Phase 2 Study of 99mTc-Trofolastat SPECT/CT to Identify and Localize Prostate Cancer in Intermediate- and High-Risk Patients Undergoing Radical Prostatectomy and Extended Pelvic LN Dissection

DNA-Targeted Complexes of Tc and Re for Biomedical Applications

Due to their favorable chemical features, Re and Tc complexes have been widely used for the development of new therapeutic agents and imaging probes to solve problems of biomedical relevance. This review provides an update of the most relevant research efforts towards the development of novel cancer theranostic agents using Re and Tc-based compounds interacting with specific DNA structures. This includes a variety of homometallic complexes, namely those containing M(CO)3 (M=Re, Tc) moieties, that exhibit different modes of interaction with DNA, such as covalent binding, intercalation, groove binding or G-quadruplex DNA binding. Additionally, heterometallic complexes, designed to potentiate synergistic effects of different metal centers to improve DNA-targeting, cytotoxicity and fluorescence properties, are also reviewed. Particular attention is also given to 99m Tc- and 188 Re-labeled oligonucleotides that have been widely explored to develop imaging and therapeutic radiopharmaceuticals through the in vivo hybridization with a specific complementary DNA or RNA target sequence to provide useful molecular tools in precision medicine for cancer diagnosis and treatment. Finally, the need for further improvement of DNA-targeted Re and Tc-based compounds as potential therapeutic and diagnostic agents is highlighted, and future directions are discussed.

[99mTc]Tc-HYNIC-ALUG SPECT/CT in the initial staging of 227 consecutive patients with newly diagnosed prostate cancer: a 5-year monocentric retrospective study

Purpose

The purpose of this study was to assess the effectiveness of [99mTc]Tc-HYNIC-ALUG SPECT/CT in the initial staging of patients with newly diagnosed PCa.

Methods

A retrospective analysis was conducted on 227 consecutive patients who underwent [99mTc]Tc-HYNIC-ALUG SPECT/CT imaging for the primary staging of newly diagnosed PCa. The presence and location of PSMA-positive lesions were determined, and the maximum standardized uptake values (SUVmax) of the primary prostate tumor were also measured. The metastatic findings and SUVmax were stratified according to International Society of Urological Pathology (ISUP) grade, prostate-specific antigen (PSA) levels, and D'Amico classification. Furthermore, the [99mTc]Tc-HYNIC-ALUG SPECT/CT findings were compared to the histopathological findings in patients who had undergone radical prostatectomy with pelvic lymph node dissection (PLND).

Results

Of the 227 patients, 92.1% (209/227) had positive [99mTc]Tc-HYNIC-ALUG SPECT/CT findings. Advanced disease was detected in 38.8% (88/227) of the patients and was positively correlated with increasing ISUP grade and PSA levels. Lymph node metastases (both pelvic and extrapelvic), bone metastases, and visceral metastases were detected in 30.0% (68/227), 25.6% (58/227), and 3.1% (7/227) of the patients, respectively. For the 129 patients who underwent radical prostatectomy with PLND, the sensitivity of [99mTc]Tc-HYNIC-ALUG SPECT/CT in the evaluation of PCa was 90.7% (117/129). The sensitivity, specificity, accuracy, and positive and negative predictive values for detecting pelvic lymph node metastases on [99mTc]Tc-HYNIC-ALUG SPECT/CT were 23.5% (12/51), 93.6% (73/78), 65.9% (85/129), 70.6% (12/17), and 65.2% (73/112), respectively. Among the 209 patients with PSMA-avid primary prostate disease, the SUVmax of the primary prostate tumor was significantly associated with ISUP grade (p<0.0001), PSA levels (p<0.0001), D'Amico classification (p<0.0001), and advanced disease (p<0.0001). Receiver operating characteristic (ROC) analysis revealed that a PSA level >19.8 ng/ml and SUVmax of the primary prostate tumor >7.4 had a sensitivity of 71.6% and 71.6% and specificity of 76.9% and 82.6%, respectively, for detecting metastatic disease.

Conclusions

[99mTc]Tc-HYNIC-ALUG SPECT/CT emerges as a valuable imaging tool for the initial staging of newly diagnosed PCa. The presence of advanced disease and the SUVmax of the primary prostate tumor were positively correlated with ISUP grade and PSA levels.

The Theranostic Optimization of PSMA-GCK01 Does Not Compromise the Imaging Characteristics of [99mTc]Tc-PSMA-GCK01 Compared to Dedicated Diagnostic [99mTc]Tc-EDDA/HYNIC-iPSMA in Prostate Cancer

PURPOSE

Radiolabeled PSMA-ligands play a major role in today's nuclear medicine. Since approval of [177Lu]Lu-PSMA-617 for therapy of metastatic prostate cancer, availability of 177Lu became bottleneck of supply due to the high demand. Recently, a theranostic PSMA-ligand, PSMA-GCK01, was developed which can be labeled either diagnostically with 99mTc or therapeutically with 188Re with both nuclides available from well-known generator systems. This novel tracer might aid to overcome aforementioned supply limitations. In this investigation, the biodistribution and general imaging characteristics of [99mTc]Tc-PSMA-GCK01 were compared with the diagnostic reference compound [99mTc]Tc-EDDA/HYNIC-iPSMA in patients with advanced stage prostate cancer. In addition, the binding of both ligands to PSMA was analyzed at the molecular level using molecular docking.

PROCEDURES

Two cohorts (n = 19 vs. n = 21) of patients with metastatic castration-resistant prostate cancer matched for age, tumor stage, and Gleason score underwent a planar gamma camera imaging with [99mTc]Tc-EDDA/HYNIC-iPSMA or [99mTc]Tc-PSMA-GCK01 prior to PSMA-ligand therapy for PSMA-phenotyping. The imaging data were retrospective analyzed for salivary gland, kidney, liver, soft tissue, and tumor uptake on a semi-automated ROI-analysis using HERMES Medical Solutions AB (HMS, Sweden).

RESULTS

The data sets were semi-automated quantified on a ROI-based analysis. The tumor-to-background presented equal results of [99mTc]Tc-PSMA-GCK01 compared to [99mTc]Tc-EDDA/HYNIC-iPSMA. The physiological PSMA-positive organs like salivary gland presented also equal uptake in counts/MBq (salivary gland median 9.48 [99mTc]Tc-PSMA-GCK01 vs. median 9.11 [99mTc]Tc-EDDA/HYNIC-iPSMA), while liver-to-kidney ratio presented a slight shift to the liver parenchyma using [99mTc]Tc-PSMA-GCK01 (0.83) compared to [99mTc]Tc-EDDA/HYNIC-iPSMA (0.55) with no statistical significance. This is in agreement with the results from the docking study revealing only a minor difference in the docking scores for both ligands.

CONCLUSIONS

The novel theranostic tracer [99mTc]Tc/[188Re]Re-PSMA-GCK01 demonstrates comparable general imaging characteristic with the reference compound [99mTc]Tc-EDDA/HYNIC-iPSMA. These results pave the way for the PSMA-targeting imaging and theranostic agents for a broader, rather low-cost, generator applied radio-ligand therapy utilization.

Diagnostic Performance of 99mTc-iPSMA SPECT/CT in the Initial Staging of Patients with Unfavorable Intermediate-, High-, and Very High-Risk Prostate Cancer: A Comparative Analysis with 18F-PSMA-1007 PET/CT

Prostate cancer is a leading cause of cancer death in men worldwide. Imaging plays a key role in disease detection and initial staging. Emerging data has shown the superiority of PSMA imaging with PET/CT over conventional imaging for primary diagnoses. Single photon emission computed tomography is more available worldwide, and the imaging agent is low in cost. The aim of this study is to compare the diagnostic accuracy of 99mTc-EDDA/HYNIC-iPSMA SPECT/CT to 18F-PSMA-1007 PET/CT in the primary diagnosis of prostate cancer and the impact on clinical staging.

