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Microfluidic synthesis techniques can offer improvement over batch syntheses which are currently used for radiopharmaceutical production. These improvements are, for example, better mixing of reactants, more efficient energy transfer, less radiolysis, faster reaction optimization, and overall improved reaction control. However, scale-up challenges hinder the routine clinical use, so the main advantage is currently the ability to optimize reactions rapidly and with low reactant consumption. Translating those results to clinical systems could be done based on calculations, if kinetic constants and diffusion coefficients were known. This study describes a microfluidic system with which it was possible to determine the kinetic association rate constants for the formation of [177Lu]Lu-DOTA-TATE under conditions currently used for clinical production. The kinetic rate constants showed a temperature dependence that followed the Arrhenius equation, allowing the determination of Arrhenius parameters for a Lu-DOTA conjugate (A = 1.24 ± 0.05 × 1019 M−1 s−1, EA = 109.5 ± 0.1 × 103 J mol−1) for the first time. The required reaction time for the formation of [177Lu]Lu-DOTA-TATE (99% yield) at 80 °C was 44 s in a microfluidic channel (100 μm). Simulations done with COMSOL Multiphysics® indicated that processing clinical amounts (3 mL reaction solution) in less than 12 min is possible in a micro- or milli-fluidic system, if the diameter of the reaction channel is increased to over 500 μm. These results show that a continuous, microfluidic system can become a viable alternative to the conventional, batch-wise radiolabelling technique. [ABSTRACT FROM AUTHOR]

In this review we give an overview of current knowledge of (68)Ga-labeled pharmaceuticals, with focus on imaging receptor-mediated processes. A major advantage of a (68)Ge/(68)Ga generator is its continuous source of (68)Ga, independently from an on-site cyclotron. The increase in knowledge of purification and concentration of the eluate and the complex ligand chemistry has led to (68)Ga-labeled pharmaceuticals with major clinical impact. (68)Ga-labeled pharmaceuticals have the potential to cover all today's clinical options with (99m)Tc, with the concordant higher resolution of positron emission tomography (PET) in comparison with single photon emission computed tomography. (68)Ga-labeled analogs of octreotide, such as DOTATOC, DOTANOC, and DOTA-TATE, are in clinical application in nuclear medicine, and these analogs are now the most frequently applied of all (68)Ga-labeled pharmaceuticals. All the above-mentioned items in favor of successful application of (68)Ga-labeled radiopharmaceuticals for imaging in patients are strong arguments for the development of a (68)Ge/(68)Ga generator with Marketing Authorization and thus to provide pharmaceutical grade eluate. Moreover, now not one United States Food and Drug Administration-approved or European Medicines Agency-approved (68)Ga-radiopharmaceutical is available. As soon as these are achieved, a whole new radiopharmacy providing PET radiopharmaceuticals might develop. [ABSTRACT FROM AUTHOR]

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., (© 2024. The Author(s).)

Purpose: Radiolabeled NeoB is a promising gastrin-releasing peptide receptor (GRPR)-targeting radiopharmaceutical for theranostics of GRPR-expressing malignancies, e.g., prostate cancer (PCa). The aim of this study was to evaluate the effect of different doses of [ 177 Lu]Lu-NeoB on the balance between therapeutic efficacy and safety in a preclinical PCa model., Procedures: To determine the efficacy of [ 177 Lu]Lu-NeoB, PC-3 xenografted mice received 3 sham injections (control group) or 3 injections of 30 MBq/300 pmol, 40 MBq/400 pmol, or 60 MBq/600 pmol [ 177 Lu]Lu-NeoB (groups 1, 2, and 3, respectively) 1 week apart. To quantify tumor uptake, single-photon emission computed tomography/computed tomography (SPECT/CT) imaging was performed 4 h after the first, second, and third injection on a separate group of animals. For safety evaluations, pancreatic and renal tissues of non-tumor-bearing mice treated with the abovementioned [ 177 Lu]Lu-NeoB doses were evaluated 12 and 24 weeks post-treatment., Results: Treatment of PC-3 tumors with all three studied [ 177 Lu]Lu-NeoB doses was effective. Median survival times were significantly (p < 0.0001) improved for treatment groups 1, 2, and 3 versus the control group (82 days, 89 days, 99 days versus 19 days, respectively). However, no significant differences were observed between treatment groups. Quantification of SPECT/CT images showed minimal differences in the average absolute radioactivity uptake, especially after the third injection. Histopathological analysis revealed no clear signs of treatment-related pancreatic toxicity. For the kidneys, atrophy and fibrosis were observed for one animal from group 1 and a chronic inflammatory response was observed for both animals from group 3 at 24 weeks post-treatment., Conclusions: Treatment with [ 177 Lu]Lu-NeoB is effective in a preclinical PCa model. Adjusting the administered dose could positively impact the risk-benefit balance as a higher dose might not lead to an increased therapeutic effect, but it may lead to an increase in toxicological effects in healthy organs such as the kidneys., (© 2023. The Author(s).)

