d. A theoretical model was developed for predicting the cumulative yield of Ac-225 eluates from a Ra-225 yield. The levels of Ra-228-derived Ac-228 impurity in Ac-225 eluates and the postelution decay times required to suppress Ac-228 to acceptable thresholds were modeled and computed. Main results. The calculated Ra-225 yields at the end of bombardment for a 15 d irradiation ranged from 2.3 +/- 0.3 MBq mu A(-1) at 100 MeV to 563.1 +/- 12.3 MBq mu A(-1) at 700 MeV. Increases in Ra-225 yield were most pronounced when the proton energy was raised from 100 to 200 MeV, with diminishing returns at higher energies. The cumulative yield of Ac-225 eluates was about 29.1% of a Ra-225 yield under combined conditions of hypothetical and empirical settings. Postelution times of at least 25.1-30.1 h should be allowed to reduce Ac-228 impurity levels to below 0.1%. Significance. The developed models provide a quantitative basis for evaluating and optimizing Ra-225 production via proton irradiation of Th-232. This study quantitatively predicted the presence of Ac-228 in Ac-225 eluates under various conditions and showed how Ac-228 can be suppressed by introducing minimal postelution decay times.

Objective. Free 225Ac can result in off-target radiation exposure if present as a result of incomplete chelation or radiolytic degradation of 225Ac radiopharmaceuticals. This study assessed absorbed doses of 229Th-derived free 225Ac to major normal tissues and organs in mouse and human models.Approach. Biodistribution data were obtainedex vivofrom SK-MEL-2 xenograft mice administered with [225Ac]AcCl3derived from a229Th/225Ac generator. Uptake fractions of225Ac in selected tissues and organs were measured at 0.5, 2, 6, and 24 h postinjection (p.i.) via its gamma-emitting daughter,221Fr, with a sample size ofn = 4-5 per p.i. time point. Time-integrated activity coefficients (TIACs) were calculated by fitting the mouse uptake profiles and extrapolated to human models using relative mass scaling. Absorbed dose coefficients (ADCs) were then calculated according to the Medical Internal Radiation Dose formalism using S values of a mouse whole-body (MOBY) phantom and anthropomorphic adult male and female phantoms implemented in the OLINDA dosimetry software. Redistribution of221Fr and213Bi from the liver to kidneys was simulated by adjusting progeny-specific TIAC ratios in the mouse model.Main results. The liver received the highest absorbed dose, with ADCs of 2860 mGy kBq-1in the MOBY phantom and 886-1080 mGy MBq-1in the anthropomorphic phantoms. The skeletal and splenic ADCs were 84.3 and 20.3 mGy kBq-1, respectively, in the MOBY phantom. ADCs in the other tissues and organs were low to moderate, with the brain receiving the least. Under simulated redistribution scenarios,221Fr showed larger changes in mouse ADCs than213Bi, increasing the renal ADCs by 7.5-30.5% and decreasing the hepatic ADCs by 8-31.8% for scale factors of 0.1-0.4.Significance. The liver, bone, and spleen were suggested as the primary dosimetric targets of229Th-derived free225Ac. The ADCs of free225Ac calculated in this work may serve as a preliminary reference for quality control of225Ac radiopharmaceuticals.

Objective. Free Ac-225 can result in off-target radiation exposure if present as a result of incomplete chelation or radiolytic degradation of Ac-225 radiopharmaceuticals. This study assessed absorbed doses of Th-229-derived free Ac-225 to major normal tissues and organs in mouse and human models. Approach. Biodistribution data were obtained ex vivo from SK-MEL-2 xenograft mice administered with [Ac-225]AcCl3 derived from a Th-229/Ac-225 generator. Uptake fractions of Ac-225 in selected tissues and organs were measured at 0.5, 2, 6, and 24 h postinjection (p.i.) via its gamma-emitting daughter, Fr-221, with a sample size of n = 4-5 per p.i. time point. Time-integrated activity coefficients (TIACs) were calculated by fitting the mouse uptake profiles and extrapolated to human models using relative mass scaling. Absorbed dose coefficients (ADCs) were then calculated according to the Medical Internal Radiation Dose formalism using S values of a mouse whole-body (MOBY) phantom and anthropomorphic adult male and female phantoms implemented in the OLINDA dosimetry software. Redistribution of Fr-221 and Bi-213 from the liver to kidneys was simulated by adjusting progeny-specific TIAC ratios in the mouse model. Main results. The liver received the highest absorbed dose, with ADCs of 2860 mGy kBq(-1) in the MOBY phantom and 886-1080 mGy MBq(-1) in the anthropomorphic phantoms. The skeletal and splenic ADCs were 84.3 and 20.3 mGy kBq(-1), respectively, in the MOBY phantom. ADCs in the other tissues and organs were low to moderate, with the brain receiving the least. Under simulated redistribution scenarios, Fr-221 showed larger changes in mouse ADCs than Bi-213, increasing the renal ADCs by 7.5-30.5% and decreasing the hepatic ADCs by 8-31.8% for scale factors of 0.1-0.4. Significance. The liver, bone, and spleen were suggested as the primary dosimetric targets of Th-229-derived free Ac-225. The ADCs of free Ac-225 calculated in this work may serve as a preliminary reference for quality control of Ac-225 radiopharmaceuticals.

