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

Prostate-specific Membrane Antigen: Alpha-labeled Radiopharmaceuticals

Novel prostate-specific membrane antigen (PSMA) ligands labeled with α-emitting radionuclides are sparking a growing interest in prostate cancer treatment. These targeted alpha therapies (TATs) have attractive physical properties that deem them effective in progressive metastatic castrate-resistant prostate cancer (mCRPC). Among the PSMA TAT radiopharmaceuticals, [225Ac]Ac-PSMA has been used extensively on a compassionate basis and is currently undergoing phase I trials. Notably, TAT has the potential to improve quality of life and has favorable antitumor activity and outcomes in multiple scenarios other than in mCRPC. In addition, resistance mechanisms to TAT may be amenable to combination therapies.

Salivary toxicity from PSMA-targeted radiopharmaceuticals: What we have learned and where we are going

Clinical trials of prostate-specific membrane antigen (PSMA) targeted radiopharmaceuticals have shown encouraging results. Some agents, like lutetium-177 [177Lu]Lu-PSMA-617 ([177Lu]Lu-PSMA-617), are already approved for late line treatment of metastatic castration-resistant prostate cancer (mCRPC). Projections are for continued growth of this treatment modality; [177Lu]Lu-PSMA-617 is being studied both in earlier stages of disease and in combination with other anti-cancer therapies. Further, the drug development pipeline is deep with variations of PSMA-targeting radionuclides, including higher energy alpha particles conjugated to PSMA-honing vectors. It is safe to assume that an increasing number of patients will be exposed to PSMA-targeted radiopharmaceuticals during the course of their cancer treatment. In this setting, it is important to better understand and mitigate the most commonly encountered toxicities. One particularly vexing side effect is xerostomia. In this review, we discuss the scope of the problem, inventories to better characterize and monitor this troublesome side effect, and approaches to preserve salivary function and effectively palliate symptoms. This article aims to serve as a useful reference for prescribers of PSMA-targeted radiopharmaceuticals, while also commenting on areas of missing data and opportunities for future research.

Discovery of Glutamate Carboxypeptidase III Ligands to Compete the Uptake of [177Lu]Lu-PSMA-617 in Healthy Organs

Prostate-specific membrane antigen (PSMA)-targeted radio ligand therapeutics (RLTs), such as [177Lu]Lu-PSMA-617 (Pluvicto), have been shown to accumulate in salivary glands and kidneys, potentially leading to undesired side effects. As unwanted accumulation in normal organs may derive from the cross-reactivity of PSMA ligands to glutamate carboxypeptidase III (GCPIII), it may be convenient to block this interaction with GCPIII-selective ligands. Parallel screening of a DNA-encoded chemical library (DEL) against GCPIII and PSMA allowed the identification of GCPIII binders. Structure-activity relationship (SAR) studies resulted in the identification of nanomolar GCPIII ligands with up to 1000-fold selectivity over PSMA. We studied the ability of GCPIII ligands to counteract the binding of [177Lu]Lu-PSMA-617 to human salivary glands by autoradiography and could demonstrate a partial radioprotection.

DNA-encoded chemical libraries enable the discovery of potent PSMA-ligands with substantially reduced affinity towards the GCPIII anti-target

Prostate-specific membrane antigen (PSMA) is a tumor-associated protein that has been successfully targeted with small organic ligands and monoclonal antibodies. Pluvicto™ is a PSMA-targeted radioligand therapeutic (RLT) recently approved by the FDA for the treatment of metastatic castration-resistant prostate cancer (2022 FDA marketing authorization). Although a large Phase III clinical trial (VISION trial) demonstrated clinical benefits in patients treated with Pluvicto™, the therapeutic window of the drug is narrowed by its undesired accumulation in healthy organs. Glutamate carboxypeptidase III (GCPIII), an enzyme sharing 70% identity with PSMA, may be responsible for the off-target accumulation of PSMA-RLTs in salivary glands and kidneys. In this work, we designed and synthesized affinity and selectivity maturation DNA-encoded chemical libraries (ASM-DELs) comprising 18'284'658 compounds that were screened in parallel against PSMA and GCPIII with the aim to identify potent and selective PSMA ligands for tumor-targeting applications. Compound A70-B104 was isolated as the most potent and selective ligand (KD of 900 pM for PSMA, KD of 40 nM for GCPIII). 177Lu-A70-B104-DOTA, a radiolabeled derivative of compound A70-B104, presented selective accumulation in PSMA-positive cancer lesions (i.e., 7.4% ID g-1, 2 hour time point) after systemic administration in tumor-bearing mice. The results of autoradiography experiments showed that 177Lu-A70-B104-DOTA selectively binds to PSMA-positive cancer tissues, while negligible binding on human salivary glands was observed.

Advances in PSMA Alpha Theragnostics

Alpha theranostics offer an attractive alternative form of therapy, which has best been investigated and documented with 225Ac-PSMA in patients with prostate cancer. Advantages offered by targeted alpha therapy include overcoming radiation resistance, oxygen independence, effecting double-stranded DNA breakages within the tumors with anticipated improved clinical outcomes and an acceptable side effect profile. The previous Seminars article on this topic, published in 2020, had to rely mostly on published case reports and small observational studies. In the last few years, however, several meta-analyses have emerged that evaluate the safety and efficacy of 225Ac-PSMA in prostate cancer patients, followed most recently by a multi-center retrospective study initiated by WARMTH. The findings of these publications, together with the exploration of TAT offered in clinical conditions other than as a last resort, is the focus of this updated overview. Unresolved clinical issues that remain, include the appropriate selection of patients that would benefit most from treatment with 225Ac-PSMA, treatment timing within the disease landscape, optimal dosing schedule, dosimetry, when and how to best use combination therapies and minimization and treatment of side effects, particularly that of xerostomia.

