Renal uptake of bismuth-213 and its contribution to kidney radiation dose following administration of actinium-225-labeled antibody

Image-based dosimetry for [225Ac]Ac-PSMA-I&T therapy and the effect of daughter-specific pharmacokinetics

PURPOSE

Although 221Fr and 213Bi have sufficient gamma emission probabilities, quantitative SPECT after [225Ac]Ac-PSMA-I&T therapy remains challenging due to low therapeutic activities. Furthermore, 221Fr and 213Bi may underlie a different pharmacokinetics due to alpha recoil. We conducted a quantitative SPECT study and a urine analysis to investigate the pharmacokinetics of 221Fr and 213Bi and the impact on image-based lesion and kidney dosimetry.

METHODS

Five patients (7.7 ± 0.2 MBq [225Ac]Ac-PSMA-I&T) underwent an abdominal SPECT/CT (1 h) at 24 and 48 h (Siemens Symbia T2, high-energy collimator, 440 keV/218 keV (width 20%), 78 keV (width 50%)). Quantitative SPECT was reconstructed using MAP-EM with attenuation and transmission-dependent scatter corrections and resolution modelling. Time-activity curves for kidneys (CT-based) and lesions (80% isocontour 24 h) were fitted mono-exponentially. Urine samples collected along with each SPECT/CT were measured in a gamma counter until secular equilibrium was reached.

RESULTS

Mean kidney and lesion effective half-lives were as follows: 213Bi, 27 ± 6/38 ± 10 h; 221Fr, 24 ± 6/38 ± 11 h; 78 keV, 23 ± 7/39 ± 13 h. The 213Bi-to-221Fr kidney SUV ratio increased by an average of 9% from 24 to 48 h. Urine analysis revealed an increasing 213Bi-to-225Ac ratio (24 h, 0.98 ± 0.15; 48 h, 1.08 ± 0.09). Mean kidney and lesion absorbed doses were 0.17 ± 0.06 and 0.36 ± 0.1 Sv RBE = 5 /MBq using 221Fr and 213Bi SPECT images, compared to 0.16 ± 0.05/0.18 ± 0.06 and 0.36 ± 0.1/0.38 ± 0.1 Sv RBE = 5 /MBq considering either the 221Fr or 213Bi SPECT.

CONCLUSION

SPECT/CT imaging and urine analysis showed minor differences of up to 10% in the daughter-specific pharmacokinetics. These variances had a minimal impact on the lesion and kidney dosimetry which remained within 8%.

Approaches to Reducing Normal Tissue Radiation from Radiolabeled Antibodies

Radiolabeled antibodies are powerful tools for both imaging and therapy in the field of nuclear medicine. Radiolabeling methods that do not release radionuclides from parent antibodies are essential for radiolabeling antibodies, and practical radiolabeling protocols that provide high in vivo stability have been established for many radionuclides, with a few exceptions. However, several limitations remain, including undesirable side effects on the biodistribution profiles of antibodies. This review summarizes the numerous efforts made to tackle this problem and the recent advances, mainly in preclinical studies. These include pretargeting approaches, engineered antibody fragments and constructs, the secondary injection of clearing agents, and the insertion of metabolizable linkages. Finally, we discuss the potential of these approaches and their prospects for further clinical application.

Towards Effective Targeted Alpha Therapy for Neuroendocrine Tumours: A Review

This review article explores the evolving landscape of Molecular Radiotherapy (MRT), emphasizing Peptide Receptor Radionuclide Therapy (PRRT) for neuroendocrine tumours (NETs). The primary focus is on the transition from β-emitting radiopharmaceuticals to α-emitting agents in PRRT, offering a critical analysis of the radiobiological basis, clinical applications, and ongoing developments in Targeted Alpha Therapy (TAT). Through an extensive literature review, the article delves into the mechanisms and effectiveness of PRRT in targeting somatostatin subtype 2 receptors, highlighting both its successes and limitations. The discussion extends to the emerging paradigm of TAT, underlining its higher potency and specificity with α-particle emissions, which promise enhanced therapeutic efficacy and reduced toxicity. The review critically evaluates preclinical and clinical data, emphasizing the need for standardised dosimetry and a deeper understanding of the dose-response relationship in TAT. The review concludes by underscoring the significant potential of TAT in treating SSTR2-overexpressing cancers, especially in patients refractory to β-PRRT, while also acknowledging the current challenges and the necessity for further research to optimize treatment protocols.

Implementing Ac-225 labelled radiopharmaceuticals: practical considerations and (pre-)clinical perspectives

BACKGROUND

In the past years, there has been a notable increase in interest regarding targeted alpha therapy using Ac-225, driven by the observed promising clinical anti-tumor effects. As the production and technology has advanced, the availability of Ac-225 is expected to increase in the near future, making the treatment available to patients worldwide.

MAIN BODY

Ac-225 can be labelled to different biological vectors, whereby the success of developing a radiopharmaceutical depends heavily on the labelling conditions, purity of the radionuclide source, chelator, and type of quenchers used to avoid radiolysis. Multiple (methodological) challenges need to be overcome when working with Ac-225; as alpha-emission detection is time consuming and highly geometry dependent, a gamma co-emission is used, but has to be in equilibrium with the mother-nuclide. Because of the high impact of alpha emitters in vivo it is highly recommended to cross-calibrate the Ac-225 measurements for used quality control (QC) techniques (radio-TLC, HPLC, HP-Ge detector, and gamma counter). More strict health physics regulations apply, as Ac-225 has a high toxicity, thereby limiting practical handling and quantities used for QC analysis.

CONCLUSION

This overview focuses specifically on the practical and methodological challenges when working with Ac-225 labelled radiopharmaceuticals, and underlines the required infrastructure and (detection) methods for the (pre-)clinical application.

212Pb-Pretargeted Theranostics for Pancreatic Cancer

Although pancreatic ductal adenocarcinoma (PDAC) is associated with limited treatment options and poor patient outcomes, targeted α-particle therapy (TAT) represents a promising development in the field. TAT shows potential in treating metastatic cancers, including those that have become resistant to conventional treatments. Among the most auspicious radionuclides stands the in vivo α-generator 212Pb. Combined with the imaging-compatible radionuclide 203Pb, this theranostic match is a promising modality rapidly translating into the clinic. Methods: Using the pretargeting approach between a radiolabeled 1,2,4,5-tetrazine (Tz) tracer and a trans-cyclooctene (TCO) modified antibody, imaging and therapy with radiolead were performed on a PDAC tumor xenograft mouse model. For therapy, 3 cohorts received a single administration of 1.1, 2.2, or 3.7 MBq of the pretargeting agent, [212Pb]Pb-DO3A-PEG7-Tz, whereby administered activity levels were guided by dosimetric analysis. Results: The treated mice were holistically evaluated; minimal-to-mild renal tubular necrosis was observed. At the same time, median survival doubled for the highest-dose cohort (10.7 wk) compared with the control cohort (5.1 wk). Conclusion: This foundational study demonstrated the feasibility and safety of pretargeted TAT with 212Pb in PDAC while considering dose limitations and potential adverse effects.

Evaluation of targeting αVβ3 in breast cancers using RGD peptide-based agents

Patients with HER2-positive and triple negative breast cancer (TNBC) are associated with increased risk to develop metastatic disease including reoccurring disease that is resistant to standard and targeted therapies. The αVβ3 has been implicated in BC including metastatic disease. The aims of this study were to investigate the potential of αVβ3-targeted peptides to deliver radioactive payloads to BC tumors expressing αVβ3 on the tumor cells or limited to the tumors' neovascular. Additionally, we aimed to assess the pharmacokinetic profile of the targeted α-particle therapy (TAT) agent [225Ac]Ac-DOTA-cRGDfK dimer peptide and the in vivo generated decay daughters. The expression of αVβ3 in a HER2-positive and a TNBC cell line were evaluated using western blot analysis. The pharmacokinetics of [111In]In-DOTA-cRGDfK dimer, a surrogate for the TAT-agent, was evaluated in subcutaneous mouse tumor models. The pharmacokinetic of the TAT-agent [225Ac]Ac-DOTA-cRGDfK dimer and its decay daughters were evaluated in healthy mice. Selective uptake of [111In]In-DOTA-cRGDfK dimer was shown in subcutaneous tumor models using αVβ3-positive tumor cells as well as αVβ3-negative tumor cells where the expression is limited to the neovasculature. Pharmacokinetic studies demonstrated rapid accumulation in the tumors with clearance from non-target organs. Dosimetric analysis of [225Ac]Ac-DOTA-cRGDfK dimer showed the highest radiation absorbed dose to the kidneys, which included the contributions from the free in vivo generated decay daughters. This study shows the potential of delivering radioactive payloads to BC tumors that have αVβ3 expression on the tumor cells as well as limited expression to the neovascular of the tumor. Furthermore, this work determines the radiation absorbed doses to normal organs/tissues and identified key organs that act as suppliers and receivers of the actinium-225 free in vivo generated α-particle-emitting decay daughters.

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.

Evaluation of the pharmacokinetics, dosimetry, and therapeutic efficacy for the α-particle-emitting transarterial radioembolization (αTARE) agent [225Ac]Ac-DOTA-TDA-Lipiodol® against hepatic tumors

BACKGROUND

The liver is a common site for metastatic disease for a variety of cancers, including colorectal cancer. Both primary and secondary liver tumors are supplied through the hepatic artery while the healthy liver is supplied by the portal vein. Transarterial radioembolization (TARE) using yttrium-90 glass or resin microspheres have shown promising results with reduced side-effects but have similar survival benefits as chemoembolization in patients with hepatocellular carcinoma (HCC). This highlights the need for new novel agents against HCC. Targeted alpha therapy (TAT) is highly potent treatment due to the short range (sparing adjacent normal tissue), and densely ionizing track (high linear energy transfer) of the emitted α-particles. The incorporation of α-particle-emitting radioisotopes into treatment of HCC has been extremely limited, with our recent publication pioneering the field of α-particle-emitting TARE (αTARE). This study focuses on an in-depth evaluation of the αTARE-agent [225Ac]Ac-DOTA-TDA-Lipiodol® as an effective therapeutic agent against HCC regarding pharmacokinetics, dosimetry, stability, and therapeutic efficacy.

