Actinium-225 Targeted Agents: Where Are We Now?

The Advancement of Targeted Alpha Therapy and the Role of Click Chemistry Therein

Recent years have seen a swift rise in the use of α-emitting radionuclides such as 225Ac and 223Ra as various radiopharmaceuticals to treat (micro)metastasized tumors. They have shown remarkable effectiveness in clinical practice owing to the highly cytotoxic α-particles that are emitted, which have a very short range in tissue, causing mainly double-stranded DNA breaks. However, it is essential that both chelation and targeting strategies are optimized for their successful translation to clinical application, as α-emitting radionuclides have distinctly different features compared to β--emitters, including their much larger atomic radius. Furthermore, upon α-decay, any daughter nuclide irrevocably breaks free from the targeting molecule, known as the recoil effect, dictating the need for faster targeting to prevent healthy tissue toxicity. In this review we provide a brief overview of the current status of targeted α-therapy and highlight innovations in α-emitter-based chelator design, focusing on the role of click chemistry to allow for fast complexation to biomolecules at mild labeling conditions. Finally, an outlook is provided on different targeting strategies and the role that pre-targeting can play in targeted alpha therapy.

Comparison of radiobiological effects induced by radiolabeled antibodies in human cancer cells and fungal cells

PURPOSE

Acute myeloid leukemia (AML) is a deadly form of leukemia, and its treatment often leaves patients immunocompromised, making them vulnerable to fungal infections. Radioimmunotherapy (RIT) is explored for both AML and fungal infections. This study compares the radiobiological effects of alpha emitter Actinium-225 (225Ac) and beta emitter Lutetium-177 (177Lu)-labeled antibodies on AML and Cryptococcus neoformans cells.

MATERIALS AND METHODS

AML OCI-AML3 and C. neoformans Cap-67 cells were treated with anti-(1-3)-beta-glucan antibody 400-2 and anti-CD33 antibody HuM-195, conjugated to DOTA and radiolabeled with 225Ac or 177Lu. Clonogenic survival, γH2A/X staining, and micronuclei assays were conducted. Antibody internalization was assessed by flow cytometry.

RESULTS

Both 225Ac- and 177Lu-enabled RIT resulted in decreased clonogenic survival in Cap-67 and OCI-AML3 cells, with Cap-67 recovering more rapidly. DNA double-strand breaks and micronuclei formation revealed DNA damage, with fewer micronuclei in OCI-AML3 cells due to radiation destruction. HuM-195 antibody internalized into OCI-AML3 cells, whereas 400-2 did not internalize into Cap-67 cells.

CONCLUSIONS

While both cell lines showed similar responses to 225Ac- and 177Lu-enabled RIT, variations were observed based on cellular structure, doubling times and DNA repair mechanisms. This study offers insights for future in vivo research on fungal infections in cancer setting.

Complete Remissions of HER2-Positive Trastuzumab-Resistant Xenografts Using a Potent [225Ac]Ac-Labeled Anti-HER2 Antibody-Drug Radioconjugate

PURPOSE

There is overwhelming interest to use actinium-225 ([225Ac]Ac) to develop targeted α therapies. Antibody-drug conjugates (ADC) are highly cytotoxic. Combining [225Ac]Ac with an ADC to develop an antibody-drug radioconjugate [225Ac]Ac-macropa-trastuzumab(T)-PEG6-emtansine (DM1), is expected to be more effective than its ADC (T-PEG6-DM1) against breast cancer.

EXPERIMENTAL DESIGN

[89Zr]Zr-p-isothiocyanatobenzyl desferrioxamine (DFO)-T-PEG6-DM1 (imaging) and [225Ac]Ac-macropa-T-PEG6-DM1 (radiotherapy) were developed. Biodistribution and safety evaluations of [225Ac]Ac-macropa-T-PEG6-DM1 were carried out in non-tumor-bearing BALB/c mice. MicroPET imaging and biodistribution were done using [89Zr]Zr-DFO-T-PEG6-DM1, and radiotherapy using [225Ac]Ac-macropa-T-PEG6-DM1 was carried out in athymic BALB/c nude mice bearing trastuzumab-resistant HCC1954 and trastuzumab-DM1 (T-DM1)/trastuzumab-resistant JIMT-1 tumor-bearing mice.

