A photon source model for alpha-emitter radionuclides

Objective.A Monte Carlo virtual source model named PHID (photon from Ion decay) that generates photons emitted in the complex decay chain process of alpha-emitter radionuclides is proposed, typically for use during the simulation of SPECT image acquisition.Approach.Given an alpha-emitter radionuclide, the PHID model extracts from Geant4 databases the photon emission lines from all decaying daughters for both isometric transition and atomic relaxation processes. According to a given time range, abundances and activities in the decay chain are considered thanks to the Bateman equations, taking into account the decay rates and the initial abundances.Main results.PHID is evaluated by comparison with analog Monte Carlo simulation. It generates photons with the correct energy and temporal distribution, avoiding the costly simulation of the complete decay chain thus decreasing the computation time. The exact time gain depends on the simulation setup. As an example, it is 30× faster for simulating 1 MBq of225Ac in water for 1 section Moreover, for225Ac, PHID was also compared to a simplified source model with the two main photon emission lines (218 and 440 keV). PHID shows that 2 times more particles are simulated and 60% more counts are detected in the images.Significance.PHID can simulate any alpha-emitter radionuclide available in the Geant4 database. As a limitation, photons emitted from Bremsstrahlung are ignored, but they represent only 0.7% of the photons above 30 keV and are not significant for SPECT imaging. PHID is open-source, available in GATE 10, and eases the investigation of imaging photon emission from alpha emitters.

Targeted alpha therapy for glioblastoma

According to the 2021 World Health Organization Classification of Tumors of the Central Nervous System, glioblastoma (GB) is a primary brain tumor and presents with the worst prognosis. Due to its infiltrating characteristic, molecular heterogeneity, and only partly preserved function of the blood-brain barrier, the median overall survival time is short (9-15 months), regardless of comprehensive treatment including surgery, radiotherapy, and chemotherapy. Several novel treatment strategies are under investigation. Unfortunately, none of them produced successful results; 90% of patients have a recurrence of the disease within 6 months. Local administration of the drug could be a promising approach to delivering treatment with minimized side effects, due to the recurrence of 95% glioblastomas in a margin of 2 cm at the primary site. Several ligand-receptor systems have been evaluated, such as targeting tenascin, the extracellular matrix protein, or radiolabeled somatostatin analogs, as it is overexpressed with the SSTR-2 receptor system in around 80% of gliomas. Moreover, this study revealed that the NK-1 receptor is overexpressed in GB, suggesting that substance P (SP) may serve as a ligand. A variety of radioisotopes, beta- (¹³¹I, ⁹⁰Y, or ¹⁷⁷ Lu) and alpha emitters (²¹³Bi, ²²⁵Ac, or ²¹¹At), with different physical properties were tested for treatment. Alpha particles have many advantages over beta radiation such as short range with higher linear energy transfer. According to that characteristic, it is extremely dose delivered to the targeted cells, while reducing harm to nearby healthy tissue. Additionally, the biological effect of alpha radiation is independent of the cell cycle phase, cell oxygenation and O-6-methylguanine-DNA methyltransferase (MGMT) gene promoter methylation status. In this article, we summarize the experience with local treatment of primary and secondary GBs with locally used radioisotopes such as [²¹³Bi]Bi-DOTA-SP or [²²⁵Ac]Ac-DOTA-SP.

Mathematical Model for Evaluation of Tumor Response in Targeted Radionuclide Therapy with 211At Using Implanted Mouse Tumor

Recent introduction of alpha-emitting radionuclides in targeted radionuclide therapy has stimulated the development of new radiopharmaceuticals. Preclinical evaluation using an animal experiment with an implanted tumor model is frequently used to examine the efficiency of the treatment method and to predict the treatment response before clinical trials. Here, we propose a mathematical model for evaluation of the tumor response in an implanted tumor model and apply it to the data obtained from the previous experiment of ²¹¹At treatment in a thyroid cancer mouse model. The proposed model is based on the set of differential equations, describing the kinetics of radiopharmaceuticals, the tumor growth, and the treatment response. First, the tumor growth rate was estimated from the control data without injection of ²¹¹At. The kinetic behavior of the injected radionuclide was used to estimate the radiation dose profile to the target tumor, which can suppress the tumor growth in a dose-dependent manner. An additional two factors, including the time delay for the reduction of tumor volume and the impaired recovery of tumor regrowth after the treatment, were needed to simulate the temporal changes of tumor size after treatment. Finally, the parameters obtained from the simulated tumor growth curve were able to predict the tumor response in other experimental settings. The model can provide valuable information for planning the administration dose of radiopharmaceuticals in clinical trials, especially to determine the starting dose at which efficacy can be expected with a sufficient safety margin.