RESULTS

[225Ac]Ac-DOTA-TDA was shown to be a highly stable with bench-top stability at ≥ 95% radiochemical purity (RCP) over a 3-day period and serum stability was ≥ 90% RCP over 5-days. The pharmacokinetic data showed retention in the tumor of [225Ac]Ac-DOTA-TDA-Lipiodol® and clearance through the normal organs. In addition, the tumor and liver acted as suppliers of the free daughters, which accumulated in the kidneys supplied via the blood. The dose limiting organ was the liver, and the estimated maximum tolerable activity based on the rodents whole-body weight: 728-3641 Bq/g (male rat), 396-1982 Bq/g (male mouse), and 453-2263 Bq/g (female mouse), depending on an RBE-value (range 1-5). Furthermore, [225Ac]Ac-DOTA-TDA-Lipiodol® showed significant improvement in survival for both the male and female mice (median survival 47-days) compared with controls (26-days untreated, and 33-35-days Lipiodol® alone).

CONCLUSIONS

This study shows that [225Ac]Ac-DOTA-TDA-Lipiodol® is a stable compound allowing for centralized manufacturing and distribution world-wide. Furthermore, the result of this study support the continue development of evaluation of the αTARE-agent [225Ac]Ac-DOTA-TDA-Lipiodol® as a potential treatment option for treating hepatic tumors.

First preclinical evaluation of [225Ac]Ac-DOTA-JR11 and comparison with [177Lu]Lu-DOTA-JR11, alpha versus beta radionuclide therapy of NETs

BACKGROUND

The [¹⁷⁷Lu]Lu-DOTA-TATE mediated peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors (NETs) is sometimes leading to treatment resistance and disease recurrence. An interesting alternative could be the somatostatin antagonist, [¹⁷⁷Lu]Lu-DOTA-JR11, that demonstrated better biodistribution profile and higher tumor uptake than [¹⁷⁷Lu]Lu-DOTA-TATE. Furthermore, treatment with alpha emitters showed improvement of the therapeutic index of PRRT due to the high LET offered by the alpha particles compared to beta emitters. Therefore, [²²⁵Ac]Ac-DOTA-JR11 can be a potential candidate to improve the treatment of NETs (Graphical abstract). DOTA-JR11 was radiolabeled with [²²⁵Ac]Ac(NO₃)₃ and [¹⁷⁷Lu]LuCl₃. Stability studies were performed in phosphate buffered saline (PBS) and mouse serum. In vitro competitive binding assay has been carried out in U2OS-SSTR2 + cells for ^natLa-DOTA-JR11, ^natLu-DOTA-JR11 and DOTA-JR11. Ex vivo biodistribution studies were performed in mice inoculated with H69 cells at 4, 24, 48 and 72 h after injection of [²²⁵Ac]Ac-DOTA-JR11. A blocking group was included to verify uptake specificity. Dosimetry of selected organs was determined for [²²⁵Ac]Ac-DOTA-JR11 and [¹⁷⁷Lu]Lu-DOTA-JR11.

RESULTS

[²²⁵Ac]Ac-DOTA-JR11 has been successfully prepared and obtained in high radiochemical yield (RCY; 95%) and radiochemical purity (RCP; 94%). [²²⁵Ac]Ac-DOTA-JR11 showed reasonably good stability in PBS (77% intact radiopeptide at 24 h after incubation) and in mouse serum (~ 81% intact radiopeptide 24 h after incubation). [¹⁷⁷Lu]Lu-DOTA-JR11 demonstrated excellent stability in both media (> 93%) up to 24 h post incubation. Competitive binding assay revealed that complexation of DOTA-JR11 with ^natLa and ^natLu did not affect its binding affinity to SSTR2. Similar biodistribution profiles were observed for both radiopeptides, however, higher uptake was noticed in the kidneys, liver and bone for [²²⁵Ac]Ac-DOTA-JR11 than [¹⁷⁷Lu]Lu-DOTA-JR11.

CONCLUSION

[²²⁵Ac]Ac-DOTA-JR11 showed a higher absorbed dose in the kidneys compared to [¹⁷⁷Lu]Lu-DOTA-JR11, which may limit further studies with this radiopeptide. However, several strategies can be explored to reduce nephrotoxicity and offer opportunities for future clinical investigations with [²²⁵Ac]Ac-DOTA-JR11.

Treatment of Prostate Cancer with CD46-targeted 225Ac Alpha Particle Radioimmunotherapy

PURPOSE

Radiopharmaceutical therapy is changing the standard of care in prostate cancer and other malignancies. We previously reported high CD46 expression in prostate cancer and developed an antibody-drug conjugate and immunoPET agent based on the YS5 antibody, which targets a tumor-selective CD46 epitope. Here, we present the preparation, preclinical efficacy, and toxicity evaluation of [225Ac]DOTA-YS5, a radioimmunotherapy agent based on the YS5 antibody.

EXPERIMENTAL DESIGN

[225Ac]DOTA-YS5 was developed, and its therapeutic efficiency was tested on cell-derived (22Rv1, DU145), and patient-derived (LTL-545, LTL484) prostate cancer xenograft models. Biodistribution studies were carried out on 22Rv1 tumor xenograft models to confirm the targeting efficacy. Toxicity analysis of the [225Ac]DOTA-YS5 was carried out on nu/nu mice to study short-term (acute) and long-term (chronic) toxicity.

RESULTS

Biodistribution study shows that [225Ac]DOTA-YS5 agent delivers high levels of radiation to the tumor tissue (11.64% ± 1.37%ID/g, 28.58% ± 10.88%ID/g, 29.35% ± 7.76%ID/g, and 31.78% ± 5.89%ID/g at 24, 96, 168, and 408 hours, respectively), compared with the healthy organs. [225Ac]DOTA-YS5 suppressed tumor size and prolonged survival in cell line-derived and patient-derived xenograft models. Toxicity analysis revealed that the 0.5 μCi activity levels showed toxicity to the kidneys, likely due to redistribution of daughter isotope 213Bi.

CONCLUSIONS

[225Ac]DOTA-YS5 suppressed the growth of cell-derived and patient-derived xenografts, including prostate-specific membrane antigen-positive and prostate-specific membrane antigen-deficient models. Overall, this preclinical study confirms that [225Ac]DOTA-YS5 is a highly effective treatment and suggests feasibility for clinical translation of CD46-targeted radioligand therapy in prostate cancer.

Biodistribution and dosimetry for combined [177Lu]Lu-PSMA-I&T/[225Ac]Ac-PSMA-I&T therapy using multi-isotope quantitative SPECT imaging

PURPOSE

Quantitative SPECT for patient-specific dosimetry is a valuable tool in the scope of radionuclide therapy, although its clinical application for ²²⁵Ac-based treatments may be limited due to low therapeutic activities. Therefore, the aim of this study was to demonstrate the feasibility of clinical quantitative low-count SPECT imaging during [¹⁷⁷Lu]Lu-PSMA-I&T/[²²⁵Ac]Ac-PSMA-I&T treatment.

METHODS

Eight prostate cancer patients (1000 MBq/8 MBq [¹⁷⁷Lu]Lu-PSMA-I&T/[²²⁵Ac]Ac-PSMA-I&T) received a single-bed quantitative ¹⁷⁷Lu/²²⁵Ac SPECT/CT acquisition (1 h) at 24 h post treatment (high-energy collimator, 16 projections p. head à 3.5 min, 128 × 128 pixel). The gamma peak at 440 keV (width: 10%) of the progeny ²¹³Bi was imaged along with the peak at 208 keV (width: 15%) of ¹⁷⁷Lu. Quantification included CT-based attenuation and window-based scatter correction plus resolution modelling. Gaussian post-filtering with a full-width-half-maximum of 30 mm and 40-45 mm was employed to match the signal-to-noise ratio of ²²⁵Ac and ¹⁷⁷Lu, respectively.

RESULTS

Kidney (r = 0.96, p < 0.01) and lesion (r = 0.94, p < 0.01) SUV for [¹⁷⁷Lu]Lu-PSMA-I&T and [²²⁵Ac]Ac-PSMA-I&T showed a strong and significant correlation. Kidney SUV were significantly higher (p < 0.01) for [²²⁵Ac]Ac-PSMA-I&T (2.5 ± 0.8 vs. 2.1 ± 0.9), while for [¹⁷⁷Lu]Lu-PSMA-I&T lesion SUV were significantly higher (p = 0.03; 1.8 ± 1.1 vs. 2.1 ± 1.5). For absorbed dose estimates, significant differences regarding the kidneys remained, while no significant differences for lesion dosimetry were found.

CONCLUSION

Quantitative low-count SPECT imaging of the peak at 440 keV during [²²⁵Ac]Ac-PSMA-I&T therapy is feasible. Multi-isotope imaging for [¹⁷⁷Lu]Lu-PSMA-I&T/[²²⁵Ac]Ac-PSMA-I&T therapy indicates accumulation of free ²¹³Bi in the kidneys.

