Lacrimal Glands May Represent Organs at Risk for Radionuclide Therapy of Prostate Cancer with [177Lu]DKFZ-PSMA-617

A review of 177Lu dosimetry workflows: how to reduce the imaging workloads?

177 Lu radiopharmaceutical therapy is a standardized systemic treatment, with a typical dose of 7.4 GBq per injection, but its response varies from patient to patient. Dosimetry provides the opportunity to personalize treatment, but it requires multiple post-injection images to monitor the radiopharmaceutical's biodistribution over time. This imposes an additional imaging burden on centers with limited resources. This review explores methods to lessen this burden by optimizing acquisition types and minimizing the number and duration of imaging sessions. After summarizing the different steps of dosimetry and providing examples of dosimetric workflows for177 Lu -DOTATATE and177 Lu -PSMA, we examine dosimetric workflows based on a reduced number of acquisitions, or even just one. We provide a non-exhaustive description of simplified methods and their assumptions, as well as their limitations. Next, we detail the specificities of each normal tissue and tumors, before reviewing dose-response relationships in the literature. In conclusion, we will discuss the current limitations of dosimetric workflows and propose avenues for improvement.

Comparative post-therapeutic dosimetry between 2D planar-based and hybrid-based methods for personalized Lu-177 treatment

PURPOSE

This study aims to compare the calculated absorbed dose in target organs and tumors obtained using the different imaging protocols and the calculation methodologies implemented by HERMES HybridViewer dosimetry software for 177Lu-PSMA I&T and 177Lu-DOTATATE therapy.

METHODS

Multiple time-point whole-body planar images and one SPECT/CT image were acquired from 18 patients including 177Lu-PSMA I&T (13 patients) and 177Lu-DOTATATE treatment (5 patients) after administration of 3.80-8.58 GBq injected activity. The regions of interest were drawn in the whole body, kidneys, liver, urinary bladder, salivary glands, and tumors to determine the time-integrated activity (TIA) in source organs. Absorbed doses in target organs were calculated according to the Medical Internal Radiation Dose (MIRD) scheme using the HERMES HybridViewer dosimetry integrated with OLINDA/EXM V.2.1 that utilizes the non-uniform rational B-splines (NURBS) for computational digital phantom.

RESULTS

The planar-based dosimetry showed a higher dose per injected activity compared to the hybrid-based dosimetry, primarily due to organ overlap. The highest difference in absorbed dose between the imaging scenarios was observed in the spleen with a variation of up to 51.6%, while the difference for other target organs and tumors was less than 40%.

CONCLUSION

The dosimetry calculation derived from the 2D planar-based method consistently demonstrates a significantly higher absorbed dose in organs and tumors compared with the hybrid-based method. However, the hybrid method outperforms the planar method in terms of tumor visualization and overlap-free organ delineation.

Dosimetry of [177Lu]Lu-PSMA-Targeted Radiopharmaceutical Therapies in Patients with Prostate Cancer: A Comparative Systematic Review and Metaanalysis

Novel theranostic approaches using radiopharmaceuticals targeting prostate-specific membrane antigen (PSMA) have emerged for treating metastatic castration-resistant prostate cancer. The physical properties and commercial availability of 177Lu make it one of the most used radionuclides for radiopharmaceutical therapy (RPT). In this literature review, we aimed at comparing the dosimetry of the most used [177Lu]Lu-PSMA RPT compounds. Methods: This was a systematic review and metaanalysis of [177Lu]Lu-PSMA RPT (617, I&T, and J591) dosimetry in patients with prostate cancer. Absorbed doses in Gy/GBq for each organ at risk (kidney, parotid and submandibular glands, bone marrow, liver, and lacrimal glands) and for tumor lesions (bone and nonbone lesions) were extracted from included articles. These were used to estimate the pooled average absorbed dose of each agent in Gy/GBq and in Gy/cycle, normalized to the injected activity (per cycle) used in the VISION (7.4 GBq), SPLASH (6.8 GBq), and PROSTACT trials (5.8 GBq). Results: Twenty-nine published articles comprising 535 patients were included in the metaanalysis. The pooled doses (weighted average across studies) of [177Lu]Lu-PSMA-617 and [177Lu]Lu-PSMA-I&T were 4.04 Gy/GBq (17 studies, 297 patients) and 4.70 Gy/GBq (10 studies, 153 patients) for the kidney (P = 0.10), 5.85 Gy/GBq (14 studies, 216 patients) and 2.62 Gy/GBq (5 studies, 86 patients) for the parotids (P < 0.01), 5.15 Gy/GBq (5 studies, 81 patients) and 4.35 Gy/GBq (1 study, 18 patients) for the submandibular glands (P = 0.56), 11.03 Gy/GBq (6 studies, 121 patients) and 19.23 Gy/GBq (3 studies, 53 patients) for the lacrimal glands (P = 0.20), 0.24 Gy/GBq (12 studies, 183 patients) and 0.19 Gy/GBq (4 studies, 68 patients) for the bone marrow (P = 0.31), and 1.11 Gy/GBq (9 studies, 154 patients) and 0.56 Gy/GBq (4 studies, 56 patients) for the liver (P = 0.05), respectively. Average tumor doses tended to be higher for [177Lu]Lu-PSMA-617 than for [177Lu]Lu-PSMA-I&T in soft tissue tumor lesions (4.19 vs. 2.94 Gy/GBq; P = 0.26). Dosimetry data of [177Lu]Lu-J591 were limited to one published study of 35 patients with reported absorbed doses of 1.41, 0.32, and 2.10 Gy/GBq to the kidney, bone marrow, and liver, respectively. Conclusion: In this metaanalysis, there was no significant difference in absorbed dose between [177Lu]Lu-PSMA-I&T and [177Lu]Lu-PSMA-617. There was a possible trend toward a higher kidney dose with [177Lu]Lu-PSMA-I&T and a higher tumor lesion dose with [177Lu]Lu-PSMA-617. It remains unknown whether this finding has any clinical impact. The dosimetry methodologies were strikingly heterogeneous among studies, emphasizing the need for standardization.

[The lacrimal apparatus as an organ at risk during radionuclide therapy]

Within the framework of the article, the authors analyzed the available information about the damage to the lacrimal apparatus during radionuclide therapy. In focus of article lesions of the lacrimal production system, the main and accessory lacrimal glands, as well as lacrimal drainage are considered. It was found that damage to the lacrimal apparatus is characteristic of 131I therapy for thyroid cancer, as well as for radioligand therapy using anti-PSMA antibodies labeled with 177Lu and 225Ac. 177Lu-PSMA and 225Ac-PSMA may damage the lacrimal gland with the formation of a clinically pronounced "dry eye syndrome". The pathogenesis of such lesions is associated with the accumulation of a radioisotope in the tissues of the lacrimal apparatus, while during therapy with 131I, accumulation is realized due to the expression of the sodium-iodine symporter in the nasolacrimal duct, and during therapy with 177Lu-PSMA and 225Ac-PSMA, the radiobiological effect is realized in connection with the expression PSMA by lacrimal tissue. An analysis of the available sources showed that to date there are no results of systematic studies on the problem, there is a lack of knowledge regarding the individual risks of developing these complications, methods for their prevention that have proven effectiveness have not been developed, and the treatment methods used, having relatively low efficiency, are not specialized. The authors concluded that the strengthening of interdisciplinary interaction, as well as the organization verification methodology and correct studies, can contribute to solving problems related to the study of the complications under consideration.

Renal and Multiorgan Safety of 177Lu-PSMA-617 in Patients with Metastatic Castration-Resistant Prostate Cancer in the VISION Dosimetry Substudy

In the VISION trial, [177Lu]Lu-PSMA-617 (177Lu-PSMA-617) plus protocol-permitted standard of care significantly improved overall survival and radiographic progression-free survival compared with standard of care alone in patients with prostate-specific membrane antigen-positive metastatic castration-resistant prostate cancer. This VISION dosimetry substudy quantified absorbed doses of 177Lu-PSMA-617 in the kidneys and other organs. Methods: Participants were a separate cohort of 30 nonrandomized patients receiving standard of care plus 177Lu-PSMA-617 at 7.4 GBq per cycle for up to 6 cycles. Blood samples, whole-body conjugate planar image scintigraphy, and abdominal SPECT/CT images were collected. SPECT/CT images were collected at 2, 24, 48, and 168 h after administration in cycle 1 and at a single time point 48 h after administration in cycles 2-6. Outcomes were absorbed dose per unit activity per cycle and cumulative absorbed dose over all cycles. Cumulative absorbed doses were predicted by extrapolation from cycle 1, and calculation of observed values was based on measurements of cycle 1 and cycles 2-6. Safety was also assessed. Results: Mean (±SD) absorbed doses per cycle in the kidneys were 0.43 ± 0.16 Gy/GBq in cycle 1 and 0.44 ± 0.21 Gy/GBq in cycles 2-6. The observed and predicted 6-cycle cumulative absorbed doses in the kidneys were 15 ± 6 and 19 ± 7 Gy, respectively. Observed and predicted cumulative absorbed doses were similar in other at-risk organs. Safety findings were consistent with those in the VISION study; no patients experienced renal treatment-emergent adverse events of a grade higher than 3. Conclusion: The renal cumulative absorbed 177Lu-PSMA-617 dose was below the established limit. 177Lu-PSMA-617 had a good overall safety profile, and low renal radiotoxicity was not a safety concern. Cumulative absorbed doses in at-risk organs over multiple cycles can be predicted by extrapolation from cycle 1 data in patients with metastatic castration-resistant prostate cancer receiving 177Lu-PSMA-617.