RESULTS

After 7 days of incubation at 37°C, [225Ac]Ac-macropa-T-PEG6-DM1 was stable in both human serum (89.2% ± 0.9%) and PBS (82.8% ± 0.4%). T-PEG6-DM1 (8 mg/kg) and [225Ac]Ac-macropa-T-PEG6-DM1 (3 × 18 kBq) administered separately in non-tumor-bearing mice 10 days apart were well tolerated biochemically and hematologically. Imaging and biodistribution showed high tumor uptake of [89Zr]Zr-DFO-T-PEG6-DM1 in tumor-bearing mice at 120 hours after injection: 38.1% ± 2.8% IA/g (HCC1954) and 14.6% ± 1% IA/g (JIMT-1). In HCC1954 tumor-bearing mice, all treatment groups had complete remission (8/8), indicative of the responsiveness of the xenograft to T-DM1-based treatments, whereas for JIMT-1 xenografts (having 1/8 complete remission) at 23 days after treatment, tumor volumes were 332.1 ± 77.5 vs. 244.6 ± 63 vs. 417.9 ± 62.1 vs. 102.4 ± 18.5 for the saline (negative control), T-DM1 (positive control), T-PEG6-DM1, and [225Ac]Ac-macropa-T-PEG6-DM1, respectively.

CONCLUSIONS

[225Ac]Ac-macropa-T-PEG6-DM1 is more potent than ADC against trastuzumab-resistant breast cancer and necessitates clinical translation.

Adverse reactions to therapeutic radiopharmaceuticals

Radiopharmaceuticals are drugs used in treatment or diagnosis that contain a radioactive part, usually a pharmaceutical part in their structure. Adverse drug reactions are harmful and unexpected responses that occur when administered at normal doses. Although radiopharmaceuticals are regarded as safe medical products, adverse reactions should not be ignored. More serious adverse reactions such as myelosuppression, pleural effusion, and death may develop in therapeutic radiopharmaceuticals due to their use at higher doses than those used in diagnosis. Therefore, monitoring adverse reactions and reporting them to health authorities is important. This review aims to provide information about adverse reactions that may be related to radiopharmaceuticals used in treatment.

Optimization Processes of Clinical Chelation-Based Radiopharmaceuticals for Pathway-Directed Targeted Radionuclide Therapy in Oncology

Targeted radionuclide therapy (TRT) for internal pathway-directed treatment is a game changer for precision medicine. TRT improves tumor control while minimizing damage to healthy tissue and extends the survival for patients with cancer. The application of theranostic-paired TRT along with cellular phenotype and genotype correlative analysis has the potential for malignant disease management. Chelation chemistry is essential for the development of theranostic-paired radiopharmaceuticals for TRT. Among image-guided TRT, 68Ga and 99mTc are the current standards for diagnostic radionuclides, while 177Lu and 225Ac have shown great promise for β- and α-TRT, respectively. Their long half-lives, potent radiobiology, favorable decay schemes, and ability to form stable chelation conjugates make them ideal for both manufacturing and clinical use. The current challenges include optimizing radionuclide production processes, coordinating chelation chemistry stability of theranostic-paired isotopes to reduce free daughters [this pertains to 225Ac daughters 221Fr and 213Bi]-induced tissue toxicity, and improving the modeling of micro dosimetry to refine dose-response evaluation. The empirical approach to TRT delivery is based on standard radionuclide administered activity levels, although clinical trials have revealed inconsistent outcomes and normal-tissue toxicities despite equivalent administered activities. This review presents the latest optimization methods for chelation-based theranostic radiopharmaceuticals, advancements in micro-dosimetry, and SPECT/CT technologies for quantifying whole-body uptake and monitoring therapeutic response as well as cytogenetic correlative analyses.

