Purpose: Initially, prostate cancer responds to hormone therapy, but eventually resistance develops. Beta emitter-based prostate-specific membrane antigen (PSMA)-targeted radionuclide therapy is approved for the treatment of metastatic castration-resistant prostate cancer. Here we introduce a targeted alpha therapy (TAT) consisting of the PSMA antibody pelgifatamab covalently linked to a macropa chelator and labeled with actinium-225 and compare its efficacy and tolerability with other TATs. Experimental Design: The in vitro characteristics and in vivo biodistribution, antitumor efficacy, and tolerability of 225Ac-macropa-pelgifatamab (225Ac-pelgi) and other TATs were investigated in cell line– and patient-derived prostate cancer xenograft models. The antitumor efficacy of 225Ac-pelgi was also investigated in combination with the androgen receptor inhibitor darolutamide. Results: Actinium-225-labeling of 225Ac-pelgi was efficient already at room temperature. Potent in vitro cytotoxicity was seen in PSMA-expressing (LNCaP, MDA-PCa-2b, and C4-2) but not in PSMA-negative (PC-3 and DU-145) cell lines. High tumor accumulation was seen for both 225Ac-pelgi and 225Ac-DOTA-pelgi in the MDA-PCa-2b xenograft model. In the C4-2 xenograft model, 225Ac-pelgi showed enhanced antitumor efficacy with a T/Cvolume (treatment/control) ratio of 0.10 compared with 225Ac-DOTA-pelgi, 225Ac-DOTA-J591, and 227Th-HOPO-pelgifatamab (227Th-pelgi; all at 300 kBq/kg) with T/Cvolume ratios of 0.37, 0.39, and 0.33, respectively. 225Ac-pelgi was less myelosuppressive than 227Th-pelgi. 225Ac-pelgi showed dose-dependent treatment efficacy in the patient-derived KuCaP-1 model and strong combination potential with darolutamide in both cell line– (22Rv1) and patient-derived (ST1273) xenograft models. Conclusions: These results provide a strong rationale to investigate 225Ac-pelgi in patients with prostate cancer. A clinical phase I study has been initiated (NCT06052306).

Glioblastoma (GB) remains the most fatal brain tumor characterized by a high infiltration rate and treatment resistance. Overexpression and/or mutation of receptor tyrosine kinases is common in GB, which subsequently leads to the activation of many downstream pathways that have a critical impact on tumor progression and therapy resistance. Therefore, receptor tyrosine kinase inhibitors (RTKIs) have been investigated to improve the dismal prognosis of GB in an effort to evolve into a personalized targeted therapy strategy with a better treatment outcome. Numerous RTKIs have been approved in the clinic and several radiopharmaceuticals are part of (pre)clinical trials as a non-invasive method to identify patients who could benefit from RTKI. The latter opens up the scope for theranostic applications. In this review, the present status of RTKIs for the treatment, nuclear imaging and targeted radionuclide therapy of GB is presented. The focus will be on seven tyrosine kinase receptors, based on their central role in GB: EGFR, VEGFR, MET, PDGFR, FGFR, Eph receptor and IGF1R. Finally, by way of analyzing structural and physiological characteristics of the TKIs with promising clinical trial results, four small molecule RTKIs were selected based on their potential to become new therapeutic GB radiopharmaceuticals.