Prediction of Response to 177Lu-PSMA Therapy Based on Tumor-to-Kidney Ratio on Pretherapeutic PSMA PET/CT and Posttherapeutic Tumor-Dose Evaluation in mCRPC

The aim of this study was to analyze the absorbed dose of 177Lu-PSMA in osseous versus lymphatic metastases in patients with metastatic castration-resistant prostate cancer across therapy cycles and to relate those data to therapeutic success. In addition, pretherapeutic prostate-specific membrane antigen (PSMA) PET/CT was evaluated for its ability to predict response behavior. Methods: The study comprised 30 patients with metastatic castration-resistant prostate cancer, each receiving at least 3 cycles of 177Lu-PSMA therapy. Prostate-specific antigen (PSA) values between baseline and 6 wk after the third therapy cycle were used to classify the patients as responders (PSA decline ≥ 50%) or nonresponders (unchanged or increasing PSA level). Quantitative SPECT/CT images were acquired 24, 48, and 168 h after application of 177Lu-PSMA. The absorbed dose for tumor lesions was calculated with dosimetry software. From the pretherapeutic PET/CT scan, the tumor-to-kidney uptake ratio was determined for different SUVs. Results: Regardless of patient response, the kidneys received a mean dose of 0.55 ± 0.20 Gy/GBq per cycle. In the first therapy cycle, the lymph node lesions received a mean dose of 3.73 ± 1.65 Gy/GBq in responders and 1.86 ± 1.25 Gy/GBq in nonresponders (P < 0.01). For bone lesions, the respective mean doses were 3.47 ± 2.00 Gy/GBq and 1.48 ± 0.95 Gy/GBq (P < 0.01). When successive therapy cycles were compared, the mean dose was found to have been reduced from the first to the second cycle by 27% for lymph nodes and by 33% for bone lesions. A significant difference (P < 0.01) in the ratio of lymph node and bone lesion uptake to kidney uptake between responders and nonresponders could be deduced from the pretherapeutic PET/CT scan. Conclusion: Significantly higher doses were achieved for lymph node and bone lesions in responders. The highest absorbed dose, for both lymphatic and osseous lesions, was achieved in the first cycle, decreasing in the second therapy cycle thereafter despite unchanged therapy activities. It may be possible to estimate the response to therapy from the ratio of tumor uptake to kidney uptake obtained from the pretherapeutic PSMA PET/CT scans.

Joint EANM/SNMMI procedure guideline for the use of 177Lu-labeled PSMA-targeted radioligand-therapy (177Lu-PSMA-RLT)

Prostate-specific membrane antigen (PSMA) is expressed by the majority of clinically significant prostate adenocarcinomas, and patients with target-positive disease can easily be identified by PSMA PET imaging. Promising results with PSMA-targeted radiopharmaceutical therapy have already been obtained in early-phase studies using various combinations of targeting molecules and radiolabels. Definitive evidence of the safety and efficacy of [177Lu]Lu-PSMA-617 in combination with standard-of-care has been demonstrated in patients with metastatic castration-resistant prostate cancer, whose disease had progressed after or during at least one taxane regimen and at least one novel androgen-axis drug. Preliminary data suggest that 177Lu-PSMA-radioligand therapy (RLT) also has high potential in additional clinical situations. Hence, the radiopharmaceuticals [177Lu]Lu-PSMA-617 and [177Lu]Lu-PSMA-I&T are currently being evaluated in ongoing phase 3 trials. The purpose of this guideline is to assist nuclear medicine personnel, to select patients with highest potential to benefit from 177Lu-PSMA-RLT, to perform the procedure in accordance with current best practice, and to prepare for possible side effects and their clinical management. We also provide expert advice, to identify those clinical situations which may justify the off-label use of [177Lu]Lu-PSMA-617 or other emerging ligands on an individual patient basis.

The "Ins and Outs" of Prostate Specific Membrane Antigen (PSMA) as Specific Target in Prostate Cancer Therapy

Prostate-specific membrane antigen (PSMA) is expressed in epithelial cells of the prostate gland and is strongly upregulated in prostatic adenocarcinoma, with elevated expression correlating with metastasis, progression, and androgen independence. Because of its specificity, PSMA is a major target of prostate cancer therapy; however, detectable levels of PSMA are also found in other tissues, especially in salivary glands and kidney, generating bystander damage of these tissues. Antibody target therapy has been used with relative success in reducing tumor growth and prostate specific antigen (PSA) levels. However, since antibodies are highly stable in plasma, they have prolonged time in circulation and accumulate in organs with an affinity for antibodies such as bone marrow. For that reason, a second generation of PSMA targeted therapeutic agents has been developed. Small molecules and minibodies have had promising clinical trial results, but concerns about their specificity had arisen with side effects due to accumulation in salivary glands and kidneys. Herein we study the specificity of small molecules and minibodies that are currently being clinically tested. We observed a high affinity of these molecules for PSMA in prostate, kidney and salivary gland, suggesting that their effect is not prostate specific. The search for specific prostate target agents must continue so as to optimally treat patients with prostate cancer, while minimizing deleterious effects in other PSMA expressing tissues.

Pharmacological Optimization of PSMA-Based Radioligand Therapy

Prostate cancer (PCa) is the most common malignancy in men of middle and older age. The standard treatment strategy for PCa ranges from active surveillance in low-grade, localized PCa to radical prostatectomy, external beam radiation therapy, hormonal treatment and chemotherapy. Recently, the use of prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) for metastatic castration-resistant PCa has been approved. PSMA is predominantly, but not exclusively, expressed on PCa cells. Because of its high expression in PCa, PSMA is a promising target for diagnostics and therapy. To understand the currently used RLT, knowledge about pharmacokinetics (PK) and pharmacodynamics (PD) of the PSMA ligand and the PSMA protein itself is crucial. PK and PD properties of the ligand and its target determine the duration and extent of the effect. Knowledge on the concentration-time profile, the target affinity and target abundance may help to predict the effect of RLT. Increased specific binding of radioligands to PSMA on PCa cells may be associated with better treatment response, where nonspecific binding may increase the risk of toxicity in healthy organs. Optimization of the radioligand, as well as synergistic effects of concomitant agents and an improved dosing strategy, may lead to more individualized treatment and better overall survival.

Lutetium Lu 177 vipivotide tetraxetan for metastatic castration-resistant prostate cancer

INTRODUCTION

¹⁷⁷Lu-vipivotide tetraxetan is a radiopharmaceutical that selectively targets prostate-specific membrane antigen (PSMA) and delivers beta-radiations to kill prostate cancer cells.

AREAS COVERED

Extensive experience outside the United States as well as randomized phase II and phase III data demonstrate that ¹⁷⁷Lu-vipivotide tetraxetan is a safe, generally well tolerated, and effective therapy for men with mCRPC. ¹⁷⁷Lu-vipivotide tetraxetan was approved by the FDA in March 2022 for the treatment of PSMA-positive metastatic castration-resistant prostate cancer (mCRPC) after androgen receptor pathway inhibition and taxane-based chemotherapy based on the results of the VISION trial.

EXPERT OPINION

This review discusses the development and studies leading to the approval of ¹⁷⁷Lu-vipivotide tetraxetan. In all, ¹⁷⁷Lu-vipivotide tetraxetan is an exciting new tool in the arsenal for men with mCRPC after novel androgen pathway inhibitors and at least one taxane chemotherapy. Optimal selection of patients, sequencing of ¹⁷⁷Lu-vipivotide tetraxetan with the other agents available to treat mCRPC, and the use of dosimetry are current areas of interest with great potential and opportunities for further individual patient optimization using the tools of theranostics.

Quantitative SPECT/CT imaging of lead-212: a phantom study

Background

Lead-212 (²¹²Pb) is a promising radionuclide for targeted therapy, as it decays to α-particle emitter bismuth-212 (²¹²Bi) via β-particle emission. This extends the problematic short half-life of ²¹²Bi. In preparation for upcoming clinical trials with ²¹²Pb, the feasibility of quantitative single photon-emission computed tomography/computed tomography (SPECT/CT) imaging of ²¹²Pb was studied, with the purpose to explore the possibility of individualised patient dosimetric estimation.

Results

Both acquisition parameters (combining two different energy windows and two different collimators) and iterative reconstruction parameters (varying the iterations x subsets between 10 × 1, 15 × 1, 30 × 1, 30 × 2, 30 × 3, 30 × 4, and 30 × 30) were investigated to evaluate visual quality and quantitative uncertainties based on phantom images. Calibration factors were determined using a homogeneous phantom and were stable when the total activity imaged exceeded 1 MBq for all the imaging protocols studied, but they increased sharply as the activity decayed below 1 MBq. Both a 20% window centred on 239 keV and a 40% window on 79 keV, with dual scatter windows of 5% and 20%, respectively, could be used. Visual quality at the lowest activity concentrations was improved with the High Energy collimator and the 79 keV energy window. Fractional uncertainty in the activity quantitation, including uncertainties from calibration factors and small volume effects, in spheres of 2.6 ml in the NEMA phantom was 16–21% for all protocols with the 30 × 4 filtered reconstruction except the High Energy collimator with the 239 keV energy window. Quantitative analysis was possible both with and without filters, but the visual quality of the images improved with a filter.

Conclusions

Only minor differences were observed between the imaging protocols which were all determined suitable for quantitative imaging of ²¹²Pb. As uncertainties generally decreased with increasing iterative updates in the reconstruction and recovery curves did not converge with few iterations, a high number of reconstruction updates are recommended for quantitative imaging.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-022-00481-z.

Individualization of Radionuclide Therapies: Challenges and Prospects

Simple Summary

Currently, patient-specific treatment plans and dosimetry calculations are not routinely performed for radionuclide therapies. In external beam radiotherapy, it is quite the opposite. As a result, a small fraction of patients receives optimal radioactivity. This conservative approach provides “radiation safety” to healthy tissues but delivers a lower than indicated absorbed dose to the tumors, resulting in a lower response rate and a higher disease relapse rate. Evidence shows that better and more predictable outcomes can be achieved with patient-individualized dose assessment. Therefore, the incorporation of individual planning into radionuclide therapies is a high priority for nuclear medicine physicians and medical physicists alike. Internal dosimetry is used in tumor therapy to optimize the absorbed dose to the target tissue. The main reasons for the difficulties in incorporating patients’ internal dosimetry into routine clinical practice are discussed. The article presents the prospects for the routine implementation of personalized radionuclide therapies.