Objective.High-energy proton irradiation of232Th produces both225Ac and its precursor225Ra, the latter being underexplored despite its potential applicability at relevant facilities. This study aims to provide a theoretical basis for232Th-based225Ra production by modeling the associated yields and radioisotopic impurity levels.Approach.Production yields of225Ra and levels of228Ra impurity were calculated using Monte Carlo simulations for proton energies of 100-800 MeV, total232Th target thicknesses of 0.125-8 mm, and an irradiation time of 15 d. A theoretical model was developed for predicting the cumulative yield of225Ac eluates from a225Ra yield. The levels of228Ra-derived228Ac impurity in225Ac eluates and the postelution decay times required to suppress228Ac to acceptable thresholds were modeled and computed.Main results.The calculated225Ra yields at the end of bombardment for a 15 d irradiation ranged from 2.3 ± 0.3 MBq µA-1at 100 MeV to 563.1 ± 12.3 MBq µA-1at 700 MeV. Increases in225Ra yield were most pronounced when the proton energy was raised from 100 to 200 MeV, with diminishing returns at higher energies. The cumulative yield of225Ac eluates was about 29.1% of a225Ra yield under combined conditions of hypothetical and empirical settings. Postelution times of at least 25.1-30.1 h should be allowed to reduce228Ac impurity levels to below 0.1%.Significance.The developed models provide a quantitative basis for evaluating and optimizing225Ra production via proton irradiation of232Th. This study quantitatively predicted the presence of228Ac in225Ac eluates under various conditions and showed how228Ac can be suppressed by introducing minimal postelution decay times.

H3tripa (H3macrotripa), a triaza-18-crown-6 macrocycle bearing three picolinate arms, enables quantitative radiolabeling of [203Pb]Pb2+ and [213Bi]Bi3+ and exhibits robust human serum stability. NMR spectroscopy, X-ray crystallography, density functional theory, and UV-potentiometry investigate its coordination geometry and thermodynamics with Bi3+ and Pb2+, highlighting H3tripa as a promising Pb/Bi theranostic chelator.

Fibroblast activation protein (FAP) is overexpressed in a variety of cancers, making it an attractive target for bifunctional chelator-based radiopharmaceuticals. This study initially aimed to assess the effect of chelator structure on the biodistribution of 203Pb/212Pb-labeled FAP inhibitor (FAPI) bioconjugates. However, suboptimal in vivo biodistribution and imaging results suggested the bioconjugate was unstable. RadioHPLC analysis of urine samples suggest the thiourea bond, formed during conjugation between an amine on the biomolecule, and an isothiocyanate-functionalized chelator, is unstable in vivo, resulting in detachment of the radiometal-chelator complex from the targeting vector, resulting in poor tumor accumulation. To determine whether this instability was specific to the FAPI system, a peptide-based (Cyclic melanocyte stimulating hormone, CycMSH) bioconjugate targeting the melanocortin-1 receptor was synthesized using the same thiourea linkage. Identical metabolites were observed, supporting the hypothesis that thiourea bonds are unstable in vivo with this theranostic isotope pair. Subsequently, the effect of bioconjugation chemistry, specifically thiourea and amide bonds, on the stability and biodistribution of 203Pb/212Pb-labeled bioconjugates was assessed. Modifying the bioconjugation linker to be an amide bond, formed by utilizing a chelate containing an active ester instead of an isothiocyanate, led to significantly improved in vitro and in vivo stability, as demonstrated by radioHPLC and biodistribution and imaging studies in both models. These findings highlight the importance of the choice of bioconjugation chemistry in the development of lead-based radiopharmaceuticals and emphasize the importance of selecting stable linkages to ensure optimal radiometal retention and tumor targeting.