225Ac-Labeled Somatostatin Analogs in the Management of Neuroendocrine Tumors: From Radiochemistry to Clinic

The widespread use of peptide receptor radionuclide therapy (PRRT) represents a major therapeutic breakthrough in nuclear medicine, particularly since the introduction of ¹⁷⁷Lu-radiolabeled somatostatin analogs. These radiopharmaceuticals have especially improved progression-free survival and quality of life in patients with inoperable metastatic gastroenteropancreatic neuroendocrine tumors expressing somatostatin receptors. In the case of aggressive or resistant disease, the use of somatostatin derivatives radiolabeled with an alpha-emitter could provide a promising alternative. Among the currently available alpha-emitting radioelements, actinium-225 has emerged as the most suitable candidate, especially regarding its physical and radiochemical properties. Nevertheless, preclinical and clinical studies on these radiopharmaceuticals are still few and heterogeneous, despite the growing momentum for their future use on a larger scale. In this context, this report provides a comprehensive and extensive overview of the development of ²²⁵Ac-labeled somatostatin analogs; particular emphasis is placed on the challenges associated with the production of ²²⁵Ac, its physical and radiochemical properties, as well as the place of ²²⁵Ac-DOTATOC and ²²⁵Ac-DOTATATE in the management of patients with advanced metastatic neuroendocrine tumors.

Radiation protection aspects for alpha therapies

The use of alpha emitting radiotherapeutics is increasing, with further growth expected due to a number of clinical trials currently running involving new alpha emitters. However, literature concerning radiation safety aspects of alpha emitting radionuclides is limited and most of the available literature concerns ²²³Ra. In general, the occupational exposure from alpha emitting radionuclides is expected to be low, as are doses to the public from external exposure. However, care must be taken to avoid skin contamination, inhalation, and ingestion. Not all alpha emitting radionuclides are identical, they often have very different associated decay chains and emissions. The decay chains and the manufacturing process should be carefully examined to identify any long-lived progeny or impurities. These may have an impact on the radiation safety processes required to limit occupational exposure and for waste management. Doses to the public must also be assessed, either arising directly from exposure to patients treated with radiotherapeutics, or via waste streams. Risk assessments should be in place when starting a new service covering all aspects of the preparation and administration, as well as any foreseeable incidents such as skin contamination or patient death, and the appropriate steps to take in these instances. It is imperative that with the increase in the use of alpha emitting radiotherapeutics more literature is published on radiation safety aspects, especially for new alpha emitting radiotherapeutics which often have very different characteristics than the currently established ones.

Prostate specific membrane antigen binding radiopharmaceuticals: Current data and new concepts

Prostate specific membrane antigen (PSMA) represents a validated target for prostate cancer therapeutics. The phase III VISION study with ¹⁷⁷lutetium (¹⁷⁷Lu)-PSMA-617 represented a pivotal step forward and the FDA has now approved this agent in advanced metastatic castrate-resistant prostate cancer (mCRPC). A number of other PSMA targeted radiopharmaceuticals are now under development. Some of these agents are targeted to PSMA via monoclonal antibodies such as J591 and TLX591. Others are targeted to PSMA via small molecules such as PSMA-617, PSMA I&T, MIP-1095, etc. In addition to the use of various ligands, multiple isotopes are now in clinical trials. Beta emitters in development include ¹⁷⁷Lu, ¹³¹iodide (¹³¹I), and ⁶⁷copper (⁶⁷Cu). Targeted alpha emitters potentially include ²²⁵actinium (²²⁵Ac), ²²⁷thorium (²²⁷Th), and ²¹²lead (²¹²Pb). Phase III trials are underway with both ¹⁷⁷Lu-PSMA-617 and ¹⁷⁷Lu-PSMA I&T in mCRPC. Single dose phase I trials are complete with ²²⁵Ac-J591 but additional data are need to launch a phase III. Data are promising with ²²⁵Ac-PSMA-617 but concerns remain over salivary and renal toxicity. Tandem therapies are also considered combining both beta and alpha-targeted therapy. Taken together the field of PSMA targeted radiopharmaceuticals is rapidly developing. The targeted alpha therapies are particularly promising and several developmental paths forward are being considered in the near future.

Preclinical Evaluation of 225Ac-Labeled Single-Domain Antibody for the Treatment of HER2pos Cancer

Human epidermal growth factor receptor type 2 (HER2) is overexpressed in various cancers; thus, HER2-targeting single-domain antibodies (sdAb) could offer a useful platform for radioimmunotherapy. In this study, we optimized the labeling of an anti-HER2-sdAb with the α-particle-emitter 225Ac through a DOTA-derivative. The formed radioconjugate was tested for binding affinity, specificity and internalization properties, whereas cytotoxicity was evaluated by clonogenic and DNA double-strand-breaks assays. Biodistribution studies were performed in mice bearing subcutaneous HER2pos tumors to estimate absorbed doses delivered to organs and tissues. Therapeutic efficacy and potential toxicity were assessed in HER2pos intraperitoneal ovarian cancer model and in healthy C57Bl/6 mice. [225Ac]Ac-DOTA-2Rs15d exhibited specific cell uptake and cell-killing capacity in HER2pos cells (EC50 = 3.9 ± 1.1 kBq/mL). Uptake in HER2pos lesions peaked at 3 hours (9.64 ± 1.69% IA/g), with very low accumulation in other organs (<1% IA/g) except for kidneys (11.69 ± 1.10% IA/g). α-camera imaging presented homogeneous uptake of radioactivity in tumors, although heterogeneous in kidneys, with a higher signal density in cortex versus medulla. In mice with HER2pos disseminated tumors, repeated administration of [225Ac]Ac-DOTA-2Rs15d significantly prolonged survival (143 days) compared to control groups (56 and 61 days) and to the group treated with HER2-targeting mAb trastuzumab (100 days). Histopathologic evaluation revealed signs of kidney toxicity after repeated administration of [225Ac]Ac-DOTA-2Rs15d. [225Ac]Ac-DOTA-2Rs15d efficiently targeted HER2pos cells and was effective in treatment of intraperitoneal disseminated tumors, both alone and as an add-on combination with trastuzumab, albeit with substantial signs of inflammation in kidneys. This study warrants further development of [225Ac]Ac-DOTA-2Rs15d.

Gamma counting protocols for the accurate quantification of 225Ac and 213Bi without the need for a secular equilibrium between parent and gamma-emitting daughter

BACKGROUND

Quantification of actinium-225 through gamma counter measurements, when there is no secular equilibrium between actinium-225 and its gamma emitting daughters bismuth-213 and/or francium-221, can provide valuable information regarding the possible relocation of recoiled daughters such that related radiotoxicity effects can be evaluated. This study proposes a multiple time-point protocol using the bismuth-213 photopeak with measurements before secular equilibrium between actinium-225 and bismuth-213, and a single time-point protocol using both the francium-221 and bismuth-213 photopeak while assuming secular equilibrium between actinium-225 and francium-221 but not between bismuth-213 and actinium-225.

RESULTS

Good agreement (i.e. 3% accuracy) was obtained when relying on a multiple time-points measurement of bismuth-213 to quantify both actinium-225 and excess of bismuth-213. Following scatter correction, actinium-225 can be accurately quantified using the francium-221 in a single time-point measurement within 3% of accuracy. The analysis performed on the stability data of [225Ac]Ac-DEPA and [225Ac]Ac-DOTA complexes, before secular equilibrium between bismuth-213 and actinium-225 was formed, revealed considerable amounts of unbound bismuth-213 (i.e. more than 90%) after 24 h of the radiolabeling most likely due to the recoiled daughter effect.

CONCLUSION

Both protocols were able to accurately estimate 225Ac-activities provided the francium-221 energy window was corrected for the down scatter of the higher-energy gamma-emissions by bismuth-213. This could prove beneficial to study the recoiled daughter effect and redistribution of free bismuth-213 by monitoring the accumulation or clearance of bismuth-213 in different tissues during biodistribution studies or in patient samples during clinical studies. On the other hand, the single gamma counter measurement protocol, although required a 30 min waiting time, is more time and cost efficient and therefore more appropriate for standardized quality control procedures of 225Ac-labeled radiopharmaceuticals.

Dosimetry in Radiopharmaceutical Therapy

The application of radiopharmaceutical therapy for the treatment of certain diseases is well established, and the field is expanding. New therapeutic radiopharmaceuticals have been developed in recent years, and more are in the research pipeline. Concurrently, there is growing interest in the use of internal dosimetry as a means of personalizing, and potentially optimizing, such therapy for patients. Internal dosimetry is multifaceted, and the current state of the art is discussed in this continuing education article. Topics include the context of dosimetry, internal dosimetry methods, the advantages and disadvantages of incorporating dosimetry calculations in radiopharmaceutical therapy, a description of the workflow for implementing patient-specific dosimetry, and future prospects in the field.

SPECT imaging of 226Ac as a theranostic isotope for 225Ac radiopharmaceutical development

Objective. The development of alpha-emitting radiopharmaceuticals using 225Ac (t ½ = 9.92 d) benefits from the quantitative determination of its biodistribution and is not always easy to directly measure. An element-equivalent matched-pair would allow for more accurate biodistribution and dosimetry estimates. 226Ac (t ½ = 29.4 h) is a candidate isotope for in vivo imaging of preclinical 225Ac radiopharmaceuticals, given its 158 keV and 230 keV gamma emissions making it suitable for quantitative SPECT imaging. This work aimed to conduct a performance assessment for 226Ac imaging and presents the first-ever 226Ac SPECT images. Approach. To establish imaging performance with regards to contrast and noise, image quality phantoms were scanned using a microSPECT/CT system. To assess the resolution, a hot rod phantom with cylindrical rods with diameters between 0.85 and 1.70 mm was additionally imaged. Two collimators were evaluated: a high-energy ultra-high resolution (HEUHR) collimator and an extra ultra-high sensitivity (UHS) collimator. Images were reconstructed from two distinct photopeaks at 158 keV and 230 keV. Main results. The HEUHR SPECT image measurements of high activity concentration regions were consistent with values determined independently via gamma spectroscopy, within 9% error. The lower energy 158 keV photopeak images demonstrated slightly better contrast recovery. In the resolution phantom, the UHS collimator only resolved rods ≥1.30 mm and ≥1.50 mm for the 158 keV and 230 keV photopeaks, respectively, while the HEUHR collimator clearly resolved all rods, with resolution <0.85 mm. Significance. Overall, the feasibility of preclinical imaging with 226Ac was demonstrated with quantitative SPECT imaging achieved for both its 158 keV and 230 keV photopeaks. The HEUHR collimator is recommended for imaging 226Ac activity distributions in small animals due to its resolution <0.85 mm. Future work will explore the feasibility of using 226Ac both as an element-equivalent isotope for 225Ac radiopharmaceuticals, or as a standalone therapeutic isotope.