Prostate Cancer Theranostics With 177Lu-PSMA

This review paper highlights the transformative role of PSMA-targeted diagnostics and therapy in prostate cancer management, particularly focusing on 177Lu-PSMA-617, approved by the FDA and EMA for metastatic castration-resistant prostate cancer (mCRPC) patients post-chemotherapy and ARPI treatment. Originating from the VISION trial's success, this paper navigates the current radioligand therapy (RLT) indications, emphasizing practical patient selection, planning, and treatment execution. It critically examines Lu-PSMA's comparative effectiveness against cabazitaxel and Ra-223, addressing decision-making dilemmas for mCRPC treatments. Furthermore, the paper discusses Lu-PSMA in chemotherapy-naïve patients and its application in hormone-sensitive prostate cancer, underlined by ongoing global studies. A significant concern is Lu-PSMA's long-term safety profile, particularly nephrotoxicity risks, necessitating further investigation. The possibility of Lu-PSMA rechallenge in responsive patients is explored, stressing the need for comprehensive analyses and real-world data to refine treatment protocols. Conclusively, PSMA-targeted therapy marks a significant advance in prostate cancer therapy, advocating for its integration into a multimodal, patient-centric treatment approach. The review underscores the imperative for additional comparative studies to optimize treatment sequences and outcomes, ultimately enhancing long-term prognosis and disease control in prostate cancer management.

225Ac-iPSMA-RGD for Alpha-Therapy Dual Targeting of Stromal/Tumor Cell PSMA and Integrins

Prostate-specific membrane antigens (PSMAs) are frequently overexpressed in both tumor stromal endothelial cells and malignant cells (stromal/tumor cells) of various cancers. The RGD (Arg-Gly-Asp) peptide sequence can specifically detect integrins involved in tumor angiogenesis. This study aimed to preclinically evaluate the cytotoxicity, biokinetics, dosimetry, and therapeutic efficacy of 225Ac-iPSMA-RGD to determine its potential as an improved radiopharmaceutical for alpha therapy compared with the 225Ac-iPSMA and 225Ac-RGD monomers. HEHA-HYNIC-iPSMA-RGD (iPSMA-RGD) was synthesized and characterized by FT-IR, UV-vis, and UPLC mass spectroscopy. The cytotoxicity of 225Ac-iPSMA-RGD was assessed in HCT116 colorectal cancer cells. Biodistribution, biokinetics, and therapeutic efficacy were evaluated in nude mice with induced HCT116 tumors. In vitro results showed increased DNA double-strand breaks through ROS generation, cell apoptosis, and death in HCT116 cells treated with 225Ac-iPSMA-RGD. The results also demonstrated in vivo cytotoxicity in cancer cells after treatment with 225Ac-iPSMA-RGD and biokinetic and dosimetric properties suitable for alpha therapy, delivering ablative radiation doses up to 237 Gy/3.7 kBq to HCT116 tumors in mice. Given the phenotype of HCT116 cancer cells, the results of this study warrant further dosimetric and clinical studies to determine the potential of 225Ac-iPSMA-RGD in the treatment of colorectal cancer.

Precision strikes: PSMA-targeted radionuclide therapy in prostate cancer - a narrative review

Introduction

Radio-ligand targeted therapy is a new and promising concept of treatment Castration resistant prostate cancer (CRPC). Only a few radio-pharmaceutics were approved for usage in treating prostate cancer, among the multiple others tested. We aimed to review and summarize the literature on the therapeutic isotopes specific for PSMA.

Methods

We performed a scoping literature review of PubMed from January 1996 to December 2022.

Results

98 publications were selected for inclusion in this review. The studies contained in publications allowed to summarize the data on pharmacokinetics, therapeutic effects, side effects and the medical use of 225Ac and 177Lu radionuclides. The review also presents new research directions for specific PSMA radionuclides.

Conclusion

Radioligand targeted therapy is a new and promising concept where Lu-177-PSMA-617 have promising outcomes in treatment according to standard of care.

225Ac-PSMA-617 radioligand therapy of de novo metastatic hormone-sensitive prostate carcinoma (mHSPC): preliminary clinical findings

PURPOSE

225Ac-PSMA-617 has demonstrated good anti-tumor effect as a treatment option for metastatic castration-resistant prostate cancer (mCRPC) patients. No study has previously assessed treatment outcome and survival following 225Ac-PSMA-617 treatment of de novo metastatic hormone-sensitive prostate carcinoma (mHSPC) patients. Based on the potential side effects that are known and explained to the patients by the oncologist, some of the patients refused the standard treatment and are seeking alternative therapies. Thus, we report our preliminary findings in a retrospective series of 21 mHSPC patients that refused standard treatment options and were treated with 225Ac-PSMA-617.

METHODS

We retrospectively reviewed patients with histologically confirmed de novo treatment-naïve bone ± visceral mHSPC that were treated with 225Ac-PSMA-617 radioligand therapy (RLT). Inclusion criteria included an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2, treatment-naive bone ± visceral mHSPC, and patients refusal for ADT ± docetaxel, abiraterone acetate, or enzalutamide. We evaluated the response to treatment using prostate-specific antigen (PSA) response and the progression-free survival (PFS) and overall survival (OS) as well as the toxicities.

RESULTS

Twenty-one mHSPC patients were included in this preliminary work. Following treatment, twenty patients (95%) had any decline in PSA and eighteen patients (86%) presented with a PSA decline of ≥ 50% including 4 patients in whom PSA became undetectable. A lower percentage decrease in PSA following treatment was associated with increased mortality and shorter progression-free survival. Overall, administration of 225Ac-PSMA-617 was well tolerated. The commonest toxicity seen was grade I/II dry mouth observed in 94% of patients.

CONCLUSIONS

Given these favorable results, randomized prospective multicenter trials assessing the clinical value of 225Ac-PSMA-617 as a therapeutic agent for mHSPC administered either as monotherapy or administered concomitant with ADT are of interest.