Objective. Conventional232Th target thicknesses used for225Ac production are substantially smaller than the proton ranges, underutilizing the225Ac production potential of proton beams. This study explores theoretical optimal thicknesses of232Th targets at widely used proton energies, 70-200 MeV.Approach. Yields of225Ac and impurity levels of227Ac were calculated for proton energies of 70-200 MeV and232Th target thicknesses of 0.05-24 mm using Monte Carlo simulations. Ranges of optimal target thicknesses were defined for each proton energy based on the relative rates of change (RROCs) of225Ac yields per target thickness of 0.25 mm and on energy-adjusted RROC thresholds. Expected225Ac yield gains from using the optimal thicknesses were also estimated.Main results. The ranges of theoretical optimal thicknesses for representative energies, 70, 100, 160, and 200 MeV, were found to be 2-2.25 mm, 4.5-6.25 mm, 7.75-14.5 mm, and 14.25-21.75 mm, respectively. All these thicknesses are markedly larger than those of the conventional thin targets used for medium- to large-scale225Ac production, 0.25 mm and 0.5 mm. By using these optimal target thicknesses,225Ac yields are expected to increase by factors of up to 8.8 at 70 MeV and 63.8 at 200 MeV. Actinium-227 impurity levels were unaffected by target thickness optimization at all proton energies.Significance. Optimizing232Th target thicknesses can provide proton accelerator facilities operating in the 70-200 MeV range with a straightforward means of increasing their225Ac production capacity. Optimal target thicknesses offer greater225Ac yield scalability at higher energies near 200 MeV, while enabling large-scale225Ac production at lower energies close to 70 MeV.

Background: Targeted alpha therapy (TAT) of somatostatin receptor-2 (SSTR2) positive neuroendocrine tumors (NETs) involving Ac-225 ([Ac-225]Ac-DOTA-TATE) has previously demonstrated improved therapeutic efficacy over conventional beta particle-emitting peptide receptor radionuclide therapy agents. DOTA-TATE requires harsh radiolabeling conditions for chelation of [Ac-225]Ac3+, which can limit the achievable molar activities and thus therapeutic efficacy of such TAT treatments. Macropa-TATE was recently highlighted as a potential alternative to DOTA-TATE, owing to the mild radiolabeling conditions and high affinity toward [Ac-225]Ac3+; however, elevated liver and kidney uptake were noted as a major limitation and a suitable imaging radionuclide is yet to be reported, which will be required for patient dosimetry studies and assessment of therapeutic benefit. Previously, [Tb-155]Tb-crown-TATE has shown highly effective imaging of NETs in preclinical SPECT/CT studies, with high tumor uptake and low non-target accumulation; these favourable properties and the versatile coordination behavior of the crown chelator may therefore show promise for combination with Ac-225 for TAT. Methods: Crown-TATE was labeled with Ac-225, and radiochemical yield was analyzed as the function of crown-TATE concentration. LogD(7.4) was measured as the indication of hydrophilicity. Free [Ac-225]Ac3+ release from [Ac-225]Ac-crown-TATE in human serum was studied. Biodistribution studies of [Ac-225]Ac-crown-TATE in mice bearing AR42J tumors was evaluated at 1, 4, 24, 48, and 120 h, and the absorbed dose to major organs calculated. Therapy-monitoring studies with AR42J tumor bearing mice were undertaken using 30 kBq and 55 kBq doses of [Ac-225]Ac-crown-TATE and compared to controls treated with PBS or crown-TATE. Results: [Ac-225]Ac-crown-TATE was successfully prepared with high molar activity (640 kBq/nmol), and characterized as a moderately hydrophilic radioligand (LogD(7.4) = -1.355 +/- 0.135). No release of bound Ac-225 was observed over 9 days in human serum. Biodistribution studies of [Ac-225]Ac-crown-TATE showed good initial tumor uptake (11.1 +/- 1.7% IA/g at 4 h) which was sustained up to 120 h p.i. (6.92 +/- 2.03% IA/g). Dosimetry calculations showed the highest absorbed dose was delivered to the tumors. Therapy monitoring studies demonstrated significant (log-rank test, P < 0.005) improved survival in both treatment groups compared to controls. Conclusions: This preclinical study demonstrated the therapeutic efficacy of [Ac-225]Ac-crown-TATE for treatment of NETs, and highlights the potential of using crown chelator for stable chelation of Ac-225 under mild conditions.