Abstract

The article presents the problems of clinical implementation of personalized radioisotope therapy. The use of radioactive drugs in the treatment of malignant and benign diseases is rapidly expanding. Currently, in the majority of nuclear medicine departments worldwide, patients receive standard activities of therapeutic radiopharmaceuticals. Intensively conducted clinical trials constantly provide more evidence of a close relationship between the dose of radiopharmaceutical absorbed in pathological tissues and the therapeutic effect of radioisotope therapy. Due to the lack of individual internal dosimetry (based on the quantitative analysis of a series of diagnostic images) before or during the treatment, only a small fraction of patients receives optimal radioactivity. The vast majority of patients receive too-low doses of ionizing radiation to the target tissues. This conservative approach provides “radiation safety” to healthy tissues, but also delivers lower radiopharmaceutical activity to the neoplastic tissue, resulting in a low level of response and a higher relapse rate. The article presents information on the currently used radionuclides in individual radioisotope therapies and on radionuclides newly introduced to the therapeutic market. It discusses the causes of difficulties with the implementation of individualized radioisotope therapies as well as possible changes in the current clinical situation.

Pretherapeutic Comparative Dosimetry of 177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T in Patients with Metastatic Castration-Resistant Prostate Cancer

Radiohybrid prostate-specific membrane antigen (rhPSMA) ligands allow for labeling with 18F and radiometals for endoradiotherapy. rhPSMA-7.3 has been designated as a lead compound with promising preclinical data for 177Lu-rhPSMA-7.3, which has shown higher tumor uptake than 177Lu-PSMA I&T. In this retrospective analysis, we compared pretherapeutic clinical dosimetry data of both PSMA ligands. Methods: Six patients with metastatic castration-resistant prostate cancer underwent both 177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T pretherapeutic dosimetry. Whole-body scintigraphy was performed at 1 h, 4 h, 24 h, 48 h, and 7 d after injection. Regions of interest covering the whole body, organs, bone marrow, and tumor lesions were drawn for each patient. Absorbed doses for individual patients and pretherapeutic applications were calculated using OLINDA/EXM. To facilitate the comparison of both ligands, we introduced the therapeutic index (TI), defined as the ratio of mean pretherapeutic doses to tumor lesions over relevant organs at risk. Results: Mean whole-body pretherapeutic effective doses for 177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T were 0.12 ± 0.07 and 0.05 ± 0.03 Sv/GBq, respectively. Mean absorbed organ doses for 177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T were, for example, 1.65 ± 0.28 and 0.73 ± 0.18 Gy/GBq for the kidneys, 0.19 ± 0.09 and 0.07 ± 0.03 Gy/GBq for the liver, 2.35 ± 0.78 and 0.80 ± 0.41 Gy/GBq for the parotid gland, and 0.67 ± 0.62 and 0.30 ± 0.27 Gy/GBq for the bone marrow, respectively. Tumor lesions received mean absorbed doses of 177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T of 6.44 ± 6.44 and 2.64 ± 2.24 Gy/GBq, respectively. The mean TIs for the kidneys were 3.7 ± 2.2 and 3.6 ± 2.2 for 177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T, respectively, and those for the bone marrow were 15.2 ± 10.2 and 15.1 ± 10.2 for 177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T, respectively. Conclusion: Pretherapeutic clinical dosimetry confirmed preclinical results of mean absorbed doses for tumors that were 2-3 times higher for 177Lu-rhPSMA-7.3 than for 177Lu-PSMA I&T. Absorbed doses to normal organs also tended to be higher for 177Lu-rhPSMA-7.3, resulting overall in similar average TIs for both radiopharmaceuticals with considerable interpatient variability. 177Lu-rhPSMA-7.3 has promise for a therapeutic efficacy similar to that of 177Lu-PSMA I&T at smaller amounts of injected activity, simplifying radiation safety measurements (especially for large patient numbers or dose escalation regimens).

Dosimetry in Lu-177-PSMA-617 prostate-specific membrane antigen targeted radioligand therapy: a systematic review

BACKGROUND

177Lu-prostate-specific membrane antigen (PSMA) gained popularity as a choice of agent in the treatment of patients with advanced prostate cancer or metastatic castration-resistant stage of prostate carcinoma (mCRPC) diseases. However, this treatment may cause fatal effects, probably due to unintended irradiation of normal organs. We performed an extensive systematic review to assess the organs at risk and the absorbed dose received by tumor lesions in 177Lu-PSMA therapy.

DESIGN

In this review, published peer-reviewed articles that cover clinical dosimetry in patients following peptide radionuclide ligand therapy using 177Lu-PSMA have been included. Two senior researchers independently checked the articles for inclusion. A systematic search in the database was made using PubMed, Publons and DOAJ. All selected articles were categorized into three groups: (1) clinical studies with the technical description of dosimetry in 177Lu-PSMA therapy (2) organ dosimetry in 177Lu-PSMA therapy or (3) tumor dosimetry in 177Lu-PSMA therapy.

RESULT

In total, 182 citations were identified on PSMA therapy and 17 original articles on 177Lu-PSMA dosimetry were recognized as eligible for review. The median absorbed dose per unit of administered activity for kidneys, salivary, liver, spleen, lacrimal and bone marrow was 0.55, 0.81, 0.1, 0.1, 2.26 and 0.03 Gy/GBq, respectively. The median absorbed dose per unit of activity for tumor lesions was found in a range of 2.71-10.94 Gy/GBq.

CONCLUSION

177Lu-PSMA systemic radiation therapy (SRT) is a well-tolerated and reliable treatment option against the management of the mCRPC stage of prostate carcinoma. Lacrimal glands and salivary glands are the major critical organs in 177Lu-PSMA SRT. Besides, tumors receive 3-6 times higher absorbed doses compared to organs at risk.

EANM dosimetry committee recommendations for dosimetry of 177Lu-labelled somatostatin-receptor- and PSMA-targeting ligands

The purpose of the EANM Dosimetry Committee is to provide recommendations and guidance to scientists and clinicians on patient-specific dosimetry. Radiopharmaceuticals labelled with lutetium-177 (177Lu) are increasingly used for therapeutic applications, in particular for the treatment of metastatic neuroendocrine tumours using ligands for somatostatin receptors and prostate adenocarcinoma with small-molecule PSMA-targeting ligands. This paper provides an overview of reported dosimetry data for these therapies and summarises current knowledge about radiation-induced side effects on normal tissues and dose-effect relationships for tumours. Dosimetry methods and data are summarised for kidneys, bone marrow, salivary glands, lacrimal glands, pituitary glands, tumours, and the skin in case of radiopharmaceutical extravasation. Where applicable, taking into account the present status of the field and recent evidence in the literature, guidance is provided. The purpose of these recommendations is to encourage the practice of patient-specific dosimetry in therapy with 177Lu-labelled compounds. The proposed methods should be within the scope of centres offering therapy with 177Lu-labelled ligands for somatostatin receptors or small-molecule PSMA.

Radiation Dosimetry in 177Lu-PSMA-617 Therapy

Radionuclide therapy using the small molecule PSMA bound to the beta-emitting radionuclide, Lutetium-177 (¹⁷⁷Lu-PSMA) has demonstrated efficacy and survival benefit castrate resistant metastatic disease and represents a novel new line of therapy. Whilst dosimetry was critical for early development, it was not incorporated into either the TheraP or VISION randomized studies, highlighting the difficulty of adopting dosimetry in routine clinical practice. Accumulated clinical experience has also shown that the common (and generally low grade) toxicities such as nausea, xerostomia, and cytopenias are not readily predicted on the basis of dosimetry estimates. The majority of dosimetry and clinical literature deals with the radiopharmaceutical ¹⁷⁷Lu-PSMA-617 which displays relatively consistent patterns of retention among normal tissues and high specificity for metastatic prostate cancer phenotypes. Population dosimetry incorporating estimates to the kidneys, salivary glands, and bone marrow have been widely reported the typical range of doses is becoming well established. There is growing interest on tumor dosimetry in ¹⁷⁷Lu-PSMA-617 therapy as an overall modest side-effect profile from primary organ retention has been observed. A focus away from normal organ dosimetry to whole body tumor dosimetry may enable early prediction of treatment failure. Given the safety of ¹⁷⁷Lu-PSMA there is also potential to escalate administered radioactivity to further improve outcomes. Importantly, the variability of uptake between individuals, both to tumor and normal organs, has also been highlighted which provides some rationale for the utility of personalized radiation analysis to optimize treatment based on potential toxicity thresholds or tumor control. Methods to perform dosimetry using serial post treatment imaging may incorporate planar, 3D SPECT, or hybrid datasets. Reliable measurements may be obtained through either method, however, continued developments in computational analysis are better suited to fully 3D imaging; particularly in conjunction with volumetric CT to assist with alignment and contouring. Dose analysis over sequential treatment cycles is vital to understand the radiobiology of these treatments which is unique compared to external beam therapy due to dose rate, fractionation scheme, and potential for intratumoral nonuniformity.

Patient-Specific Dosimetry in Radioligand Therapy (RLT) for Metastatic Prostate Cancer Using 177Lu-DKFZ-PSMA-617

Purpose

¹⁷⁷Lu-DKFZ-PSMA-617 is a promising treatment for patients with metastatic prostate cancer. Specific dosimetry for each patient is an important factor in planning the patient's treatment process. This study aimed to perform an image-based absorbed dose calculation for the treatment of metastatic prostate cancer with ¹⁷⁷Lu-DKFZ-PSMA-617.

Methods

The individualized patient dosimetry calculations were based on whole-body planar scintigraphy images acquired in 10 patients with a mean age of 71.4 ± 6.07 years (range 63-85 years) at approximately 0-2 h, 4-6 h, 18-24 h, and 36-48 h after administration of the mean 6253 ± 826.4 MBq (range 5500-7400 MBq) of ¹⁷⁷Lu-DKFZ-PSMA-617. Time-activity curves were generated for various organs. For count conversion to activities, calibration factors were calculated. Finally, the absorbed dose for an individual cycle was calculated using IDIAC-DOSE 2.1 software.

