Radioimmunotherapy using 225Ac, a highly cytotoxic α-particle emitter, has potential for treating advanced breast cancer, especially human epidermal growth factor receptor 2 (HER2)-positive cases. We use a pretargeted radioimmunotherapy (PRIT) approach consisting of a 3-step intravenous regimen (step 1: bispecific anti-HER2/anti-DOTA antibody; step 2: clearing agent; step 3: 225Ac-radiolabeled Proteus DOTA, or [225Ac]Ac-Pr). Our goal was to establish curative 225Ac-PRIT with high therapeutic indices. Methods: The impact of [225Ac]Ac-Pr specific activity was evaluated in the BT-474 breast xenograft model. We tested the effects of [225Ac]Ac-Pr dosing during PRIT on tumor-targeting efficiency and tissue biodistribution. Using a 225Ac-PRIT regimen consisting of a ratio of 1.19 nmol of bispecific antibody to 0.60-0.66 nmol of [225Ac]Ac-Pr, we evaluated therapy in the BT-474 model and a patient-derived xenograft model. BT-474-tumor-bearing mice were treated with 1 or 2 cycles of 225Ac-PRIT (37 kBq/cycle) separated by 1 wk. A dose escalation study was performed on the BT-474 model to establish an absorbed radiation dose of approximately 40 Gy (relative biological effectiveness [RBE], 5) as a nephrotoxic dose, as no such histologic findings were observed in prior studies at the 20.7-Gy (RBE, 5) renal dose level. Results: In the BT-474 model, 100% (20/20) achieved complete responses and histologic cure in 17 of 20 (85%) of the treated animals. One-cycle and 2-cycle treatments were equally effective. Treatments were well tolerated, with no chronic radiation toxicity documented during necropsy at 175 d. Dosimetry estimates (RBE, 5) per 37 kBq administered for tumors and kidneys were 210 and 3.5 Gy, respectively. In the patient-derived xenograft model, a single 225Ac-PRIT treatment led to 60% (3/5) complete response and prolonged survival (>93 d) versus no treatment (30 d; P = 0.0185). Lastly, a 225Ac-PRIT regimen was identified that induces severe chronic nephrotoxicity (41.4 Gy/592 kBq; RBE, 5). Conclusion: Safe and effective 225Ac-PRIT regimens were developed in 2 preclinical models of advanced HER2-positive human breast cancer with tumor cure without dose-limiting nephrotoxicity. This study establishes crucial preclinical dosimetry benchmarks for 225Ac-PRIT and provides a compelling rationale for its advancement into the clinic.

S1. Hierarchical clustering based on Euclidean distance of RNA-seq data demonstrates separation of EBRT-treated (B1-3) and untreated (C1-3) samples.S2. Multi-dimensional scaling plot of RNA-seq data demonstrates separation of EBRT-treated (B1-3) and untreated (C1-3) samples.

The use of radionuclides for targeted endoradiotherapy is a rapidly growing field in oncology. In particular, the focus on the biological effects of different radiation qualities is an important factor in understanding and implementing new therapies. Together with the combined approach of imaging and therapy, therapeutic nuclear medicine has recently made great progress. A particular area of research is the use of alpha-emitting radionuclides, which have unique physical properties associated with outstanding advantages, e.g., for single tumor cell targeting. Here, recent results and open questions regarding the production of alpha-emitting isotopes as well as their chemical combination with carrier molecules and clinical experience from compassionate use reports and clinical trials are discussed.

Epithelial ovarian cancer (EOC) is often asymptomatic and presents clinically in an advanced stage as widespread peritoneal microscopic disease that is generally considered to be surgically incurable. Targeted alpha-therapy with the alpha-particle-emitting radionuclide Ac-225 (half-life, 9.92 d) is a high-linear-energy-transfer treatment approach effective for small-volume disease and even single cells. Here, we report the use of human epidermal growth factor receptor 2 (HER2) Ac-225-pretargeted radioimmunotherapy (PRIT) to treat a mouse model of human EOC SKOV3 xenografts growing as peritoneal carcinomatosis (PC). Methods: On day 0,10(5) SKOV3 cells transduced with a luciferase reporter gene were implanted intraperitoneally in nude mice, and tumor engraftment was verified by bioluminescent imaging (BLI). On day 15, treatment was started using 1 or 2 cycles of 3-step anti-HER2 Ac-225-PRIT (37 kBq/cycle as Ac-225-Proteus DOTA), separated by a 1-wk interval. Efficacy and toxicity were monitored for up to 154 d. Results: Untreated PC-tumor-bearing nude mice showed a median survival of 112 d. We used 2 independent measures of response to evaluate the efficacy of Ac-225-PRIT. First, a greater proportion of the treated mice (9/10 1-cycle and 8/10 2-cycle; total, 17/20; 85%) survived long-term compared with controls (9/27, 33%), and significantly prolonged survival was documented (log-rank [Mantel-Cox] P = 0.0042). Second, using BLI, a significant difference in the integrated BLI signal area to 98 d was noted between controls and treated groups (P = 0.0354). Of a total of 8 mice from the 2-cycle treatment group (74 kBq total) that were evaluated by necropsy, kidney radiotoxicity was mild and did not manifest itself clinically (normal serum blood urea nitrogen and creatinine). Dosimetry estimates (relative biological effectiveness-weighted dose, where relative bio-logical effectiveness = 5) per 37 kBq administered for tumors and kidneys were 56.9 and 16.1 Gy, respectively. One-cycle and 2-cycle treatments were equally effective. With immunohistology, mild tubular changes attributable to a-toxicity were observed in both therapeutic groups. Conclusion: Treatment of EOC PC-tumor-bearing mice with anti-HER2 Ac-225-PRIT resulted in histologic cures and prolonged survival with minimal toxicity. Targeted alpha-therapy using the anti-HER2 Ac-225-PRIT system is a potential treatment for otherwise incurable EOC.