METHODS

In this prospective controlled study, 18 patients with histologically confirmed prostate cancer with unfavorable intermediate-, high-, and very high-risk characteristics were recruited to undergo 18F-PSMA-PET/CT and 99mTc-iPSMA SPECT/CT. The median age of the patients was 71 years old, and the median PSA level was 23.3 ng/mL. Lesions were divided into the prostate, seminal vesicles, lymph nodes, bone, and visceral metastases. Volumetric analysis was also performed between the two imaging modalities and correlated with PSA levels.

RESULTS

A total of 257 lesions were detected on 18F-PSMA-PET/CT: prostate (n = 18), seminal vesicles (n = 12), locoregional lymph nodes (n = 62), non-locoregional (n = 67), bone (n = 90), and visceral (n = 8). Of these, 99mTc-iPSMA-SPECT/CT detected 229 lesions, while both reviewers detected 100% of the lesions in the prostate (18/18), seminal vesicles (12/12), and visceral (8/8); LN LR (56/62; 90%), NLR (57/67; 85%), and bone (78/90; 86%). There were no statistically significant differences between volumetric parameters (t = -0.02122; p = 0.491596).

CONCLUSIONS

99mTc-iPSMA SPECT/CT is useful in the primary diagnosis of prostate cancer. Despite it showing a slightly lower lesion detection rate compared to 18F-PSMA PET/CT, it exhibited no impact on clinical staging and, consequently, the initial treatment intention.

Radiometals in Imaging and Therapy: Highlighting Two Decades of Research

The present article highlights the important progress made in the last two decades in the fields of molecular imaging and radionuclide therapy. Advancements in radiometal-based positron emission tomography, single photon emission computerized tomography, and radionuclide therapy are illustrated in terms of their production routes and ease of radiolabeling. Applications in clinical diagnostic and radionuclide therapy are considered, including human studies under clinical trials; their current stages of clinical translations and findings are summarized. Because the metalloid astatine is used for imaging and radionuclide therapy, it is included in this review. In regard to radionuclide therapy, both beta-minus (β-) and alpha (α)-emitting radionuclides are discussed by highlighting their production routes, targeted radiopharmaceuticals, and current clinical translation stage.

Synthesis and Radiolabeling of Glu-Urea-Lys with 99mTc-Tricarbonyl-Imidazole-Bathophenanthroline Disulfonate Chelation System and Biological Evaluation as Prostate-Specific Membrane Antigen Inhibitor

Background: The Glu-Urea-Lys (EUK) pharmacophore as prostate-specific membrane antigen (PSMA)-targeted ligand was synthesized, radiolabeled with 99mTc-tricarbonyl-imidazole-BPS chelation system, and biological activities were evaluated. The strategy [2 + 1] ligand is applied for tricarbonyl labeling. (5-imidazole-1-yl)pentanoic acid as a monodentate ligand and bathophenanthroline disulfonate (BPS) as a bidentate ligand formed a chelate system with 99mTc-tricarbonyl. EUK-pentanoic acid-imidazole and EUK were evaluated for PSMA active site using AutoDock 4 software. Materials and Methods: EUK-pentanoic acid-imidazole was synthesized in two steps. BPS was radiolabeled with 99mTc-tricarbonyl at 100°C for 30 min. The purified 99mTc(CO)3(H2O)BPS was used to radiolabel EUK-pentanoic acid-imidazole at 100°C, 30 min. Radiochemical purity, Log P, and stability studies were carried out within 24 h. Affinity of 99mTc(CO)3BPS-imidazole-EUK was performed in the saturation binding studies using LNCaP cells at 37°C for 1 h with a range of 0.001-1000 nM radiolabeled compound range. Internalization studies were performed in LNCaP cells with 1000 nM radiolabeled compound incubated for (0-2) h at 37°C. Biodistribution was studied in normal male Balb/c mice. The artificial intelligence predicts the uptake of radiolabeled compound in tumor. Results: The structures of synthesized compounds were confirmed by mass spectroscopy. Radiochemical purity, Log P, and protein binding were ≥95%, -0.2%, and 23%, respectively. The radiolabeled compound was stable in saline and human plasma within 24 h with radiochemical purity ≥90%. There was no release of 99mTc within 4 h in competition with histidine. The affinity was 82 ± 26.38 nM, and the activity increased inside the cells over time. Biodistribution studies showed radioactivity accumulation in kidneys less than 99mTc-HYNIC-PSMA. There was a moderate accumulation of radioactivity in the liver and intestine. Conclusion: Based on the results, 99mTc(CO)3BPS-imidazole-EUK can potentially be used as an imaging agent for studies at prostate bed and distal areas. The chelate system can be potentially labeled with rhenium for imaging studies (fluorescent or scintigraphy) and therapy.

An Australian experience using Tc-PSMA SPECT/CT in the primary diagnosis of prostate cancer and for staging at biochemical recurrence after local therapy

BACKGROUND

Technetium 99 prostate-specific membrane antigen (Tc-PSMA) single-photon emission computed tomography/computed tomography (SPECT/CT) has the potential to provide greater accessibility globally than gallium 68 (Ga)-PSMA positron emission tomography (PET)/CT but has not been studied as extensively in primary diagnosis, staging, or relapse of prostate cancer (PC). We instituted a novel SPECT/CT reconstruction algorithm using Tc-PSMA and established a database to prospectively accumulate data on all patients referred with PC. This study extracts data on all patients referred over a 3.5-year period with the primary aim of comparing the diagnostic accuracy of Tc-PSMA and multiparametric magnetic resonance imaging (mpMRI) in the primary diagnosis of PC. The secondary aim was to assess the sensitivity of Tc-PSMA in detecting disease with relapse after either radical prostatectomy or primary radiotherapy.

METHODS

A total of 425 men referred for primary staging (PS) of PC and 172 men referred with biochemical relapse (BCR) were evaluated. We evaluated diagnostic accuracy and correlations between Tc-PSMA SPECT/CT, magnetic resonance imaging (MRI), prostate biopsy, prostate-specific antigen (PSA), and age in the PS group and positivity rates at different PSA levels in the BCR group.

RESULTS

Taking the biopsy's grade according to the International Society of Urological Pathology protocol as a reference, the sensitivity (true positive rate), specificity (true negative rate), accuracy (positive and negative predictive value), and precision (positive predictive value) for Tc-PSMA in the PS group were 99.7%, 83.3%, 99.4%, and 99.7%, respectively. Comparison rates for MRI in this group were 96.4%, 71.4%, 95.7%, and 99.1%. We found moderate correlations between Tc-PSMA uptake in the prostate and biopsy grade, the presence of metastases, and PSA. In BCR, the Tc-PSMA positive rates were 38.9%, 53.2%, 62.5%, and 84.6% at PSA levels of <0.2, 0.2 to <0.5, 0.5 to <1.0, and > 1.0 ng/mL respectively.

CONCLUSIONS

We have shown that Tc-PSMA SPECT/CT using an enhanced reconstruction algorithm has a diagnostic performance similar to Ga-PSMA PET/CT and mpMRI in an everyday clinical setting. It may have some advantages in cost, sensitivity for primary lesion detection, and the ability for intraoperative localization of lymph nodes.

Eligibility for 177Lu-PSMA Therapy Depends on the Choice of Companion Diagnostic Tracer: A Comparison of 68Ga-PSMA-11 and 99mTc-MIP-1404 in Metastatic Castration-Resistant Prostate Cancer

¹⁷⁷Lu-prostate-specific membrane antigen-617 (¹⁷⁷Lu-PSMA-617) is an effective therapy for metastatic castration-resistant prostate cancer (mCRPC), with evidence of improved survival over standard care. The VISION trial inclusion criteria required a metastatic lesion-to-liver ratio of greater than 1 on ⁶⁸Ga-PSMA-11 PET scans. We aimed to determine whether an equivalent ratio is suitable for a SPECT tracer, ^99mTc-MIP-1404, and to compare lesion and lesion-to-normal-organ ratios between the 2 radiotracers. Methods: Two cohorts of patients with mCRPC matched for age, prostate-specific antigen level, and total Gleason score, with either ^99mTc-MIP-1404 SPECT/CT (n = 25) or ⁶⁸Ga-PSMA-11 PET/CT (n = 25) scans, were included for analysis. Up to 3 lesions in each site (prostate/prostate bed, lymph nodes, bone and soft-tissue metastases) as well as normal liver, parotid gland, spleen, and mediastinal blood-pool SUV_max were measured. Results: ^99mTc-MIP-1404 SPECT lesion SUV_max was not significantly different from ⁶⁸Ga-PSMA-11 PET (median, 18.2 vs. 17.3; P = 0.93). However, ^99mTc-MIP-1404 liver SUV_max was higher (median, 8.5 vs. 5.8; P = 0.002) and lesion-to-liver ratios were lower (median, 2.7 vs. 3.5; P = 0.009). There was no significant difference in parotid gland or splenic SUV_max or lesion-to-parotid gland ratios between the 2 tracers although there was a small difference in lesion-to-spleen ratios (P = 0.034). Conclusion: There are differences in biodistribution and, in particular, liver activity, between ⁶⁸Ga-PSMA-11 and ^99mTc-MIP-1404. Therefore, if ^99mTc-MIP-1404 is used to assess eligibility for ¹⁷⁷Lu-PSMA-617 therapy, a lower adjusted lesion-to-liver ratio should be used.