Parkinson's disease (PD) is associated with aggregation of misfolded α-synuclein and other proteins, including tau. We designed a cross-sectional study to quantify the brain binding of [11C]PBB3 (a ligand known to bind to misfolded tau and possibly α-synuclein) as a proxy of misfolded protein aggregation in Parkinson's disease (PD) subjects with and without cognitive impairment and healthy controls (HC). In this cross-sectional study, nineteen cognitively normal PD subjects (CN-PD), thirteen cognitively impaired PD subjects (CI-PD) and ten HC underwent [11C]PBB3 PET. A subset of the PD subjects also underwent PET imaging with [11C](+)DTBZ to assess dopaminergic denervation and [11C]PBR28 to assess neuroinflammation. Compared to HC, PD subjects showed higher [11C]PBB3 binding in the posterior putamen but not the substantia nigra. There was no relationship across subjects between [11C]PBB3 and [11C]PBR28 binding in nigrostriatal regions. [11C]PBB3 binding was increased in the anterior cingulate in CI-PD compared to CN-PD and HC, and there was an inverse correlation between cognitive scores and [11C]PBB3 binding in this region across all PD subjects. Our results support a primary role of abnormal protein deposition localized to the posterior putamen in PD. This suggests that striatal axonal terminals are preferentially involved in the pathophysiology of PD. Furthermore, our findings suggest that anterior cingulate pathology might represent a significant in vivo marker of cognitive impairment in PD, in agreement with previous neuropathological studies.

226Ac (t½ = 29.37 h) has been proposed as a theranostic radioisotope leveraging both its diagnostic γ-emissions and therapeutic α-emissions. 226Ac emits 158 and 230 keV γ-photons ideal for quantitative SPECT imaging and acts as an in vivo generator of 4 high-energy α-particles. Because of these nuclear decay properties, 226Ac has potential to act as a standalone theranostic isotope. In this proof-of-concept study, we evaluated a preclinical 226Ac-radiopharmaceutical for its theranostic efficacy and present the first 226Ac-targeted α-therapy study. Methods: 226Ac was produced at TRIUMF and labeled with the chelator-peptide bioconjugate crown-TATE. [226Ac]Ac-crown-TATE was selected to target neuroendocrine tumors in male NRG mice bearing AR42J tumor xenografts for SPECT imaging, biodistribution, and therapy studies. A preclinical SPECT/CT scanner acquired quantitative images reconstructed from both the 158 and the 230 keV emissions. Mice in the biodistribution study were euthanized at 1, 3, 5, 24, and 48 h after injection, and internal radiation dosimetry was derived for the tumor and organs of interest to establish appropriate therapeutic activity levels. Mice in the therapy study were administered 125, 250, or 375 kBq treatments and were monitored for tumor size and body condition. Results: We present quantitative SPECT images of the in vivo biodistribution of [226Ac]Ac-crown-TATE, which showed agreement with ex vivo measurements. Biodistribution studies demonstrated high uptake (>30%IA/g at 5 h after injection) and retention in the tumor, with an estimated mean absorbed dose coefficient of 222 mGy/kBq. [226Ac]Ac-crown-TATE treatments significantly extended the median survival from 7 d in the control groups to 16, 24, and 27 d in the 125, 250, and 375 kBq treatment groups, respectively. Survival was prolonged by slowing tumor growth, and no weight loss or toxicities were observed. Conclusion: This study highlights the theranostic potential of 226Ac as a standalone therapeutic isotope in addition to its demonstrated diagnostic capabilities to assess dosimetry in matched 225Ac-radiopharmaceuticals. Future studies will investigate maximum dose and toxicity to further explore the therapeutic potential of 226Ac-radiopharmaceuticals.