Results

On average, the calculated absorbed dose for the kidneys and salivary glands were 0.46 ± 0.09 mGy/MBq and 0.62 ± 0.07 mGy/MBq, respectively.

Conclusions

Based on the results, the¹⁷⁷Lu-PSMA-617 therapy is a safe method for the treatment of castration-resistant prostate cancer patients. Large inter-individual variations in organ dose were found, demonstrating the need for patient-specific dosimetry and treatment planning.

Patient dosimetry of 177Lu-PSMA I&T in metastatic prostate cancer treatment: the experience in Thailand

PURPOSE

This study aimed to determine the radiation dosimetry for 177Lu-PSMA imaging and therapy (I&T) in Thai patients who were treated for metastatic prostate cancer.

METHODS

Whole-body planar images acquired at immediately, 4 and 24 h after 177Lu-PSMA I&T injection (range 4.44-8.51 GBq) were collected from 12 treatment cycles of 8 prostate cancer patients. Region of interests (ROIs) were manually contoured on the whole-body, liver, spleen, urinary bladder, lacrimal glands, parotid, and submandibular glands to determine time-integrated activity (TIA) in source organs and fitted time-activity curves using mono-exponential extrapolation. The S values calculated utilizing non-uniform rational B-splines (NURBS) computational phantoms were extracted from the OLINDA/EXM v. 2.0 to calculate the absorbed dose coefficient in target organs according to the Medical Internal Radiation Dose (MIRD) scheme. The absorbed doses to bone marrow were estimated using the planar two-compartment image-based method by separating the high-uptake and low-uptake compartment. The spherical model was used to calculate the lacrimal gland absorbed doses.

RESULTS

Mean absorbed dose coefficients to the kidneys, bone marrow, liver, urinary bladder, spleen, lacrimal glands, parotid, and submandibular glands were 0.81 ± 0.24, 0.02 ± 0.01, 0.13 ± 0.10, 0.27 ± 0.25, 0.16 ± 0.07, 3.62 ± 1.78, 0.21 ± 0.14, and 0.09 ± 0.07 Gy/GBq, respectively. Dose constraints for the kidneys (23 Gy) and bone marrow (2 Gy) were not reached in any patients. The absorbed dose in lacrimal glands calculated by the NURBS computational phantoms was slightly lower than the calculation based on the Cristy-Eckerman computational phantoms using OLINDA/EXM v. 1.0 by 6.37 ± 0.14%.

CONCLUSION

Dosimetry results in this study suggested that 177Lu-PSMA I&T treatment with higher activities and more cycles is possible without the risk of damaging normal organs in prostate cancer patients.

Streamlined Schemes for Dosimetry of 177Lu-Labeled PSMA Targeting Radioligands in Therapy of Prostate Cancer

Simple Summary

In patients with progressive metastasized castration-resistance prostate cancer PSMA radioligand therapies have shown promising results regarding clinical safety and efficacy. Dosimetry is mandatory due to legal regulations and also required for the estimation of doses to organs at risk allowing for individual tailoring of treatment in PSMA-RLT. Due to those factors and the often poor health status of patients which restricts intense dosimetric imaging protocols, there is a clear need for simplified dosimetric approaches in mCRPC patients treated with [¹⁷⁷Lu]Lu-PSMA-617. In this study, we evaluated different dosimetric methodologies and found that a streamlined dosimetric approach is feasible and valid. This approach is based on single time-point imaging at 48 h p.i. in cycle 2 to 6 taking into account kinetic results of a full dosimetric scheme performed only in cycle1. These results might have a relevant impact on patients handling regarding dosimetry during [¹⁷⁷Lu]Lu-PSMA-617 radioligand therapy.

Abstract

(Background) Aim of this retrospective analysis was to investigate in mCRPC patients treated with [¹⁷⁷Lu]Lu-PSMA-617 whether the absorbed dose (AD) in organs at risk (OAR, i.e., kidneys and parotid glands) can be calculated using simplified methodologies with sufficient accuracy. For this calculation, results and kinetics of the first therapy cycle were used. (Methods) 46 patients treated with 2 to 6 cycles of [¹⁷⁷Lu]Lu-PSMA-617 were included. As reference (current clinical standard) full dosimetry of the OAR based on quantitative imaging (whole body scintigraphy and quantitative SPECT/CT at 2, 24, 48 and 72 h p.i.) for every cycle was used. Alternatively, two dosimetry schemes, simplified in terms of image acquisition and dose calculation, were established, both assuming nearly unchanged kinetics of the radiopharmaceutical for subsequent cycles. (Results) In general, for both OAR the simplified methods provided results that were consistent with the dosimetric reference method, both per cycle and in terms of cumulative AD. Best results were obtained when imaging was performed at 48 h p.i. in each of the subsequent cycles. However, both simplified methods tended to underestimate the cumulative AD. (Conclusion) Simplified dosimetry schemes are feasible to tailor multi-cycle [¹⁷⁷Lu]Lu-PSMA-targeted therapies.

Kidney doses in 177Lu-based radioligand therapy in prostate cancer: Is dose estimation based on reduced dosimetry measurements feasible?

The radiation dose to the kidneys should be monitored in prostate cancer patients treated with radioligand therapy (RLT) targeting the prostate-specific membrane antigen (PSMA). We analyzed whether pretherapeutic kidney function is predictive of subsequent kidney dose and to what extend the cumulative kidney dose after multiple therapy cycles at the end of treatment can be predicted from a dosimetry based on the first cycle. Methods: Data of 59 patients treated with at least 2 cycles of ¹⁷⁷Lu-PSMA-617 (PSMA-RLT) were analyzed. Treatment (median: 6 GBq/cycle) was performed at 6-8 week intervals, accompanied by voxel-based 3D-dosimetry (measured kidney dose) with SPECT/CT on each of days 0-3 and once during days 6-9. Pretherapeutic kidney function (eGFR, MAG3-clearance) was correlated to the kidney doses. Cumulative kidney doses at the end of treatment were compared to a dose estimation based on the population-based mean kidney dose, individual first cycle kidney dose and mean kidney doses of cycles 1, 3 and 5 per administered activity. Results: A total of 176 PSMA-RLT cycles were performed with a median of 3 cycles per patient. The average kidney dose per administered activity of all 176 cycles was 0.67 ± 0.24 Gy/GBq (range 0.21 - 1.60). MAG3-clearance and eGFR were no reliable predictors of subsequent absorbed kidney dose and showed only small effect sizes (R² = 0.080 and 0.014, P = 0.039 and 0.375). All simplified estimations of cumulative kidney dose correlated significantly (P < 0.001) with measured kidney doses: Estimations based on the individual first-cycle dose were more accurate than the use of the population-based average kidney dose (R² = 0.853 vs. R² = 0.560). Dose estimation was best when the doses of cycles 3 and 5 were included as well (R² = 0.960). Conclusion: Pretherapeutic renal function was not predictive for subsequent kidney dose during therapy. Extrapolation of individual data from dosimetry of the first cycle was highly predictive for the cumulative kidney dose at the end of treatment. This is further improved by the integration of dose information from every other cycle. In any case, because of a high interindividual variance, an individual dosimetry is advisable.

177Lu-PSMA Therapy in Metastatic Castration-Resistant Prostate Cancer

The aim of this narrative review is to evaluate the current status of ¹⁷⁷Lu-PSMA (prostate specific membrane antigen) therapy for metastatic castration-resistant prostate cancer (mCRPC) in the light of the current literature. We also addressed patient preparation, therapy administration and side effect profiles. ¹⁷⁷Lu-PSMA therapy efficacy was assessed by using prospective trials, meta-analyses and major retrospective trials. Predictors of efficacy were also mentioned. Although there are some different approaches regarding the use of ¹⁷⁷Lu-PSMA therapy in different countries, this type of therapy is generally safe, with a low toxicity profile. From the oncological point of view, a PSA (prostate specific antigen) decline of ≥50% was seen in 10.6-69% of patients with mCRPC; whereas progression-free survival (PFS) was reported to be 3-13.7 months in different studies. Consequently, ¹⁷⁷Lu-PSMA therapy is a promising treatment in patients with mCRPC, with good clinical efficacy, even in heavily pretreated patients with multiple lines of systemic therapy. Currently, there are ongoing clinical trials in the United States, including a phase III multicenter FDA registration trial.

Preclinical Dosimetry, Imaging, and Targeted Radionuclide Therapy Studies of Lu-177-Labeled Albumin-Binding, PSMA-Targeted CTT1403

PURPOSE

Prostate-specific membrane antigen (PSMA) continues to be the hallmark biomarker for prostate cancer as it is expressed on nearly all prostatic tumors. In addition, increased PSMA expression correlates with castration resistance and progression to the metastatic stage of the disease. Recently, we combined both an albumin-binding motif and an irreversible PSMA inhibitor to develop the novel PSMA-targeted radiotherapeutic agent, CTT1403. This molecule was novel in the field of PSMA-targeted agents as its key motifs resulted in extended blood circulation time and tumor uptake, rapid and extensive internalization into PSMA+ cells, and promising therapeutic efficacy. The objective of this study was to perform IND-enabling translational studies on CTT1403 in rodent models.

PROCEDURES

A dose optimization study was performed in PC3-PIP (PSMA+) tumor-bearing mice. Treatment groups were randomly selected to receive one to three 14-MBq injections of CTT1403. Control groups included (1) saline, (2) non-radioactive [¹⁷⁵Lu]CTT1403, or (3) two injections of 14 MBq CTT1751, a Lu-177-labeled non-targeted albumin-binding moiety. Tumor growth was monitored up to 120 days. Small-animal single photon emission tomography/X-ray computed tomography imaging was performed with CTT1403 and CTT1751 in PC3-PIP tumor-bearing mice to visualize infiltration of the Lu-177-labeled agent into the tumor. In preparation for a first-in-human study, human absorbed doses were estimated based on rat biodistribution out to 5 weeks to determine a safe CTT1403 therapy dose in humans.