Effect to Therapy of Sodium-Iodine Symporter Expression by Alpha-Ray Therapeutic Agent via Sodium/Iodine Symporter

This study confirmed the effect of sodium/iodine symporter (NIS) expression on existing drugs by in vitro and in vivo tests using cultured cell lines. The tumor growth inhibitory effect of sodium astatide ([²¹¹At]NaAt) was evaluated by in vitro and in vivo tests using human thyroid cancer cells (K1, K1/NIS and K1/NIS-DOX). NIS expression in cancer cells was controlled using the Tet-On system. [¹³¹I]NaI was used as control existing drug. From the results of the in vitro studies, the mechanism of [²¹¹At]NaAt uptake into thyroid cancer cells is mediated by NIS, analogous to [¹³¹I]NaI, and the cellular uptake rate correlates with the expression level of NIS. [²¹¹At]NaAt's ability to inhibit colony formation was more than 10 times that of [¹³¹I]NaI per becquerel (Bq), and [²¹¹At]NaAt's DNA double-strand breaking (DSB) induction was more than ten times that of [¹³¹I]NaI per Bq, and [²¹¹At]NaAt was more than three times more cytotoxic than [¹³¹I]NaI (at 1000 kBq each). In vivo studies also showed that the tumor growth inhibitory effect of [²¹¹At]NaAt depended on NIS expression and was more than six times that of [¹³¹I]NaI per Bq.

AlphaBet: Combination of Radium-223 and [17 7Lu]Lu-PSMA-I&T in men with metastatic castration-resistant prostate cancer (clinical trial protocol)

BACKGROUND

[177Lu]Lu-PSMA is a radioligand therapy used in metastatic castration-resistant prostate cancer (mCRPC). Despite a survival benefit, the responses for many patients receiving [177Lu]Lu-PSMA are not durable, and all patients eventually develop progressive disease. The bone marrow is the most common site of progression. Micrometastases in this area likely receive an inadequate dose of radiation, as the emitted beta-particles from 177Lu travel an average range of 0.7 mm in soft tissue, well beyond the diameter of micrometastases. Radium-223 (223Ra) is a calcium-mimetic and alpha-emitting radionuclide approved for use in men with mCRPC with bone metastases. The range of emitted alpha particles in soft tissue is much shorter (≤100 μm) with high linear energy transfer, likely more lethal for osseous micrometastases. We anticipate that combining a bone-specific alpha-emitter with [177Lu]Lu-PSMA will improve eradication of micrometastatic osseous disease, and thereby lead to higher and longer responses.

METHODS

This is a single-center, single-arm phase I/II trial evaluating the combination of 223Ra and [177Lu]Lu-PSMA-I&T in men with mCRPC. Thirty-six patients will receive 7.4 GBq of [177Lu]Lu-PSMA-I&T, concurrently with 223Ra in escalating doses (28 kBq/kg - 55kBq/kg), both given intravenously every six weeks for up to six cycles. Eligible patients will have at least two untreated bone metastases visible on bone scintigraphy, and PSMA-positive disease on PSMA PET scan. Patients must have adequate bone marrow and organ function and be willing to undergo tumor biopsies. Patients with discordant disease visible on FDG PET scan (defined as FDG positive disease with minimal or no PSMA expression and no uptake on bone scan) will be excluded. Other key exclusion criteria include the presence of diffuse marrow disease, prior treatment with 223Ra or [177Lu]Lu-PSMA, or more than one prior line of chemotherapy for prostate cancer. The co-primary objectives of this study are to determine the maximum tolerated dose of 223Ra when combined with [177Lu]Lu-PSMA-I&T and the 50% PSA response rate.