H3trica is a nonadentate chelating ligand intended for coordinating large radiometal ions, such as those used in nuclear medicine. This chelator, featuring a triaza-18-crown-6 macrocycle with three pendant carboxylic acid functional groups, was synthesized and characterized. Complementary nuclear magnetic resonance (NMR) spectroscopy and high-resolution electrospray-ionization mass spectroscopy (HR-ESI-MS) studies were used to explore the coordination of H(3)trica with metal ions such as La3+, Y3+ (as a model for Tb3+), and Lu3+ at the bulk scale. Thermodynamic solution studies provided valuable insights, highlighting robust metal complexation of H3trica with La3+, Tb3+, and Lu3+, with the most noteworthy log K ML value observed for Tb3+ (log K-TbL = 17.08), followed by La3+ (log K-LaL = 16.64) and Lu3+ (log K-LuL = 16.25). Concentration-dependent radiolabeling studies with [225Ac]Ac3+, [155Tb]Tb3+, and [161Tb]Tb3+ demonstrated rapid complexation (5-30 min) under mild conditions (pH 6-7, 25 degrees C). Importantly, the radiolabeled complexes exhibited stability during incubation in human serum for one-half-life of the corresponding radiometal. Thus, H3trica emerges as a valuable chelator, demonstrating its potential to coordinate the theranostic couple [225Ac]Ac3+/[155Tb]Tb3+ as well as the powerful terbium quartet ([149/152/155/161Tb]Tb3+) with efficiency and stability.

BackgroundTargeted radionuclide therapy is established as a highly effective strategy for the treatment of metastatic tumors; however, the co-development of suitable imaging companions to therapy remains significant challenge. Theranostic isotopes of terbium (149Tb, 152Tb, 155Tb, 161Tb) have the potential to provide chemically identical radionuclidic pairs, which collectively encompass all modes of nuclear decay relevant to nuclear medicine. Herein, we report the first radiochemistry and preclinical studies involving 155Tb- and 161Tb-labeled crown-αMSH, a small peptide-based bioconjugate suitable for targeting melanoma.Methods155Tb was produced via proton induced spallation of Ta targets using the isotope separation and acceleration facility at TRIUMF with isotope separation on-line (ISAC/ISOL). The radiolabeling characteristics of crown-αMSH with 155Tb and/or 161Tb were evaluated by concentration-dependence radiolabeling studies, and radio-HPLC stability studies. LogD7.4 measurements were obtained for [161Tb]Tb-crown-αMSH. Competitive binding assays were undertaken to determine the inhibition constant for [natTb]Tb-crown-αMSH in B16-F10 cells. Pre-clinical biodistribution and SPECT/CT imaging studies of 155Tb and 161Tb labeled crown-αMSH were undertaken in male C57Bl/6 J mice bearing B16-F10 melanoma tumors to evaluate tumor specific uptake and imaging potential for each radionuclide.ResultsQuantitative radiolabeling of crown-αMSH with [155Tb]Tb3+ and [161Tb]Tb3+ was demonstrated under mild conditions (RT, 10 min) and low chelator concentrations; achieving high molar activities (23–29 MBq/nmol). Radio-HPLC studies showed [161Tb]Tb-crown-αMSH maintains excellent radiochemical purity in human serum, while gradual metabolic degradation is observed in mouse serum. Competitive binding assays showed the high affinity of [natTb]Tb-crown-αMSH toward MC1R. Two different methods for preparation of the [155Tb]Tb-crown-αMSH radiotracer were investigated and the impacts on the biodistribution profile in tumor bearing mice is compared. Preclinical in vivo studies of 155Tb- and 161Tb- labeled crown-αMSH were performed in parallel, in mice bearing B16-F10 tumors; where the biodistribution results showed similar tumor specific uptake (6.06–7.44 %IA/g at 2 h pi) and very low uptake in nontarget organs. These results were further corroborated through a series of single-photon emission computed tomography (SPECT) studies, with [155Tb]Tb-crown-αMSH and [161Tb]Tb-crown-αMSH showing comparable uptake profiles and excellent image contrast.ConclusionsCollectively, our studies highlight the promising characteristics of [155Tb]Tb-crown-αMSH and [161Tb]Tb-crown-αMSH as theranostic pair for nuclear imaging (155Tb) and radionuclide therapy (161Tb).