Background: Life expectancy of patients with metastatic castration-resistant prostate cancer (mCRPC) is still limited despite several systemic treatments. Within five years after diagnosis of primary prostate cancer, 10-20% of the patients have mCRPC and curation is not an option. Radionuclide therapy (RNT) targeted against prostate-specific membrane antigen (PSMA) emerged as a new treatment option and showed effective results in patients with mCRPC. Survival benefit after [ 177 Lu]Lu-PSMA RNT has already been demonstrated in several clinical trials. However, [ 225 Ac]Ac-PSMA ( 225 Ac-PSMA) appears to be an even more promising radiopharmaceutical for the treatment of mCRPC. The use of alpha emitting radionuclides offers advantages over beta emitting radionuclides due to the high linear energy transfer effective for killing tumor cells and the limited range to reduce the radiation effects on the healthy tissue. However, these results are based on retrospective data and safety data of 225 Ac-PSMA are still limited. Therefore, a prospective trial is needed to determine the optimal amount of activity that can be administered., Methods: The 225 Ac-PSMA-Imaging & Therapy (I&T) trial is an investigator-initiated phase I, single-center, open label, repeated dose-escalation and expansion trial. Patient with PSMA-positive mCRPC after at least one line of chemotherapy and/or one line of nonsteroidal antiandrogen will be treated with 225 Ac-PSMA-I&T in increasing amount of activity per cycle. Dose-escalation following an accelerated 3 + 3 design which allows to open the next dose-level cohort in the absence of dose limiting toxicity while the previous one is still ongoing. Up to 4 treatment cohorts will be explored including up to 3 dose-escalation cohorts and one expansion cohort where patients will be administered with the recommended dose. A total of up to 30 patients will be enrolled in this trial. All patients will be evaluated for safety. Additionally, dosimetry was performed for the patients in the dose-escalation cohorts after the first 225 Ac-PSMA-I&T administration., Discussion: This trial will assess the safety and tolerability of 225 Ac-PSMA-I&T in patients with mCRPC to recommend the optimal dose for the phase II trial., Trial Registration: ClinicalTrials.gov, (NCT05902247). Retrospectively registered 13 June 2023., (© 2024. The Author(s).)

Background: Dynamic glucose-enhanced (DGE) chemical exchange saturation transfer (CEST) has the potential to characterize glucose metabolism in brain metastases. Since the effect size of DGE CEST is small at 3 T (< 1%), measurements of signal-to-noise ratios are challenging. To improve DGE detection, we developed an acquisition pipeline and extended image analysis for DGE CEST on a hybrid 3-T positron emission tomography/magnetic resonance imaging system., Methods: This cross-sectional study was conducted after local ethical approval. Static Z-spectra (from -100 to 100 ppm) were acquired to compare the use of 1.2 versus 2 ppm to calculate static glucose-enhanced (glucoCEST) maps in 10 healthy volunteers before and after glucose infusion. Dynamic CEST images were acquired during glucose infusion. Image analysis was optimized using motion correction, dynamic B 0 correction, and principal component analysis (PCA) to improve the detection of DGE CEST in the sagittal sinus, cerebrospinal fluid, and grey and white matter. The developed DGE CEST pipeline was applied to four patients diagnosed with brain metastases., Results: GlucoCEST was strongest in healthy tissues at 2 ppm. Correcting for motion, B 0, and use of PCA locally improved DGE maps. A larger contrast between healthy tissues and enhancing regions in brain metastases was found when dynamic B 0 correction and PCA denoising were applied., Conclusion: We demonstrated the feasibility of DGE CEST with our developed acquisition and analysis pipeline at 3 T in patients with brain metastases. This work enables a direct comparison of DGE CEST to 18F-fluoro-deoxy-D-glucose positron emission tomography of glucose metabolism in patients with brain metastases., Relevance Statement: Contrast between brain metastasis and healthy brain tissue in DGE CEST MR images is improved by including principle component analysis and dynamic magnetic field correction during postprocessing. This approach enables the detection of increased DGE CEST signal in brain metastasis, if present., Key Points: • Despite the low signal-to-noise ratio, dynamic glucose-enhanced CEST MRI is feasible at 3 T. • Principal component analyses and dynamic magnetic field correction improve DGE CEST MRI. • DGE CEST MRI does not consequently show changes in brain metastases compared to healthy brain tissue. • Increased DGE CEST MRI in brain metastases, if present, shows overlap with contrast enhancement on T1-weighted images., (© 2023. The Author(s).)

Aim: To explore the dosimetric effect of substituting Lu-177 with Tb-161 in targeted radionuclide therapy (TRT) using the registered tracers DOTA-TATE and PSMA-617., Methods: Using established kinetic data for [ 177 Lu]Lu-DOTA-TATE and [ 177 Lu]Lu-PSMA-617, radiation absorbed doses to typical tumour lesion as well as non-target tissues ([ 177 Lu]Lu-DOTA-TATE: kidneys, spleen and liver, [ 177 Lu]Lu-PSMA-617: kidneys, liver and salivary glands) were calculated for Lu-177 and Tb-161., Results: For both DOTA-TATE and PSMA-617, the substitution of Lu-177 with Tb-161 results in an increase in the delivered dose per unit of activity to tumour tissue by 40%. If an equivalent non-target delivered dose is strived for in order not to increase toxicity, based on kidney absorbed dose, 7400 MBq Lu-177 per cycle should be substituted with 5400 MBq Tb-161 for DOTA-TATE and 5300 MBq of Tb-161 for PSMA-617., Conclusion: When substituting Lu-177 with Tb-161, activity conversion is necessary in order not to exceed non-target dose limits., (© 2023. The Author(s).)