CONCLUSION

The AlphaBet trial is a phase I/II study combining 223Ra with [177Lu]Lu-PSMA-I&T in patients with mCRPC. We aim to enroll the first patient in Q3 2022, and recruitment is anticipated to continue for 24 months.

STUDY REGISTRATION

NCT05383079.

[225Ac]Ac-SibuDAB for Targeted Alpha Therapy of Prostate Cancer: Preclinical Evaluation and Comparison with [225Ac]Ac-PSMA-617

In the present study, SibuDAB, an albumin-binding PSMA ligand, was investigated in combination with actinium-225 and the data were compared with those of [²²⁵Ac]Ac-PSMA-617. In vitro, [²²⁵Ac]Ac-SibuDAB and [²²⁵Ac]Ac-PSMA-617 showed similar tumor cell uptake and PSMA-binding affinities as their ¹⁷⁷Lu-labeled counterparts. The in vitro binding to serum albumin in mouse and human blood plasma, respectively, was 2.8-fold and 1.4-fold increased for [²²⁵Ac]Ac-SibuDAB as compared to [¹⁷⁷Lu]Lu-SibuDAB. In vivo, this characteristic was reflected by the longer retention of [²²⁵Ac]Ac-SibuDAB in the blood than previously seen for [¹⁷⁷Lu]Lu-SibuDAB. Similar to [²²⁵Ac]Ac-PSMA-617, [²²⁵Ac]Ac-SibuDAB was well tolerated at 30 kBq per mouse. Differences in blood cell counts were observed between treated mice and untreated controls, but no major variations were observed between values obtained for [²²⁵Ac]Ac-SibuDAB and [²²⁵Ac]Ac-PSMA-617. [²²⁵Ac]Ac-SibuDAB was considerably more effective to treat PSMA-positive tumor xenografts than [²²⁵Ac]Ac-PSMA-617. Only 5 kBq per mouse were sufficient to eradicate the tumors, whereas tumor regrowth was observed for mice treated with 5 kBq [²²⁵Ac]Ac-PSMA-617 and only one out of six mice survived until the end of the study. The enhanced therapeutic efficacy of [²²⁵Ac]Ac-SibuDAB as compared to that of [²²⁵Ac]Ac-PSMA-617 and reasonable safety data qualify this novel radioligand as a candidate for targeted α-therapy of prostate cancer.

Intraperitoneal alpha therapy with 224Ra-labeled microparticles combined with chemotherapy in an ovarian cancer mouse model

A novel alpha-therapy consisting of ²²⁴Ra-labeled calcium carbonate microparticles (²²⁴Ra-CaCO₃-MP) has been designed to treat micrometastatic peritoneal disease via intraperitoneal (IP) administration. This preclinical study aimed to evaluate its efficacy and tolerability when given as a single treatment or in combination with standard of care chemotherapy regimens, in a syngeneic model of ovarian cancer in immune competent mice. Female C57BL/6 mice bearing ID8-fLuc ovarian cancer were treated with ²²⁴Ra-CaCO₃-MP 1 day after IP tumor cell inoculation. The activity dosages of ²²⁴Ra ranged from 14 to 39 kBq/mouse. Additionally, ²²⁴Ra-CaCO₃-MP treatment was followed by either carboplatin (80 mg/kg)-pegylated liposomal doxorubicin (PLD, 1.6 mg/kg) or carboplatin (60 mg/kg)-paclitaxel (10 mg/kg) on day 14 post tumor cell inoculation. All treatments were administered via IP injections. Readouts included survival, clinical signs, and body weight development over time. There was a slight therapeutic benefit after single treatment with ²²⁴Ra-CaCO₃-MP compared to the vehicle control, with median survival ratios (MSRs) ranging between 1.1 and 1.3. The sequential administration of ²²⁴Ra-CaCO₃-MP with either carboplatin-paclitaxel or carboplatin-PLD indicated a synergistic effect on overall survival at certain ²²⁴Ra activities. Moreover, the combinations tested appeared well tolerated in terms of weight assessment in the first 4 weeks after treatment. Overall, this research supports the further evaluation of ²²⁴Ra-CaCO₃-MP in patients with ovarian cancer. However, the most optimal chemotherapy regimen to combine with ²²⁴Ra-CaCO₃-MP should be identified to fully exploit its therapeutic potential.