Feasibility of 99mTc-MIP-1404 for SPECT/CT Imaging and Subsequent PSMA-Radioguided Surgery in Early Biochemically Recurrent Prostate Cancer: A Case Series of 9 Patients

This case series evaluated the feasibility of prostate-specific membrane antigen (PSMA)-radioguided surgery (RGS) with ^99mTc-MIP-1404 in recurrent prostate cancer. Methods: Nine patients with PSMA-positive lesions on PET/CT received ^99mTc-MIP-1404 (median, 747 MBq; interquartile range [IQR], 710-764 MBq) 17.2 h (IQR, 16.9-17.5 h) before SPECT/CT and 22.3 h (IQR, 20.8-24.0 h) before RGS. Results: Seventeen PSMA-positive lesions were detected on PET/CT (median short-axis diameter, 4 mm; IQR, 3-6 mm; median SUV_max, 8.9; IQR, 5.2-12.6). Nine of 17 (52.9%) were visible on SPECT/CT (median SUV_max, 13.8; IQR, 8.0-17.9). Except for 2 foci, all PET/CT-positive findings demonstrated intraoperative count rates above the background level (median count, 31; IQR, 17-89) and were lymph node metastases. Moreover, PSMA-RGS identified 2 additional metastases compared with PET/CT. Prostate-specific antigen values decreased after RGS in 6 of 9 patients (67%). Conclusion: PSMA-RGS with ^99mTc-MIP-1404 identified lymph node metastases in all patients, including 2 additional lesions compared with PET/CT.

Detection of prostate cancer bone metastases with fast whole-body 99mTc-HMDP SPECT/CT using a general-purpose CZT system

BACKGROUND

We evaluated the effects of acquisition time, energy window width, and matrix size on the image quality, quantitation, and diagnostic performance of whole-body ^99mTc-HMDP SPECT/CT in the primary metastasis staging of prostate cancer.

METHODS

Thirty prostate cancer patients underwent ^99mTc-HMDP SPECT/CT from the top of the head to the mid-thigh using a Discovery NM/CT 670 CZT system with list-mode acquisition, 50-min acquisition time, 15% energy window width, and 128 × 128 matrix size. The acquired list-mode data were resampled to produce data sets with shorter acquisition times of 41, 38, 32, 26, 20, and 16 min, narrower energy windows of 10, 8, 6, and 4%, and a larger matrix size of 256 × 256. Images were qualitatively evaluated by three experienced nuclear medicine physicians and quantitatively evaluated by noise, lesion contrast and SUV measurements. Diagnostic performance was evaluated from the readings of two experienced nuclear medicine physicians in terms of patient-, region-, and lesion-level sensitivity and specificity.

RESULTS

The originally acquired images had the best qualitative image quality and lowest noise. However, the acquisition time could be reduced to 38 min, the energy window narrowed to 8%, and the matrix size increased to 256 × 256 with still acceptable qualitative image quality. Lesion contrast and SUVs were not affected by changes in acquisition parameters. Acquisition time reduction had no effect on the diagnostic performance, as sensitivity, specificity, accuracy, and area under the receiver-operating characteristic curve were not significantly different between the 50-min and reduced acquisition time images. The average patient-level sensitivities of the two readers were 88, 92, 100, and 96% for the 50-, 32-, 26-, and 16-min images, respectively, and the corresponding specificities were 78, 84, 84, and 78%. The average region-level sensitivities of the two readers were 55, 58, 59, and 56% for the 50-, 32-, 26-, and 16-min images, respectively, and the corresponding specificities were 95, 98, 96, and 95%. The number of equivocal lesions tended to increase as the acquisition time decreased.

CONCLUSION

Whole-body ^99mTc-HMDP SPECT/CT can be acquired using a general-purpose CZT system in less than 20 min without any loss in diagnostic performance in metastasis staging of high-risk prostate cancer patients.

A Review on the Current State and Future Perspectives of [99mTc]Tc-Housed PSMA-i in Prostate Cancer

Recently, prostate-specific membrane antigen (PSMA) has gained momentum in tumor nuclear molecular imaging as an excellent target for both the diagnosis and therapy of prostate cancer. Since 2008, after years of preclinical research efforts, a plentitude of radiolabeled compounds mainly based on low molecular weight PSMA inhibitors (PSMA-i) have been described for imaging and theranostic applications, and some of them have been transferred to the clinic. Most of these compounds include radiometals (e.g., ⁶⁸Ga, ⁶⁴Cu, ¹⁷⁷Lu) for positron emission tomography (PET) imaging or endoradiotherapy. Nowadays, although the development of new PET tracers has caused a significant drop in single-photon emission tomography (SPECT) research programs and the development of new technetium-99m (^99mTc) tracers is rare, this radionuclide remains the best atom for SPECT imaging owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry. Indeed, ^99mTc still plays a relevant role in diagnostic nuclear medicine, as the number of clinical examinations based on ^99mTc outscores that of PET agents and ^99mTc-PSMA SPECT/CT may be a cost-effective alternative for ⁶⁸Ga-PSMA PET/CT. This review aims to give an overview of the specific features of the developed [^99mTc]Tc-tagged PSMA agents with particular attention to [^99mTc]Tc-PSMA-i. The chemical and pharmacological properties of the latter will be compared and discussed, highlighting the pros and cons with respect to [⁶⁸Ga]Ga-PSMA11.

Trends in coordination of rhenium organometallic complexes in the Protein Data Bank

Radiopharmaceutical development has similar overall characteristics to any biomedical drug development requiring a compound's stability, aqueous solubility and selectivity to a specific disease site. However, organometallic complexes containing ^188/186Re or ^99mTc involve a d-block transition-metal radioactive isotope and therefore bring additional factors such as metal oxidation states, isotope purity and half life into play. This topical review is focused on the development of radiopharmaceuticals containing the radioisotopes of rhenium and technetium and, therefore, on the occurrence of these organometallic complexes in protein structures in the Worldwide Protein Data Bank (wwPDB). The purpose of incorporating the group 7 transition metals of rhenium/technetium in the protein and the reasons for study by protein crystallography are described, as certain PDB studies were not aimed at drug development. Technetium is used as a medical diagnostic agent and involves the ^99mTc isotope which decays to release gamma radiation, thereby employed for its use in gamma imaging. Due to the periodic relationship among group 7 transition metals, the coordination chemistry of rhenium is similar (but not identical) to that of technetium. The types of reactions the potential model radiopharmaceutical would prefer to partake in, and by extension knowing which proteins and biomolecules the compound would react with in vivo, are needed. Crystallography studies, both small molecule and macromolecular, are a key aspect in understanding chemical coordination. Analyses of bonding modes, coordination to particular residues and crystallization conditions are presented. In our Forward look as a concluding summary of this topical review, the question we ask is: what is the best way for this field to progress?