RESULTS

Two to 3 injections of 14 MBq CTT1403 yielded significant tumor growth inhibition and increased survival compared with all control groups and mice receiving 1 injection of 14 MBq CTT1403. Five of 12 mice receiving 2 or 3 injections of CTT1403 survived to the 120-day post-treatment study endpoint. Dosimetry identified the kidneys as the dose-limiting organ, with an equivalent dose of 5.18 mSv/MBq, resulting in a planned maximum dose of 4.4 GBq for phase 1 clinical trials.

CONCLUSIONS

The preclinical efficacy and dosimetry of CTT1403 suggest that this agent has significant potential to be safe and effective in humans.

Posttherapeutic Critical Organ Dosimetry of Extensive 177Lu-PSMA Inhibitor Therapy With Metastatic Castration-Resistant Prostate Cancer: One Center Results

PURPOSE

Lu-PSMA inhibitor peptide receptor radioligand therapy (RLT) is playing an increasing role in metastatic castration-resistant prostate cancer. We aimed to estimate the absorbed radiation doses for critical organs (eg, kidneys, parotid glands, submandibular glands, and lacrimal glands) of patients treated with 4 to 6 cycles by Lu-PSMA inhibitor RLT, retrospectively, and to evaluate the findings extensively in order to determine the critical organ radiation-absorbed limitations and the number of prospective RLT.

MATERIALS AND METHODS

A total of 51 cycles Lu-PSMA inhibitor RLT in 10 patients was analyzed. Therapies have been applied in 4 to 6 cycles with 8 to 10 weeks' intervals. Dosimetric estimates of kidneys, parotid glands, submandibular glands, and lacrimal glands have been calculated based on MIRD scheme pamphlet no. 16. Regions of interest were drawn with GE Xeleris Functional Imaging Workstation. OLINDA/EXM 1.1 simulation software was used to calculate radiation-absorbed doses.

RESULTS

Mean radiation-absorbed doses were 0.70 ± 0.24 Gy/GBq for kidneys, 1.34 ± 0.78 Gy/GBq for parotid glands, 0.94 ± 0.45 Gy/GBq for submandibular glands, and 2.28 ± 1.29 Gy/GBq for lacrimal glands.

CONCLUSIONS

Due to the critical target organ risks and the optimal therapy doses, patient-specific dosimetry is a deterministic factor in radionuclide therapy. Even when the absorbed kidney doses were above the ICRP critical dose limits in patients who had 4 to 6 cycles of therapy, mortality due to nephrotoxicity has not been observed. Mild increased tolerated radiation dose is acceptable for the patient groups with very low survival rate.

Internal Radiation Therapy

Targeted therapies are applied to increase the efficiency of antitumor treatment by simultaneously decreasing side effects. This can be achieved using carrier molecules which specifically bind to target structures or areas with remodeling activity. These carrier molecules may be coupled to chemotherapeutic drugs or to radioactive isotopes. In most cases, these carrier molecules are antibodies against tumor antigens, peptides, or small molecules which are binders for overexpressed receptors on tumor cells. The paradigm of endoradiotherapy is exemplified by the peptidic tracer DOTATOC which binds to somatostatin receptors and recently also small molecule inhibitors with high affinity for the prostate-specific membrane antigen.

Dosimetry and safety of 177Lu PSMA-617 along with polyglutamate parotid gland protector: preliminary results in metastatic castration-resistant prostate cancer patients

PURPOSE

Radioligand therapy (RLT) with ¹⁷⁷Lu-PSMA-617 is a promising option for patients with metastatic castration-resistant prostate cancer (mCRPC). The present study was designed to define the safety and initial response to a minimal effective injected activity/cycle of ¹⁷⁷Lu-PSMA-617 in mCRPC patients. New protective agents for salivary glands and kidney were co-administered and dosimetry was carried out.

PATIENTS AND METHODS

A prospective single-arm, open label phase II study on mCRPC was activated at our institute in April 2017. Patients with histologically confirmed advanced mCRPC previously treated with standard life-prolonging agents were enrolled. Folic polyglutamate tablets were orally administered as parotid gland protectors and 500 mL of a 10% mannitol solution was intravenously infused to reduce kidney uptake before the injection of 3.7-5.5 GBq of ¹⁷⁷Lu-PSMA-617 repeated four times at interval of 8 weeks. The adsorbed dose calculation was performed with MIRD formalism (OLINDA/EXM software). The Bryant and Day design was used to estimate the sample size taking account of both activity and toxicity.

RESULTS

Forty-three eligible patients were evaluated for toxicity and initial response. Dosimetry was carried out in 13 patients. Two (4.8%) patients had G3 and 8 (19.5%) had G2 hematological toxicity. Only 3 (6.9%) patients had mild G1 salivary gland toxicity and 8 (19.5%) had G1 renal toxicity. A decrease of ≥ 30% in prostate-specific antigen (PSA) was achieved after the first cycle in 17 (40.5%) patients, of whom 13 had a PSA decline of >50% after the second cycle. The median adsorbed doses were 0.65 mGy/MBq (range 0.33-2.63) for parotid glands, 0.42 mGy/MBq (0.14-0.81) for kidneys, 0.036 mGy/MBq (0.023-0.067) for red marrow, and 0.038 mGy/MBq (0.018-0.135) for the whole body.

CONCLUSION

In advanced, heavily pre-treated mCRPC patients, 3.7 GBq/cycle of ¹⁷⁷Lu-PSMA-617 was safe and produced early biochemical and imaging responses at PSMA whole-body scan post injection. Dosimetry of salivary glands suggests that the co-administration of polyglutamate tablets may reduce salivary gland uptake.

CLINICAL TRIAL REGISTRATION

EU Clinical Trials Register No.: 2016-002732-32; NCT03454750. Collection and assembly of data: April 2017 and February 2019.

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.

Radiation Dosimetry in 177Lu-PSMA-617 Therapy Using a Single Posttreatment SPECT/CT Scan: A Novel Methodology to Generate Time- and Tissue-Specific Dose Factors

Calculation of radiation dosimetry in targeted nuclear medicine therapies is traditionally resource-intensive, requiring multiple posttherapy SPECT acquisitions. An alternative approach is to take advantage of existing pharmacokinetic data from these smaller cohorts to enable dose computation from a single posttreatment scan in a manner that may be applied to a much broader patient population. Methods: In this work, a technical description of simplified dose estimation is presented and applied to the assessment of ¹⁷⁷Lu-prostate-specific membrane antigen (PSMA)-617 therapy for metastatic prostate cancer. By normalizing existing time-activity curves to a single measurement time, it is possible to calculate a mean and range of time-integrated activity values that relate to absorbed radiation dose. To assist with accurate pharmacokinetic modeling of the training cohort, a method for contour-guided image registration was developed. Results: Tissue-specific dose conversion factors for common posttreatment imaging times are reported along with a characterization of added uncertainty in comparison to a traditional serial imaging protocol. Single-time-point dose factors for tumor were determined to be 11.0, 12.1, 13.6, and 15.2 Gy per MBq/mL at image times of 24, 48, 72, and 96 h, respectively. For normal tissues, parotid gland factors were 6.7, 9.4, 13.3, and 19.3 Gy per MBq/mL at those times, and kidneys were 7.1, 10.3, 15.0, and 22.0 Gy per MBq/mL. Tumor dose estimates were most accurate using delayed scanning at times beyond 72 h. Dose to healthy tissues is best characterized by scanning patients in the first 2 d of treatment because of the larger degree of tracer clearance in this early phase. Conclusion: This work demonstrates a means for efficient dose estimation in ¹⁷⁷Lu-PSMA-617 therapy. By providing methods to simplify and potentially automate radiation dosimetry, we hope to accelerate the understanding of radiobiology and development of dose-response models in this unique therapeutic context.

Long-Term Follow-up and Outcomes of Retreatment in an Expanded 50-Patient Single-Center Phase II Prospective Trial of 177Lu-PSMA-617 Theranostics in Metastatic Castration-Resistant Prostate Cancer

¹⁷⁷Lu-PSMA-617 is a radioligand with high affinity for prostate-specific membrane antigen (PSMA), enabling targeted β-irradiation of prostate cancer. We have previously reported favorable activity with low toxicity in a prospective phase II trial involving 30 men with metastatic castration-resistant prostate cancer. We now report their longer-term outcomes, including a 20-patient extension cohort and outcomes of subsequent systemic treatments after completion of trial therapy. Methods: Fifty patients with PSMA-avid metastatic castration-resistant prostate cancer who had progressed after standard therapies received up to 4 cycles of ¹⁷⁷Lu-PSMA every 6 wk. Endpoints included prostate-specific antigen (PSA) response (Prostate Cancer Working Group 2), toxicity (Common Terminology Criteria for Adverse Events, version 4.03), imaging response, patient-reported health-related quality of life, progression-free survival, and overall survival. We also describe, as a novel finding, outcomes of men who subsequently progressed and had further systemic therapies, including ¹⁷⁷Lu-PSMA. Results: Seventy-five men were screened to identify 50 patients eligible for treatment. Adverse prognostic features of the cohort included short median PSA doubling time (2.3 mo) and extensive prior treatment, including prior docetaxel (84%), cabazitaxel (48%), and abiraterone or enzalutamide (92%). The mean administered radioactivity was 7.5 GBq/cycle. A PSA decline of at least 50% was achieved in 32 of 50 patients (64%; 95% confidence interval [CI], 50%-77%), including 22 patients (44%; 95% CI, 30%-59%) with at least an 80% decrease. Of 27 patients with measurable soft-tissue disease, 15 (56%) achieved an objective response by RECIST 1.1. The most common toxicities attributed to ¹⁷⁷Lu-PSMA were self-limiting G1-G2 dry mouth (66%), transient G1-G2 nausea (48%), G3-G4 thrombocytopenia (10%), and G3 anemia (10%). Brief Pain Inventory severity and interference scores decreased at all time points, including at the 3-mo follow-up, with a decrease of -1.2 (95% CI, -0.5 to -1.9; P = 0.001) and -1.0 (95% CI, -0.2 to -0.18; P = 0.013), respectively. At a median follow-up of 31.4 mo, median overall survival was 13.3 mo (95% CI, 10.5-18.7 mo), with a significantly longer survival of 18.4 mo (95% CI, 13.8-23.8 mo) in patients achieving a PSA decline of at least 50%. At progression after prior response, further ¹⁷⁷Lu-PSMA was administered to 15 (30%) patients (median of 2 cycles commencing 359 d from enrollment), with a PSA decline of at least 50% in 11 patients (73%). Four of 21 patients (19%) receiving other systemic therapies on progression experienced a PSA decline of at least 50%. There were no unexpected adverse events with ¹⁷⁷Lu-PSMA retreatment. Conclusion: This expanded 50-patient cohort of men with extensive prior therapy confirms our earlier report of high response rates, low toxicity, and improved quality of life with ¹⁷⁷Lu-PSMA radioligand therapy. On progression, rechallenge ¹⁷⁷Lu-PSMA demonstrated higher response rates than other systemic therapies.