Comparison of the Therapeutic Effects of [211At]NaAt and [131I]NaI in an NIS-Expressing Thyroid Cancer Mouse Model

Astatine (²¹¹At) is an alpha-emitter with a better treatment efficacy against differentiated thyroid cancer compared with iodine (¹³¹I), a conventional beta-emitter. However, its therapeutic comparison has not been fully evaluated. In this study, we compared the therapeutic effect between [²¹¹At]NaAt and [¹³¹I]NaI. In vitro analysis of a double-stranded DNA break (DSB) and colony formation assay were performed using K1-NIS cells. The therapeutic effect was compared using K1-NIS xenograft mice administered with [²¹¹At]NaAt (0.4 MBq (n = 7), 0.8 MBq (n = 9), and 1.2 MBq (n = 4)), and [¹³¹I]NaI (1 MBq (n = 4), 3 MBq (n = 4), and 8 MBq (n = 4)). The [²¹¹At]NaAt induced higher numbers of DSBs and had a more reduced colony formation than [¹³¹I]NaI. In K1-NIS mice, dose-dependent therapeutic effects were observed in both [²¹¹At]NaAt and [¹³¹I]NaI. In [²¹¹At]NaAt, a stronger tumour-growth suppression was observed, while tumour regrowth was not observed until 18, 25, and 46 days after injection of 0.4, 0.8, and 1.2 MBq of [²¹¹At]NaAt, respectively. While in [¹³¹I]NaI, this was observed within 12 days after injection (1, 3, and 8 MBq). The superior therapeutic effect of [²¹¹At]NaAt suggests the promising clinical applicability of targeted alpha therapy using [²¹¹At]NaAt in patients with differentiated thyroid cancer refractory to standard [¹³¹I]NaI treatment.

A Novel Single-Step-Labeled 212Pb-CaCO3 Microparticle for Internal Alpha Therapy: Preparation, Stability, and Preclinical Data from Mice

Lead-212 is recognized as a promising radionuclide for targeted alpha therapy for tumors. Many studies of ²¹²Pb-labeling of various biomolecules through bifunctional chelators have been conducted. Another approach to exploiting the cytotoxic effect is coupling the radionuclide to a microparticle acting as a carrier vehicle, which could be used for treating disseminated cancers in body cavities. Calcium carbonate may represent a suitable material, as it is biocompatible, biodegradable, and easy to synthesize. In this work, we explored ²¹²Pb-labeling of various CaCO₃ microparticles and developed a protocol that can be straightforwardly implemented by clinicians. Vaterite microparticles stabilized by pamidronate were effective as ²¹²Pb carriers; labeling yields of ≥98% were achieved, and ²¹²Pb was strongly retained by the particles in an in vitro stability assessment. Moreover, the amounts of ²¹²Pb reaching the kidneys, liver, spleen, and skeleton of mice following intraperitoneal (i.p.) administration were very low compared to i.p. injection of unbound ²¹²Pb²⁺, indicating that CaCO₃-bound ²¹²Pb exhibited stability when administered intraperitoneally. Therapeutic efficacy was observed in a model of i.p. ovarian cancer for all the tested doses, ranging from 63 to 430 kBq per mouse. Lead-212-labeled CaCO₃ microparticles represent a promising candidate for treating intracavitary cancers.