Radiolabeled PSMA Inhibitors

PSMA has shown to be a promising target for diagnosis and therapy (theranostics) of prostate cancer. We have reviewed developments in the field of radio- and fluorescence-guided surgery and targeted photodynamic therapy as well as multitargeting PSMA inhibitors also addressing albumin, GRPr and integrin αvβ3. An overview of the regulatory status of PSMA-targeting radiopharmaceuticals in the USA and Europe is also provided. Technical and quality aspects of PSMA-targeting radiopharmaceuticals are described and new emerging radiolabeling strategies are discussed. Furthermore, insights are given into the production, application and potential of alternatives beyond the commonly used radionuclides for radiolabeling PSMA inhibitors. An additional refinement of radiopharmaceuticals is required in order to further improve dose-limiting factors, such as nephrotoxicity and salivary gland uptake during endoradiotherapy. The improvement of patient treatment achieved by the advantageous combination of radionuclide therapy with alternative therapies is also a special focus of this review.

Molecular Imaging in Primary Staging of Prostate Cancer Patients: Current Aspects and Future Trends

Accurate primary staging is the cornerstone in all malignancies. Different morphological imaging modalities are employed in the evaluation of prostate cancer (PCa). Regardless of all developments in imaging, invasive histopathologic evaluation is still the standard method for the detection and staging of the primary PCa. Magnetic resonance imaging (MRI) and computed tomography (CT) play crucial roles; however, functional imaging provides additional valuable information, and it is gaining ever-growing acceptance in the management of PCa. Targeted imaging with different radiotracers has remarkably evolved in the past two decades. [111In]In-capromab pendetide scintigraphy was a new approach in the management of PCa. Afterwards, positron emission tomography (PET) tracers such as [11C/18F]choline and [11C]acetate were developed. Nevertheless, none found a role in the primary staging. By introduction of the highly sensitive small molecule prostate-specific membrane antigen (PSMA) PET/CT, as well as recent developments in MRI and hybrid PET/MRI systems, non-invasive staging of PCa is being contemplated. Several studies investigated the role of these sophisticated modalities in the primary staging of PCa, showing promising results. Here, we recapitulate the role of targeted functional imaging. We briefly mention the most popular radiotracers, their diagnostic accuracy in the primary staging of PCa, and impact on patient management.

The utility of radiolabeled PSMA ligands for tumor imaging

Prostate-specific membrane antigen (PSMA) is a glycosylated type-II transmembrane protein expressed in prostatic tissue and significantly overexpressed in several prostate cancer cells. Despite its name, PSMA has also been reported to be overexpressed in endothelial cells of benign and malignant non-prostate disease. So its clinical use was extended to detection, staging, and therapy of various tumor types. Recently small molecules targeting PSMA have been developed as imaging probes for diagnosis of several malignancies. Preliminary studies are emerging improved diagnostic sensitivity and specificity of PSMA imaging, leading to a change in patient management. In this review, we evaluated the first preclinical and clinical studies on PSMA ligands resulting future perspectives radiolabeled PSMA in staging and molecular characterization, based on histopathologic examinations of PSMA expression.

PSMA Theranostics: A "Must Have" in Every Prostate Cancer Center. Illustration of Two Clinical Cases and Review of the Literature

This manuscript describes the history of 2 patients with prostate cancer (PCa) and the role of prostate-specific membrane antigen (PSMA) theranostics in their clinical management. In the first patient, PSMA-directed positron emission tomography (PET)/computed tomography (CT) imaging was used for primary staging of high-risk PCa before initial therapy. Then after biochemical relapse it was used to plan the scope of further treatment, in which it allowed among others to perform precise target volume delineation for salvage radiotherapy for pathologic lymph nodes. In the second patient with metastatic castration-resistant prostate cancer (mCRPC), PSMA-guided imaging played a key role in the qualification for PSMA-directed radioligand therapy (RLT) with lutetium-177. We also present a review of the current literature concerning PSMA theranostics in the 2 clinical settings, ie, primary staging of PCa and PSMA RLT of mCRPC. In the first part of the review, we report on the diagnostic efficacy of various PSMA imaging radiotracers labeled with gallium-68, fluorine-18, and technetium-99m. In the second part, we describe the limitations and future perspectives of PSMA therapeutic radiopharmaceuticals, including various beta(-) and alpha emitters.

Combination of 99mTc-Labeled PSMA-SPECT/CT and Diffusion-Weighted MRI in the Prediction of Early Response After Carbon Ion Therapy in Prostate Cancer: A Non-Randomized Prospective Pilot Study

Purpose

The purpose of this study was to assess the potential of ^99mTc-labeled PSMA-SPECT/CT and diffusion-weighted image (DWI) for predicting treatment response after carbon ion radiotherapy (CIRT) in prostate cancer.

Patients and Methods

We prospectively registered 26 patients with localized prostate cancer treated with CIRT. All patients underwent ^99mTc-labeled PSMA-SPECT/CT and multiparametric magnetic resonance imaging (MRI) before and after CIRT. The tumor/background ratio (TBR) and mean apparent diffusion coefficient (ADC_mean) were measured on the tumor and the percentage changes before and after therapy (ΔTBR and ΔADC_mean) were calculated. Patients were divided into two groups: good response and poor response according to clinical follow-up.

Results

The median follow up time was 38.3months. The TBR was significantly decreased (p=0.001), while the ADC_mean was significantly increased compared with the pretreatment value (p<0.001). The ΔTBR and ΔADC_mean were negatively correlated with each other (p = 0.002). On ROC curve analysis for predicting treatment response, the area under the ROC curve (AUC) of ΔTBR (0.867) for predicting good response was higher than that of ΔADC_mean (0.819). The AUC of combined with ΔTBR and ΔADC_mean (0.895) was higher than that of either ΔADC_mean or ΔTBR alone. The combined use of ΔTBR and ΔADC_mean showed 91.4% sensitivity and 95.2% specificity.

Conclusion

Our preliminary data indicate that the changes of TBR and ADC_mean maybe an early bio-marker for predicting prognosis after CIRT in localized prostate cancer patients. In addition, the ΔTBR seems to be a more powerful prognostic factor than ΔADC_mean in prostate cancer treated with CIRT.

Substitution reactivity and structural variability induced by tryptamine on the biomimetic rhenium tricarbonyl complex

A series of seven fac-[Re(CO)₃(5Me-Sal-Trypt)(L)] complexes containing tryptamine on the N,O 5-methyl-salicylidene bidentate ligand backbone and where L is MeOH, Py, Imi, DMAP, PPh₃ coordinated to the 6th position have been studied, including the formation of a dinuclear Re₂ cluster. The crystallographic solid state structures show marked similarity in structural tendency, in particular the rigidity of the Re core and the hydrogen bond interactions similar to those found in protein structures. The rates of formation and stability of the complexes were evaluated by rapid time-resolved stopped-flow techniques and the methanol substitution reaction indicates the significant activation induced by the use of the N,O salicylidene bidentate ligand as manifested by the second-order rate constants for the entering nucleophiles. Both linear and limiting kinetics were observed and a systematic evaluation of the kinetics is reported clearly indicating an interchange type of intimate mechanism for the methanol substitution. The anticancer activity of compounds 1–7 was tested on HeLa cells and it was found that all compounds showed similar cytotoxicity where solubility allowed. IC₅₀-values between ca. 11 and 22 μM indicate that some cytotoxicity resides most likely on the salicylidene–tryptamine ligand. The photoluminescence of the seven complexes is similar in maximum emission wavelength with little variation despite the broad range of ligands coordinated to the 6th position on the metal centre.

The biomimetic tryptamine rhenium tricarbonyl complex shows rapid substitution reactivity on the 6th position as well as cytotoxicity and photoluminescence capability induced by the salicylidene bidentate ligand.

The emerging role of cell surface receptor and protein binding radiopharmaceuticals in cancer diagnostics and therapy

Targeting specific cell membrane markers for both diagnostic imaging and radionuclide therapy is a rapidly evolving field in cancer research. Some of these applications have now found a role in routine clinical practice and have been shown to have a significant impact on patient management. Several molecular targets are being investigated in ongoing clinical trials and show promise for future implementation. Advancements in molecular biology have facilitated the identification of new cancer-specific targets for radiopharmaceutical development.