Peptide receptor radionuclide therapy (PRRT) has been applied to the treatment of neuroendocrine tumors (NETs) for over two decades. However, improvement is still needed, and targeted alpha therapy (TAT) with alpha emitters such as lead-212 ( 212 Pb) represents a promising avenue. A series of ligands based on octreotate was developed. Lead-203 was used as an imaging surrogate for the selection of the best candidate for the studies with lead-212. 203/212 Pb radiolabeling and in vitro assays were carried out, followed by SPECT/CT imaging and ex vivo biodistribution in NCI-H69 tumor-bearing mice. High radiochemical yields (≥99%) and purity (≥96%) were obtained for all ligands. [ 203 Pb]Pb-eSOMA-01 and [ 203 Pb]Pb-eSOMA-02 showed high stability in PBS and mouse serum up to 24 h, whereas [ 203 Pb]Pb-eSOMA-03 was unstable in those conditions. All compounds exhibited a nanomolar affinity (2.5-3.1 nM) for SSTR2. SPECT/CT images revealed high tumor uptake at 1, 4, and 24 h post-injection of [ 203 Pb]Pb-eSOMA-01/02. Ex vivo biodistribution studies confirmed that the highest uptake in tumors was observed with [ 212 Pb]Pb-eSOMA-01. [ 212 Pb]Pb-eESOMA-01 displayed the highest absorbed dose in the tumor (35.49 Gy/MBq) and the lowest absorbed dose in the kidneys (121.73 Gy/MBq) among the three tested radioligands. [ 212 Pb]Pb-eSOMA-01 is a promising candidate for targeted alpha therapy of NETs. Further investigations are required to confirm its potential.

Background: The [ 177 Lu]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, [ 177 Lu]Lu-DOTA-JR11, that demonstrated better biodistribution profile and higher tumor uptake than [ 177 Lu]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, [ 225 Ac]Ac-DOTA-JR11 can be a potential candidate to improve the treatment of NETs (Graphical abstract). DOTA-JR11 was radiolabeled with [ 225 Ac]Ac(NO 3 ) 3 and [ 177 Lu]LuCl 3 . 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 nat La-DOTA-JR11, nat Lu-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 [ 225 Ac]Ac-DOTA-JR11. A blocking group was included to verify uptake specificity. Dosimetry of selected organs was determined for [ 225 Ac]Ac-DOTA-JR11 and [ 177 Lu]Lu-DOTA-JR11., Results: [ 225 Ac]Ac-DOTA-JR11 has been successfully prepared and obtained in high radiochemical yield (RCY; 95%) and radiochemical purity (RCP; 94%). [ 225 Ac]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). [ 177 Lu]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 nat La and nat Lu 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 [ 225 Ac]Ac-DOTA-JR11 than [ 177 Lu]Lu-DOTA-JR11., Conclusion: [ 225 Ac]Ac-DOTA-JR11 showed a higher absorbed dose in the kidneys compared to [ 177 Lu]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 [ 225 Ac]Ac-DOTA-JR11., (© 2023. The Author(s).)

Background: Peptide receptor radionuclide therapy (PRRT) increases progression-free survival and quality of life of neuroendocrine tumor (NET) patients, however complete cures are rare and dose-limiting toxicity has been reported. PRRT induces DNA damage of which DNA double strand breaks (DSBs) are the most cytotoxic. DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a key player in DSB repair and its inhibition therefore is a potential way to enhance PRRT efficacy without increasing the dosage. Methods: We analyzed effects of combining PRRT and DNA-PKcs inhibitor AZD7648 on viability, cell death and clonogenic survival on SSTR2-expressing cell lines BON1-SSTR2, GOT1 and NCI-H69. Therapy-induced DNA damage response was assessed by analyzing DSB foci levels and cell cycle distributions. In vivo efficacy was investigated in BON1-SSTR2 and NCI-H69 xenografted mice and hematologic and renal toxicity were monitored by blood counts, creatinine levels and analyzing renal morphology. Results: Combining PRRT and AZD7648 significantly decreased viability of BON1-SSTR2, GOT1 and NCI-H69 cells and induced cell death in GOT1 and BON1-SSTR2 cells. A strong effect of AZD7648 on PRRT-induced DSB repair was found. In GOT1 cells, this was accompanied by induction of cell cycle blocks. However, BON1-SSTR2 cells were unable to fully arrest their cell cycle and polyploid cells with high DNA damage levels were detected. In vivo , AZD7648 significantly sensitized BON1-SSTR2 and NCI-H69 xenograft models to PRRT. In addition, combination therapy did not induce significant changes in body weight, blood composition, plasma creatinine levels and renal morphology, indicating the absence of severe acute hematologic and renal toxicity. Conclusion: These results highlight that the potentiation of the therapeutic effect of PRRT by DNA-PKcs inhibition is a highly effective and well-tolerated therapeutic strategy. Based on our findings, we recommend initiation of phase I/II studies in patients to find a safe and effective combination regimen., Competing Interests: Competing Interests: The authors have declared that no competing interest exists., (© The author(s).)

Targeted radionuclide therapy (TRT) is a promising strategy to treat different types of cancer. TRT relies on a targeting vector used to deliver a therapeutic radionuclide specifically to the tumour site. Several low molecular weight ligands targeting the prostate-specific membrane antigen (PSMA) have been synthesized, but their pharmacokinetic properties still need to be optimized. Hereby, we describe the synthesis of new conjugates, featuring the cleavable linkers Gly-Tyr-Lys (GYK) and Met-Val-Lys (MVK), to reduce the dose delivered to the kidneys. Compounds were synthesized by solid-phase peptide synthesis (SPPS) and obtained in greater than 95% chemical purity. Radiolabelling was performed with both In-111 and Lu-177 to validate potential use of the compounds as both imaging and therapeutic agents. Radiochemical purity greater than 80% was obtained for both nuclides, but significant radiolysis was observed for the methionine-containing analogue. The results obtained thus far with the GYK-PSMA conjugate could warrant further biological investigations., (© 2023 The Authors.)