Improved Formulation of 224Ra-Labeled Calcium Carbonate Microparticles by Surface Layer Encapsulation and Addition of EDTMP

Radium-224-labeled CaCO₃ microparticles have been developed to treat peritoneal carcinomatosis. The microparticles function as carriers of ²²⁴Ra, facilitating intraperitoneal retention of the alpha-emitting radionuclide. It was necessary to control the size of microparticles in suspension over time and introduce a sterilization process for the clinical use of the radiopharmaceutical. Ethylenediamine tetra(methylene phosphonic acid) (EDTMP) was investigated as a stabilizing additive. The possibility of encapsulating the radiolabeled microparticles with an outer surface layer of CaCO₃ for the improved retention of radioactivity by the carrier was studied. This work evaluated these steps of optimization and their effect on radiochemical purity, the biodistribution of radionuclides, and therapeutic efficacy. An EDTMP concentration of >1% (w/w) relative to CaCO₃ stabilized the particle size for at least one week. Without EDTMP, the median particle size increased from ~5 µm to ~25 µm immediately after sterilization by autoclaving, and the larger microparticles sedimented rapidly in suspension. The percentage of adsorbed ²²⁴Ra progeny ²¹²Pb increased from 56% to 94% at 2.4-2.5% (w/w) EDTMP when the ²²⁴Ra-labeled microparticles were layer-encapsulated. The improved formulation also resulted in a suitable biodistribution of radionuclides in mice, as well as a survival benefit for mice with intraperitoneal ovarian or colorectal tumors.

Targeted alpha therapy using astatine (211At)-labeled phenylalanine: A preclinical study in glioma bearing mice

Phenylalanine derivatives, which target tumors especially through L-type amino acid transporter-1 (LAT1), have elicited considerable attention. In this study, we evaluated the treatment effect of phenylalanine labeled with the alpha emitter astatine (²¹¹At-PA) in tumor bearing mice. The C6 glioma, U-87MG, and GL261 cell lines were subjected to a cellular ²¹¹At-PA uptake analysis that included an evaluation of the uptake inhibition by the system L amino acid transporter inhibitor 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH). BCH significantly inhibited para-²¹¹At-PA uptake in C6 glioma (12.2 ± 0.8%), U-87MG (27.6 ± 1.1%), and GL261 (12.6 ± 2.0%) cells compared to baseline, suggesting an uptake contribution by system L amino acid transporters. Subsequently, xenograft and allograft models were prepared by subcutaneously injecting C6 glioma (n = 12) or GL-261 cells (n = 12), respectively. C6 glioma mice received three ²¹¹At-PA doses (0.1, 0.5, or 1 MBq, n = 3/dose), while GL261 mice received one high dose (1 MBq, n = 7). ²¹¹At-PA exhibited a tumor growth suppression effect in C6 glioma models in a dose-dependent manner as well as in GL-261 models. This phenylalanine derivative labeled with astatine may be applicable as an alpha therapy that specifically targets system L amino acid transporters.

Enhancement of 211At Uptake via the Sodium Iodide Symporter by the Addition of Ascorbic Acid in Targeted α-Therapy of Thyroid Cancer

²¹¹At is an α-emitter that has similar chemical properties to iodine and is used in targeted α-therapy. In the present study, we added ascorbic acid (AA) to ²¹¹At solution to increase the radiochemical purity of astatide and evaluated its efficacy against differentiated thyroid cancer, which is characterized by the expression of sodium/iodide symporter (NIS). Methods: Crude ²¹¹At solution (AA(-)) and ²¹¹At solution treated with AA (AA(+)) were prepared. Uptake by the thyroid was compared between the 2 solutions in normal male Wistar rats (n = 6). Cellular uptake in K1-NIS cells was analyzed under the AA(+) and AA(-) conditions. AA(+) was injected at 3 doses into K1-NIS xenograft mice: 1 MBq (n = 6), 0.4 MBq (n = 6), and 0.1 MBq (n = 6), and vehicle was injected into control mice (n = 6). The treatment effects were compared among the 4 groups. Results: Uptake by the thyroid was significantly enhanced in rats injected with the AA(+) as compared with those injected with AA(-). Cellular uptake analysis showed significantly increased uptake of ²¹¹At by the K1-NIS cells under the AA(+) condition as compared with the AA(-) condition. In the mouse xenograft model, the K1-NIS tumors showed significant accumulation of ²¹¹At at 3 and 24 h after administration (22.5 ± 10.4 and 12.9 ± 6.8 percentage injected dose, respectively). Tumor growth was immediately inhibited in a dose-dependent manner after administration of ²¹¹At. In the survival analysis, the ²¹¹At groups (0.1, 0.4, and 1 MBq) showed significantly better survival than the control group. Conclusion: Uptake of ²¹¹At was enhanced in differentiated thyroid cancer cells as well as the normal thyroid using ²¹¹At solution treated with AA. The method also showed dose-dependent efficacy against the K1-NIS xenografts, suggesting its potential applicability to targeted α-therapy.