99mTc-MIP-1404 SPECT/CT for Assessment of Whole-Body Tumor Burden and Treatment Response in Patients With Biochemical Recurrence of Prostate Cancer

OBJECTIVE

This study aims to investigate the value of Tc-MIP-1404 SPECT/CT for assessment of whole-body tumor burden and treatment response in patients with biochemical recurrence of prostate cancer who undergo androgen deprivation therapy (ADT) or external beam radiation therapy (EBRT).

METHODS

A total of 125 patients with biochemical recurrence of prostate cancer underwent Tc-MIP-1404 SPECT/CT. All 364 prostate-specific membrane antigen (PSMA)-positive lesions in the field of view were assessed quantitatively to calculate PSMA-derived metabolic tumor parameters, including whole-body PSMA tumor volume and whole-body total lesion PSMA. These metrics were correlated with serum prostate-specific antigen (PSA) levels and Gleason scores. In a subset of 50 patients who underwent Tc-MIP-1404 SPECT/CT before the initiation of ADT or EBRT, TL-PSMA and SUVmax were compared with radiographic response assessment by CT based on RECIST 1.1 and to biochemical response (BR) determined by changes in serum PSA levels.

RESULTS

Serum PSA levels correlated with SUVmax, whole-body PSMA tumor volume, and whole-body total lesion PSMA in patients with 1 and in those with more than 1 PSMA-positive lesion (P < 0.05). The correlations were significant for both well-differentiated (Gleason score ≤7) and poorly differentiated tumors (Gleason score ≥8) (P < 0.05). The agreement between TL-PSMA derived from SPECT and BR in patients who underwent Tc-MIP-1404 SPECT/CT before and after initiation of ADT was 80% (95% confidence interval [CI], 0.43-0.91; Cohen κ = 0.68; P < 0.05); in these patients, the agreement between TL-PSMA and CT was 60% (95% CI, 0.20-0.72; Cohen κ = 0.46; P < 0.05) and the agreement between BR and CT was 52% (0.07-0.61; Cohen κ = 0.34; P < 0.05). Comparable results were found for patients who underwent SPECT/CT before and after initiation of EBRT, with the strongest agreement between TL-PSMA and BR (80%; 95% CI, 0.38-0.93; Cohen κ = 0.66; P < 0.05) compared with the agreement between TL-PSMA and CT (60%; 95% CI, 0.13-0.69; Cohen κ = 0.69; P < 0.05) and between BR and CT (48%; 95% CI, 0-0.54; Cohen κ = 0.26; P = 0.11). Discordant findings between SPECT and CT were most likely due to limitations in the assessment of small lymph node metastases and bone involvement, which were detectable on SPECT but not on CT.

CONCLUSIONS

The results of our study show that Tc-MIP-1404 SPECT/CT is a promising method for the evaluation of treatment response in patients with biochemical recurrence of prostate cancer who undergo either ADT or EBRT. TL-PSMA for assessment of treatment response has the strongest correlation with serum PSA levels, superior to SUVmax-based evaluation and response assessment based on CT data and RECIST 1.1.

Fluorine-18 labelled prostate-specific membrane antigen (PSMA)-1007 positron-emission tomography-computed tomography: normal patterns, pearls, and pitfalls

Prostate-specific membrane antigen (PSMA)-based positron-emission tomography (PET)-computed tomography (CT) has shown great promise in prostate cancer imaging. This technique has demonstrated particular utility in the staging of high-risk primary cancer and in the localisation of recurrent disease. The use of fluorine-18 PSMA-1007 is advantageous, as it is excreted via the hepatobiliary system rather than urinary and the longer half-life of fluorine-18 compared to gallium tracers, allows for PSMA imaging in centres without a gallium generator. However, imaging with this tracer is not without flaws and areas of ambiguity remain. In this article, the biodistribution, clinical indications, and pearls of ¹⁸F-PSMA-1007 PET-CT in patients with prostate cancer will be discussed, as well as the potential pitfalls in the reporting of these studies.

SPECT/CT: Standing on the Shoulders of Giants, It Is Time to Reach for the Sky!

Twenty years ago, SPECT/CT became commercially available, combining the strengths of both techniques: the diagnostic sensitivity of SPECT and the anatomic detail of CT. Other benefits initially included attenuation correction of SPECT reconstructions, ultimately evolving to correction techniques that would enable absolute tracer uptake quantification. Recent developments in SPECT hardware include solid-state digital systems with higher sensitivity and resolution, using novel collimator designs based on tungsten. Similar advances in CT technology have been introduced in hybrid SPECT/CT systems, replacing low-end x-ray tubes with high-end multislice CT scanners equipped with iterative reconstruction, metal artifact reduction algorithms, and dual-energy capabilities. More recently, the design of whole-body SPECT/CT systems has taken another major leap with the introduction of a ring-shaped gantry equipped with multiple movable detectors surrounding the patient. These exciting developments have fueled efforts to develop novel SPECT radiopharmaceuticals, creating new chelators and prosthetic groups for radiolabeling. Innovative SPECT radionuclide pairs have now become available for radiolabeling with the potential for use as theranostic agents. The growth of precision medicine and the associated need for accurate radionuclide treatment dosimetry will likely drive the use of SPECT/CT in the near future. In addition, expanding clinical applications of SPECT/CT in other areas such as orthopedics offer exciting opportunities. Although it is true that the SPECT/CT ecosystem has seen several challenges during its development over the past 2 decades, it is now a feature-rich and mature tool ready for clinical prime time.

[99cmTc]Tc-PSMA-I&S-SPECT/CT: experience in prostate cancer imaging in an outpatient center

BACKGROUND

Prostate-specific membrane antigen (PSMA) SPECT imaging in prostate cancer (PCa) could be a valuable alternative in regions where access to PSMA-PET imaging is restricted. [99mTc]Tc-PSMA-I&S is a new 99mTc-labeled PSMA-targeting SPECT agent, initially developed for radio-guided surgery. We report on the diagnostic use of [99mTc]Tc-PSMA-I&S-SPECT/CT in PCa.

RESULTS

[99mTc]Tc-PSMA-I&S-SPECT/CT was performed and evaluated in 210 outpatients with PCa at a single center. Patients were imaged for biochemical recurrence (BCR, n = 152, mean PSA 8.7 ng/ml), for primary staging of high-risk PCa (n = 12, mean PSA 393 ng/ml), and restaging in advanced recurrent PCa (n = 46, mean PSA 101.3 ng/ml). Number and location of positive lesions were determined for the different subgroups. For BCR, detection rates were calculated, defined as the proportion of scans with at least one PSMA-positive lesion. PSMA positive lesions were detected in 65.2% of all 210 patients. Tumor tissue was mainly detected in lymph nodes (59%), in the bone (42%), and in the prostate (fossa) (28%). In the subgroup of patients referred for detection of BCR the detection rate increased from 20% at a PSA level < 1 ng/ml to 82.9% and 100% at PSA levels > 4 ng/ml and > 10 ng/ml, respectively. In the subgroup of high-risk patients referred for primary staging, 42% demonstrated metastatic disease. Restaging of advanced recurrent PCa revealed detectability of PSMA positive tumor lesions in 85% of the scans.

CONCLUSIONS

[99mTc]Tc-PSMA-I&S-SPECT/CT was useful in PSMA-targeted imaging of PCa at various clinical stages. At low PSA levels (< 4 ng/ml), detection rates of [99mTc]Tc-PSMA-I&S-SPECT/CT in BCR are clearly inferior to data reported for PET-imaging and should thus only be considered for lesion detection if imaging with PET is unavailable. However, at higher PSA levels (> 4 ng/ml) [99mTc]Tc-PSMA-I&S-SPECT/CT provides high detection rates in BCR. [99mTc]Tc-PSMA-I&S-SPECT/CT can also be used for primary staging and for restaging of advanced recurrent PCa. However, further studies are needed to assess the clinical value in these indications.