Abstract Background Element-equivalent matched theranostic pairs facilitate quantitative in vivo imaging to establish pharmacokinetics and dosimetry estimates in the development of preclinical radiopharmaceuticals. Terbium radionuclides have significant potential as matched theranostic pairs for multipurpose applications in nuclear medicine. In particular, 155Tb (t1/2 = 5.32 d) and 161Tb (t1/2 = 6.89 d) have been proposed as a theranostic pair for their respective applications in single photon emission computed tomography (SPECT) imaging and targeted beta therapy. Our study assessed the performance of preclinical quantitative SPECT imaging with 155Tb and 161Tb. A hot rod resolution phantom with rod diameters ranging between 0.85 and 1.70 mm was filled with either 155Tb (21.8 ± 1.7 MBq/mL) or 161Tb (23.6 ± 1.9 MBq/mL) and scanned with the VECTor preclinical SPECT/CT scanner. Image performance was evaluated with two collimators: a high energy ultra high resolution (HEUHR) collimator and an extra ultra high sensitivity (UHS) collimator. SPECT images were reconstructed from photopeaks at 43.0 keV, 86.6 keV, and 105.3 keV for 155Tb and 48.9 keV and 74.6 keV for 161Tb. Quantitative SPECT images of the resolution phantoms were analyzed to report inter-rod contrast, recovery coefficients, and contrast-to-noise metrics. Results Quantitative SPECT images of the resolution phantom established that the HEUHR collimator resolved all rods for 155Tb and 161Tb, and the UHS collimator resolved rods ≥ 1.10 mm for 161Tb and ≥ 1.30 mm for 155Tb. The HEUHR collimator maintained better quantitative accuracy than the UHS collimator with recovery coefficients up to 92%. Contrast-to-noise metrics were also superior with the HEUHR collimator. Conclusions Both 155Tb and 161Tb demonstrated potential for applications in preclinical quantitative SPECT imaging. The high-resolution collimator achieves < 0.85 mm resolution and maintains quantitative accuracy in small volumes which is advantageous for assessing sub organ activity distributions in small animals. This imaging method can provide critical quantitative information for assessing and optimizing preclinical Tb-radiopharmaceuticals.

Background: Targeted radionuclide therapy is established as a highly effective strategy for the treatment of metastatic tumors; however, the co-development of suitable imaging companions to therapy remains significant challenge. Theranostic isotopes of terbium (149Tb, 152Tb, 155Tb, 161Tb) have the potential to provide chemically identical radionuclidic pairs, which collectively encompass all modes of nuclear decay relevant to nuclear medicine. Herein, we report the first radiochemistry and preclinical studies involving 155Tb- and 161Tb-labeled crown-alpha MSH, a small peptide-based bioconjugate suitable for targeting melanoma. Methods: 155Tb was produced via proton induced spallation of Ta targets using the isotope separation and acceleration facility at TRIUMF with isotope separation on-line (ISAC/ISOL). The radiolabeling characteristics of crown-alpha MSH with 155Tb and/or 161Tb were evaluated by concentration-dependence radiolabeling studies, and radio-HPLC stability studies. LogD7.4 measurements were obtained for [161Tb]Tb-crown-alpha MSH. Competitive binding assays were undertaken to determine the inhibition constant for [natTb]Tb-crown-alpha MSH in B16-F10 cells. Pre-clinical biodistribution and SPECT/CT imaging studies of 155Tb and 161Tb labeled crown-alpha MSH were undertaken in male C57Bl/6 J mice bearing B16-F10 melanoma tumors to evaluate tumor specific uptake and imaging potential for each radionuclide. Results: Quantitative radiolabeling of crown-alpha MSH with [155Tb]Tb3+ and [161Tb]Tb3+ was demonstrated under mild conditions (RT, 10 min) and low chelator concentrations; achieving high molar activities (23-29 MBq/ nmol). Radio-HPLC studies showed [161Tb]Tb-crown-alpha MSH maintains excellent radiochemical purity in human serum, while gradual metabolic degradation is observed in mouse serum. Competitive binding assays showed the high affinity of [natTb]Tb-crown-alpha MSH toward MC1R. Two different methods for preparation of the [155Tb]Tbcrown-alpha MSH radiotracer were investigated and the impacts on the biodistribution profile in tumor bearing mice is compared. Preclinical in vivo studies of 155Tb- and 161Tb- labeled crown-alpha MSH were performed in parallel, in mice bearing B16-F10 tumors; where the biodistribution results showed similar tumor specific uptake (6.06-7.44 %IA/g at 2 h pi) and very low uptake in nontarget organs. These results were further corroborated through a series of single-photon emission computed tomography (SPECT) studies, with [155Tb]Tb-crown-alpha MSH and [161Tb]Tb-crown-alpha MSH showing comparable uptake profiles and excellent image contrast.