Auger electron emitters are considered to be a promising strategy for targeted radionuclide therapy of metastatic diseases, given their high linear energy transfer (LET) and short range in tissue which could potentially limit normal tissue toxicity. Particularly Auger electron emitters that can be targeted into the DNA of tumor cells have been considered as an attractive cancer therapy in the past decade. In this study, the efficiency of the Auger electron emitter ¹²³I (half-life 13.2 h) to induce chromosomal damage was investigated by using the cytokinesis-block micronucleus assay. A stannylated deoxyuridine was synthesized and radiolabeled with ¹²³I, resulting in ¹²³IUdR that carried the Auger electron emitter across the nuclear membrane and allowed its incorporation into newly synthesized DNA. The DNA damage caused by the ¹²³I Auger cascade was estimated by evaluating the induced micronuclei frequencies in human peripheral blood lymphocytes obtained from three different donors. The isolated lymphocytes were stimulated with phytohemagglutinin (1 mg/ml) for 48 h before pulse labeling with ¹²³IUdR and the S-phase fraction was determined using flow cytometry. Geant4 Monte Carlo calculations were performed to determine the absorbed dose in cells by the Auger emitter. The relative biological effectiveness (RBE) was calculated by comparing the dose response curves for ¹²³IUdR with the reference dose response curves, obtained with ⁶⁰Co γ-ray irradiation in this study, for lymphocytes of the same donors. This resulted in a range of individual RBE values from 3 up to 10, depending on the donor and the radiation dose. In addition, dose limiting RBE values (RBE_Max) were calculated for each donor and ranged from 5 to 11, dependent on the inherent radiosensitivity of the donors. This study provides valuable information on the RBE of Auger electron emitter ¹²³I, which is identified as a promising theranostic radionuclide for future targeted radionuclide therapy.

Despite numerous innovative treatment strategies, the treatment of glioblastoma (GB) remains challenging. With the current state-of-the-art therapy, most GB patients succumb after about a year. In the evolution of personalized medicine, targeted radionuclide therapy (TRT) is gaining momentum, for example, to stratify patients based on specific biomarkers. One of these biomarkers is deficiencies in DNA damage repair (DDR), which give rise to genomic instability and cancer initiation. However, these deficiencies also provide targets to specifically kill cancer cells following the synthetic lethality principle. This led to the increased interest in targeted drugs that inhibit essential DDR kinases (DDRi), of which multiple are undergoing clinical validation. In this review, the current status of DDRi for the treatment of GB is given for selected targets: ATM/ATR, CHK1/2, DNA-PK, and PARP. Furthermore, this review provides a perspective on the use of radiopharmaceuticals targeting these DDR kinases to (1) evaluate the DNA repair phenotype of GB before treatment decisions are made and (2) induce DNA damage via TRT. Finally, by applying in-house selection criteria and analyzing the structural characteristics of the DDRi, four drugs with the potential to become new therapeutic GB radiopharmaceuticals are suggested.

Children have an increased risk of developing radiation-induced secondary malignancies compared to adults, due to their high radiosensitivity and longer life expectancy. In contrast to the epidemiological evidence, there is only a handful of radiobiology studies which investigate the difference in radiosensitivity between children and adults at a cellular level. In this study, the previous results on the potential age dependency in chromosomal radiosensitivity were validated again by means of the cytokinesis-block micronucleus (CBMN) assay in T-lymphocytes isolated from the umbilical cord and adult peripheral blood of a South African population. The isolated cells were irradiated with ⁶⁰Co γ-rays at doses ranging from 0.5 Gy to 4 Gy. Increased radiosensitivities of 34%, 42%, 29%, 26% and 16% were observed for newborns compared to adults at 0.5, 1, 2, 3 and 4 Gy, respectively. An immunophenotypic evaluation with flow cytometry revealed a significant change in the fraction of naïve (CD45RA⁺) T-lymphocytes in CD4⁺ and CD8⁺ T-lymphocytes with age. Newborns co-expressed an average of 91.05% CD45RA⁺ (range: 80.80–98.40%) of their CD4⁺ cells, while this fraction decreased to an average of 39.08% (range: 12.70–58.90%) for adults. Similar observations were made for CD8⁺ cells. This agrees with previous published results that the observed differences in chromosomal radiosensitivity between newborn and adult T-lymphocytes could potentially be linked to their immunophenotypic profiles.

Table S1. Cytotoxicity of 225Ac-pelgi (2 kBq/μg) in PSMA-positive and PSMA-negative prostate cancer cell lines in vitro.