Cross-reactivity to glutamate carboxypeptidase III causes undesired salivary gland and kidney uptake of PSMA-targeted small-molecule radionuclide therapeutics

PURPOSE

Recently, Pluvicto™ ([177Lu]Lu-PSMA-617), a small-molecule prostate-specific membrane antigen (PSMA) radioligand therapeutic, has been approved by the FDA in metastatic castration-resistant prostate cancer. Pluvicto™ and other PSMA-targeting radioligand therapeutics (RLTs) have shown side effects due to accumulation in certain healthy tissues, such as salivary glands and kidney. Until now, the molecular mechanism underlying the undesired accumulation of PSMA-targeting RLTs had not been elucidated.

METHODS

We compared the sequence of PSMA with the entire human proteome to identify proteins closely related to the target. We have identified glutamate carboxypeptidase III (GCPIII), N-acetylated alpha-linked acidic dipeptidase like 1 (NAALADL-1), and transferrin receptor 1 (TfR1) as extracellular targets with the highest similarity to PSMA. The affinity of compound 1 for PSMA, GCPIII, NAALADL-1, and TfR1 was measured by fluorescence polarization. The expression of the putative anti-target GCPIII was assessed by immunofluorescence on human salivary glands and kidney, using commercially available antibodies.

RESULTS

A fluorescent derivative of Pluvicto™ (compound 1) bound tightly to PSMA and to GCPIII in fluorescence polarization experiments, while no interaction was observed with NAALADL-1 and TfR1. Immunofluorescence analysis revealed abundant expression of GCPIII both in healthy human kidney and salivary glands.

CONCLUSION

We conclude that the membranous expression of GCPIII in kidney and salivary gland may be the underlying cause for unwanted accumulation of Pluvicto™ and other Glu-ureido PSMA radio pharmaceuticals in patients.

177Lu-PSMA Therapy for Metastatic Castration-Resistant Prostate Cancer: A Mini-Review of State-of-the-Art

BACKGROUND

Prostate specific membrane antigen (PSMA) ligand labeled with Lutetium-177 (177Lu) is a promising therapeutic option for metastatic castration-resistant prostate cancer (mCRPC). Several prospective and retrospective studies as well as clinical trials are completed or underway. This has ultimately led to the approval of this therapy by the US Food and Drug Administration (FDA) on March 23 2022. Our work aims to present a mini-review of the most recent research performed and the potential future directions of 177Lu-PSMA-radioligand therapy (RLT) for mCRPC patients.

MAIN BODY

For patients with mCRPCwho have met the eligibility criteria for 177Lu-PSMA RLT, numerous studies and trials are either ongoing or have been completed. The studies included in this review have reported overall biochemical response, defined as a prostate-specific antigen (PSA) decline of at least 50%, in at least 44% of patients with mCRPC. The median ranges of overall survival (OS) and radiographic progression-free survival (rPFS) were reported within 10.7-56 and 3.6-16 months, respectively. With data from several retrospective and prospective studies published, the safety of 177Lu-PSMA RLT in mCRPC has been confirmed and demonstrated by its low toxicity profile. Various studies have published pharmacokinetic/pharmacodynamic models to better understand the absorption, distribution, metabolism, and excretion of the RLT in this patient population. Findings have been published for 177Lu-PSMA RLT alone and in combination with other agents. We summarize their findings in our review.

CONCLUSIONS

The efficacy of 177Lu-PSMA RLT for patients with mCRPC has been proven thus far with promising results: PSA response, OS and rPFS when used alone or in combination with other treatment options, relative to the standard treatment options alone. The low toxicity profile noted also proves the safety of 177Lu-PSMA RLT in these patients.

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.

Salivary Gland Toxicity of PSMA-Targeted Radioligand Therapy with 177Lu-PSMA and Combined 225Ac- and 177Lu-Labeled PSMA Ligands (TANDEM-PRLT) in Advanced Prostate Cancer: A Single-Center Systematic Investigation

Purpose: PSMA-targeted radioligand therapy (PRLT) is a promising treatment option for patients with metastatic castration-resistant prostate cancer (mCRPC). However, a high uptake of the radiopharmaceutical in the salivary glands (SG) can lead to xerostomia and becomes dose-limiting for 225Ac-PSMA-617. This study investigated the sialotoxicity of 177Lu-PSMA-I&T/-617 monotherapy and co-administered 225Ac-PSMA-617 and 177Lu-PSMA-617 (Tandem-PPRLT). Methods: Three patient cohorts, that had undergone 177Lu-PSMA-I&T/-617 monotherapy or Tandem-PRLT, were retrospectively analyzed. In a short-term cohort (91 patients), a xerostomia assessment (CTCAE v.5.0), a standardized questionnaire (sXI), salivary gland scintigraphy (SGS), and SG SUVmax and the metabolic volume (MV) on 68Ga-PSMA-11-PET/CT were obtained before and after two cycles of 177Lu-PSMA-I&T/-617. In a long-term cohort, 40 patients were similarly examined. In a Tandem cohort, the same protocol was applied to 18 patients after one cycle of Tandem-PRLT. Results: Grade 1 xerostomia in the short-term follow-up was observed in 22 (24.2%) patients with a worsening of sXI from 7 to 8 at (p < 0.05). In the long-term cohort, xerostomia grades 1 to 2 occurred in 16 (40%) patients. SGS showed no significant changes, but there was a decline of the MV of all SGs. After Tandem-PRLT, 12/18 (66.7%) patients reported xerostomia grades 1 to 2, and the sXI significantly worsened from 9.5 to 14.0 (p = 0.005), with a significant reduction in the excretion fraction (EF) and MV of all SGs. Conclusion: 177Lu-PSMA-I&T/-617 causes only minor SG toxicity, while one cycle of Tandem-PRLT results in a significant SG impairment. This standardized protocol may help to objectify and quantify SG dysfunction.