Herein we report the automation and scale-up of a photofluorination process key to the production of branched-chain aliphatic radiotracers such as (S)-5-[18F]fluorohomoleucine ((S)-5-[18F]]FHL). (S)-5-[18F]FHL is a leucine analogue that is primarily taken up by the L-type amino acid transporter (LAT or System L). LAT1 expression levels correlate closely with tumor proliferation, angiogenesis, and treatment outcomes, making it an attractive target for molecular imaging of cancer. We have previously synthesized (S)-5-[18F]FHL and tested this tracer in mice bearing PC3 (prostate) or U87 (glioma) xenografts in order to establish its feasibility for detecting and monitoring treatment for a broad range of cancers. In this study, the radiosynthesis of 5-[18F]FHL is demonstrated on an automated DT-PhotoFluor module with a radiochemical yield of 20.1 ± 4.8

Objective . 225 Ac radiopharmaceuticals have tremendous potential for targeted alpha therapy, however, 225 Ac ( t 1/2 = 9.9 d) lacks direct gamma emissions for in vivo imaging. 226 Ac ( t 1/2 = 29.4 h) is a promising element-equivalent matched diagnostic radionuclide for preclinical evaluation of 225 Ac radiopharmaceuticals. 226 Ac has two gamma emissions (158 keV and 230 keV) suitable for SPECT imaging. This work is the first feasibility study for in vivo quantitative 226 Ac SPECT imaging and validation of activity estimation. Approach . 226 Ac was produced at TRIUMF (Vancouver, Canada) with its Isotope Separator and Accelerator (ISAC) facility. [ 226 Ac]Ac 3+ was radiolabelled with the bioconjugate crown-TATE developed for therapeutic targeting of neuroendocrine tumours. Mice with AR42J tumour xenografts were injected with either 2 MBq of [ 226 Ac]Ac-crown-TATE or 4 MBq of free [ 226 Ac]Ac 3+ activity and were scanned at 1, 2.5, 5, and 24 h post injection in a preclinical microSPECT/CT. Quantitative SPECT images were reconstructed from the 158 keV and 230 keV photopeaks with attenuation, background, and scatter corrections. Image-based 226 Ac activity measurements were assessed from volumes of interest within tumours and organs of interest. Imaging data was compared with ex vivo biodistribution measured via gamma counter. Main results . We present, to the best of our knowledge, the first ever in vivo quantitative SPECT images of 226 Ac activity distributions. Time-activity curves derived from SPECT images quantify the in vivo biodistribution of [ 226 Ac]Ac-crown-TATE and free [ 226 Ac]Ac 3+ activity. Image-based activity measurements in the tumours and organs of interest corresponded well with ex vivo biodistribution measurements. Significance . Here in, we established the feasibility of in vivo 226 Ac quantitative SPECT imaging for accurate measurement of actinium biodistribution in a preclinical model. This imaging method could facilitate more efficient development of novel actinium labelled compounds by providing accurate quantitative in vivo pharmacokinetic information essential for estimating toxicities, dosimetry, and therapeutic potency.

Superior bifunctional chelating ligands, which can sequester both α-emitting radionuclides (225Ac, 213Bi) and their diagnostic companions (155Tb, 111In), remain a formidable challenge to translating targeted alpha therapy, with complementary diagnostic imaging, to the clinic. H4noneupaX, a chelating ligand with an unusual diametrically opposed arrangement of pendant donor groups, has been developed to this end. H4noneunpaX preferentially complexes Ln3+ and An3+ ions, forming thermodynamically stable (pLa = 17.8, pLu = 21.3) and kinetically inert complexes─single isomeric species by nuclear magnetic resonance and density functional theory. Metal binding versatility demonstrated in radiolabeling [111In]In3+, [155Tb]Tb3+, [177Lu]Lu3+, and [225Ac]Ac3+ achieved high molar activities under mild conditions. Efficient, scalable synthesis enabled in vivo evaluation of bifunctional H4noneunpaX conjugated to two octreotate peptides targeting neuroendocrine tumors. Single photon emission computed tomography/CT and biodistribution studies of 155Tb-radiotracers in AR42J tumor-bearing mice showed excellent image contrast, good tumor uptake, and high in vivo stability. H4noneunpaX shows significant potential for theranostic applications involving 225Ac/155Tb or 177Lu/155Tb.