First-in-human dosimetry of gastrin-releasing peptide receptor antagonist [177Lu]Lu-RM2: a radiopharmaceutical for the treatment of metastatic castration-resistant prostate cancer

PURPOSE

Besides PSMA, prostate cancer cells also express gastrin-releasing peptide receptor (GRPr) which is therefore a promising target for theranostic approaches. The high affinity GRPr antagonist RM2 can be labeled with beta-emitting radiometals for therapeutic purposes. The aim of this study was to calculate absorbed doses for critical organs and tumor lesions for [¹⁷⁷Lu]Lu-RM2 therapy administered in a group of metastatic castration-resistant prostate cancer (mCRPC) patients who had insufficient PSMA expression or showed lower PSMA accumulation after previous cycles of [¹⁷⁷Lu]Lu-PSMA-617 therapy.

METHODS

Thirty-five patients suffering from mCRPC without further treatment options for approved therapies were examined with [⁶⁸Ga]Ga-RM2-PET/CT. Out of these, 4 patients (mean age 68 years) were treated with [¹⁷⁷Lu]Lu-RM2; two of these also received a 2nd therapy cycle. Mean activity was 4.5 ± 0.9 GBq. For dosimetry, patients underwent planar WB-scintigraphy and SPECT/CT imaging of the upper and lower abdomen at approximately 1, 24, 48, and 72 h p.i. along with blood sampling. Absorbed doses for kidneys, pancreas, liver, spleen, gallbladder wall, and tumor lesions were derived based on quantitative SPECT/CT according to RADAR dosimetry scheme; individual organ masses were extracted from CT. Absorbed dose to bone marrow was calculated based on serial whole-body images and blood sampling according to the EANM guideline.

RESULTS

Therapy was well tolerated by all patients and no side effects were observed. An increased uptake in tumor lesions and the pancreas was seen within the first 1 h. Mean absorbed organ doses were 1.08 ± 0.44 Gy/GBq in the pancreas, 0.35 ± 0.14 Gy/GBq in the kidneys, 0.05 ± 0.02 Gy/GBq in the liver, 0.07 ± 0.02 Gy/GBq in the gallbladder wall, 0.10 ± 0.06 Gy/GBq in the spleen, and 0.02 ± 0.01 Gy/GBq for the red bone marrow. The mean dose for tumor lesions was 6.20 ± 3.00 Gy/GBq.

CONCLUSIONS

Application of GRPr antagonist [¹⁷⁷Lu]Lu-RM2 is suitable for targeted radiotherapy of mCRPC as it shows high tumor uptake and rapid clearance from normal organs. Absorbed doses in tumor lesions are therapeutically relevant. The critical organ receiving the highest absorbed dose was the pancreas. Results suggest that the activity administered for each cycle could be increased to maximize the absorbed dose of tumors and metastases.

Pretherapeutic 68Ga-PSMA-617 PET May Indicate the Dosimetry of 177Lu-PSMA-617 and 177Lu-EB-PSMA-617 in Main Organs and Tumor Lesions

AIM

Combined Ga-PSMA-617 PET imaging and Lu-PSMA-617 therapy is a precise targeted theranostic approach for patients with metastatic castration-resistant prostate cancer (mCRPC). The purpose of this study was to determine whether pretherapeutic standard uptake value (SUV) in Ga-PSMA-617 PET could indicate the effective dose in the main organs and absorbed dose in tumor lesions.

METHODS

After institutional review board approval and informed consent, 9 patients with mCRPC were recruited and underwent Ga-PSMA-617 PET/CT scans. Five patients received Lu-PSMA-617 (1.30-1.42 GBq, 35-38.4 mCi) and then underwent serial whole-body planar imaging and SPECT/CT imaging of both thoracic and abdominal regions at 0.5-, 2-, 24-, 48-, and 72-hour time points. The other 4 patients received Lu-EB-PSMA-617 (0.80-1.1 GBq, 21.5-30 mCi) and then underwent the same imaging procedures at 2-, 24-, 72-, 120-, and 168-hour time points. The effective dose in the main organs and the absorbed dose in tumor lesions were calculated. Detailed correlations between the pretherapeutic SUV in Ga-PSMA-617 PET and effective dose in the main organs as well as absorbed dose in the tumor lesions were analyzed.

RESULTS

SUV of Ga-PSMA-617 PET was moderately correlated with effective dose in main organs (r = 0.610 for Lu-PSMA-617, r = 0.743 for Lu-EB-PSMA-617, both P < 0.001). SUV of tumor lesions in Ga-PSMA-617 PET had high correlation with those in Lu-PSMA-617 (r = 0.915, P < 0.001) and moderate correlation with those in Lu-EB-PSMA-617 (r = 0.611, P = 0.002).

CONCLUSIONS

Pretherapeutic Ga-PSMA-617 PET may indicate the dosimetry of Lu-PSMA-617 and Lu-EB-PSMA-617. Both the effective dose in main organs and absorbed dose in tumor lesions correlate with SUV of Ga-PSMA-617 PET. This relationship may help select appropriate candidates for peptide receptor radionuclide therapy. Further investigations of larger cohorts are needed to confirm these initial findings.

Recent Advances in Prostate-Specific Membrane Antigen-Based Radiopharmaceuticals

BACKGROUND

Prostate cancer (PCa) is the most common sex-related malignancy with high mortality in men worldwide. Prostate-specific membrane antigen (PSMA) is overexpressed on the surface of most prostate tumor cells and considered a valuable target for both diagnosis and therapy of prostate cancer. A series of radiolabeled agents have been developed based on the featured PSMA ligands in the previous decade and have demonstrated promising outcomes in clinical research of primary and recurrent PCa. Furthermore, the inspiring response and safety of lutetium-177-PSMA-617 (177Lu-PSMA-617) radiotherapy represent the potential for expanded therapeutic options for metastatic castration-resistant PCa. Retrospective cohort studies have revealed that radiolabeled PSMA agents are the mainstays of the current success, especially in detecting prostate cancer with metastasis and biochemical recurrence.

OBJECTIVE

This review is intended to present a comprehensive overview of the current literature on PSMA ligand-based agents for both radionuclide imaging and therapeutic approaches, with a focus on those that have been clinically adopted.

CONCLUSION

PSMA-based diagnosis and therapy hold great promise for improving the clinical management of prostate cancer.

Effect of External Cooling on 177Lu-PSMA Uptake by the Parotid Glands

Recent years have seen the start of treatment of metastatic castration-resistant prostate cancer with prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (PRLT), especially ¹⁷⁷Lu-PSMA-617. However, PRLT has side effects on the salivary glands that limit the safety of the treatment. The current study aimed to show the effect of external cooling with ice packs on ¹⁷⁷Lu-PSMA-617 uptake by the parotid glands (PGs). Methods: The study included 19 patients (mean age, 72.9 y) with metastatic castration-resistant prostate cancer who had been referred for the first time for ¹⁷⁷Lu-PSMA-617 treatment and underwent pretreatment ⁶⁸Ga-PSMA-11 PET/CT. Before the initiation of PRLT, the SUV_max and SUV_mean of the right and left PGs were measured on ⁶⁸Ga-PSMA PET/CT. Frozen ice packs were then affixed over the right PG of each patient for approximately 5 h; 1 h after they were affixed, PRLT was administered. At 4 h after PRLT, head-and-neck SPECT/CT was performed, and at both 4 and 24 h after PRLT, whole-body planar scintigraphy was performed. Regions and volumes of interest were applied for the right and left PGs, and the counts and volumes were determined. Results: Before PRLT, ⁶⁸Ga-PSMA-11 PET/CT showed no significant difference in SUV_max or SUV_mean between the right and left PGs (P > 0.05). At 4 and 24 h after PRLT, planar imaging showed no significant difference in counts between the cooled and noncooled PGs (P > 0.05). Furthermore, at 4 h after PRLT, SPECT/CT showed no significant difference in counts or volumes between the cooled and noncooled PGs (P > 0.05). Conclusion: External cooling does not reduce uptake of ¹⁷⁷Lu-PSMA-617 by the PGs.

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.

Radiation dosimetry of [68Ga]PSMA-11 in low-risk prostate cancer patients

BACKGROUND

68Ga-labeled Glu-NH-CO-NH-Lys(Ahx)-HBED-CC ([68Ga]PSMA-11) has been increasingly used to image prostate cancer using positron emission tomography (PET)/computed tomography (CT) both during diagnosis and treatment planning. It has been shown to be of clinical value for patients both in the primary and secondary stages of prostate cancer. The aim of this study was to determine the effective dose and organ doses from injection of [68Ga]PSMA-11 in a cohort of low-risk prostate cancer patients.

METHODS

Six low-risk prostate cancer patients were injected with 133-178 MBq [68Ga]PSMA-11 and examined with four PET/CT acquisitions from injection to 255 min post-injection. Urine was collected up to 4 h post-injection, and venous blood samples were drawn at 45 min, 85 min, 175 min, and 245 min post-injection. Kidneys, liver, lungs, spleen, salivary and lacrimal glands, and total body where delineated, and cumulated activities and absorbed organ doses calculated. The software IDAC-Dose 2.1 was used to calculate absorbed organ doses according to the International Commission on Radiological Protection (ICRP) publication 107 using specific absorbed fractions published in ICRP 133 and effective dose according to ICRP Publication 103. We also estimated the absorbed dose to the eye lenses using Monte Carlo methods.