Carbonic Anhydrase IX-Targeted α-Radionuclide Therapy with 225Ac Inhibits Tumor Growth in a Renal Cell Carcinoma Model

In this study, we compared the tumor-targeting properties, therapeutic efficacy, and tolerability of the humanized anti-CAIX antibody (hG250) labeled with either the α-emitter actinium-225 (225Ac) or the β--emitter lutetium-177 (177Lu) in mice. BALB/c nude mice were grafted with human renal cell carcinoma SK-RC-52 cells and intravenously injected with 30 µg [225Ac] Ac-DOTA-hG250 (225Ac-hG250) or 30 µg [177Lu] Lu-DOTA-hG250 (177Lu-hG250), followed by ex vivo biodistribution studies. Therapeutic efficacy was evaluated in mice receiving 5, 15, and 25 kBq of 225Ac-hG250; 13 MBq of 177Lu-hG250; or no treatment. Tolerability was evaluated in non-tumor-bearing animals. High tumor uptake of both radioimmunoconjugates was observed and increased up to day 7 (212.8 ± 50.2 %IA/g vs. 101.0 ± 18.4 %IA/g for 225Ac-hG250 and 177Lu-hG250, respectively). Survival was significantly prolonged in mice treated with 15 kBq 225Ac-hG250, 25 kBq 225Ac-hG250, and 13 MBq 177Lu-hG250 compared to untreated control (p < 0.05). Non-tumor-bearing mice that received single-dose treatment with 15 or 25 kBq 225Ac-hG250 showed weight loss at the end of the experiment (day 126), and immunohistochemical analysis suggested radiation-induced nephrotoxicity. These results demonstrate the therapeutic potential of CAIX-targeted α-therapy in renal cell carcinoma. Future studies are required to find an optimal balance between therapeutic efficacy and toxicity.

Impact of metallic coating on the retention of 225Ac and its daugthers within core-shell nanocarriers

Currently, alpha-emitting radionuclide 225Ac is one of the most promising isotopes in alpha therapy due to its high linear energy transfer during four sequential alpha decays. However, the main obstacle preventing the full introduction of 225Ac into clinical practice is the lack of stable retention of radionuclides, leading to free circulation of toxic isotopes in the body. In this work, the surface of silica nanoparticles (SiO2 NPs) has been modified with metallic shells composed of titanium dioxide (TiO2) and gold (Au) nanostructures to improve the retention of 225Ac and its decay products within the developed nanocarriers. In vitro and in vivo studies in healthy mice show that the metallic surface coating of SiO2 NPs promotes an enhanced sequestering of radionuclides (225Ac and its daughter isotopes) compared to non-modified SiO2 NPs for a prolonged period of time. Histological analysis reveals that for the period of 3-10 d after the injections, the developed nanocarriers have no significant toxic effects in mice. At the same time, almost no accumulation of leaked radionuclides can be detected in non-target organs (e.g., in the kidneys). In contrast, non-modified carriers (SiO2 NPs) demonstrate the release of free radionuclides, which are distributed over the whole animal body with the consequent morphological changes in the lung, liver and kidney tissues. These results highlight the potential of the developed nanocarriers to be utilized as radionuclide delivery systems and offer an insight into design rules for the fabrication of new nanotherapeutic agents.

Implications of physics, chemistry and biology for dosimetry calculations using theranostic pairs

Theranostics is an emerging paradigm that combines imaging and therapy in order to personalize patient treatment. In nuclear medicine, this is achieved by using radiopharmaceuticals that target identical molecular targets for both imaging (using emitted gamma rays) and radiopharmaceutical therapy (using emitted beta, alpha or Auger-electron particles) for the treatment of various diseases, such as cancer. If the therapeutic radiopharmaceutical cannot be imaged quantitatively, a “theranostic pair” imaging surrogate can be used to predict the absorbed radiation doses from the therapeutic radiopharmaceutical. However, theranostic dosimetry assumes that the pharmacokinetics and biodistributions of both radiopharmaceuticals in the pair are identical or very similar, an assumption that still requires further validation for many theranostic pairs. In this review, we consider both same-element and different-element theranostic pairs and attempt to determine if factors exist which may cause inaccurate dose extrapolations in theranostic dosimetry, either intrinsic (e.g. chemical differences) or extrinsic (e.g. injecting different amounts of each radiopharmaceutical) to the radiopharmaceuticals. We discuss the basis behind theranostic dosimetry and present common theranostic pairs and their therapeutic applications in oncology. We investigate general factors that could create alterations in the behavior of the radiopharmaceuticals or the quantitative accuracy of imaging them. Finally, we attempt to determine if there is evidence showing some specific pairs as suitable for theranostic dosimetry. We show that there are a variety of intrinsic and extrinsic factors which can significantly alter the behavior among pairs of radiopharmaceuticals, even if they belong to the same chemical element. More research is needed to determine the impact of these factors on theranostic dosimetry estimates and on patient outcomes, and how to correctly account for them.

Dosing 225Ac-DOTATOC in patients with somatostatin-receptor-positive solid tumors: 5-year follow-up of hematological and renal toxicity

PURPOSE

The aim of this retrospective analysis is to estimate the most appropriate single cycle and cumulative doses of 225Ac-DOTATOC in patients treated for somatostatin-receptor-expressing cancers.

METHODS

225Ac-DOTATOC was administered to thirty-nine patients with various somatostatin-receptor-positive tumors. Baseline and follow-up 68Ga-DOTATOC PET/CT, lab tests, and renal scintigraphy were obtained. Patients received long-term follow-up either at the local cancer center or in close collaboration with external oncologists. Acute and chronic hematological toxicity was evaluated quantitatively over time. Long-term follow-up of creatinine was used to approximate the annual loss of estimated GFR (eGFR).

RESULTS

Dose-dependent acute hematological toxicity was seen at single doses above 40 MBq or repeated doses greater than approximately 20 MBq 225Ac-DOTATOC at 4 month intervals. Treatment-related kidney failure occurred in 2 patients after a delay of >4 years but was independent of administered radioactivity, and other clinical risk factors were important contributors to renal decline. In general, the annual decline of eGFR among patients did not follow a clear dose-effect relationship even in patients with previous β-therapy. An average eGFR-loss of 8.4ml/min (9.9%) per year was observed which is similar to the experience with β-therapy studies.

CONCLUSION

Treatment activities of approx. 20 MBq per cycle (4 monthly repetition) and cumulative doses up to 60-80 MBq generally avoided both acute and chronic grade 3/4 hematotoxicity in patients with advanced stage malignancies. Chronic renal toxicity was observed at these doses, but pre-existing renal risk factors were important co-factors. These data represent a starting point for additional research to more precisely define safety thresholds of 225Ac-DOTATOC.

Two diverse carriers are better than one: A case study in α‐particle therapy for prostate specific membrane antigen‐expressing prostate cancers

Partial and/or heterogeneous irradiation of established (i.e., large, vascularized) tumors by α‐particles that exhibit only a 4–5 cell‐diameter range in tissue, limits the therapeutic effect, since regions not being hit by the high energy α‐particles are likely not to be killed. This study aims to mechanistically understand a delivery strategy to uniformly distribute α‐particles within established solid tumors by simultaneously delivering the same α‐particle emitter by two diverse carriers, each killing a different region of the tumor: (1) the cancer‐agnostic, but also tumor‐responsive, liposomes engineered to best irradiate tumor regions far from the vasculature, and (2) a separately administered, antibody, targeting any cancer‐cell's surface marker, to best irradiate the tumor perivascular regions. We demonstrate that on a prostate specific membrane antigen (PSMA)‐expressing prostate cancer xenograft mouse model, for the same total injected radioactivity of the α‐particle emitter Actinium‐225, any radioactivity split ratio between the two carriers resulted in better tumor growth inhibition compared to the tumor inhibition when the total radioactivity was delivered by any of the two carriers alone. This finding was due to more uniform tumor irradiation for the same total injected radioactivity. The killing efficacy was improved even though the tumor‐absorbed dose delivered by the combined carriers was lower than the tumor‐absorbed dose delivered by the antibody alone. Studies on spheroids with different receptor‐expression, used as surrogates of the tumors' avascular regions, demonstrated that our delivery strategy is valid even for as low as 1+ (ImmunoHistoChemistry score) PSMA‐levels. The findings presented herein may hold clinical promise for those established tumors not being effectively eradicated by current α‐particle radiotherapies.

A perspective on the radiopharmaceutical requirements for imaging and therapy of glioblastoma

Despite numerous clinical trials and pre-clinical developments, the treatment of glioblastoma (GB) remains a challenge. The current survival rate of GB averages one year, even with an optimal standard of care. However, the future promises efficient patient-tailored treatments, including targeted radionuclide therapy (TRT). Advances in radiopharmaceutical development have unlocked the possibility to assess disease at the molecular level allowing individual diagnosis. This leads to the possibility of choosing a tailored, targeted approach for therapeutic modalities. Therapeutic modalities based on radiopharmaceuticals are an exciting development with great potential to promote a personalised approach to medicine. However, an effective targeted radionuclide therapy (TRT) for the treatment of GB entails caveats and requisites. This review provides an overview of existing nuclear imaging and TRT strategies for GB. A critical discussion of the optimal characteristics for new GB targeting therapeutic radiopharmaceuticals and clinical indications are provided. Considerations for target selection are discussed, i.e. specific presence of the target, expression level and pharmacological access to the target, with particular attention to blood-brain barrier crossing. An overview of the most promising radionuclides is given along with a validation of the relevant radiopharmaceuticals and theranostic agents (based on small molecules, peptides and monoclonal antibodies). Moreover, toxicity issues and safety pharmacology aspects will be presented, both in general and for the brain in particular.