Ra-224 labeling of calcium carbonate microparticles for internal α-therapy: Preparation, stability, and biodistribution in mice

Internal therapy with α‐emitters should be well suited for micrometastatic disease. Radium‐224 emits multiple α‐particles through its decay and has a convenient 3.6 days of half‐life. Despite its attractive properties, the use of ²²⁴Ra has been limited to bone‐seeking applications because it cannot be stably bound to a targeting molecule. Alternative delivery systems for ²²⁴Ra are therefore of considerable interest. In this study, calcium carbonate microparticles are proposed as carriers for ²²⁴Ra, designed for local therapy of disseminated cancers in cavitary regions, such as peritoneal carcinomatosis. Calcium carbonate microparticles were radiolabeled by precipitation of ²²⁴Ra on the particle surface, resulting in high labeling efficiencies for both ²²⁴Ra and daughter ²¹²Pb and retention of more than 95% of these nuclides for up to 1 week in vitro. The biodistribution after intraperitoneal administration of the ²²⁴Ra‐labeled CaCO₃ microparticles in immunodeficient mice revealed that the radioactivity mainly remained in the peritoneal cavity. In addition, the systemic distribution of ²²⁴Ra was found to be strongly dependent on the amount of administered microparticles, with a reduced skeletal uptake of ²²⁴Ra with increasing dose. The results altogether suggest that the ²²⁴Ra‐labeled CaCO₃ microparticles have promising properties for use as a localized internal α‐therapy of cavitary cancers.

Quality of life in patients with brain tumors in the course of alpha therapy

The aim of the study was to assess subjective quality of life in patients receiving alpha therapy for malignant tumors (glioblastoma multiforme, GBM) of the brain. No significant differences in self-assessed quality of life were found between GBM patients in the course of α-therapy and healthy controls, the two groups differed only as regards somatic symptoms. Moreover, the stronger the respondents' sense of self-efficacy, the higher their subjective quality of life. The findings may have practical implications for clinical psychology, namely, it seems worthwhile to build up the patient's sense of control of his own health.

Encapsulation of α-Particle-Emitting 225Ac3+ Ions Within Carbon Nanotubes

(225)Ac(3+) is a generator of α-particle-emitting radionuclides with 4 net α-particle decays that can be used therapeutically. Targeting (225)Ac(3+) by use of ligands conjugated to traditional bifunctional chelates limits the amount of (225)Ac(3+) that can be delivered. Ultrashort, single-walled carbon nanotubes (US-tubes), previously demonstrated as sequestering agents of trivalent lanthanide ions and small molecules, also successfully incorporate (225)Ac(3+).

METHODS

Aqueous loading of both (225)Ac(3+) ions and Gd(3+) ions via bath sonication was used to construct (225)Ac@gadonanotubes ((225)Ac@GNTs). The (225)Ac@GNTs were subsequently challenged with heat, time, and human serum.

RESULTS

US-tubes internally loaded with both (225)Ac(3+) ions and Gd(3+) ions show 2 distinct populations of (225)Ac(3+) ions: one rapidly lost in human serum and one that remains bound to the US-tubes despite additional challenge with heat, time, and serum. The presence of the latter population depended on cosequestration of Gd(3+) and (225)Ac(3+) ions.

CONCLUSION

US-tubes successfully sequester (225)Ac(3+) ions in the presence of Gd(3+) ions and retain them after a human serum challenge, rendering (225)Ac@GNTs candidates for radioimmunotherapy for delivery of (225)Ac(3+) ions at higher concentrations than is currently possible for traditional ligand carriers.