JNMT continuing education: 177Lu PSMA therapy

Radiopharmaceutical therapy utilizing ¹⁷⁷Lu-PSMA is an effective treatment for prostate cancer which has recently been approved by the United States Food and Drug Administration. This method leverages the success of PSMA targeted PET imaging, enabling the delivery of targeted radiopharmaceutical therapy, This agent has demonstrated a clear benefit in large prospective clinical trials, and promises to become part of the standard armamentarium of treatment for patients with prostate cancer. In this review, the evidence supporting the use of this agent is highlighted, along with important areas now under investigation. Practical information on technology aspects, dose administration, nursing, and the role of the treating physician is highlighted. Overall, ¹⁷⁷Lu-PSMA treatment requires close collaboration between referring physicians, nuclear medicine, technologists, radiopharmacy, and nursing, to enable streamlined patient care.

Sublingual Atropine Administration as a Tool to Decrease Salivary Glands' PSMA-Ligand Uptake: A Preclinical Proof of Concept Study Using [68Ga]Ga-PSMA-11

Prostate Specific Membrane Antigen (PSMA)-directed radionuclide therapy has gained an important role in the management of advanced castration-resistant prostate cancer. Although extremely promising, the prolongation in survival and amelioration of disease-related symptoms must be balanced against the direct toxicities of the treatment. Xerostomia is amongst the most common and debilitating of these, particularly when using an alpha emitter. It is therefore of main importance to develop new preventive strategies. This preclinical study has evaluated the effect of α-adrenergic and anticholinergic drugs on [99mTc]TcO4− Single Photon Emission Computed Tomography/Computed Tomography (SPECT/CT) and [68Ga]Ga-PSMA-11 Positron Emission Tomography (PET/CT). Methods: The effects of phenylephrine, scopolamine, atropine, and ipratropium on salivary glands uptake were evaluated in non-tumor-bearing mice by [99mTc]TcO4− microSPECT/CT. The most efficient identified strategy was evaluated in non-tumor-bearing and xenografted mice by [68Ga]Ga-PSMA-11 PET/CT. Results: Scopolamine and atropine showed a significant decrease in the parotid glands’ uptake on SPECT/CT whereas phenylephrine and ipratropium failed. Atropine premedication (sublingual route), which was the most effective strategy, also showed a drastic decrease of [68Ga]Ga-PSMA-11 salivary glands’ uptake in both non-tumor-bearing mice (−51.6% for the parotids, p < 0.0001) and human prostate adenocarcinoma xenografted mice (−26.8% for the parotids, p < 0.0001). Conclusion: Premedication with a local administration of atropine could represent a simple, safe, and efficient approach for reducing salivary glands’ uptake.

Advances in PSMA theranostics

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PSMA is an appealing target for theranostic because it is a transmembrane protein with a known substrate that is overexpessed on prostate cancer cells and internalizes upon ligand binding.

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There are a number of PSMA theranostic ligands in clinical evaluation, clinical trial, or clinically approved.

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PSMA theranostic ligands increase progression-free survival, overall survival, and pain in patients with metastatic castration resistant prostate cancer.

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A major obstacle to PSMA-targeted radioligand therapy is off-target toxicity in salivary glands.

The validation of prostate specific membrane antigen (PSMA) as a molecular target in metastatic castration-resistant prostate cancer has stimulated the development of multiple classes of theranostic ligands that specifically target PSMA. Theranostic ligands are used to image disease or selectively deliver cytotoxic radioactivity to cells expressing PSMA according to the radioisotope conjugated to the ligand. PSMA theranostics is a rapidly advancing field that is now integrating into clinical management of prostate cancer patients. In this review we summarize published research describing the biological role(s) and activity of PSMA, highlight the most clinically advanced PSMA targeting molecules and biomacromolecules, and identify next generation PSMA ligands that aim to further improve treatment efficacy. The goal of this review is to provide a comprehensive assessment of the current state-of-play and a roadmap to achieving further advances in PSMA theranostics.

Global experience with PSMA-based alpha therapy in prostate cancer

PURPOSE

This review discusses the current state of prostate-specific membrane antigen (PSMA)-based alpha therapy of metastatic castration-resistant prostate cancer (mCRPC). With this in-depth discussion on the growing field of PSMA-based alpha therapy (PAT), we aimed to increase the interactions between basic scientists and physician-scientists in order to advance the field.

METHODS

To achieve this, we discuss the potential, current status, and opportunities for alpha therapy and strategies, attempted to date, and important questions that need to be addressed. The paper reviews important concepts, including whom to treat, how to treat, what to expect regarding treatment outcome, and toxicity, and areas requiring further investigations.

RESULTS

There is much excitement about the potential of this field. Much of the potential exists because these therapies utilize unique mechanisms of action, difficult to achieve with other conventional therapies.

CONCLUSION

A better understanding of the strengths and limitations of PAT may help in creating an effective therapy for mCRPC and design a rational combinatorial approach to treatment by targeting different tumor pathways.

The effect of eating on the uptake of PSMA ligands in the salivary glands

Rationale

PSMA-directed therapy for metastatic prostate cancer is gaining adoption as a treatment option. However, accumulation of ¹⁷⁷Lu/²²⁵Ac-PSMA in the salivary glands remains a problem, with risk of dose-limiting xerostomia and potentially severe effect on the quality of life. Gustatory stimulation is an approach that has commonly been used in radioactive iodine therapy to reduce accumulation in the salivary glands. However, based on theoretical differences in biodistribution, it was hypothesized that this could potentially lead to adverse increased toxicity for PSMA-ligand therapy. The primary objective of this work was to determine if gustatory stimulation by eating an assortment of sweet/fatty/acidic foods during the biodistribution phase of [¹⁸F]DCFPyl could result in a clinically relevant (> 30%) change in the uptake of the tracer in the salivary glands.