Preliminary results of targeted prostate-specific membrane antigen imaging in evaluating the efficacy of a novel hormone agent in metastatic castration-resistant prostate cancer

To investigate the feasibility and effectiveness of prostate‐specific membrane antigen (PSMA) imaging to make response assessment regarding novel hormone treatment and to predict the outcomes for metastatic castration‐resistant prostate cancer (mCRPC) patients. This retrospective study enrolled 68 mCRPC patients who had daily received a novel hormone agent named abiraterone. Tc‐99m PSMA single‐photon emission computed tomography (SPECT/CT) was performed at the baseline (SPECT/CT1) and after 3‐6 months of treatment (SPECT/CT2). The treatment response was determined by visual analysis based on molecular imaging PSMA (miPSMA) scores framework and was compared with conventional biochemical analysis. We chose either the hottest lesion (target A) or five of the hottest lesions (target B) to calculate the tumor/background ratio (TBR) and the maximum standardized uptake value (SUVmax) and compared their performances in predicting progression‐free survival (PFS). Changes in PSMA expression between SPECT/CT1 and SPECT/CT2 were well associated with the results of the visual analysis. The TBR and the SUVmax of both targets were significantly associated with the baseline serum PSA level (P < .0001). The biochemical and radiological responses were concordant in 56 of the 68 patients (P < .001). The median PFS of the nonresponse group patients was significantly shorter than that of the patients in the response group (6.8 vs 12.1 months, P = .012). For predicting PFS, most of the indexes tested were significant on SPECT/CT2, with %ΔTBR being the most significant prognostic factor. Our preliminary results suggest that molecular imaging‐targeted PSMA is of great value for treatment response assessment and clinical outcome prediction in mCRPC patients with long‐term abiraterone treatment.

99mTc-MIP-1404 SPECT/CT for Patients With Metastatic Prostate Cancer: Interobserver and Intraobserver Variability in Treatment-Related Longitudinal Tracer Uptake Assessments of Prostate-Specific Membrane Antigen-Positive Lesions

BACKGROUND

Tc-MIP-1404 is a SPECT-suitable prostate-specific membrane antigen (PSMA) ligand for detection of prostate cancer. In patients with metastatic prostate cancer, there are no data as yet on interobserver and intraobserver variability when assessing PSMA-positive lesions for longitudinal changes of tracer uptake.

METHODS

Tc-MIP-1404 SPECT/CT scans of 22 patients with metastatic prostate cancer were analyzed, and each subject was imaged at 2 separate points in time, before and after treatment. Mean interval between scans was 10 months. Three independent observers visually assessed a total of 96 PSMA-positive metastases (bone, 69; lymph node, 22; viscera, 3) or local recurrences (n = 2) for longitudinal changes in tracer uptake on planar scintigraphy and SPECT/CT. All lesions were categorized as regressive, stable, or progressive based on visual findings and on peak SUV (SUVpeak) of quantitative SPECT/CT (progressive, >30% SUVpeak increase; regressive, <30% SUVpeak decrease; or stable, all others).

RESULTS

Quantitative analysis of PSMA-positive lesions yielded significantly higher interobserver agreement (90.6%; 95% confidence interval [CI], 0.83%-0.96%) than visual assessments by either SPECT/CT (76.0%; 95% CI, 0.66%-0.84%) or planar scintigraphy (56.3%; 95% CI, 0.46%-0.66%). Intermethod comparison of aggregated results yielded significantly higher agreement between quantitative and visual SPECT/CT (85.1%; 95% CI, 0.80%-0.89%), as opposed to quantitative SPECT/CT and planar scintigraphy (53.1%; 95% CI, 0.47%-0.59%) or visual SPECT/CT and planar scintigraphy (54.9%; 95% CI, 0.49%-0.61%). In visual and quantitative analysis of 96 PSMA-positive lesions, the number of discrepancies ranged from 9 (9.4%) for quantitative SPECT/CT to 42 (43.8%) for planar scintigraphy. Overall reader confidence was higher for SPECT/CT than for planar scintigraphy (P < 0.001). Intraobserver agreement was near-perfect for all methods, whether SPECT/CT (visual, all κ = 0.94-0.97; quantitative κ = 0.94-0.98) or planar scintigraphy (all κ = 0.90-0.94).

CONCLUSIONS

Quantitative evaluation of longitudinal change in tracer uptake by PSMA-positive lesions measured via SPECT/CT is superior to visual interpretation of images by planar scintigraphy or SPECT/CT. Compared with visual evaluation, quantitative SPECT/CT is highly reproducible, showing near-perfect agreement among observers and higher reader confidence.

Optimum Imaging Strategies for Advanced Prostate Cancer: ASCO Guideline

PURPOSE

Provide evidence- and expert-based recommendations for optimal use of imaging in advanced prostate cancer. Due to increases in research and utilization of novel imaging for advanced prostate cancer, this guideline is intended to outline techniques available and provide recommendations on appropriate use of imaging for specified patient subgroups.

METHODS

An Expert Panel was convened with members from ASCO and the Society of Abdominal Radiology, American College of Radiology, Society of Nuclear Medicine and Molecular Imaging, American Urological Association, American Society for Radiation Oncology, and Society of Urologic Oncology to conduct a systematic review of the literature and develop an evidence-based guideline on the optimal use of imaging for advanced prostate cancer. Representative index cases of various prostate cancer disease states are presented, including suspected high-risk disease, newly diagnosed treatment-naïve metastatic disease, suspected recurrent disease after local treatment, and progressive disease while undergoing systemic treatment. A systematic review of the literature from 2013 to August 2018 identified fully published English-language systematic reviews with or without meta-analyses, reports of rigorously conducted phase III randomized controlled trials that compared ≥ 2 imaging modalities, and noncomparative studies that reported on the efficacy of a single imaging modality.

RESULTS

A total of 35 studies met inclusion criteria and form the evidence base, including 17 systematic reviews with or without meta-analysis and 18 primary research articles.

RECOMMENDATIONS

One or more of these imaging modalities should be used for patients with advanced prostate cancer: conventional imaging (defined as computed tomography [CT], bone scan, and/or prostate magnetic resonance imaging [MRI]) and/or next-generation imaging (NGI), positron emission tomography [PET], PET/CT, PET/MRI, or whole-body MRI) according to the clinical scenario.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

PSMA SPECT/CT with 99mTc-MIP-1404 in biochemical recurrence of prostate cancer: predictive factors and efficacy for the detection of PSMA-positive lesions at low and very-low PSA levels

BACKGROUND

The in vivo expression of the prostate-specific membrane antigen (PSMA) can be investigated using the SPECT-suitable tracer ^99mTc-MIP-1404. We investigated the performance of ^99mTc-MIP-1404 PSMA SPECT/CT in the detection of PSMA-positive tumor lesions in patients suffering from biochemical recurrence of prostate cancer presenting with serum levels of the prostate-specific antigen (PSA) below 1 ng/mL.

METHODS

We retrospectively analyzed ^99mTc-MIP-1404-SPECT/CT scans of 50 patients (25 with low PSA levels between > 0.5 and 1 ng/mL and 25 with very low PSA levels between 0.2 and 0.5 ng/mL) that had undergone whole-body planar scintigraphy and SPECT/CT of the thorax, abdomen and pelvis 3-4 h p.i. of 691 ± 72 MBq ^99mTc-MIP-1404. All datasets were evaluated for the presence and location of PSMA-positive tumor lesions, in which maximal standardized uptake values (SUV_max) were also measured. Based on the results of the quantitative evaluation as well as on biochemical and histological parameters, predictive factors for a positive ^99mTc-MIP-1404 scan result were determined. The influence of ^99mTc-MIP-1404 PSMA SPECT/CT on further therapy planning was assessed, based on the decision-making of the interdisciplinary tumor board.

RESULTS

Pathological ^99mTc-MIP-1404 uptake was detected in a total of 25 patients (50%). In the very low PSA subgroup, detection rates of PSMA-positive lesions suggestive of tumor recurrence were 44%, in the low-PSA subgroup 56%. Gleason scores ≥ 8 and the presence of antiandrogen deprivation therapy were further significant predictors of pathological ^99mTc-MIP-1404 uptake. This was paralleled by significantly higher lesional SUV_max patients with PSA levels > 0.5 ng/mL and Gleason scores ≥ 8 compared to those without these two features. Changes in therapeutic strategy following MIP-1404 imaging were recommended by the interdisciplinary tumor board in 25/50 of patients.