Peptide receptor radionuclide therapy (PRRT), a form of internal targeted radiation treatment using [ 177 Lu]Lu [DOTA 0 -Tyr 3 ]octreotate, is used to treat patients with metastasized neuroendocrine tumors (NETs). Even though PRRT is now the second line of treatment for patients with metastasized NETs, the majority of patients will not be cured by the treatment. PRRT functions by inducing DNA damage upon radioactive decay and inhibition of DNA damage repair proteins could therefore be used as a strategy to potentiate PRRT. Previous work has shown promising results on the combination of PRRT with the PARP inhibitor olaparib in cell lines and mice and we have been taken the next step for further in vivo validation using two different xenografted mouse models. We observed that this combination therapy resulted in increased therapeutic efficacy only in one model and not the other. Overall, our findings indicate a tumor-type dependent anti-tumor response to the combination of PRRT and olaparib. These data emphasize the unmet need for the molecular stratification of tumors to predetermine the potential clinical value of combining PARP inhibition with PRRT.

Background: Radiopharmaceuticals are considered as regular medicinal products and therefore the same regulations as for non-radioactive medicinal products apply. However, specific aspects should be considered due to the radiochemical properties. Radiopharmaceutical dedicated monographs are developed in the European Pharmacopoeia to address this. Currently, different quality control methods for non-registered radiopharmaceuticals are utilized, often focusing on radio-TLC only, which has its limitations. When the radiochemical yield (RCY) is measured by radio-TLC analysis, degradation products caused by radiolysis are frequently not detected. In contrast, HPLC analysis defines the radiochemical purity (RCP), allowing for detection of peak formation related to radiolysis. During the introduction and optimization phase of therapeutic radiopharmaceuticals, significant percentages of impurities, like radiolysed construct formation, may have consequential impact on patient treatment. Since more hospitals and institutes are offering radiopharmaceutical therapies, such as [ 177 Lu]Lu-PSMA with an in-house production, the demand for adequate quality control is increasing. Here we show the optimization and implementation of a therapeutic radiopharmaceutical, including the comparison of ITLC and HPLC quality control., Results: Downscaled conditions (74 MBq/μg) were in concordance to clinical conditions (18 GBq/250 µg, 5 mL syringe/100 mL flacon); all results were consistent with an > 98% RCY (radio-TLC) and stability of > 95% RCP (HPLC). Radio-TLC did not identify radiolysis peaks, while clear identification was performed by HPLC analysis. Decreasing the RCP with 50%, reduced the cell-binding capacity with 27%., Conclusion: This research underlines the importance of the radiolabeling and optimization including clinical implementation and clarifies the need for cross-validation of the RCY and RCP for quality control measurements. Only HPLC analysis is suitable for identification of radiolysis. Here we have proven that radiolysed [ 177 Lu]Lu-PSMA has less binding affinity and thus likely will influence treatment efficacy. HPLC analysis is therefore essential to include in at least the validation phase of radiopharmaceutical implementation to ensure clinical treatment quality., (© 2022. The Author(s).)

Purpose: The radiolabeled gastrin-releasing peptide receptor (GRPR)-targeting antagonist NeoB is a promising radioligand for imaging and therapy of GRPR-expressing malignancies. In the current study, we aimed to discover the target organs of toxicity and the radiotoxic effects to these organs, when repeated dosages of [ 177 Lu]Lu-NeoB are administered to healthy female and male mice., Methods: Animals received either 3 injections, with a 7-day interval, of vehicle (control group 1), 1200 pmol [ 175 Lu]Lu-NeoB (control group 2) or 40 MBq/400 pmol, 80 MBq/800 pmol, and 120 MBq/1200 pmol [ 177 Lu]Lu-NeoB (treatment groups 1, 2, and 3, respectively). At week 5, 19, and 43 after the first injection acute, early, and late organ toxicity, respectively, was determined. For this, histopathological and blood analyses were performed. To correlate the observed toxicity to absorbed dose, we also performed extensive biodistribution and dosimetry studies., Results: The biodistribution study showed the highest absorbed doses in GRPR-expressing pancreas, the liver, and the kidneys (the main organs of excretion). Both control groups and almost all animals of treatment group 1 did not show any treatment-related toxicological effects. Despite the high absorbed doses, no clear microscopic signs of toxicity were found in the pancreas and the liver. Histological analysis indicated kidney damage in the form of hydronephrosis and nephropathy in treatment groups 2 and 3 that were sacrificed at the early and late time point. In the same groups, increased blood urea nitrogen levels were found., Conclusion: In general, repeated administration of [ 177 Lu]Lu-NeoB was tolerated. The most significant radiotoxic effects were found in the kidneys, similar to other clinically applied radioligands. The results of this study underline the potential of [ 177 Lu]Lu-NeoB as a promising option for clinical therapy., (© 2022. The Author(s).)

For patients with metastatic castration-resistant prostate cancer (mCRPC), the survival benefit of classic treatment options with chemotherapy and drugs targeting androgen signaling is limited. Therefore, beta and alpha radionuclide therapy (RNT) have emerged as novel treatment options for patients with mCRPC. Radioligands target the prostate-specific membrane antigen (PSMA) epitopes, which are upregulated up to a thousand times more in prostate cancer cells compared to the cells in normal tissues. For this reason, PSMA is an excellent target for both imaging and therapy. Over the past years, many studies have investigated the treatment effects of lutetium-177 labeled PSMA ( 177 Lu-PSMA) and actinium-225 labeled PSMA ( 225 Ac-PSMA) RNT in patients with mCRPC. While promising results have been achieved, this field is still in development. In this review, we have summarized and discussed the clinical data of 177 Lu-PSMA and 225 Ac-PSMA RNT in patients with mCRPC.