Methods

10 patients who already received a whole-body [¹⁸F]DCFPyl PET/CT scan for evaluation of prostate cancer, underwent a repeat (intervention) PET/CT scan within a month of the first (control) scan. During the intervention scan, patients chose from an assortment of sweet/fatty/acidic foods, which they then chewed and swallowed for a period of time starting 1 min before tracer administration to 10 min thereafter. Data from both scans were analyzed by placing VOIs on the major salivary glands and segmenting them using relative thresholds.

Results

A slight increase in PSMA uptake in the parotid glands was observed on the intervention scan when compared to the baseline scan (+ 7.1% SUL_mean and + 9.2% SUL_max, p < 0.05). No significant difference in PSMA uptake in the submandibular glands was seen.

Conclusions

Eating only slightly increases uptake of [¹⁸F]DCFPyl in the parotid glands. We nonetheless recommend refraining from gustatory stimulation during the administration and early biodistribution phase of radionuclide therapy with PSMA-ligands to reduce the risk of avoidable additional toxicity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-021-00838-y.

The Impact of Monosodium Glutamate on 68Ga-PSMA-11 Biodistribution in Men with Prostate Cancer: A Prospective Randomized, Controlled Imaging Study

The prostate-specific membrane antigen (PSMA) has been targeted for PET imaging and radioligand therapy (RLT) in patients with prostate cancer. Xerostomia is a common side effect of RLT because of the high salivary gland uptake of PSMA radioligands. Here, we aimed to determine the impact of monosodium glutamate (MSG) administration on PSMA-radioligand biodistribution within healthy organs and tumor lesions by using ⁶⁸Ga-PSMA-11 PET imaging. Methods: Sixteen men with prostate cancer were randomized (1:1) into oral ingestion and oral topical application ("swishing") arms. Each subject underwent 2 ⁶⁸Ga-PSMA-11 PET/CT scans within 14 d under baseline and MSG conditions. The salivary glands and whole-body tumor lesions were segmented using qPSMA software. We quantified tracer uptake via SUV_mean and SUV_max and compared parameters within each patient. Results: For the oral ingestion arm, salivary gland SUV_mean and SUV_max decreased on average from the control scan to the MSG scan by 45% ± 15% (P = 0.004) and 53% ± 11% (P < 0.001), respectively. Tumor lesion SUV_mean and SUV_max also decreased by 38% (interquartile range, -67% to -33%) and -52% (interquartile range, -70% to -49%), respectively (P = 0.018). Swishing had no significant effect on ⁶⁸Ga-PSMA-11 accumulation in normal organs or tumor lesions. Conclusion: Oral ingestion but not topical application of MSG reduced ⁶⁸Ga-PSMA-11 uptake in salivary glands. Tumor uptake also declined; therefore, the clinical application of MSG is unlikely to be useful in the framework of RLT.

PSMA-Targeting Radiopharmaceuticals for Prostate Cancer Therapy: Recent Developments and Future Perspectives

Simple Summary

One of the most frequently diagnosed cancer in men is adenocarcinoma of the prostate. Once the disease is metastatic, only very limited treatment options are available, resulting in a very short median survival time of 13 months; however, this reality is gradually changing due to the discovery of prostate-specific membrane antigen (PSMA), a protein that is present in cancerous prostate tissue. Researchers have developed pharmaceuticals specific for PSMA, ranging from antibodies (mAb) to low-molecular weight molecules coupled to beta minus and alpha-emitting radionuclides for their use in targeted radionuclide therapy (TRT). TRT offers the possibility of selectively removing cancer tissue via the emission of radiation or radioactive particles within the tumour. In this article, the major milestones in PSMA ligand research and the therapeutic developments are summarised, together with a future perspective on the enhancement of current therapeutic approaches.

Abstract

Prostate cancer (PC) is the second most common cancer among men, with 1.3 million yearly cases worldwide. Among those cancer-afflicted men, 30% will develop metastases and some will progress into metastatic castration-resistant prostate cancer (mCRPC), which is associated with a poor prognosis and median survival time that ranges from nine to 13 months. Nevertheless, the discovery of prostate specific membrane antigen (PSMA), a marker overexpressed in the majority of prostatic cancerous tissue, revolutionised PC care. Ever since, PSMA-targeted radionuclide therapy has gained remarkable international visibility in translational oncology. Furthermore, on first clinical application, it has shown significant influence on therapeutic management and patient care in metastatic and hormone-refractory prostate cancer, a disease that previously had remained immedicable. In this article, we provide a general overview of the main milestones in the development of ligands for PSMA-targeted radionuclide therapy, ranging from the firstly developed monoclonal antibodies to the current state-of-the-art low molecular weight entities conjugated with various radionuclides, as well as potential future efforts related to PSMA-targeted radionuclide therapy.

PSMA Theranostics: Science and Practice

Simple Summary

A significant number of prostate cancer patients will progress to metastatic castrate resistant prostate cancer despite optimal therapies. There is a growing need for alternative therapeutic strategies for this category of patients. Theragnostic refers to the ability to use an organ specific ligand and label it to both a diagnostic/imaging and therapeutic agent. Several prostate specific membrane antigen radioligands have been developed for imaging and treating PCa. Beta and alpha emitting radionuclides have been used with great success. Xerostomia is the greatest adverse event associated with radioligand therapy. More trials are necessary to determine the timing of introducing these novel therapies and to assess the efficacy as monotherapy as well as in combination with other novel agents to improve efficacy and reduce side effects to other organs.