CONCLUSION

^99mTc-MIP-1404 PSMA-SPECT/CT demonstrated a high performance in detecting PSMA-positive lesions suggestive of tumor recurrence in patients with biochemical recurrence of prostate cancer and low and very low serum PSA levels. Results from MIP-1404 PSMA SPECT/CT have therapeutic impact in one-half of the patients examined by this technology.

A multifunctional lipid that forms contrast-agent liposomes with dual-control release capabilities for precise MRI-guided drug delivery

Monitoring of nanoparticle-based therapy in vivo and controlled drug release are urgently needed for the precise treatment of disease. We have synthesized a multifunctional Gd-DTPA-ONB (GDO) lipid by introducing the Gd-DTPA contrast agent moiety into an o-nitro-benzyl ester lipid. By design, liposomes formed from the GDO lipid combine MRI tracking ability and dual-trigger release capabilities with maximum sensitivity (because all lipids bear the cleavable moiety) without reducing the drug encapsulation rate. We first confirmed that both photo-treatment and pH-triggered hydrolysis are able to cleave the GDO lipid and lyse GDO liposomes. We then investigated the efficiency of drug release via the combined release processes for GDO liposomes loaded with doxorubicin (DOX). Relative to neutral pH, the release efficiency in acidic environment increased by 10.4% (at pH = 6.5) and 13.3% (at pH = 4.2). This pH-dependent release response is conducive to distinguishing pathological tissue such as tumors and endolysosomal compartments. The photo-induced release efficiency increases with illumination time as well as with distance of the pH from neutral. Photolysis increased the release efficiency by 13.8% at pH = 4.2, which is remarkable considering the already increased amount of drug release in the acidic environment. In addition, the relaxation time of GDO liposomes was 4.1 times that of clinical Gd-DTPA, with brighter T₁-weighted imaging in vitro and in vivo. Real-time MRI imaging and in vivo fluorescence experiments demonstrated tumor targeting and MRI guided release. Furthermore, significant tumor growth inhibition in a treatment experiment using DOX-loaded GDO liposomes clearly demonstrated the benefit of photo-treatment for efficacy: the tumor size in the photo-treatment group was 3.7 times smaller than in the control group. The present study thus highlights the benefit of the design idea of combining efficient imaging/guiding, targeting, and triggerable release functions in one lipid molecule for drug delivery applications.

Integration of PSMA-targeted PET imaging into the armamentarium for detecting clinically significant prostate cancer

PURPOSE OF REVIEW

To explore the current state of using prostate-specific membrane antigen (PSMA)-targeted PET imaging to aid in the diagnosis of clinically significant prostate cancer.

RECENT FINDINGS

Prostate-specific antigen screening remains controversial, as it is associated with the underdetection of clinically significant prostate cancer as well as the overdetection and subsequent overtreatment of clinically insignificant disease. A diagnostic test that can accurately assess the presence of clinically significant prostate cancer and avoid detection of low-risk tumors is needed. Multiparametric magnetic resonance imaging (mpMRI) can aid in the detection of clinically significant prostate cancer and can be used with fusion-based biopsy platforms to target biopsies to specific lesions. However, there are several limitations of mpMRI including a modest negative predictive value for high-grade cancer. PSMA-targeted PET imaging has shown promise as a noninvasive test to aid in the detection of clinically significant prostate cancer while providing anatomical information to guide targeted biopsies. PSMA-targeted PET in combination with mpMRI offers a higher degree of diagnostic accuracy for imaging localized prostate cancer than either modality alone.

SUMMARY

PSMA-targeted PET imaging can aid in the identification of men with clinically significant prostate cancer. Further research is needed to determine the full potential of PSMA-targeted imaging in both the detection and treatment of localized prostate cancer.

A Picture of Modern Tc-99m Radiopharmaceuticals: Production, Chemistry, and Applications in Molecular Imaging

Even today, techentium-99m represents the radionuclide of choice for diagnostic radio-imaging applications. Its peculiar physical and chemical properties make it particularly suitable for medical imaging. By the use of molecular probes and perfusion radiotracers, it provides rapid and non-invasive evaluation of the function, physiology, and/or pathology of organs. The versatile chemistry of technetium-99m, due to its multi-oxidation states, and, consequently, the ability to produce a variety of complexes with particular desired characteristics, are the major advantages of this medical radionuclide. The advances in technetium coordination chemistry over the last 20 years, in combination with recent advances in detector technologies and reconstruction algorithms, make SPECT’s spatial resolution comparable to that of PET, allowing 99mTc radiopharmaceuticals to have an important role in nuclear medicine and to be particularly suitable for molecular imaging. In this review the most efficient chemical methods, based on the modern concept of the 99mTc-metal fragment approach, applied to the development of technetium-99m radiopharmaceuticals for molecular imaging, are described. A specific paragraph is dedicated to the development of new 99mTc-based radiopharmaceuticals for prostate cancer.

PSMA-Targeted Radiopharmaceuticals for Imaging and Therapy

As described in more detail in other contributions in this issue of Seminars in Nuclear Medicine, prostate-specific membrane antigen (PSMA) has become one of the most promising molecular targets in nuclear medicine. Due to its overexpression on prostate cancer cells in proportion to the stage and grade of tumour progression, especially in androgen-independent, advanced and metastatic disease, various tracers for the detection and treatment of prostate cancer by means of radioligand imaging, radioligand therapy or radioguided surgery have been developed and transferred to clinical applications. Even though monoclonal antibodies were investigated and introduced as first PSMA-targeted probes, the inherent advantage of fast tumour uptake and rapid excretion of small molecules has shifted the research focus during the last decade to low molecular weight inhibitors with high affinity to PSMA, such as [¹⁸F]FDCFPyL, [¹⁸F]PSMA-1007, [⁶⁸Ga]PSMA-HBED, [¹⁷⁷Lu]PSMA-617, [¹⁷⁷Lu]PSMA-I&T, [^99mTc]MIP-1404 or [^99mTc]PSMA I&S, to mention only a few. Due to the plethora of such PSMA probes described during the last years, this review aims to give an overview over the specific characteristics of those radiopharmaceuticals that have already found widespread clinical application. In addition, recently introduced concepts such as PSMA-tracers with increased plasma protein binding, are discussed.

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.

Two decades of SPECT/CT - the coming of age of a technology: An updated review of literature evidence

PURPOSE

Single-photon emission computed tomography (SPECT) combined with computed tomography (CT) was introduced as a hybrid SPECT/CT imaging modality two decades ago. The main advantage of SPECT/CT is the increased specificity achieved through a more precise localization and characterization of functional findings. The improved diagnostic accuracy is also associated with greater diagnostic confidence and better inter-specialty communication.

METHODS

This review presents a critical assessment of the relevant literature published so far on the role of SPECT/CT in a variety of clinical conditions. It also includes an update on the established evidence demonstrating both the advantages and limitations of this modality.

CONCLUSIONS

For the majority of applications, SPECT/CT should be a routine imaging technique, fully integrated into the clinical decision-making process, including oncology, endocrinology, orthopaedics, paediatrics, and cardiology. Large-scale prospective studies are lacking, however, on the use of SPECT/CT in certain clinical domains such as neurology and lung disorders. The review also presents data on the complementary role of SPECT/CT with other imaging modalities and a comparative analysis, where available.

Relationship between PSA kinetics and Tc-99m HYNIC PSMA SPECT/CT detection rates of biochemical recurrence in patients with prostate cancer after radical prostatectomy

BACKGROUND

Prostate-specific antigen (PSA) levels should reflect or be proportional to the size and the metabolic activity of prostatic metastases. Moreover, a rapid change in PSA kinetics, either before or after treatment, is an indicator of poor prognosis after radical prostatectomy. Therefore, the purpose of this study was to investigate the effect of total PSA at the time of Tc-99m HYNIC PSMA SPECT/CT (trigger PSA), PSA velocity (PSAvel), and PSA doubling time (PSAdt) on the Tc-99m HYNIC PSMA SPECT/CT detection rate in prostate cancer patients who showed biochemical recurrence after radical prostatectomy during follow-up.