RESULTS

[68Ga]PSMA-11 was rapidly cleared from the blood and accumulated preferentially in the kidneys and the liver. The substance has a biological half-life in blood of 6.5 min (91%) and 4.4 h (9%). The effective dose was calculated to 0.022 mSv/MBq. The kidneys received approximately 40 mGy after an injection with 160 MBq [68Ga]PSMA-11 while the lacrimal glands obtained an absorbed dose of 0.12 mGy per administered MBq. Regarding the eye lenses, the absorbed dose was low (0.0051 mGy/MBq).

CONCLUSION

The effective dose for [68Ga]PSMA-11 is 0.022 mSv/MBq, where the kidneys and lacrimal glands receiving the highest organ dose.

Uptake in non-affected bone tissue does not differ between [18F]-DCFPyL and [68Ga]-HBED-CC PSMA PET/CT

Introduction

[⁶⁸Ga]PSMA-HBED-CC and [¹⁸F]DCFPyL show a high potential for the detection of recurrent prostate cancer. While ¹⁸F-based tracers have several advantages in availability and image resolution, their sensitivity in the skeleton might be impaired by released [¹⁸F]fluoride due to its high bone affinity. In turn, chemically unbound trivalent ⁶⁸Ga might also accumulate in osseous tissue, in cases of occupied binding sites of plasma proteins and thereby influence bone signal.

Methods

A comparison of average bone SUV was performed in 17 bone-negative and 4 bone-positive patients. All patients underwent PET/CT 125 minutes after application of [¹⁸F]DCFPyL and 73 minutes after application of [⁶⁸Ga]PSMA-HBED-CC at another date.

Results

Native SUVs in unaffected bone tissue and SUVs relative to liver uptake were lower in [¹⁸F]DCFPyL (0.49) than in [⁶⁸Ga]PSMA-HBED-CC scans (0.52). SUVs relative to gluteal muscles did not differ between the two tracers. Average lesional SUVs did not differ between tracers.

Conclusion

No difference of average bone signal intensity was observed for [¹⁸F]DCFPyL-PET/CT in comparison to [⁶⁸Ga]PSMA-HBED-CC scans indicating that diagnostic assessment of the skeleton is not affected by non-specific accumulation of free [¹⁸F]fluoride or ⁶⁸Ga.

Dosimetry of 177Lu-PSMA-617 in Metastatic Castration-Resistant Prostate Cancer: Correlations Between Pretherapeutic Imaging and Whole-Body Tumor Dosimetry with Treatment Outcomes

¹⁷⁷Lu-prostate-specific membrane antigen (PSMA)-617 enables targeted delivery of β-particle radiation to prostate cancer. We determined its radiation dosimetry and relationships to pretherapeutic imaging and outcomes. Methods: Thirty patients with prostate cancer receiving ¹⁷⁷Lu-PSMA-617 within a prospective clinical trial (ACTRN12615000912583) were studied. Screening ⁶⁸Ga-PSMA-11 PET/CT demonstrated high PSMA expression in all patients. After therapy, patients underwent quantitative SPECT/CT at 4, 24, and 96 h. Pharmacokinetic uptake and clearance at a voxel level were calculated and translated into absorbed dose using voxel S values. Volumes of interest were drawn on normal tissues and tumor to assess radiation dose, and a whole-body tumor dose was defined. Correlations between PSMA PET/CT parameters, dosimetry, and biochemical and therapeutic response were analyzed to identify relationships between absorbed dose, tumor burden, and patient physiology. Results: Mean absorbed dose to kidneys, submandibular and parotid glands, liver, spleen, and bone marrow was 0.39, 0.44, 0.58, 0.1, 0.06, and 0.11 Gy/MBq, respectively. Median whole-body tumor-absorbed dose was 11.55 Gy and correlated with prostate-specific antigen (PSA) response at 12 wk. A median dose of 14.1 Gy was observed in patients achieving a PSA decline of at least 50%, versus 9.6 Gy for those achieving a PSA decline of less than 50% (P < 0.01). Of 11 patients receiving a tumor dose of less than 10 Gy, only one achieved a PSA response of at least 50%. On screening PSMA PET, whole-body tumor SUV_mean correlated with mean absorbed dose (r = 0.62), and SUV_max of the parotids correlated with absorbed dose (r = 0.67). There was an inverse correlation between tumor volume and mean dose to the parotids (r = -0.41) and kidneys (r = -0.43). The mean parotid dose was also reduced with increasing body mass (r = -0.41) and body surface area (r = -0.37). Conclusion: ¹⁷⁷Lu-PSMA-617 delivers high absorbed doses to tumor, with a significant correlation between whole-body tumor dose and PSA response. Patients receiving less than 10 Gy were unlikely to achieve a fall in PSA of at least 50%. Significant correlations between aspects of screening ⁶⁸Ga-PET/CT and tumor and normal tissue dose were observed, providing a rationale for patient-specific dosing. Reduced salivary and kidney doses were observed in patients with a higher tumor burden. The parotid dose also reduced with increasing body mass and body surface area.

Evaluation of 111In-DOTA-5D3, a Surrogate SPECT Imaging Agent for Radioimmunotherapy of Prostate-Specific Membrane Antigen

5D3 is a new high-affinity murine monoclonal antibody specific for prostate-specific membrane antigen (PSMA). PSMA is a target for the imaging and therapy of prostate cancer. ¹¹¹In-labeled antibodies have been used as surrogates for ¹⁷⁷Lu/⁹⁰Y-labeled therapeutics. We characterized ¹¹¹In-DOTA-5D3 by SPECT/CT imaging, tissue biodistribution studies, and dosimetry. Methods: Radiolabeling, stability, cell uptake, and internalization of ¹¹¹In-DOTA-5D3 were performed by established techniques. Biodistribution and SPECT imaging were done on male nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice bearing human PSMA(+) PC3 PIP and PSMA(-) PC3 flu prostate cancer xenografts on the upper right and left flanks, respectively, at 2, 24, 48, 72, and 192 h after injection. Biodistribution was also evaluated in tumor-free, healthy male CD-1 mice. Blocking studies were performed by coinjection of a 10-fold and 50-fold excess of 5D3 followed by biodistribution at 24 h to determine PSMA binding specificity. The absorbed radiation doses were calculated on the basis of murine biodistribution data, which were translated to a human adult man using organ weights as implemented in OLINDA/EXM. Results: ¹¹¹In-DOTA-5D3 was synthesized with specific activity of approximately 2.24 ± 0.74 MBq/μg (60.54 ± 20 μCi/μg). Distribution of ¹¹¹In-DOTA-5D3 in PSMA(+) PC3 PIP tumor peaked at 24 h after injection and remained high until 72 h. Uptake in normal tissues, including the blood, spleen, liver, heart, and lungs, was highest at 2 h after injection. Coinjection of ¹¹¹In-DOTA-5D3 with a 10- and 50-fold excess of nonradiolabeled antibody significantly reduced PSMA(+) PC3 PIP tumor and salivary gland uptake at 24 h but did not reduce uptake in kidneys and lacrimal glands. Significant clearance of ¹¹¹In-DOTA-5D3 from all organs occurred at 192 h. The highest radiation dose was received by the liver (0.5 mGy/MBq), followed by the spleen and kidneys. Absorbed radiation doses to the salivary and lacrimal glands and bone marrow were low. Conclusion: ¹¹¹In-DOTA-5D3 is a new radiolabeled antibody for imaging and a surrogate for therapy of malignant tissues expressing PSMA.

Meeting report from the Prostate Cancer Foundation PSMA-directed radionuclide scientific working group

INTRODUCTION

The Prostate Cancer Foundation (PCF) convened a PSMA-Directed Radionuclide Scientific Working Group on November 14, 2017, at Weill Cornell Medicine, New York, NY.

METHODS

The meeting was attended by 35 global investigators with expertise in prostate cancer biology, radionuclide therapy, molecular imaging, prostate-specific membrane antigen (PSMA)-targeted agents, drug development, and prostate cancer clinical trials. The goal of this meeting was to discuss the potential for using PSMA-targeted radionuclide agents for the treatment of advanced prostate cancer and to define the studies and clinical trials necessary for validating and optimizing the use of these agents.

RESULTS

Several major topic areas were discussed including the overview of PSMA biology, lessons and applications of PSMA-targeted PET imaging, the nuances of designing PSMA-targeted radionuclide agents, clinical experiences with PSMA-targeted radionuclides, PCF-funded projects to accelerate PSMA-targeted radionuclide therapy, and barriers to the use of radionuclide treatments in widespread clinical practice.

DISCUSSION

This article reviews the major topics discussed at the meeting with the goal of promoting research that will validate and optimize the use of PSMA-targeted radionuclide therapies for the treatment of advanced prostate cancer.

Theranostics of prostate cancer: from molecular imaging to precision molecular radiotherapy targeting the prostate specific membrane antigen

Alterations at the molecular level are a hallmark of cancer. Prostate cancer is associated with the overexpression of prostate-specific membrane antigen (PSMA) in a majority of cases, predominantly in advanced tumors, increasing with the grade or Gleason's score. PSMA can be selectively targeted using radiolabeled PSMA ligands. These small molecules binding the PSMA can be radiolabeled with γ-emitters like ^99mTc and ¹¹¹In or positron emitters like ⁶⁸Ga and ¹⁸F for diagnosis as well as with their theranostic pairs such as ¹⁷⁷Lu (β-emitter) or ²²⁵Ac (α-emitter) for therapy. This review summarizes the theranostic role of PSMA ligands for molecular imaging and targeted molecular radiotherapy, moving towards precision oncology.