2-step SADA-PRIT imaging by PET/CT

Rationale: The in vivo dynamics of CAR-T cells remain incompletely understood. Novel methods are urgently needed to longitudinally monitor transferred cells non-invasively for biodistribution, functionality, proliferation, and persistence in vivo and for improving their cytotoxic potency in case of treatment failure. Methods: Here we engineered CD19 CAR-T cells ("Thor"-cells) to express a membrane-bound scFv, huC825, that binds DOTA-haptens with picomolar affinity suitable for labeling with imaging or therapeutic radionuclides. We assess its versatile utility for serial tracking studies with PET and delivery of α-radionuclides to enhance anti-tumor killing efficacy in sub-optimal adoptive cell transfer in vivo using Thor-cells in lymphoma models. Results: We show that this reporter gene/probe platform enables repeated, sensitive, and specific assessment of the infused Thor-cells in the whole-body using PET/CT imaging with exceptionally high contrast. The uptake on PET correlates with the Thor-cells on a cellular and functional level. Furthermore, we report the ability of Thor-cells to accumulate cytotoxic alpha-emitting radionuclides preferentially at tumor sites, thus increasing therapeutic potency. Conclusion: Thor-cells are a new theranostic agent that may provide crucial information for better and safer clinical protocols of adoptive T cell therapies, as well as accelerated development strategies.

summary of toxicity data from 225Ac SADA-PRIT experiment against a neuroblastoma PDX

Summary of immunogenicity experiment.

summary of toxicity data from 177Lu SADA-PRIT experiment against a neuroblastoma cell line

AbstractNoninvasive biomarkers for androgen receptor (AR) pathway activation are urgently needed to better monitor patient response to prostate cancer therapies. AR is a critical driver and mediator of resistance of prostate cancer but currently available noninvasive prostate cancer biomarkers to monitor AR activity are discordant with downstream AR pathway activity. External beam radiotherapy (EBRT) remains a common treatment for all stages of prostate cancer, and DNA damage induced by EBRT upregulates AR pathway activity to promote therapeutic resistance. [89Zr]11B6-PET is a novel modality targeting prostate-specific protein human kallikrein 2 (hK2), which is a surrogate biomarker for AR activity. Here, we studied whether [89Zr]11B6-PET can accurately assess EBRT-induced AR activity.Genetic and human prostate cancer mouse models received EBRT (2–50 Gy) and treatment response was monitored by [89Zr]11B6-PET/CT. Radiotracer uptake and expression of AR and AR target genes was quantified in resected tissue.EBRT increased AR pathway activity and [89Zr]11B6 uptake in LNCaP-AR and 22RV1 tumors. EBRT increased prostate-specific [89Zr]11B6 uptake in prostate cancer–bearing mice (Hi-Myc x Pb_KLK2) with no significant changes in uptake in healthy (Pb_KLK2) mice, and this correlated with hK2 protein levels.Implications:hK2 expression in prostate cancer tissue is a proxy of EBRT-induced AR activity that can noninvasively be detected using [89Zr]11B6-PET; further clinical evaluation of hK2-PET for monitoring response and development of resistance to EBRT in real time is warranted.

Supplementary Figure S2. Autoradiography of whole mounted KLK3_Hi-MYC GEMM prostate sections obtained 120h p.i. of [89Zr]hu5A10 (A) or [90Y]hu5A10 (B). Radioimmunoconjugate activity was detected in regions expressing PSA, but not in adjacent tissues lacking PSA expression, such as seminal vesicles and urethra.

Supplementary Figure S1. Ex-vivo biodistribution of [89Zr]hu5A10.

2-step IgG-PRIT imaging by PET/CT (no CA)

summary of toxicity data from 177Lu SADA-PRIT experiment against a neuroblastoma PDX

Figure S2 shows the BRCA1 qMSP results and HR functional status of the patient samples at multiple time points.