METHODS

In total, 208 patients who showed an increase in PSA were evaluable for this retrospective analysis covering November 2015 to March 2017. Data were available for calculation of PSAvel in 112 patients and for PSAdt in 157 patients. Logistic regression analysis was used to determine whether there was a relationship between the PSA levels and PSA kinetics and the rate of detection of relapse using Tc-99m HYNIC PSMA SPECT/CT.

RESULTS

Tc-99m HYNIC PSMA SPECT/CT detected disease relapse in 151 of 208 patients (72.6%). The PSA level (P < 0.0001) and PSAdt (P = 0.0036) were significantly different between SPECT-positive patients (higher PSA level, shorter PSAdt) and SPECT-negative patients (lower PSA, longer PSAdt). ROC analysis showed that a PSA level of 1.30 ng/mL and a PSAdt of 2.9 months were optimal cut-off values. Patients with purely local recurrence had lower PSAvel and longer PSAdt values (P < 0.001). According to the multivariate analysis, a pathological positive SPECT/CT scan was associated with the PSA level (P < 0.001), PSAdt <6 months (P < 0.05), and Gleason scores (GSC) >7 (P < 0.05).

CONCLUSION

The Tc-99m HYNIC PSMA SPECT/CT detection rate is influenced by trigger PSA, PSAdt, and PSAvel. Like PSA, PSAdt is an independent predictor of Tc-99m HYNIC PSMA SPECT/CT. PSAdt should be taken into account by physicians especially when PSA <1 ng/mL.

The Value of 99mTc-PSMA SPECT/CT-Guided Surgery for Identifying and Locating Lymph Node Metastasis in Prostate Cancer Patients

BACKGROUND

This study evaluated the effect of technetium-99m (^99mTc)-labeled prostate-specific membrane antigen (PSMA)-based image-guided surgery on the oncologic outcomes for patients with primary or recurrent prostate cancer (PCa).

METHODS

This study retrospectively analyzed 54 consecutive patients with PCa who underwent ^99mTc-labeled PSMA-based image-guided surgery between January 2016 and September 2017. These patients received a radical prostatectomy (RP) with pelvic lymph node dissection (PLND) or salvage lymph node dissection (sLND). The resected specimens were compared with findings of postoperative histologic analysis. The responses to the treatment were recorded during the follow-up period.

RESULTS

In 31 patients, PSMA single-photon emission computerized tomography (SPECT) and computed tomography (CT) could find 52 suspicious lymph node metastases (LNMs). With the help of PSMA SPECT/CT, 12 patients with recurrence received sLND, 19 primary PCa patients received RP with extended PLND, and 23 primary PCa patients received RP with standard PLND. The findings showed that PSMA SPECT/CT could detect LNMs with high sensitivity and specificity. In six patients, PSMA SPECT/CT could find more LNMs that were not found by MRI and help to modify the extent of lymphadenectomy. At the latest follow-up evaluation, 39 patients showed a biochemical response (BR), 9 patients showed a biochemical recurrence (BCR) after BR, and 6 patients never exhibited BR. The patients who received RP with standard PLND or extended PLND had a better prostate-specific antigen (PSA) response than the patients who received sLND. The patients with pelvic LNMs also had a better PSA response than the patients with retroperitoneal LNMs.

CONCLUSIONS

This study showed that ^99mTc-PSMA SPECT/CT-guided surgery can remove more LNMs than conventional imaging with high sensitivity and specificity and delay disease progression in PCa patients.

Keeping up with the prostate-specific membrane antigens (PSMAs): an introduction to a new class of positron emission tomography (PET) imaging agents

Prostate-specific membrane antigen (PSMA) targeted positron emission tomography (PET) is an emerging prostate cancer imaging method, which has been reported to have a higher sensitivity and specificity than the currently approved PET imaging agents. Multiple PSMA ligands are being investigated around the world and applications range from primary tumor characterization, to local staging, biochemical recurrence, metastasis, and image-guided interventions. The most investigated PET tracers are labelled with 68-Gallium or 18-Fluoride and are discussed in this review. Additionally, ^99mTc labeled PSMA agents for single photon emission computed tomography (SPECT) imaging are elucidated as an alternative method of PSMA image acquisition.

68Ga-PSMA-PET: added value and future applications in comparison to the current use of choline-PET and mpMRI in the workup of prostate cancer

Positron emission tomography (PET) has been commonly and successfully used, in combination with computed tomography (CT) and more recently magnetic resonance (MRI), in the workup of intermediate or high-risk prostate cancer (PCa). Nowadays, new specific receptor targeted PET tracers in prostate cancer imaging have been introduced; one of the most used is ⁶⁸Ga-PSMA, that evaluates the expression of prostate-specific membrane antigen (PSMA). This tracer has been rapidly taken into account for its better sensitivity and specificity compared to lipid metabolism tracers, such as ¹¹C/¹⁸F labelled fluorocholine. Besides, in the era of theranostics, this tracer is having a useful application not only for imaging but also for therapeutic purposes. The aim of this review article is, in the first part, to give an overview of the main indications and future development of ⁶⁸Ga-PSMA imaging, using PET/CT or PET/MRI, according to the clinical course of the disease and in view of the current use of multiparametric MRI (mpMRI) and choline PET in the management of PCa. In the second part, a brief overview of the promising ¹⁸F-labelled PSMA tracers and the current use of PSMA radionuclide therapy will be provided.

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.

The emerging role of imaging in prostate cancer secondary screening: multiparametric magnetic resonance imaging and the incipient incorporation of molecular imaging

An increasingly robust body of evidence indicates that multiparametric MRI (mpMRI) prior to prostate biopsy can improve the detection of clinically significant prostate cancer while avoiding unnecessary biopsies. As a result, the use of mpMRI and biopsy platforms that allow for the real-time fusion of mpMRI and transrectal ultrasound images is now routinely used in clinical practice. On the horizon, molecular imaging offers the promise of improved sensitivity relative to mpMRI and early data would suggest that the combination of mpMRI and positron emission tomography using radiotracers targeting prostate-specific membrane antigen provide a more accurate assessment than either modality alone. In this review, we examine the current role of imaging to aid in the initial diagnosis of prostate cancer.

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.

Intraindividual Comparison of 99mTc-Methylene Diphosphonate and Prostate-Specific Membrane Antigen Ligand 99mTc-MIP-1427 in Patients with Osseous Metastasized Prostate Cancer

The objective of this study was to evaluate the rate of detection of bone metastases obtained with the prostate-specific membrane antigen (PSMA)-targeting tracer ^99mTc-MIP-1427, as opposed to conventional bone scanning with ^99mTc-methylene diphosphonate (^99mTc-MDP), in a collective of patients with known advanced-stage osseous metastasized prostate cancer. Methods: Twenty-one patients with known metastatic disease were staged with both conventional bone scanning and PSMA ligand scintigraphy within a time frame of less than 10 d. Imaging included planar whole-body scanning and SPECT or SPECT/CT with 2 bed positions 3 h after injection of either 500-750 MBq of ^99mTc-MIP-1427 or 600-750 MBq of ^99mTc-MDP. Lesions were scored as typical tumor, equivocal (benign/malignant), or normal within a standard reporting schema divided into defined anatomic regions. Masked and consensus readings were performed with sequential unmasking: planar scans first, then SPECT/CT, the best evaluable comparator (including MRI), PET/CT, and follow-up examinations. Results: Eleven patients had PSMA-positive visceral metastases that were predictably not diagnosed with conventional bone scanning. However, SPECT/CT was required to distinguish between soft-tissue uptake and overlapping bone. Four patients had extensive ^99mTc-MDP-negative bone marrow lesions. Seven patients had superscan characteristics on bone scans; in contrast, the extent of red marrow involvement was more evident on PSMA scans. Only 3 patients had equivalent results on bone scans and PSMA scans. In 16 patients, more suspect lesions were detected with PSMA scanning than with bone scanning. In 2 patients (10%), a PSMA-negative tumor phenotype was present. Conclusion: PSMA scanning provided a clear advantage over bone scanning by reducing the number of equivocal findings in most patients. SPECT/CT was pivotal for differentiating bone metastases from extraosseous tumor lesions.