Abstract

Prostate cancer (PCa) causes significant morbidity and mortality in men globally. While localized PCa may be managed with curative intent by surgery and/or radiation therapy, the management of advanced hormone resistant metastatic disease (mCRPC) is more challenging. Theranostics is a principle based on the ability to use an organ specific ligand and label it to both a diagnostic and a therapeutic agent. The overexpression of prostate specific membrane antigen (PSMA) on prostate cancer cells creates a unique opportunity for development of targeted radionuclide therapy. The use of both beta and alpha emitting particles has shown great success. Several clinical trials have been initiated assessing the efficacy and safety profile of these radionuclide agents. The results are encouraging with PSMA directed radioligand therapy performing well in patients who have exhausted all other standard treatment options. Future studies need to assess the timing of introduction of these radionuclide therapies in the management schema of mCRPC. Drugs or therapies are not without side effects and targeted radionuclide therapies presents a new set of toxicities including xerostomia and myelosuppression. New therapeutic strategies are being explored to improve outcomes while keeping toxicities to a minimum. This review aims to look at the various PSMA labelled tracers that form part of the theragnostic approach and subsequently delve into the progress made in the area of radionuclide therapy.

Safety of PSMA-targeted molecular radioligand therapy with 177Lu-PSMA-617: results from the prospective multicenter phase 2 trial RESIST-PC NCT03042312

Purpose of the study: To report the safety evaluation of ¹⁷⁷Lu-PSMA-617 derived from the cohort of 64 patients exposed to ¹⁷⁷Lu-PSMA-617 in the RESIST-PC trial NCT03042312 Methods: RESIST-PC was a prospective multicenter phase 2 trial. Patients with progressive mCRPC after ≥1 novel androgen-axis drug, either chemotherapy naïve or post-chemotherapy, with sufficient bone marrow reserve, normal kidney function, sufficient PSMA expression by PSMA PET and no visceral PSMA-negative lesions were eligible. Patients were randomized (1:1) into two activity groups (6.0 or 7.4 GBq per cycle) and received up to 4 cycles every 8 weeks. The primary safety endpoint was assessed by collecting and grading Adverse Events (AE) using the CTCAE. Patients were followed until disease progression, death, serious or intolerable AE, study termination by sponsor, patient withdrawal, lost to follow-up or 24 months after the first cycle. Results: The study was closed at enrollment of 71/200 planned patients because of sponsorship transfer. A total of 64 (90.1%) patients received at least one cycle of ¹⁷⁷Lu-PSMA-617: 28 (36%) in Arm 1 (6.0 GBq) and 41 (64%) in Arm 2 (7.4GBq). There were 10 (43.5%), 19 (46.5%) and 29 (45.3%) patients who completed 4 cycles of ¹⁷⁷Lu-PSMA-617 in the 6.0 GBq arm, 7.4 GBq arm, and overall, respectively. The most common treatment-emergent adverse events (TEAEs) of any grade in the 6.0 GBq arm, the 7.4 GBq arm and overall, were dry mouth (47.8%; 63.4%; 57.8%, respectively), fatigue (56.5%; 51.2%; 53.1%), nausea (52.2%; 43.9%; 46.9%), and diarrhea (13.0%; 31.7%; 25.0%). Frequencies of all other TEAEs were comparable among the 2 groups (within 10% difference). Serious possibly drug-related TEAEs were reported for 5 (7.8%) patients overall (none were considered as probably or definitely related to treatment): one subdural hematoma Grade 4, one anemia grade 3, one thrombocytopenia grade 4, one gastrointestinal hemorrhage grade 3, and one acute kidney injury grade 3. There were no clinically significant changes in vital signs in ECGs in the 2 treatment groups. No trend to creatinine increase, or increasing frequency of shifts from normal to abnormal over time for any hematologic parameter was noted. Conclusion: ¹⁷⁷Lu-PSMA-617 was safe and well-tolerated at 6.0 and 7.4 GBq per cycle given at 8-week intervals with side effects easily managed with standard medical support. With established safety, further clinical trials applying individualized dosimetry and testing different ¹⁷⁷Lu-PSMA-617 administration schemes (activity levels, time intervals) are needed to optimize tumor dose delivery and treatment efficacy.

68Ga-PSMA PET/CT: effect of external cooling on salivary gland uptake

Objective

To evaluate the effect that external cooling of the salivary glands (ECSG) has on the uptake of gallium-68-labeled prostate-specific membrane antigen (⁶⁸Ga-PSMA), as an indirect assessment of the capacity of ECSG to reduce the local dose in lutetium-177-PSMA-617 radioligand therapy.

Materials and Methods

Ten patients with prostate cancer were submitted to ⁶⁸Ga-PSMA positron emission tomography/computed tomography with unilateral ECSG. The ECSG was started at 30 min before the injection of the radiotracer and maintained until the end of image acquisition (1 h after injection). Each salivary gland was assessed by determining the maximum, mean, and peak standardized uptake values (SUVmax, SUVmean, and SUVpeak, respectively). The volume of each gland was determined in a volume of interest delineated by a threshold SUVmax of 10%. Paired Student's t-tests were used in order to compare the results.

Results

In terms of the SUV parameters, there were no statistically significant differences between the cooled and contralateral salivary glands. However, the mean volume was 27% lower in the cooled parotid glands than in the contralateral parotid glands (p = 0.004).

Conclusion

The use of ECSG does not appear to reduce ⁶⁸Ga-PSMA uptake by the salivary glands. In addition, there is yet no evidence that ECSG is effective in preventing salivary gland toxicity.

Muscarinic inhibition of salivary glands with glycopyrronium bromide does not reduce the uptake of PSMA-ligands or radioiodine

RATIONALE

Salivary glands are highly perfused and express the prostate-specific membrane antigen (PSMA) receptor as well as the sodium-iodide symporter. As a consequence, treatment with ¹⁷⁷Lu/²²⁵Ac-PSMA for prostate cancer or ¹³¹I for thyroid cancer leads to a high radiation dose in the salivary glands, and patients can be confronted with persistent xerostomia and reduced quality of life. Salivation can be inhibited using an antimuscarinic pharmaceutical, such as glycopyrronium bromide (GPB), which may also reduce perfusion. The primary objective of this work was to determine if inhibition with GPB could provide a considerable (> 30%) reduction in the accumulation of administered ¹²³I or ⁶⁸Ga-PSMA-11 in salivary glands.