In vivoquantitative SPECT imaging of actinium-226: feasibility and proof-of-concept

Objective.225Ac radiopharmaceuticals have tremendous potential for targeted alpha therapy, however,225Ac (t1/2= 9.9 d) lacks direct gamma emissions forin vivoimaging.226Ac (t1/2= 29.4 h) is a promising element-equivalent matched diagnostic radionuclide for preclinical evaluation of225Ac radiopharmaceuticals.226Ac has two gamma emissions (158 keV and 230 keV) suitable for SPECT imaging. This work is the first feasibility study forin vivoquantitative226Ac SPECT imaging and validation of activity estimation.Approach.226Ac was produced at TRIUMF (Vancouver, Canada) with its Isotope Separator and Accelerator (ISAC) facility. [226Ac]Ac3+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 [226Ac]Ac-crown-TATE or 4 MBq of free [226Ac]Ac3+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-based226Ac activity measurements were assessed from volumes of interest within tumours and organs of interest. Imaging data was compared withex vivobiodistribution measured via gamma counter.Main results. We present, to the best of our knowledge, the first everin vivoquantitative SPECT images of226Ac activity distributions. Time-activity curves derived from SPECT images quantify thein vivobiodistribution of [226Ac]Ac-crown-TATE and free [226Ac]Ac3+activity. Image-based activity measurements in the tumours and organs of interest corresponded well withex vivobiodistribution measurements.Significance. Here in, we established the feasibility ofin vivo226Ac 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 quantitativein vivopharmacokinetic information essential for estimating toxicities, dosimetry, and therapeutic potency.

Brachytherapy at the nanoscale with protein functionalized and intrinsically radiolabeled [169Yb]Yb2O3 nanoseeds

PURPOSE

Classical brachytherapy of solid malignant tumors is an invasive procedure which often results in an uneven dose distribution, while requiring surgical removal of sealed radioactive seed sources after a certain period of time. To circumvent these issues, we report the synthesis of intrinsically radiolabeled and gum Arabic glycoprotein functionalized [169Yb]Yb2O3 nanoseeds as a novel nanoscale brachytherapy agent, which could directly be administered via intratumoral injection for tumor therapy.

METHODS

169Yb (T½ = 32 days) was produced by neutron irradiation of enriched (15.2% in 168Yb) Yb2O3 target in a nuclear reactor, radiochemically converted to [169Yb]YbCl3 and used for nanoparticle (NP) synthesis. Intrinsically radiolabeled NP were synthesized by controlled hydrolysis of Yb3+ ions in gum Arabic glycoprotein medium. In vivo SPECT/CT imaging, autoradiography, and biodistribution studies were performed after intratumoral injection of radiolabeled NP in B16F10 tumor bearing C57BL/6 mice. Systematic tumor regression studies and histopathological analyses were performed to demonstrate therapeutic efficacy in the same mice model.

RESULTS

The nanoformulation was a clear solution having high colloidal and radiochemical stability. Uniform distribution and retention of the radiolabeled nanoformulation in the tumor mass were observed via SPECT/CT imaging and autoradiography studies. In a tumor regression study, tumor growth was significantly arrested with different doses of radiolabeled NP compared to the control and the best treatment effect was observed with ~ 27.8 MBq dose. In histopathological analysis, loss of mitotic cells was apparent in tumor tissue of treated groups, whereas no significant damage in kidney, lungs, and liver tissue morphology was observed.

CONCLUSIONS

These results hold promise for nanoscale brachytherapy to become a clinically practical treatment modality for unresectable solid cancers.

Actinium-225 in Targeted Alpha Therapy

The utilization of actinium-225 (225Ac) radionuclides in targeted alpha therapy for cancer was initially outlined in 1993. Over the past two decades, substantial research has been conducted, encompassing the establishment of 225Ac production methods, various preclinical investigations, and several clinical studies. Currently, there is a growing number of compounds labeled with 225Ac that are being developed and tested in clinical trials. In response to the increasing demand for this nuclide, production facilities are either being built or have already been established. This article offers a concise summary of the present state of clinical advancements in compounds labeled with 225Ac. It outlines various processes involved in the production and purification of 225Ac to cater to the growing demand for this radionuclide. The article examines the merits and drawbacks of different procedures, delves into preclinical trials, and discusses ongoing clinical trials.