Purpose: Targeting the prostate-specific membrane antigen (PSMA) using lutetium-177-labeled PSMA-specific tracers has become a very promising novel therapy option for prostate cancer (PCa). The efficacy of this therapy might be further improved by replacing the β-emitting lutetium-177 with the α-emitting actinium-225. Actinium-225 is thought to have a higher therapeutic efficacy due to the high linear energy transfer (LET) of the emitted α-particles, which can increase the amount and complexity of the therapy induced DNA double strand breaks (DSBs). Here we evaluated the relative biological effectiveness of [ 225 Ac]Ac-PSMA-I&T and [ 177 Lu]Lu-PSMA-I&T by assessing in vitro binding characteristics, dosimetry, and therapeutic efficacy., Methods and Results: The PSMA-expressing PCa cell line PC3-PIP was used for all in vitro assays. First, binding and displacement assays were performed, which revealed similar binding characteristics between [ 225 Ac]Ac-PSMA-I&T and [ 177 Lu]Lu-PSMA-I&T. Next, the assessment of the number of 53BP1 foci, a marker for the number of DNA double strand breaks (DSBs), showed that cells treated with [ 225 Ac]Ac-PSMA-I&T had slower DSB repair kinetics compared to cells treated with [ 177 Lu]Lu-PSMA-I&T. Additionally, clonogenic survival assays showed that specific targeting with [ 225 Ac]Ac-PSMA-I&T and [ 177 Lu]Lu-PSMA-I&T caused a dose-dependent decrease in survival. Lastly, after dosimetric assessment, the relative biological effectiveness (RBE) of [ 225 Ac]Ac-PSMA-I&T was found to be 4.2 times higher compared to [ 177 Lu]Lu-PSMA-I&T., Conclusion: We found that labeling of PSMA-I&T with lutetium-177 or actinium-225 resulted in similar in vitro binding characteristics, indicating that the distinct biological effects observed in this study are not caused by a difference in uptake of the two tracers. The slower repair kinetics of [ 225 Ac]Ac-PSMA-I&T compared to [ 177 Lu]Lu-PSMA-I&T correlates to the assumption that irradiation with actinium-225 causes more complex, more difficult to repair DSBs compared to lutetium-177 irradiation. Furthermore, the higher RBE of [ 225 Ac]Ac-PSMA-I&T compared to [ 177 Lu]Lu-PSMA-I&T underlines the therapeutic potential for the treatment of PCa., (© 2022. The Author(s).)

Prostate specific membrane antigen targeted radionuclide therapy (PSMA-TRT) is a promising novel treatment for prostate cancer (PCa) patients. However, PSMA-TRT cannot be used for curative intent yet, thus additional research on how to improve the therapeutic efficacy is warranted. A potential way of achieving this, is combining TRT with poly ADP-ribosylation inhibitors (PARPi), which has shown promising results for TRT of neuroendocrine tumor cells. Currently, several clinical trials have been initiated for this combination for PCa, however so far, no evidence of synergism is available for PCa. Therefore, we evaluated the combination of PSMA-TRT with three classes of PARPi in preclinical PCa models. In vitro viability and survival assays were performed using PSMA-expressing PCa cell lines PC3-PIP and LNCaP to assess the effect of increasing concentrations of PARPi veliparib, olaparib or talazoparib in combination with PSMA-TRT compared to single PARPi treatment. Next, DNA damage analyses were performed by quantifying the number of DNA breaks by immunofluorescent stainings. Lastly, the potential of the combination treatments was studied in vivo in mice bearing PC3-PIP xenografts. Our results show that combining PSMA-TRT with PARPi did not synergistically affect the in vitro clonogenic survival or cell viability. DNA-damage analysis revealed only a significant increase in DNA breaks when combining PSMA-TRT with veliparib and not in the other combination treatments. Moreover, PSMA-TRT with PARPi treatment did not improve tumor control compared to PSMA-TRT monotherapy. Overall, the data presented do not support the assumption that combining PSMA-TRT with PARPi leads to a synergistic antitumor effect in PCa. These results underline that extensive preclinical research using various PCa models is imperative to validate the applicability of the combination strategy for PCa, as it is for other cancer types.

Purpose: To assess our improved NACA for the detection of tumor necrosis., Methods: We increased the blood circulation time of our NACA by adding an albumin-binding domain to the molecular structure. We tested the necrosis avidity on dead or alive cultured cells and performed SPECT and fluorescence imaging of both spontaneous and treatment-induced necrosis in murine breast cancer models. We simultaneously recorded [ 18 F]FDG-PET and bioluminescence images for complementary detection of tumor viability., Results: We generated two albumin-binding IRDye800CW derivatives which were labeled with indium-111 with high radiochemical purity. Surprisingly, both albumin-binding NACAs had >10x higher in vitro binding towards dead cells. We selected [ 111 In] 3 for in vivo experiments which showed higher dead cell binding in vitro and in vivo stability. The doxorubicin-treated tumors showed increased [ 111 In] 3 -uptake (1.74 ± 0.08%ID/g after saline treatment, 2.25 ± 0.16%ID/g after doxorubicin treatment, p = 0.044) and decreased [ 18 F]FDG-uptake (3.02 ± 0.51%ID/g after saline treatment, 1.79 ± 0.11%ID/g after doxorubicin treatment, p = 0.040), indicating therapy efficacy. Moreover, we detected increased [ 111 In] 3- uptake and tumor necrosis in more rapidly growing EMT6 tumors., Conclusions: Our albumin-binding NACA based on IRDye800CW facilitates tumor-necrosis imaging for assessment of therapy efficacy and aggressiveness in solid tumors using both fluorescence and SPECT imaging.