METHODS

Ten patients who already received a whole-body ⁶⁸Ga-PSMA-11 PET/CT scan for (re)staging of prostate cancer underwent a repeat PET/CT scan with tracer administration at 90 min after intravenous injection of 0.2 mg GPB. Four patients in follow-up after thyroid cancer, who had been treated with one round of ablative ¹³¹I therapy with curative intent and had no signs of recurrence, received ¹²³I planar scintigraphy at 4 h after tracer administration without GPB and a repeated scan at least one week later, with tracer administration at 30 min after intramuscular injection of 0.4 mg GPB. Tracer uptake in the salivary glands was quantified on PET and scintigraphy, respectively, and values with and without GPB were compared.

RESULTS

No significant difference in PSMA uptake in the salivary glands was seen without or with GPB (Mean SUL_mean parotid glands control 5.57, intervention 5.72, p = 0.50. Mean SUL_mean submandibular glands control 6.25, intervention 5.89, p = 0.12). Three out of 4 patients showed increased ¹²³I uptake in the salivary glands after GPB (Mean counts per pixel control 8.60, intervention 11.46).

CONCLUSION

Muscarinic inhibition of salivation with GPB did not significantly reduce the uptake of PSMA-ligands or radioiodine in salivary glands, and can be dismissed as a potential strategy to reduce toxicity from radionuclide therapies.

Design of PSMA ligands with modifications at the inhibitor part: an approach to reduce the salivary gland uptake of radiolabeled PSMA inhibitors?

AIM

To investigate whether modifications of prostate-specific membrane antigen (PSMA)-targeted radiolabeled urea-based inhibitors could reduce salivary gland uptake and thus improve tumor-to-salivary gland ratios, several analogs of a high affinity PSMA ligand were synthesized and evaluated in in vitro and in vivo studies.

METHODS

Binding motifs were synthesized 'on-resin' or, when not practicable, in solution. Peptide chain elongations were performed according to optimized standard protocols via solid-phase peptide synthesis. In vitro experiments were performed using PSMA⁺ LNCaP cells. In vivo studies as well as μSPECT/CT scans were conducted with male LNCaP tumor xenograft-bearing CB17-SCID mice.

RESULTS

PSMA ligands with A) modifications within the central Zn²⁺-binding unit, B) proinhibitor motifs and C) substituents & bioisosteres of the P1'-γ-carboxylic acid were synthesized and evaluated. Modifications within the central Zn²⁺-binding unit of PSMA-10 (Glu-urea-Glu) provided three compounds. Thereof, only ^natLu-carbamate I (^natLu-3) exhibited high affinity (IC₅₀ = 7.1 ± 0.7 nM), but low tumor uptake (5.31 ± 0.94% ID/g, 1 h p.i. and 1.20 ± 0.55% ID/g, 24 h p.i.). All proinhibitor motif-based ligands (three in total) exhibited low binding affinities (> 1 μM), no notable internalization and very low tumor uptake (< 0.50% ID/g). In addition, four compounds with P1'-ɣ-carboxylate substituents were developed and evaluated. Thereof, only tetrazole derivative ^natLu-11 revealed high affinity (IC₅₀ = 16.4 ± 3.8 nM), but also this inhibitor showed low tumor uptake (3.40 ± 0.63% ID/g, 1 h p.i. and 0.68 ± 0.16% ID/g, 24 h p.i.). Salivary gland uptake in mice remained at an equally low level for all compounds (between 0.02 ± 0.00% ID/g and 0.09 ± 0.03% ID/g), wherefore apparent tumor-to-submandibular gland and tumor-to-parotid gland ratios for the modified peptides were distinctly lower (factor 8-45) than for [¹⁷⁷Lu]Lu-PSMA-10 at 24 h p.i.

CONCLUSIONS

The investigated compounds could not compete with the in vivo characteristics of the EuE-based PSMA inhibitor [¹⁷⁷Lu]Lu-PSMA-10. Although two derivatives (3 and 11) were found to exhibit high affinities towards LNCaP cells, tumor uptake at 24 h p.i. was considerably low, while uptake in salivary glands remained unaffected. Optimization of the established animal model should be envisaged to enable a clear identification of PSMA-targeting radioligands with improved tumor-to-salivary gland ratios in future studies.

A simple strategy to reduce the salivary gland and kidney uptake of PSMA-targeting small molecule radiopharmaceuticals

PURPOSE

Peptide-based prostate-specific membrane antigen (PSMA) targeted radionuclide therapy (TRT) agent [¹⁷⁷Lu]-PSMA-617 has emerged as leading TRT candidate for treatment of castration-resistant prostate cancer (mCRPC). [¹⁷⁷Lu]-PSMA-617 and other small molecule-based PSMA ligands have shown efficacy in reducing the tumor burden in mCRPC patients but irradiation to the salivary gland and kidneys is a concern and dose-limiting factor. Therefore, methods to reduce non-target organ toxicity are needed to safely treat patients and preserve their quality of life. Herein, we report that addition of cold PSMA ligand PSMA-11 can aid in reducing the uptake of [¹⁷⁷Lu]-PSMA-617 in the salivary glands and kidneys.

METHODS

Groups of athymic nude mice (n = 4) bearing PC3-PIP (PSMA+) tumor xenografts were administered with [¹⁷⁷Lu]-PSMA-617 along with 0, 5, 100, 500, 1000, and 2000 pmoles of PSMA-11 and biodistribution studies were performed at 1 h.