A Whole-Body Physiologically Based Pharmacokinetic Model for Alpha Particle Emitting Bismuth in Rats

Introduction/Aim: α particle emitting bismuth (²¹²Bi) as decay product of ²¹²Pb-labeled pharmaceuticals has been effective in targeted α particle therapy (TAT). Estimating the contribution of ²¹²Bi released from its chelator to the absorbed doses in nontarget tissues is challenging in TAT. Physiologically based pharmacokinetic (PBPK) modeling can help overcome this limitation. Therefore, a whole-body ²¹²Bi-PBPK model was developed to describe the pharmacokinetics (PKs) of ²¹²Bi in rats. Materials and Methods: The rat ²¹²Bi-PBPK model was implemented using the modeling software SAAM II with data and parameter values from the literature. Besides other mechanisms, ²¹²Bi interactions with red blood cells, high molecular weight plasma protein, and intracellular biological thiols are described. Important PK parameters were fitted to time-activity data. Absorbed dose coefficients (ADCs) were calculated for injecting 0.774 fmol of ²¹²Bi. Results: ²¹²Bi uptake rates of liver, bone, small intestine, bone marrow, skin, and muscle were (0.86 ± 0.13), (3.85 ± 0.63), (0.27 ± 0.05), (1.44 ± 0.29), (0.04 ± 0.01), and (0.007 ± 0.007) per min with corresponding ADCs of 0.09, 0.03, 0.03, 0.07, 0.01, and 0.003 mGy/kBq, respectively. An ADC of 0.70 mGy/kBq was determined for kidneys. Conclusion: Kidneys are the dose-limiting organs in ²¹²Bi-based TAT. The ²¹²Bi-PBPK model is an effective tool to investigate the ²¹²Bi biodistribution in murine models. Integrating the ²¹²Bi-PBPK model into other murine and human PBPK models of α particle generators can help study the efficacy and safety of TAT.

Transport-driven engineering of liposomes for delivery of α-particle radiotherapy to solid tumors: effect on inhibition of tumor progression and onset delay of spontaneous metastases

PURPOSE

Highly cytotoxic α-particle radiotherapy delivered by tumor-selective nanocarriers is evaluated on metastatic Triple Negative Breast Cancer (TNBC). On vascularized tumors, the limited penetration of nanocarriers (<50-80 μm) combined with the short range of α-particles (40-100 μm) may, however, result in only partial tumor irradiation, compromising efficacy. Utilizing the α-particle emitter Actinium-225 (²²⁵Ac), we studied how the therapeutic potential of a general delivery strategy using nanometer-sized engineered liposomes was affected by two key transport-driven properties: (1) the release from liposomes, when in the tumor interstitium, of the highly diffusing ²²⁵Ac-DOTA that improves the uniformity of tumor irradiation by α-particles and (2) the adhesion of liposomes on the tumors' ECM that increases liposomes' time-integrated concentrations within tumors and, therefore, the tumor-delivered radioactivities.

METHODS

On an orthotopic MDA-MB-231 TNBC murine model forming spontaneous metastases, we evaluated the maximum tolerated dose (MTD), biodistributions, and control of tumor growth and/or spreading after administration of ²²⁵Ac-DOTA-encapsulating liposomes, with different combinations of the two transport-driven properties.

RESULTS

At 83% of MTD, ²²⁵Ac-DOTA-encapsulating liposomes with both properties (1) eliminated formation of spontaneous metastases and (2) best inhibited the progression of orthotopic xenografts, compared to liposomes lacking one or both properties. These findings were primarily affected by the extent of uniformity of the intratumoral microdistributions of ²²⁵Ac followed by the overall tumor uptake of radioactivity. At the MTD, long-term toxicities were not detected 9.5 months post administration.

CONCLUSION

Our findings demonstrate the potential of a general, transport-driven strategy enabling more uniform and prolonged solid tumor irradiation by α-particles without cell-specific targeting.

In situ Generated ²¹²Pb-PSMA Ligand in a ²²⁴Ra-Solution for Dual Targeting of Prostate Cancer Sclerotic Stroma and PSMA-positive Cells

Background: New treatments combating bone and extraskeletal metastases are needed for patients with metastatic castration-resistant prostate cancer. The majority of metastases overexpress prostate-specific membrane antigen (PSMA), making it an ideal candidate for targeted radionuclide therapy.

Objective: The aim of this study was to test a novel liquid 224Ra/212Pb-generator for the rapid preparation of a dual-alpha targeting solution. Here, PSMA-targeting ligands are labelled with 212Pb in the 224Ra-solution in transient equilibrium with daughter nuclides. Thus, natural bone-seeking 224Ra targeting sclerotic bone metastases and 212Pb-chelated PSMA ligands targeting PSMA-expressing tumour cells are obtained.

Methods: Two PSMA-targeting ligands, the p-SCN-Bn-TCMC-PSMA ligand (NG001), specifically developed for chelating 212Pb, and the most clinically used DOTA-based PSMA-617 were labelled with 212Pb. Radiolabelling and targeting potential were investigated in situ, in vitro (PSMA-positive C4-2 human prostate cancer cells) and in vivo (athymic mice bearing C4-2 xenografts).

Results: NG001 was rapidly labelled with 212Pb (radiochemical purity >94% at concentrations of ≥15 µg/ml) using the liquid 224Ra/212Pb-generator. The high radiochemical purity and stability of [212Pb]Pb-NG001 were demonstrated over 48 hours in the presence of ascorbic acid and albumin. Similar binding abilities of the 212Pb-labelled ligands were observed in C4-2 cells. The PSMA ligands displayed comparable tumour uptake after 2 hours, but NG001 showed a 3.5-fold lower kidney uptake than PSMA-617. Radium-224 was not chelated and, hence, showed high uptake in bones.

Conclusion: A fast method for the labelling of PSMA ligands with 212Pb in the 224Ra/212Pb-solution was developed. Thus, further in vivo studies with dual tumour targeting by alpha-particles are warranted.

Targeted alpha immunotherapy of CD20-positive B-cell lymphoma model: dosimetry estimate of 225Ac-DOTA-rituximab using 64Cu-DOTA-rituximab

OBJECTIVE

The aim of this study was to evaluate the radiation dosimetry of alpha-emitter ²²⁵Ac-DOTA-rituximab using Monte Carlo simulation of ⁶⁴Cu-DOTA-rituximab.

METHODS

CD20 expression was evaluated in lymphoma cell lines (Jurkat and Raji). DOTA-rituximab was conjugated and then chelated by ⁶⁴Cu. Tumor xenograft models were established in BALB/c-nu mice. Animal PET/CT imaging was obtained after tail vein injection with and without a pre-dose of 2 mg of cold rituximab. Specific binding of tumors was evaluated by an organ distribution assay and autoradiography. CD20 expression in tumor tissues was evaluated by immunohistochemistry. The residence time was calculated using ⁶⁴Cu-DOTA-rituximab PET/CT acquisition data using OLINDA/EXM software. ²²⁵Ac-DOTA-rituximab tumor dosimetry was performed using Monte Carlo simulation with ⁶⁴Cu-DOTA-rituximab PET/CT images.

RESULTS

Specific binding of Raji cells (CD20 positive) was 90 times that of Jurkat cells (CD20 negative) (p < 0.0001). Immunoreactivity was more than 75%. PET/CT imaging with ⁶⁴Cu-DOTA-rituximab was specifically observed in tumors. The radioactivity of the tumor was much higher than that of other organs, and tumor uptake was related to CD20 expression. The predicted human dose for the administration of ⁶⁴Cu-DOTA-rituximab was measured as the effective dose (1.07E-02 mSv/MBq). In the tumor region, equivalent doses of ²²⁵Ac-DOTA-rituximab (14 Sv_RBE5/MBq) were much higher (74-fold) than those of ⁶⁴Cu-DOTA-rituximab (0.19 Sv_RBE5/MBq) (p < 0.01).

CONCLUSION

Tumor dosimetry of ²²⁵Ac-DOTA-rituximab can be estimated via the Monte Carlo simulation of ⁶⁴Cu-DOTA-rituximab. ²²⁵Ac-DOTA-rituximab can be employed for lymphoma as targeted alpha therapy.

Recent Advances in Radiometals for Combined Imaging and Therapy in Cancer

Nuclear medicine is defined as the use of radionuclides for diagnostic and therapeutic applications. The imaging modalities positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are based on γ-emissions of specific energies. The therapeutic technologies are based on β^- -particle-, α-particle-, and Auger electron emitters. In oncology, PET and SPECT are used to detect cancer lesions, to determine dosimetry, and to monitor therapy effectiveness. In contrast, radiotherapy is designed to irreparably damage tumor cells in order to eradicate or control the disease's progression. Radiometals are being explored for the development of diagnostic and therapeutic radiopharmaceuticals. Strategies that combine both modalities (diagnostic and therapeutic), referred to as theranostics, are promising candidates for clinical applications. This review provides an overview of the basic concepts behind therapeutic and diagnostic radiopharmaceuticals and their significance in contemporary oncology. Select radiometals that significantly impact current and upcoming cancer treatment strategies are grouped as clinically suitable theranostics pairs. The most important physical and chemical properties are discussed. Standard production methods and current radionuclide availability are provided to indicate whether a cost-efficient use in a clinical routine is feasible. Recent preclinical and clinical developments and outline perspectives for the radiometals are highlighted in each section.

Targeted Alpha Therapy: Progress in Radionuclide Production, Radiochemistry, and Applications

This review outlines the accomplishments and potential developments of targeted alpha (α) particle therapy (TAT). It discusses the therapeutic advantages of the short and highly ionizing path of α-particle emissions; the ability of TAT to complement and provide superior efficacy over existing forms of radiotherapy; the physical decay properties and radiochemistry of common α-emitters, including ²²⁵Ac, ²¹³Bi, ²²⁴Ra, ²¹²Pb, ²²⁷Th, ²²³Ra, ²¹¹At, and ¹⁴⁹Tb; the production techniques and proper handling of α-emitters in a radiopharmacy; recent preclinical developments; ongoing and completed clinical trials; and an outlook on the future of TAT.