Necrosis only occurs in pathological situations and is directly related to disease severity and, therefore, is an important biomarker. Tumor necrosis occurs in most solid tumors due to improperly functioning blood vessels that cannot keep up with the rapid growth, especially in aggressively growing tumors. The amount of necrosis per tumor volume is often correlated to rapid tumor proliferation and can be used as a diagnostic tool. Furthermore, efficient therapy against solid tumors will directly or indirectly lead to necrotic tumor cells, and detection of increased tumor necrosis can be an early marker for therapy efficacy. We propose the application of necrosis avid contrast agents to detect therapy-induced tumor necrosis. Herein, we advance gallium-68-labeled IRDye800CW, a near-infrared fluorescent dye that exhibits excellent necrosis avidity, as a potential PET tracer for in vivo imaging of tumor necrosis. We developed a reliable labeling procedure to prepare [ 68 Ga]Ga-DOTA-PEG 4 -IRDye800CW ([ 68 Ga]Ga-1) with a radiochemical purity of >96% (radio-HPLC). The prominent dead cell binding of fluorescence and radioactivity from [ 68 Ga]Ga-1 was confirmed with dead and alive cultured 4T1-Luc2 cells. [ 68 Ga]Ga-1 was injected in 4T1-Luc2 tumor-bearing mice, and specific fluorescence and PET signal were observed in the spontaneously developing tumor necrosis. The ip injection of D-luciferin enabled simultaneous bioluminescence imaging of the viable tumor regions. Tumor necrosis binding was confirmed ex vivo by colocalization of fluorescence uptake with TUNEL dead cell staining and radioactivity uptake in dichotomized tumors and frozen tumor sections. Our presented study shows that [ 68 Ga]Ga-1 is a promising PET tracer for the detection of tumor necrosis., Competing Interests: The authors declare that there are no conflicts of interest., (Copyright © 2021 Marcus C.M. Stroet et al.)

Background: Imaging Somatostatin Subtype Receptor 2 (SST 2 ) expressing macrophages by [DOTA,Tyr 3 ]-octreotate (DOTATATE) has proven successful for plaque detection. DOTA-JR11 is a SST 2 targeting ligand with a five times higher tumor uptake than DOTATATE, and holds promise to improve plaque imaging. The aim of this study was to evaluate the potential of DOTA-JR11 for plaque detection., Methods and Results: Atherosclerotic ApoE -/- mice (n = 22) fed an atherogenic diet were imaged by SPECT/CT two hours post injection of [ 111 In]In-DOTA-JR11 (~ 200 pmol, ~ 50 MBq). In vivo plaque uptake of [ 111 In]In-DOTA-JR11 was visible in all mice, with a target-to-background-ratio (TBR) of 2.23 ± 0.35. Post-mortem scans after thymectomy and ex vivo scans of the arteries after excision of the arteries confirmed plaque uptake of the radioligand with TBRs of 2.46 ± 0.52 and 3.43 ± 1.45 respectively. Oil red O lipid-staining and ex vivo autoradiography of excised arteries showed [ 111 In]In-DOTA-JR11 uptake at plaque locations. Histological processing showed CD68 (macrophages) and SST 2 expressing cells in plaques. SPECT/CT, in vitro autoradiography and immunohistochemistry performed on slices of a human carotid endarterectomy sample showed [ 111 In]In-DOTA-JR11 uptake at plaque locations containing CD68 and SST 2 expressing cells., Conclusions: The results of this study indicate DOTA-JR11 as a promising ligand for visualization of atherosclerotic plaque inflammation., (© 2020. The Author(s).)

The gastrin-releasing peptide receptor (GRPr) is overexpressed in prostate cancer (PCa) cells, making it an excellent tool for targeted imaging. The 68 Ga-labeled GRPr antagonist SB3 has shown excellent results in preclinical and clinical studies and was selected for further clinical investigation. The aims of this phase I study were to investigate 68 Ga-SB3 PET/CT imaging of primary PCa tumors and assess safety. More aims included an investigation of biodistribution and dosimetry and a comparison with pathology and GRPr expression. Methods: Ten therapy-naïve, biopsy-confirmed PCa patients planned for prostatectomy were included. A 3-h extensive PET/CT imaging protocol was performed within 2 wk before prostatectomy. Prostate tissue was evaluated for tumor localization and Gleason score, and in vitro autoradiography was performed to determine GRPr expression. Available MRI scans performed within 3 mo before the study were matched. For dosimetry, residence times were estimated and effective dose to the body as well as absorbed doses to organs were calculated using the IDAC dose model, version 2.1. Results: Administration of 68 Ga-SB3 (187.4 ± 40.0 MBq, 40 ± 5 μg) was well tolerated; no significant changes in vital signs or laboratory results were observed. 68 Ga-SB3 PET/CT showed lesions in 8 of 10 patients. Pathologic analysis revealed a total of 16 tumor lesions, of which PET/CT showed 14, resulting in a sensitivity of 88%. 68 Ga-SB3 PET/CT imaging showed uptake in 2 large prostatic intraepithelial neoplasia foci, considered a precursor to PCa, resulting in an 88% specificity. Autoradiography of tumor lesions revealed heterogeneous GRPr expression and was negative in 4 patients. Both PET/CT-negative patients had a GRPr-negative tumor. In autoradiography-positive tumors, the level of GRPr expression showed a significant correlation to tracer uptake on PET/CT. Dosimetry calculations estimated the effective dose to be 0.0144 mSv/MBq, similar to other 68 Ga-labeled radiopeptides. The highest absorbed dose was detected in the physiologic GRPr-expressing pancreas (0.198 mGy/MBq), followed by the bladder wall and kidneys. Conclusion: 68 Ga-SB3 PET/CT is a safe imaging method and a promising tool for early PCa imaging., (© 2021 by the Society of Nuclear Medicine and Molecular Imaging.)