RESULTS

Biodistribution studies at 1 h post-administration revealed that [¹⁷⁷Lu]-PSMA-617 uptake in PC3-PIP tumors was 21.71 ± 6.13, 18.7 ± 2.03, 26.44 ± 2.94, 16.21 ± 3.5, 13.52 ± 3.68, and 12.03 ± 1.96 %ID/g when 0, 5, 100, 500, 1000, and 2000 pmoles of PSMA-11 were added, respectively. Corresponding uptake values in kidney were 123.14 ± 52.52, 132.31 ± 47.4, 84.29 ± 78.25, 2.12 ± 1.88, 1.16 ± 0.36, and 0.64 ± 0.23 %ID/g, respectively. Corresponding salivary gland uptake values were 0.48 ± 0.11, 0.45 ± 0.15, 0.38 ± 0.3, 0.08 ± 0.03, 0.09 ± 0.07, and 0.05 ± 0.02 % ID/g, respectively.

CONCLUSION

The uptake of [¹⁷⁷Lu]-PSMA-617 in the salivary gland and kidney can be substantially reduced without significantly impacting tumor uptake by adding cold PSMA-11.

The 68Ga/177Lu-theragnostic concept in PSMA-targeting of metastatic castration-resistant prostate cancer: impact of post-therapeutic whole-body scintigraphy in the follow-up

INTRODUCTION

A new therapeutic option for metastatic castration-resistant prostate cancer (mCRPC) of heavily pre-treated patients lies in ¹⁷⁷Lu-PSMA-617 radioligand therapy.

METHODS

On the basis of PSMA-targeted ⁶⁸Ga-PSMA-11 PET/CT, 32 consecutive mCRPC patients were selected for ¹⁷⁷Lu-PSMA-617 therapy (6 GBq/cycle, 2 to 6 cycles, 6-10 weeks apart) and followed until death. Post-therapy whole-body (WB) dosimetry and ⁶⁸Ga-PSMA-11 PET/CT data were compared and related to progression free and overall survival.

RESULTS

¹⁷⁷Lu-PSMA-617 dosimetry after the first cycle indicated high tumor doses for skeletal (4.01 ± 2.64; range 1.10-13.00 Gy/GBq), lymph node (3.12 ± 2.07; range 0.70-8.70 Gy/GBq), and liver (2.97 ± 1.38; range 0.76-5.00 Gy/GBq) metastases whereas the dose for tissues/organs was acceptable in all patients for an intention-to-treat activity of 24 GBq. Any PSA decrease after the first cycle was found in 23/32 (72%), after the second cycle in 22/32 (69%), after the third cycle in 16/28 (57%), and after the fourth cycle in 8/18 (44%) patients. Post-therapy 24 h WB scintigraphy showed decreased tumor-to-background ratios in 24/32 (75%) after the first therapy cycle, after the second cycle in 17/29 (59%), and after the third cycle in 13/21 (62%) patients. The median PFS was 7 months and the median OS 12 months. In the group of PSA responders (n = 22) the median OS was 17 months versus 11 months in the group of non-responders (n = 10), p < 0.05. Decreasing SUV_max values were found for parotid (15.93 ± 6.23 versus 12.33 ± 4.07) and submandibular glands (17.65 ± 7.34 versus 13.12 ± 4.62) following treatment, along with transient (n = 6) or permanent (n = 2) xerostomia in 8/32 (25%) patients. In 3/32 patients, nephrotoxicity changed from Grade 2 to 3, whereas neither Grade 4 nephrotoxicity nor hematotoxicity was found. In most patients a good agreement was observed for the visual interpretation of the tracer accumulation between 24 h WB and PET/CT scans. However, no significance could be calculated for baseline-absorbed tumor doses and SUV_max values of tumor lesions. 5/32 (16%) patients showed a mixed response pattern, which resulted in disease progression over time.

CONCLUSION

Serial PSA measurements and post-therapy 24 h WB scintigraphy seems to allow a sufficiently accurate follow-up of ¹⁷⁷Lu-PSMA-617-treated mCRPC patients whereas ⁶⁸Ga-PSMA-11 PET/CT should be performed for patient selection and final response assessment.

Lutetium-177 prostate-specific membrane antigen (PSMA) theranostics: practical nuances and intricacies

Theranostic principles utilize a molecular biomarker specific for a tumor target, initially for imaging to assess target expression and, if deemed suitable, for targeted therapy. This presents an exciting opportunity for a highly personalized treatment strategy in the era of precision medicine. Prostate-specific membrane antigen (PSMA) theranostics has attracted increasing attention as a promising targeted treatment in metastatic prostate cancer (PC). ¹⁷⁷Lu-DOTA-PSMA-617 (¹⁷⁷Lu-PSMA-617) is a PSMA-targeted small molecule with favorable properties and is the most extensively investigated PSMA radioligand for radionuclide therapy (RNT) in PC. Since 2014 multiple retrospective studies and more recently a phase II prospective study demonstrated safety and impressive efficacy of ¹⁷⁷Lu-PSMA RNT. The evidence generated by these trials led to two currently underway randomized trials in metastatic castrate-resistant PC: TheraP (NCT03392428) and VISION (NCT03511664). While we wait for these pivotal trials to read out, nuclear medicine physicians, medical oncologists, radiation oncologists, and urologists are facing a steep learning curve to master the intricacies and nuances of this novel therapeutic strategy. This review article aims to share and discuss the evolving experience in practical aspects of PSMA theranostics.

PSMA-Based Theranostics: A Step-by-Step Practical Approach to Diagnosis and Therapy for mCRPC Patients

To date, several papers have been published about prostate-specific membrane antigen (PSMA)-based radioligand diagnostic and therapeutic approaches. This paper mainly provides information for nuclear medicine physicians that are clinically engaged in the diagnosis and treatment of prostate cancer patients. It aims to present the utility of PSMA imaging and therapy in a step-by-step practical approach; thus, it does not discuss radiochemistry and the molecular basics of PSMA.