Evaluation of Actinium-225 Labeled Minigastrin Analogue [225Ac]Ac-DOTA-PP-F11N for Targeted Alpha Particle Therapy

The overexpression of cholecystokinin B receptor (CCKBR) in human cancers led to the development of radiolabeled minigastrin analogues for targeted radionuclide therapy, which aims to deliver cytotoxic radiation specifically to cancer cells. Alpha emitters (e.g., actinium-225) possess high potency in cancer cell-killing and hold promise for the treatment of malignant tumors. In these preclinical studies, we developed and evaluated CCKBR-targeted alpha particle therapy. The cellular uptake and cytotoxic effect of actinium-225 labeled and HPLC-purified minigastrin analogue [²²⁵Ac]Ac-PP-F11N were characterized in the human squamous cancer A431 cells transfected with CCKBR. Nude mice bearing A431/CCKBR tumors were used for biodistribution and therapy studies followed by histological analysis and SPECT/CT imaging. In vitro, [²²⁵Ac]Ac-PP-F11N showed CCKBR-specific and efficient internalization rate and potent cytotoxicity. The biodistribution studies of [²²⁵Ac]Ac-PP-F11N revealed CCKBR-specific uptake in tumors, whereas the therapeutic studies demonstrated dose-dependent inhibition of tumor growth and extended mean survival time, without apparent toxicity. The histological analysis of kidney and stomach indicated no severe adverse effects after [²²⁵Ac]Ac-PP-F11N administration. The post-therapy SPECT-CT images with [¹¹¹In]In-PP-F11N confirmed no CCKBR-positive tumor left in the mice with complete remission. In conclusion, our study demonstrates therapeutic efficacy of [²²⁵Ac]Ac-PP-F11N without acute radiotoxicity in CCKBR-positive cancer model.

Biodistribution and Multicompartment Pharmacokinetic Analysis of a Targeted α Particle Therapy

Targeted α particle therapy (TAT) is ideal for treating disease while minimizing damage to surrounding nontargeted tissues due to short path length and high linear energy transfer (LET). We developed a TAT for metastatic uveal melanoma, targeting the melanocortin-1 receptor (MC1R), which is expressed in 94% of uveal melanomas. Two versions of the therapy are being investigated: ²²⁵Ac-DOTA-Ahx-MC1RL (²²⁵Ac-Ahx) and ²²⁵Ac-DOTA-di-d-Glu-MC1RL (²²⁵Ac-di-d-Glu). The biodistribution (BD) from each was studied and a multicompartment pharmacokinetic (PK) model was developed to describe drug distribution rates. Two groups of 16 severe combined immunodeficient (SCID) mice bearing high MC1R expressing tumors were intravenously injected with ²²⁵Ac-Ahx or ²²⁵Ac-di-d-Glu. After injection, four groups (n = 4) were euthanized at 24, 96, 144, and 288 h time points for each cohort. Tumors and 13 other organs were harvested at each time point. Isomeric γ spectra were measured in tissue samples using a scintillation γ detector and converted to α activity using factors for γ ray abundance per α decay. Time activity curves were calculated for each organ. A five-compartment PK model was built with the following compartments: blood, tumor, normal tissue, kidney, and liver. This model is characterized by a system of five ordinary differential equations using mass action kinetics, which describe uptake, intercompartmental transitions, and clearance rates. The ordinary differential equations were simultaneously solved and fit to experimental data using a genetic algorithm for optimization. The BD data show that both compounds have minimal distribution to organs at risk other than the kidney and liver. The PK parameter estimates had less than 5% error. From these data, ²²⁵Ac-Ahx showed larger and faster uptake in the liver. Both compounds had comparable uptake and clearance rates for other compartments. The BD and PK behavior for two targeted radiopharmaceuticals were investigated. The PK model fit the experimental data and provided insight into the kinetics of the compounds systematically.

Alpha radioimmunotherapy using 225Ac-proteus-DOTA for solid tumors - safety at curative doses

This is the initial report of an α-based pre-targeted radioimmunotherapy (PRIT) using ²²⁵Ac and its theranostic pair, ¹¹¹In. We call our novel tumor-targeting DOTA-hapten PRIT system "proteus-DOTA" or "Pr." Herein we report the first results of radiochemistry development, radiopharmacology, and stoichiometry of tumor antigen binding, including the role of specific activity, anti-tumor efficacy, and normal tissue toxicity with the Pr-PRIT approach (as α-DOTA-PRIT). A series of α-DOTA-PRIT therapy studies were performed in three solid human cancer xenograft models of colorectal cancer (GPA33), breast cancer (HER2), and neuroblastoma (GD2), including evaluation of chronic toxicity at ~20 weeks of select survivors. Methods: Preliminary biodistribution experiments in SW1222 tumor-bearing mice revealed that ²²⁵Ac could not be efficiently pretargeted with current DOTA-Bn hapten utilized for ¹⁷⁷Lu or ⁹⁰Y, leading to poor tumor uptake in vivo. Therefore, we synthesized Pr consisting of an empty DOTA-chelate for ²²⁵Ac, tethered via a short polyethylene glycol linker to a lutetium-complexed DOTA for picomolar anti-DOTA chelate single-chain variable fragment (scFv) binding. Pr was radiolabeled with ²²⁵Ac and its imaging surrogate, ¹¹¹In. In vitro studies verified anti-DOTA scFv recognition of [²²⁵Ac]Pr, and in vivo biodistribution and clearance studies were performed to evaluate hapten suitability and in vivo targeting efficiency. Results: Intravenously (i.v.) administered ²²⁵Ac- or ¹¹¹In-radiolabeled Pr in mice showed rapid renal clearance and minimal normal tissue retention. In vivo pretargeting studies show high tumor accumulation of Pr (16.71 ± 5.11 %IA/g or 13.19 ± 3.88 %IA/g at 24 h p.i. for [²²⁵Ac]Pr and [¹¹¹In]Pr, respectively) and relatively low uptake in normal tissues (all average ≤ 1.4 %IA/g at 24 h p.i.). Maximum tolerated dose (MTD) was not reached for either [²²⁵Ac]Pr alone or pretargeted [²²⁵Ac]Pr at administered activities up to 296 kBq/mouse. Single-cycle treatment consisting of α-DOTA-PRIT with either huA33-C825 bispecific anti-tumor/anti-DOTA-hapten antibody (BsAb), anti-HER2-C825 BsAb, or hu3F8-C825 BsAb for targeting GPA33, HER2, or GD2, respectively, was highly effective. In the GPA33 model, no complete responses (CRs) were observed but prolonged overall survival of treated animals was 42 d for α-DOTA-PRIT vs. 25 d for [²²⁵Ac]Pr only (P < 0.0001); for GD2, CRs (7/7, 100%) and histologic cures (4/7, 57%); and for HER2, CRs (7/19, 37%) and histologic cures (10/19, 56%) with no acute or chronic toxicity. Conclusions: [²²⁵Ac]Pr and its imaging biomarker [¹¹¹In]Pr demonstrate optimal radiopharmacologic behavior for theranostic applications of α-DOTA-PRIT. For this initial evaluation of efficacy and toxicity, single-cycle treatment regimens were performed in all three systems. Histologic toxicity was not observed, so MTD was not observed. Prolonged overall survival, CRs, and histologic cures were observed in treated animals. In comparison to RIT with anti-tumor IgG antibodies, [²²⁵Ac]Pr has a much improved safety profile. Ultimately, these data will be used to guide clinical development of toxicity and efficacy studies of [²²⁵Ac]Pr, with the goal of delivering massive lethal doses of radiation to achieve a high probability of cure without toxicity.

Nanoparticles in Targeted Alpha Therapy

Recent advances in the field of nanotechnology application in nuclear medicine offer the promise of better therapeutic options. In recent years, increasing efforts have been made on developing nanoconstructs that can be used as carriers for immobilising alpha (α)-emitters in targeted drug delivery. In this publication, we provide a comprehensive overview of available information on functional nanomaterials for targeted alpha therapy. The first section describes why nanoconstructs are used for the synthesis of α-emitting radiopharmaceuticals. Next, we present the synthesis and summarise the recent studies demonstrating therapeutic applications of α-emitting labelled radiobioconjugates in targeted therapy. Finally, future prospects and the emerging possibility of therapeutic application of radiolabelled nanomaterials are discussed.

TAG-72-Targeted α-Radionuclide Therapy of Ovarian Cancer Using 225Ac-Labeled DOTAylated-huCC49 Antibody

Radioimmunotherapy, an approach using radiolabeled antibodies, has had minimal success in the clinic with several β-emitting radionuclides for the treatment of ovarian cancer. Alternatively, radioimmunotherapy with α-emitters offers the advantage of depositing much higher energy over shorter distances but was thought to be inappropriate for the treatment of solid tumors, for which antibody penetration is limited to a few cell diameters around the vascular system. However, the deposition of high-energy α-emitters to tumor markers adjacent to a typical leaky tumor vascular system may have large antitumor effects at the tumor vascular level, and their reduced penetration in normal tissue would be expected to lower off-target toxicity. Methods: To evaluate this concept, DOTAylated-huCC49 was labeled with the α-emitter ²²⁵Ac to target tumor-associated glycoprotein 72-positive xenografts in a murine model of ovarian cancer. Results: ²²⁵Ac-labeled DOTAylated-huCC49 radioimmunotherapy significantly reduced tumor growth in a dose-dependent manner (1.85, 3.7, and 7.4 kBq), with the 7.4-kBq dose extending survival by more than 3-fold compared with the untreated control. Additionally, a multitreatment regime (1.85 kBq followed by 5 weekly doses of 0.70 kBq for a total of 5.4 kBq) extended survival almost 3-fold compared with the untreated control group, without significant off-target toxicity. Conclusion: These results establish the potential for antibody-targeted α-radionuclide therapy for ovarian cancer, which may be generalized to α-radioimmunotherapy in other solid tumors.