Recently, promising results of the antitumor effects were observed in patients with metastatic castration-resistant prostate cancer treated with 177 Lu-labeled PSMA-ligands. Radionuclide therapy efficacy may even be improved by using the alpha emitter Ac-225. Higher efficacy is claimed due to high linear energy transfer specifically towards PSMA positive cells, causing more double-strand breaks. This study aims to manufacture [ 225 Ac]Ac-PSMA-I&T according to good manufacturing practice guidelines for the translation of [ 225 Ac]Ac-PSMA-I&T into a clinical phase 1 dose escalation study. Quencher addition during labeling was investigated. Quality control of [ 225 Ac]Ac-PSMA-I&T was based on measurement of Fr-221 (218 keV), in equilibrium with Ac-225 in approximately six half-lives of Fr-221 (T½ = 4.8 min). Radio-(i)TLC methods were utilized for identification of the different radiochemical forms, gamma counter for concentration determination, and HPGe-detector for the detection of the radiochemical yield. Radiochemical purity was determined by HPLC. The final patient dose was prepared and diluted with an optimized concentration of quenchers as during labeling, with an activity of 8-12 MBq (±5%), pH > 5.5, 100 ± 20 μg/dose, PSMA-I&T, radiochemical yield >95%, radiochemical purity >90% (up to 3 h), endotoxin levels of <5 EU/mL, osmolarity of 2100 mOsmol, and is produced according to current guidelines. The start of the phase I dose escalation study is planned in the near future.

Purpose: Various radiolabeled prostate-specific membrane antigen (PSMA)-targeting tracers are clinically applied for prostate cancer (PCa) imaging and targeted radionuclide therapy. The PSMA binding affinities, biodistribution, and DNA-damaging capacities of these radiotracers have not yet been compared in detail. A major concern of PSMA-targeting radiotracers is the toxicity in other PSMA-expressing organs, such as the salivary glands, thus demanding careful evaluation of the most optimal and safest radiotracer. In this extensive preclinical study, we evaluated the clinically applied PSMA-targeting small molecule inhibitors DOTA-PSMA-617 (PSMA-617) and DOTAGA-PSMA-I&T (PSMA-I&T) and the PSMA nanobody DOTA-JVZ-007 (JVZ-007) using PSMA-expressing cell lines, a unique set of PCa patient-derived xenografts (PDX) and healthy human tissues., Methods and Results: In vitro displacement studies on PSMA-expressing cells and cryosections of a PSMA-positive PDX revealed high and specific binding affinity for all three tracers labeled with lutetium-177 with IC 50 values in the nanomolar range. Interestingly, [ 177 Lu]Lu-JVZ-007 could not be displaced by PSMA-617 or PSMA-I&T, suggesting that this tracer targets an alternative binding site. Autoradiography assays on cryosections of human salivary and renal tissues revealed [ 177 Lu]Lu-PSMA-617 to have the lowest binding to these healthy organs compared with [ 177 Lu]Lu-PSMA-I&T. In vivo biodistribution assays confirmed the in vitro results with comparable tumor uptake of [ 177 Lu]Lu-PSMA-617 and [ 177 Lu]Lu-PSMA-I&T at all timepoints, resulting in induction of similar levels of DNA double-strand breaks in the tumors. However, [ 177 Lu]Lu-PSMA-I&T demonstrated approximately 40× higher renal uptake at 4 and 8 h post injection resulting in an unfavorable tumor-to-kidney ratio., Conclusion: [ 177 Lu]Lu-PSMA-617 has the most favorable biodistribution in mice as well as more favorable binding characteristics in vitro in PSMA-positive cells and human kidney and salivary gland specimens compared with [ 177 Lu]Lu-PSMA-I&T and [ 177 Lu]Lu-JVZ-007. Based on our preclinical evaluation, [ 177 Lu]Lu-PSMA-617 is the best performing tracer to be taken further into clinical evaluation for PSMA-targeted radiotherapeutic development although with careful evaluation of the tracer binding to PSMA-expressing organs.