Targeting prostate cancer: Prostate-specific membrane antigen based diagnosis and therapy

The high incidence rates of prostate cancer (PCa) raise demand for improved therapeutic strategies. Prostate tumors specifically express the prostate-specific membrane antigen (PSMA), a membrane-bound protease. As PSMA is highly overexpressed on malignant prostate tumor cells and as its expression rate correlates with the aggressiveness of the disease, this tumor-associated biomarker provides the possibility to develop new strategies for diagnostics and therapy of PCa. Major advances have been made in PSMA targeting, ranging from immunotherapeutic approaches to therapeutic small molecules. This review elaborates the diversity of PSMA targeting agents while focusing on the radioactively labeled tracers for diagnosis and endoradiotherapy. A variety of radionuclides have been shown to either enable precise diagnosis or efficiently treat the tumor with minimal effects to nontargeted organs. Most small molecules with affinity for PSMA are based on either a phosphonate or a urea-based binding motif. Based on these pharmacophores, major effort has been made to identify modifications to achieve ideal pharmacokinetics while retaining the specific targeting of the PSMA binding pocket. Several tracers have now shown excellent clinical usability in particular for molecular imaging and therapy as proven by the efficiency of theranostic approaches in current studies. The archetypal expression profile of PSMA may be exploited for the treatment with alpha emitters to break radioresistance and thus to bring the power of systemic therapy to higher levels.

External radiation exposure, excretion, and effective half-life in 177Lu-PSMA-targeted therapies

Background

Prostate-specific membrane antigen (PSMA)-targeted therapy with ¹⁷⁷Lu-PSMA-617 is a therapeutic option for patients with metastatic castration-resistant prostate cancer (mCRPC). To optimize the therapy procedure, it is necessary to determine relevant parameters to define radiation protection and safety necessities. Therefore, this study aimed at estimating the ambient radiation exposure received by the patient. Moreover, the excreted activity was quantified.

Results

In total, 50 patients with mCRPC and treated with ¹⁷⁷Lu-PSMA-617 (mean administered activity 6.3 ± 0.5 GBq) were retrospectively included in a bi-centric study. Whole-body dose rates were measured at a distance of 2 m at various time points after application of ¹⁷⁷Lu-PSMA-617, and effective half-lives for different time points were calculated and compared. Radiation exposure to the public was approximated using the dose integral. For the estimation of the excreted activity, whole body measurements of 25 patients were performed at 7 time points.

Unbound ¹⁷⁷Lu-PSMA-617 was rapidly cleared from the body. After 4 h, approximately 50% and, after 12 h, approximately 70% of the administered activity were excreted, primarily via urine. The mean dose rates were the following: 3.6 ± 0.7 μSv/h at 2 h p. i., 1.6 ± 0.6 μSv/h at 24 h, 1.1 ± 0.5 μSv/h at 48 h, and 0.7 ± 0.4 μSv/h at 72 h. The mean effective half-life of the cohort was 40.5 ± 9.6 h (min 21.7 h; max 85.7 h). The maximum dose to individual members of the public per treatment cycle was ~ 250 ± 55 μSv when the patient was discharged from the clinic after 48 h and ~ 190 ± 36 μSv when the patient was discharged after 72 h.

Conclusions

In terms of the radiation exposure to the public, ¹⁷⁷Lu-PSMA is a safe option of radionuclide therapy. As usually four (sometimes more) cycles of the therapy are performed, it must be conducted in a way that ensures that applicable legal requirements can be followed. In other words, the radiation exposure to the public and the concentration of activity in wastewater must be sub-marginal. Therefore, in certain countries, hospitalization of these patients is mandatory.

Combined model-based and patient-specific dosimetry for 18F-DCFPyL, a PSMA-targeted PET agent

PURPOSE

Prostate-specific membrane antigen (PSMA), a type-II integral membrane protein highly expressed in prostate cancer, has been extensively used as a target for imaging and therapy. Among the available PET radiotracers, the low molecular weight agents that bind to PSMA are proving particularly effective. We present the dosimetry results for ¹⁸F-DCFPyL in nine patients with metastatic prostate cancer.

METHODS

Nine patients were imaged using sequential PET/CT scans at approximately 1, 12, 35 and 70 min, and a final PET/CT scan at approximately 120 min after intravenous administration of 321 ± 8 MBq (8.7 ± 0.2 mCi) of¹⁸F-DCFPyL. Time-integrated-activity coefficients were calculated and used as input in OLINDA/EXM software to obtain dose estimates for the majority of the major organs. The absorbed doses (AD) to the eye lens and lacrimal glands were calculated using Monte-Carlo models based on idealized anatomy combined with patient-specific volumes and activity from the PET/CT scans. Monte-Carlo based models were also developed for calculation of the dose to two major salivary glands (parotid and submandibular) using CT-based patient-specific gland volumes.

RESULTS

The highest calculated mean AD per unit administered activity of ¹⁸F was found in the lacrimal glands, followed by the submandibular glands, kidneys, urinary bladder wall, and parotid glands. The S-values for the lacrimal glands to the eye lens (0.42 mGy/MBq h), the tear film to the eye lens (1.78 mGy/MBq h) and the lacrimal gland self-dose (574.10 mGy/MBq h) were calculated. Average S-values for the salivary glands were 3.58 mGy/MBq h for the parotid self-dose and 6.78 mGy/MBq h for the submandibular self-dose. The resultant mean effective dose of ¹⁸F-DCFPyL was 0.017 ± 0.002 mSv/MBq.

CONCLUSIONS

¹⁸F-DCFPyL dosimetry in nine patients was obtained using novel models for the lacrimal and salivary glands, two organs with potentially dose-limiting uptake for therapy and diagnosis which lacked pre-existing models.

Uptake of PSMA-ligands in normal tissues is dependent on tumor load in patients with prostate cancer

Radioligand therapy (RLT) with Lu-177-labeled PSMA-ligands is a new therapy option for prostate cancer. Biodistribution in normal tissues is of interest for therapy planning. We evaluated if the biodistribution of Ga-68-PSMA-11 is influenced by tumor load.

177Lu-PSMA Radioligand Therapy for Prostate Cancer

¹⁷⁷Lu-prostate-specific membrane antigen (PSMA) radioligand therapy (RLT) using inhibitors of PSMA is a novel therapeutic option in patients with metastatic castration-resistant prostate cancer. The current literature suggests that this therapy is well tolerated and effective. On the basis of clinical need and current evidence, the therapy is being implemented in a growing number of centers worldwide. Here, we review important aspects of ¹⁷⁷Lu-PSMA RLT, including patient stratification, the therapy protocol, concomitant medication, and follow-up, to inform medical staff involved in the RLT and care of patients with metastatic castration-resistant prostate cancer.

Current Status of Prostate-Specific Membrane Antigen Targeting in Nuclear Medicine: Clinical Translation of Chelator Containing Prostate-Specific Membrane Antigen Ligands Into Diagnostics and Therapy for Prostate Cancer

The prostate-specific membrane antigen (PSMA) is expressed by approximately 90% of prostate carcinomas. The expression correlates with unfavorable prognostic factors, such as a high Gleason score, infiltrative growth, metastasis, and hormone-independence. The high specificity, especially in the undifferentiated stage, makes it an excellent target for diagnosis and therapy. Therefore, antibodies and small molecule inhibitors have been developed for imaging and therapy. In 2011 PSMA-11, a ligand that consists of the Glu-urea-motif and the chelator HBED-CC, which can be exclusively radiolabeled with (68)Ga for PET imaging, presented the clinical breakthrough for prostate cancer diagnostics. In two large diagnostic studies (n = 319 and n = 248) PET/CT with PSMA-11 successfully localized the recurrent tumor in approximately 90% of patients with biochemical relapse. Integrating PSMA-PET/CT into the planning phase of radiotherapy, the treatment concept is changed in 30%-50% of the patients. The combination of the Glu-urea-motif with DOTA, which can be labeled with several diagnostic and therapeutic radionuclides, opened new avenues for therapeutic usage of the small-molecule PSMA ligands. In the beginning of 2016, there are four confirmative reports (n = 19, n = 24, n = 30, and n = 56) from four different centers reporting a PSA response in approximately 70% of patients treated with (177)Lu-labeled PSMA ligands. In conclusion, the data available up to now indicate a widespread use of PSMA ligands for diagnostic applications with respect to staging, detection of recurrence, or metastases in patients with rising tumor markers and for therapy in case of failure of guideline-compliant treatment.

Targeted Radionuclide Therapy: An Evolution Toward Precision Cancer Treatment

OBJECTIVE

This article reviews recent developments in targeted radionuclide therapy (TRT) approaches directed to malignant liver lesions, bone metastases, neuroendocrine tumors, and castrate-resistant metastatic prostate cancer and discusses challenges and opportunities in this field.

CONCLUSION

TRT has been employed since the first radioiodine thyroid treatment almost 75 years ago. Progress in the understanding of the complex underlying biology of cancer and advances in radiochemistry science, multimodal imaging techniques including the concept of "see and treat" within the framework of theranostics, and universal traction with the notion of precision medicine have all contributed to a resurgence of TRT.

The Rise of PSMA Ligands for Diagnosis and Therapy of Prostate Cancer

The prostate-specific membrane antigen (PSMA) has received increased consideration during the past few years as an excellent target for both imaging and therapy of prostate cancer. After many years of outstanding preclinical research, the first significant step forward in clinical use was achieved in 2008 with the first human experience with the small-molecule PSMA inhibitors ¹²³I-MIP-1972 and ¹²³I-MIP-1095. A clinical breakthrough followed in 2011 with ⁶⁸Ga-PSMA-11 for PET imaging and ¹³¹I-MIP-1095 for endoradiotherapy of metastatic prostate cancer. Since then, PET/CT with ⁶⁸Ga-PSMA-11 has rapidly spread worldwide, and endoradiotherapy with PSMA ligands has been conducted at increasing numbers of centers. ⁶⁸Ga-PSMA-11 is currently the subject of multicenter studies in different countries. Since 2013, ¹³¹I-related PSMA therapy has been replaced by ¹⁷⁷Lu-labeled ligands, such as PSMA-617, which is also the subject of multicenter studies. Alternative PSMA ligands for both imaging and therapy are available. Among them is ^99mTc-MIP-1404, which has recently entered a phase 3 clinical trial. This article focuses on the highlights of the development and clinical application of PSMA ligands.