Targeted Alpha Therapy: Current Clinical Applications

α-Emitting radionuclides have been approved for cancer treatment since 2013, with increasing degrees of success. Despite this clinical utility, little is known regarding the mechanisms of action of α particles in this setting, and accurate assessments of the dosimetry underpinning their effectiveness are lacking. However, targeted alpha therapy (TAT) is gaining more attention as new targets, synthetic chemistry approaches, and α particle emitters are identified, constructed, developed, and realized. From a radiobiological perspective, α particles are more effective at killing cells compared to low linear energy transfer radiation. Also, from these direct effects, it is now evident from preclinical and clinical data that α emitters are capable of both producing effects in nonirradiated bystander cells and stimulating the immune system, extending the biological effects of TAT beyond the range of α particles. The short range of α particles makes them a potent tool to irradiate single-cell lesions or treat solid tumors by minimizing unwanted irradiation of normal tissue surrounding the cancer cells, assuming a high specificity of the radiopharmaceutical and good stability of its chemical bonds. Clinical approval of ²²³RaCl₂ in 2013 was a major milestone in the widespread application of TAT as a safe and effective strategy for cancer treatment. In addition, ²²⁵Ac-prostate specific membrane antigen treatment benefit in metastatic castrate-resistant prostate cancer patients, refractory to standard therapies, is another game-changing piece in the short history of TAT clinical application. Clinical applications of TAT are growing with different radionuclides and combination therapies, and in different clinical settings. Despite the remarkable advances in TAT dosimetry and imaging, it has not yet been used to its full potential. Labeled ²²⁷Th and ²²⁵Ac appear to be promising candidates and could represent the next generation of agents able to extend patient survival in several clinical scenarios.

Preclinical evaluation of an 111In/225Ac theranostic targeting transformed MUC1 for triple negative breast cancer

Rationale: Transformed MUC1 (tMUC1) is a cancer-associated antigen that is overexpressed in >90% of triple-negative breast cancers (TNBC), a highly metastatic and aggressive subtype of breast cancer. TAB004, a murine antibody targeting tMUC1, has shown efficacy for the targeted delivery of therapeutics to cancer cells. Our aim was to evaluate humanized TAB004 (hTAB004) as a potential theranostic for TNBC. Methods: The internalization of hTAB004 in tMUC1 expressing HCC70 cells was assessed via fluorescent microscopy. hTAB004 was DOTA-conjugated and radiolabeled with Indium-111 or Actinium-225 and tested for stability and tMUC1 binding (ELISA, flow cytometry). Lastly, in vivo biodistribution (SPECT-CT), dosimetry, and efficacy of hTAB004 were evaluated using a TNBC orthotopic mouse model. Results: hTAB004 was shown to bind and internalize into tMUC1-expressing cells. A production method of ²²⁵Ac-DOTA-hTAB004 (yield>97%, RCP>97% SA=5 kBq/µg) and ¹¹¹In-DOTA-hTAB004 (yield>70%, RCP>99%, SA=884 kBq/µg) was developed. The labeled molecules retained their affinity to tMUC1 and were stable in formulation and mouse serum. In NSG female mice bearing orthotopic HCC70 xenografts, the in vivo tumor concentration of ¹¹¹In-DOTA-hTAB004 was 65 ± 15 %ID/g (120 h post injection). A single ²²⁵Ac-DOTA-hTAB004 dose (18.5 kBq) caused a significant reduction in tumor volume (P<0.001, day 22) and increased survival compared to controls (P<0.007). The human dosimetry results were comparable to other clinically used agents. Conclusion: The results obtained with hTAB004 suggest that the ¹¹¹In/²²⁵Ac-DOTA-hTAB004 combination has significant potential as a theranostic strategy in TNBC and merits further development toward clinical translation.

Targeted alpha therapy: a critical review of translational dosimetry research with emphasis on actinium-225

This review provides a general overview of the current achievements and challenges in translational dosimetry for targeted alpha therapy (TAT). The concept of targeted radionuclide therapy (TRNT) is described with an overview of its clinical applicability and the added value of TAT is discussed. For TAT, we focused on actinium-225 (225Ac) as an example for alpha particle emitting radionuclides and their features, such as limited range within tissue and high linear energy transfer, which make alpha particle emissions more effective in targeted killing of tumour cells compared to beta radiation. Starting with the state-of-the-art dosimetry for TRNT and TAT, we then describe the challenges that still need to be met in order to move to a personalized dosimetry approach for TAT. Specifically for 225Ac, we discuss the recoiled daughter effect which may provoke significant damage to healthy tissue or organs and should be considered. Next, a broad overview is given of the pre-clinical research on 225Ac-TAT with an extensive description of tools which are only available in a pre-clinical setting and their added value. In addition, we review the preclinical biodistribution and dosimetry studies that have been performed on TAT-agents and more specifically of 225Ac and its multiple progeny, and describe their potential role to better characterize the pharmacokinetic (PK) profile of TAT-agents and to optimize the use of theranostic approaches for dosimetry. Finally, we discuss the support pre-clinical studies may provide in understanding dose-effect relationships, linking radiation dose quantities to biological endpoints and even moving away from macro- to microdosimetry. As such, the translation of pre-clinical findings may provide valuable information and new approaches for improved clinical dosimetry, thus paving the way to personalized TAT.

Targeted Radionuclide Therapy of Prostate Cancer-From Basic Research to Clinical Perspectives

Prostate cancer is the most commonly diagnosed malignancy in men and the second leading cause of cancer-related deaths in Western civilization. Although localized prostate cancer can be treated effectively in different ways, almost all patients progress to the incurable metastatic castration-resistant prostate cancer. Due to the significant mortality and morbidity rate associated with the progression of this disease, there is an urgent need for new and targeted treatments. In this review, we summarize the recent advances in research on identification of prostate tissue-specific antigens for targeted therapy, generation of highly specific and selective molecules targeting these antigens, availability of therapeutic radionuclides for widespread medical applications, and recent achievements in the development of new-generation small-molecule inhibitors and antibody-based strategies for targeted prostate cancer therapy with alpha-, beta-, and Auger electron-emitting radionuclides.

Trastuzumab Conjugated Superparamagnetic Iron Oxide Nanoparticles Labeled with 225Ac as a Perspective Tool for Combined α-Radioimmunotherapy and Magnetic Hyperthermia of HER2-Positive Breast Cancer

It has been proven and confirmed in numerous repeated tests, that the use of a combination of several therapeutic methods gives much better treatment results than in the case of separate therapies. Particularly promising is the combination of ionizing radiation and magnetic hyperthermia in one drug. To achieve this objective, magnetite nanoparticles have been modified in their core with α emitter ²²⁵Ac, in an amount affecting only slightly their magnetic properties. By 3-phosphonopropionic acid (CEPA) linker nanoparticles were conjugated covalently with trastuzumab (Herceptin^®), a monoclonal antibody that recognizes ovarian and breast cancer cells overexpressing the HER2 receptors. The synthesized bioconjugates were characterized by transmission electron microscopy (TEM), Dynamic Light Scattering (DLS) measurement, thermogravimetric analysis (TGA) and application of ¹³¹I-labeled trastuzumab for quantification of the bound biomolecule. The obtained results show that one ²²⁵Ac@Fe₃O₄-CEPA-trastuzumab bioconjugate contains an average of 8–11 molecules of trastuzumab. The labeled nanoparticles almost quantitatively retain ²²⁵Ac (>98%) in phosphate-buffered saline (PBS) and physiological salt, and more than 90% of ²²¹Fr and ²¹³Bi over 10 days. In human serum after 10 days, the fraction of ²²⁵Ac released from ²²⁵Ac@Fe₃O₄ was still less than 2%, but the retention of ²²¹Fr and ²¹³Bi decreased to 70%. The synthesized ²²⁵Ac@Fe₃O₄-CEPA-trastuzumab bioconjugates have shown a high cytotoxic effect toward SKOV-3 ovarian cancer cells expressing HER2 receptor in-vitro. The in-vivo studies indicate that this bioconjugate exhibits properties suitable for the treatment of cancer cells by intratumoral or post-resection injection. The intravenous injection of the ²²⁵Ac@Fe₃O₄-CEPA-trastuzumab radiobioconjugate is excluded due to its high accumulation in the liver, lungs and spleen. Additionally, the high value of a specific absorption rate (SAR) allows its use in a new very perspective combination of α radionuclide therapy with magnetic hyperthermia.

Radioimmunotherapy of Pancreatic Ductal Adenocarcinoma: A Review of the Current Status of Literature

Pancreatic ductal adenocarcinoma (PDAC) has long been associated with low survival rates. A lack of accurate diagnostic tests and limited treatment options contribute to the poor prognosis of PDAC. Radioimmunotherapy using α- or β-emitting radionuclides has been identified as a potential treatment for PDAC. By harnessing the cytotoxicity of α or β particles, radioimmunotherapy may overcome the anatomic and physiological factors which traditionally make PDAC resistant to most conventional treatments. Appropriate selection of target receptors and the development of selective and cytotoxic radioimmunoconjugates are needed to achieve the desired results of radioimmunotherapy. The aim of this review is to examine the growing preclinical and clinical trial evidence regarding the application of α and β radioimmunotherapy for the treatment of PDAC. A systematic search of MEDLINE® and Scopus databases was performed to identify 34 relevant studies conducted on α or β radioimmunotherapy of PDAC. Preclinical results demonstrated α and β radioimmunotherapy provided effective tumour control. Clinical studies were limited to investigating β radioimmunotherapy only. Phase I and II trials observed disease control rates of 11.2%-57.9%, with synergistic effects noted for combination therapies. Further developments and optimisation of treatment regimens are needed to improve the clinical relevance of α and β radioimmunotherapy in PDAC.