Supplementary Methods, Figures 1-3 from Selective Killing of Tumor Neovasculature Paradoxically Improves Chemotherapy Delivery to Tumors

External beam radiotherapy (EBRT) remains a common treatment for all stages of PCa, but DNA damage induced by EBRT upregulates androgen receptor (AR) pathway activity to promote therapeutic resistance. [ 89 Zr]11B6-PET is a novel modality targeting prostate-specific protein human kallikrein 2 (hK2), which is a surrogate biomarker for AR activity. Here, we studied if [ 89 Zr]11B6-PET can accurately assess EBRT-induced AR activity. PCa mouse models received EBRT (2-50 Gy) and treatment response was monitored by [ 89 Zr]11B6-PET/CT. Radiotracer uptake and expression of AR and AR target genes was quantified in resected tissue. EBRT increased AR pathway activity in LNCaP-AR tumors. EBRT increased prostate-specific [ 89 Zr]11B6 uptake and hK2 levels in PCa-bearing mice (Hi -Myc x Pb_ KLK2 ) with no significant changes in uptake in healthy (Pb_ KLK2 ) mice. Thus, [ 89 Zr]11B6-PET specifically detects activation of AR pathway activity after EBRT in PCa. Further clinical evaluation of hK2-PET for monitoring EBRT is warranted.

The growing interest and clinical translation of alpha particle (α) therapies brings with it new challenges to assess target cell engagement and to monitor therapeutic effect. Noninvasive imaging has great potential to guide α-treatment and to harness the potential of these agents in the complex environment of disseminated disease. Poly(ADP) ribose polymerase 1 (PARP-1) is among the most abundantly expressed DNA repair enzymes with key roles in multiple repair pathways—such as those induced by irradiation. Here, we used a third-generation PARP1-specific radiotracer, [ 18 F]-PARPZ, to delineate castrate resistant prostate cancer xenografts. Following treatment with the clinically applied [ 225 Ac]-PSMA-617, positron emission tomography was performed and correlative autoradiography and histology acquired. [ 18 F]-PARPZ was able to distinguish treated from control (saline) xenografts by increased uptake. Kinetic analysis of tracer accumulation also suggests that the localization of the agent to sites of increased PARP-1 expression is a consequence of DNA damage response. Together, these data support expanded investigation of [ 18 F]-PARPZ to facilitate clinical translation in the ⍺-therapy space.

Visual Abstract CE credit: For CE credit, you can access the test for this article, as well as additional JNMT CE tests, online at https://www.snmmilearningcenter.org. Complete the test online no later than March 2025. Your online test will be scored immediately. You may make 3 attempts to pass the test and must answer 75% of the questions correctly to receive Continuing Education Hour (CEH) credit. Credit amounts can be found in the SNMMI Learning Center Activity. SNMMI members will have their CEH credit added to their VOICE transcript automatically; nonmembers will be able to print out a CE certificate upon successfully completing the test. The online test is free to SNMMI members; nonmembers must pay $15.00 by credit card when logging onto the website to take the test. α-emitting radionuclides provide an effective means of delivering large radiation doses to targeted treatment locations. 223RaCl2 is Food and Drug Administration–approved for treatment of metastatic castration-resistant prostate cancer, and 225Ac (225Ac-lintuzumab) radiolabeled antibodies have been shown to be beneficial for patients with acute myeloid leukemia. In recent years, there has been increasing use of α-emitters in theranostic agents with both small- and large-molecule constructs. The proper precautionary means for their use and surveying documentation of these isotopes in a clinical setting are an essential accompaniment to these treatments. Methods: Patient treatment data collected over a 3-y period, as well as regulatory requirements and safety practices, are described. Commonly used radiation instruments were evaluated for their ability to identify potential radioactive material spills and contamination events during a clinical administration of 225Ac. These instruments were placed at 0.32 cm from a 1.0-cm 225Ac disk source for measurement purposes. Radiation background values, efficiencies, and minimal detectable activities were measured and calculated for each type of detector. Results: The median external measured dose rate from 223RaCl2 patients (n = 611) was 2.5 μSv h−1 on contact and 0.2 μSv h−1 at 1 m immediately after administration. Similarly, 225Ac-lintuzumab (n = 19) patients had median external dose rates of 2.0 μSv h−1 on contact and 0.3 μSv h−1 at 1 m. For the measurement of 225Ac samples, a liquid scintillation counter was found to have the highest overall efficiency (97%), whereas a ZnS α-probe offered the lowest minimal detectable activity at 3 counts per minute. Conclusion: In this article, we report data from 630 patients who were undergoing treatment with the α-emitting isotopes 223Ra and 225Ac. Although α-emitters have the ability to deliver a higher internal radiation dose to the exposed tissues than can other unsealed radionuclides, they typically present minimal concerns about external dose rate. Additionally, α-radiation can be efficiently detected with appropriate radiation instrumentation, such as a liquid scintillation counter or ZnS probe, which should be prioritized when surveying for spills of α-emitters.