Purpose: Many radioligands have been developed for the visualization of atherosclerosis by targeting inflammation. However, interpretation of in vivo signals is often limited to plaque identification. We evaluated binding of some promising radioligands in an in vitro approach in atherosclerotic plaques with different phenotypes., Methods: Tissue sections of carotid endarterectomy tissue were characterized as early plaque, fibro-calcific plaque, or phenotypically vulnerable plaque. In vitro binding assays for the radioligands [ 111 In]In-DOTATATE; [ 111 In]In-DOTA-JR11; [ 67 Ga]Ga-Pentixafor; [ 111 In]In-DANBIRT; and [ 111 In]In-EC0800 were conducted, the expression of the radioligand targets was assessed via immunohistochemistry. Radioligand binding and expression of radioligand targets was investigated and compared., Results: In sections characterized as vulnerable plaque, binding was highest for [ 111 In]In-EC0800; followed by [ 111 In]In-DANBIRT; [ 67 Ga]Ga-Pentixafor; [ 111 In]In-DOTA-JR11; and [ 111 In]In-DOTATATE (0.064 ± 0.036; 0.052 ± 0.029; 0.011 ± 0.003; 0.0066 ± 0.0021; 0.00064 ± 0.00014 %Added activity/mm 2 , respectively). Binding of [ 111 In]In-DANBIRT and [ 111 In]In-EC0800 was highest across plaque phenotypes, binding of [ 111 In]In-DOTA-JR11 and [ 67 Ga]Ga-Pentixafor differed most between plaque phenotypes. Binding of [ 111 In]In-DOTATATE was the lowest across plaque phenotypes. The areas positive for cells expressing the radioligand's target differed between plaque phenotypes for all targets, with lowest percentage area of expression in early plaque sections and highest in phenotypically vulnerable plaque sections., Conclusions: Radioligands targeting inflammatory cell markers showed different levels of binding in atherosclerotic plaques and among plaque phenotypes. Different radioligands might be used for plaque detection and discerning early from vulnerable plaque. [ 111 In]In-EC0800 and [ 111 In]In-DANBIRT appear most suitable for plaque detection, while [ 67 Ga]Ga-Pentixafor and [ 111 In]In-DOTA-JR11 might be best suited for differentiation between plaque phenotypes.

Purpose: Atherosclerotic plaque development and progression signifies a complex inflammatory disease mediated by a multitude of proinflammatory leukocyte subsets. Using single photon emission computed tomography (SPECT) coupled with computed tomography (CT), this study tested a new dual-isotope acquisition protocol to assess each radiotracer's capability to identify plaque phenotype and inflammation levels pertaining to leukocytes expressing leukocyte function-associated antigen-1 (LFA-1) and the leukocyte subset of proinflammatory macrophages expressing somatostatin receptor subtype-2 (SST 2 ). Individual radiotracer uptake was quantified and the presence of corresponding immunohistological cell markers was assessed., Methods: Human symptomatic carotid plaque segments were obtained from endarterectomy. Segments were incubated in dual-isotope radiotracers [ 111 In]In-DOTA-butylamino-NorBIRT ([ 111 In]In-Danbirt) and [ 99m Tc]Tc-[N 0-1 4 ,Asp 0 ,Tyr 3 ]-octreotate ([ 99m Tc]Tc-Demotate 2) before scanning with SPECT/CT. Plaque phenotype was classified as pathological intimal thickening, fibrous cap atheroma or fibrocalcific using histology sections based on distinct morphological characteristics. Plaque segments were subsequently immuno-stained with LFA-1 and SST 2 and quantified in terms of positive area fraction and compared against the corresponding SPECT images., Results: Focal uptake of co-localising dual-radiotracers identified the heterogeneous distribution of inflamed regions in the plaques which co-localised with positive immuno-stained regions of LFA-1 and SST 2 . [ 111 In]In-Danbirt and [ 99m Tc]Tc-Demotate 2 uptake demonstrated a significant positive correlation (r = 0.651; p = 0.001). Fibrous cap atheroma plaque phenotype correlated with the highest [ 111 In]In-Danbirt and [ 99m Tc]Tc-Demotate 2 uptake compared with fibrocalcific plaques and pathological intimal thickening phenotypes, in line with the immunohistological analyses., Conclusion: A dual-isotope acquisition protocol permits the imaging of multiple leukocyte subsets and the pro-inflammatory macrophages simultaneously in atherosclerotic plaque tissue. [ 111 In]In-Danbirt may have added value for assessing the total inflammation levels in atherosclerotic plaques in addition to classifying plaque phenotype.

Purpose: Current clinical measurements for tumor treatment efficiency rely often on changes in tumor volume measured as shrinkage by CT or MRI, which become apparent after multiple lines of treatment and pose a physical and psychological burden on the patient. Detection of therapy-induced cell death in the tumor can be a fast measure for treatment efficiency. However, there are no reliable clinical tools for detection of tumor necrosis. Previously, we studied the necrosis avidity of cyanine-based fluorescent dyes, which suffered long circulation times before tumor necrosis could be imaged due to low hydrophilicity. We now present the application of radiolabeled 800CW, a commercially available cyanine with high hydrophilicity, to image tumor necrosis in a mouse model., Procedures: We conjugated 800CW to DOTA via a PEG linker, for labeling with single-photon emission-computed tomography isotope indium-111, yielding [ 111 In]In-DOTA-PEG 4 -800CW. We then investigated specific [ 111 In]In-DOTA-PEG 4 -800CW uptake by dead cells in vitro, using both fluorescence and radioactivity as detection modalities. Finally, we investigated [ 111 In]In-DOTA-PEG 4 -800CW uptake into necrotic tumor regions of a 4T1 breast tumor model in mice., Results: We successfully prepared a precursor and developed a reliable procedure for labeling 800CW with indium-111. We detected specific [ 111 In]In-DOTA-PEG 4 -800CW uptake by dead cells, using both fluorescence and radioactivity. Albeit with a tumor uptake of only 0.37%ID/g at 6 h post injection, we were able to image tumor necrosis with a tumor to background ratio of 7:4. Fluorescence and radioactivity in cryosections from the dissected tumors were colocalized with tumor necrosis, confirmed by TUNEL staining., Conclusions: [ 111 In]In-DOTA-PEG 4 -800CW can be used to image tumor necrosis in vitro and in vivo. Further research will elucidate the application of [ 111 In]In-DOTA-PEG 4 -800CW or other radiolabeled hydrophilic cyanines for the detection of necrosis caused by chemotherapy or other anti-cancer therapies. This can provide valuable prognostic information in treatment of solid tumors.