225Ac-PSMA-617 for Therapy of Prostate Cancer

Prostate-specific membrane antigen (PSMA)-targeting radio-ligand therapy with beta-emitting ¹⁷⁷Lutetium has already been investigated in several early phase dosimetry studies, demonstrated promising results in phase-2, and recently the first phase-3 trial finished recruitment. In contrast, PSMA-targeting alpha-particle therapy (TAT) has only been evaluated in few preclinical experiments, preliminary dosimetry attempts and some retrospective observational studies, yet. First clinical experience with ²²⁵Ac-PSMA-617 demonstrates promising antitumor activity with a 63%-70% PSA_>50%-response rate, 10-15 months duration of response and complete remissions in approximately ten percent of patients, some of them with enduring relapse-free survival. Nevertheless, without comparative trials there is no prove whether, applied in identical clinical situations, ²²⁵Ac-PSMA-617 is really more efficiently than ¹⁷⁷Lu-PSMA-617 or vice versa. However, there is some good rationale, that PSMA-TAT might have advantages in particular clinical indications. This includes patients with diffuse type red-marrow infiltration by reducing off-target radiation to surrounding cells; ablation of micrometastases after favorable response to other previous therapy or someday in early stage disease. Also treatment escalation of patients, either with poor response to ¹⁷⁷Lu-PSMA or harboring adverse prognostic biomarkers, appears promising. In preclinical research, alpha-radiation demonstrated stronger induction of abscopal effects than beta-radiation; favoring its usage as a combination partner with immunotherapies. So, further evaluation of PSMA-TAT is definitely warranted. Recently, de-escalated treatment protocols and application of ²²⁵Ac/¹⁷⁷Lu-PSMA "cocktail"-regimens improved the tolerability of ²²⁵Ac-PSMA-617 TAT, reducing the risk for development dry-mouth syndrome. This opens new avenues for future application in earlier stage disease.

Development of Targeted Alpha Particle Therapy for Solid Tumors

Targeted alpha-particle therapy (TAT) aims to selectively deliver radionuclides emitting α-particles (cytotoxic payload) to tumors by chelation to monoclonal antibodies, peptides or small molecules that recognize tumor-associated antigens or cell-surface receptors. Because of the high linear energy transfer (LET) and short range of alpha (α) particles in tissue, cancer cells can be significantly damaged while causing minimal toxicity to surrounding healthy cells. Recent clinical studies have demonstrated the remarkable efficacy of TAT in the treatment of metastatic, castration-resistant prostate cancer. In this comprehensive review, we discuss the current consensus regarding the properties of the α-particle-emitting radionuclides that are potentially relevant for use in the clinic; the TAT-mediated mechanisms responsible for cell death; the different classes of targeting moieties and radiometal chelators available for TAT development; current approaches to calculating radiation dosimetry for TATs; and lead optimization via medicinal chemistry to improve the TAT radiopharmaceutical properties. We have also summarized the use of TATs in pre-clinical and clinical studies to date.

Cationic radionuclides and ligands for targeted therapeutic radiopharmaceuticals

This review considers the already used and potential α- and β-emitting cationic radionuclides for targeted radionuclide therapy. Recent results of laboratory, preclinical and clinical applications of these radionuclides are discussed. As opposed to β-emitters, which are already used in nuclear medicine, α-emitters involved in targeted radiopharmaceuticals were subjected to clinical trials only recently and were found to be therapeutically effective. The review summarizes recent trends in the development of ligands as components of radiopharmaceuticals addressing specific features of short-lived cationic radionuclides applied in medicine. Despite a steadily growing number of chelating ligands, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and diethylenetriaminepentaacetic acid (DTPA) remain the most widely used agents in nuclear medicine. The drawbacks of these compounds restrict the application of radionuclides in medicine. Variations in the macrocycle size, the introduction and modification of substituents can significantly improve the chelating ability of ligands, enhance stability of radionuclide complexes with these ligands and eliminate the influence of ligands on the affinity of biological targeting vectors.

The bibliography includes 189 references.

The in vivo fate of 225Ac daughter nuclides using polymersomes as a model carrier

Increasing attention is given to personalized tumour therapy, where α-emitters can potentially play an important role. Alpha particles are ideal for localized cell killing because of their high linear energy transfer and short ranges. However, upon the emission of an α particle the daughter nuclide experiences a recoil energy large enough to ensure decoupling from any chemical bond. These 'free' daughter nuclides are no longer targeted to the tumour and can accumulate in normal tissue. In this paper, we used polymersomes as model carrier to evaluate the retention of recoiling daughters of ²²⁵Ac in vivo, and assessed their suitability as therapeutic agents. Vesicles containing ²²⁵Ac were injected intravenously in healthy mice, and intratumourally in tumour-bearing mice, and the relocation of free ²¹³Bi was assessed in different organs upon the injection [²²⁵Ac]Ac-polymersomes. The therapeutic effect of ²²⁵Ac-containing vesicles was studied upon intratumoural injection, where treatment groups experienced no tumour-related deaths over a 115 day period. While polymersomes containing ²²⁵Ac could be suitable agents for long-term irradiation of tumours without causing significant renal toxicity, there is still a significant re-distribution of daughter nuclides throughout the body, signifying the importance of careful evaluation of the effect of daughter nuclides in targeted alpha therapy.

Radionuclide Therapy of Metastatic Prostate Cancer

The current mainstay of treatment in metastatic prostate cancer is based on hormonal manipulations. Standard androgen deprivation therapy and novel androgen axis drugs are commonly well tolerable and can stabilize metastatic hormone-sensitive prostate cancers for years. However, metastatic castration-resistant prostate cancer is still challenging to treat. Except taxanes, prostate cancer presents intrinsic resistance against conventional chemotherapies. The typically elderly patient population excludes more aggressive treatment regimens. First clinical trials evaluating immunotherapy or tyrosine-kinase-inhibitors against prostate cancer failed. In contrast, prostate cancer can be radiosensitive and external beam radiotherapy is effective in localized prostate cancer, thus providing a good rationale for the use of systemic radiopharmaceuticals in the metastatic setting. Beta-particle emitting "bone-seekers" have a long history and are effective as analgesics but do not improve survival because they are limited by red-marrow dose. Alpha emitting ²²³Radium can be used in a dose that prolongs survival but is restricted to bone-confined patients. Currently radiolabeled high-affinity ligands to the prostate-specific membrane antigen are in clinical evaluation. While radioimmunotherapy approaches were limited by the long circulation time and slow tumor-accumulation of antibodies, low molecular weight PSMA-specific ligands offer an approx. ten-fold improved tumor to red-marrow ratio in comparison to the unspecific bone-seekers. Early clinical studies demonstrate that regarding surrogate markers, such as >50% PSA reduction (60%) and radiologic response (80%), PSMA-therapy exceeds the antitumor activity of all approved or other recently tested compounds; for example, PSA-response was only observed in approx. a total of 10% of patients treated with ipilimumab, sunitinib, cabozantinib, or xofigo, respectively and in approx. 30, 40, 50% of patients treated with abiraterone, cabazitaxel, or enzalutamide. Also progression free and overall survivals of these single-arm studies appear promising when compared to historical controls. Consecutively, the first PSMA-RLT recently advanced into phase-3 (¹⁷⁷Lu-PSMA-617; VISION-trial). Future developments aim to avoid off-target radiation by ligand-optimization and to outperform the antitumor activity of beta-emitter PSMA-RLT by labeling with highly focused, high energy transferring alpha-nuclides; however the latter potentially also increasing the risk of side-effects and additional early phase studies are needed to improve treatment protocols. Academically clinical research is developing prognostic tools to improve treatment benefit by selecting the most appropriate patients in advance.

Leveraging Bioorthogonal Click Chemistry to Improve 225Ac-Radioimmunotherapy of Pancreatic Ductal Adenocarcinoma

PURPOSE

Interest in targeted alpha-therapy has surged due to α-particles' high cytotoxicity. However, the widespread clinical use of this approach could be limited by on-/off-target toxicities. Here, we investigated the inverse electron-demand Diels-Alder ligation between an 225Ac-labeled tetrazine radioligand and a trans-cyclooctene-bearing anti-CA19.9 antibody (5B1) for pretargeted α-radioimmunotherapy (PRIT) of pancreatic ductal adenocarcinoma (PDAC). This alternative strategy is expected to reduce nonspecific toxicities as compared with conventional radioimmunotherapy (RIT).Experimental Design: A side-by-side comparison of 225Ac-PRIT and conventional RIT using a directly 225Ac-radiolabeled immunoconjugate evaluates the therapeutic efficacy and toxicity of both methodologies in PDAC murine models.

RESULTS

A comparative biodistribution study of the PRIT versus RIT methodology underscored the improved pharmacokinetic properties (e.g., prolonged tumor uptake and increased tumor-to-tissue ratios) of the PRIT approach. Cerenkov imaging coupled to PRIT confirmed the in vivo biodistribution of 225Ac-radioimmunoconjugate but-importantly-further allowed for the ex vivo monitoring of 225Ac's radioactive daughters' redistribution. Human dosimetry was extrapolated from the mouse biodistribution and confirms the clinical translatability of 225Ac-PRIT. Furthermore, longitudinal therapy studies performed in subcutaneous and orthotopic PDAC models confirm the therapeutic efficacy of 225Ac-PRIT with the observation of prolonged median survival compared with control cohorts. Finally, a comparison with conventional RIT highlighted the potential of 225Ac-PRIT to reduce hematotoxicity while maintaining therapeutic effectiveness.

CONCLUSIONS

The ability of 225Ac-PRIT to deliver a radiotherapeutic payload while simultaneously reducing the off-target toxicity normally associated with RIT suggests that the clinical translation of this approach will have a profound impact on PDAC therapy.