Histologically Confirmed Diagnostic Efficacy of 18F-rhPSMA-7 PET for N-Staging of Patients with Primary High-Risk Prostate Cancer

Diagnostic Accuracy of 18F-Prostate Specific Membrane Antigen (PSMA) PET/CT Radiotracers in Staging and Restaging of Patients With High-Risk Prostate Cancer or Biochemical Recurrence: An Overview of Reviews

The aim of this overview was to consolidate existing evidence syntheses and provide a comprehensive overview of the evidence for 18F-prostate specific membrane antigen (PSMA) PET/CT in the staging of high-risk prostate cancer and restaging after biochemical recurrence. An overview of reviews was performed and reported in line with the preferred reporting items for overview of reviews (PRIOR) statement and synthesis without meta-analysis (SWiM) reporting guidelines. A comprehensive database and grey literature search were conducted up to July 18, 2023. Systematic reviews were assessed using the risk of bias in systematic reviews (ROBIS) tool. The certainty of the evidence was assessed using grading of recommendations, assessment, development and evaluations (GRADE). 11 systematic reviews were identified; 10 were at high or unclear risk of bias. Evidence reported on a per-patient, per-lymph node, and per-lesion basis for sensitivity, specificity and overall accuracy was identified. There was a lack of data on dose, adverse events and evidence directly comparing 18F-PSMA PET/CT to other imaging modalities. Evidence with moderate to very low certainty indicated high sensitivity, specificity and accuracy of 18F-PSMA PET/CT in patients with high-risk prostate cancer and biochemical recurrence. There was considerably lower certainty evidence and greater variability in effect estimates for outcomes for the combined intermediate/high-risk cohort. While evidence gaps remain for some outcomes, and most systematic reviews were at high or unclear risk of bias, the current evidence base is broadly supportive of 18F-PSMA PET/CT imaging in the staging and restaging of patients with high-risk prostate cancer and biochemical recurrence.

18 F-prostate specific membrane antigen positron emission tomography/computerized tomography for lymph node staging in medium/high risk prostate cancer: A systematic review and meta-analysis

BACKGROUND

Lymph node staging of prostate cancer (PCa) is important for planning and monitoring of treatment. 18 F-prostate specific membrane antigen positron emission tomography/computerized tomography ( 18 F-PSMA PET/CT) has several advantages over 68 Ga-PSMA PET/CT, but its diagnostic value requires further investigation. This meta-analysis focused on establishing the diagnostic utility of 18 F-PSMA PET/CT for lymph node staging in medium/high-risk PCa.

METHODS

We searched the EMBASE, PubMed, Cochrane library, and Web of Science databases from inception to October 1, 2022. Prostate cancer, 18 F, lymph node, PSMA, and PET/CT were used as search terms and the language was limited to English. We additionally performed a manual search using the reference lists of key articles. Patients and study characteristics were extracted and the QUADAS-2 tool was employed to evaluate the quality of included studies. Sensitivity, specificity, the positive and negative likelihood ratio (PLR and NLR), diagnostic odds ratio (DOR), area under the curve (AUC), and 95% confidence interval (CI) were used to evaluate the diagnostic value of 18 F-PSMA PET/CT. Stata 17 software was employed for calculation and statistical analyses.

RESULTS

A total of eight diagnostic tests including 734 individual samples and 6346 lymph nodes were included in this meta-analysis. At the patient level, the results of each consolidated summary were as follows: sensitivity of 0.57 (95% CI 0.39-0.73), specificity of 0.95 (95% CI 0.92-0.97), PLR of 11.2 (95% CI 6.6-19.0), NLR of 0.46 (95% CI 0.31-0.68), DOR of 25 (95% CI 11-54), and AUC of 0.94 (95% CI 0.92-0.96). At the lesion level, the results of each consolidated summary were as follows: sensitivity of 0.40 (95% CI 0.21-0.62), specificity of 0.99 (95% CI 0.95-1.00), PLR of 40.0 (95% CI 9.1-176.3), NLR of 0.61 (95% CI 0.42-0.87), DOR of 66 (95% CI 14-311), and AUC of 0.86 (95% CI 0.83-0.89).

CONCLUSIONS

18 F-PSMA PET/CT showed moderate sensitivity but high specificity in lymph node staging of medium/high-risk PCa. The diagnostic efficacy was almost equivalent to that reported for 68 Ga-PSMA PET/CT.

REGISTRATION

International Prospective Register of Systematic Reviews (PROSPERO), No. CRD42023391101.

Head-to-head Comparison of the Diagnostic Accuracy of Prostate-specific Membrane Antigen Positron Emission Tomography and Conventional Imaging Modalities for Initial Staging of Intermediate- to High-risk Prostate Cancer: A Systematic Review and Meta-analysis

CONTEXT

Whether prostate-specific membrane antigen positron emission tomography (PSMA-PET) should replace conventional imaging modalities (CIM) for initial staging of intermediate-high risk prostate cancer (PCa) requires definitive evidence on their relative diagnostic abilities.

OBJECTIVE

To perform head-to-head comparisons of PSMA-PET and CIM including multiparametric magnetic resonance imaging (mpMRI), computed tomography (CT) and bone scan (BS) for upfront staging of tumour, nodal, and bone metastasis.

EVIDENCE ACQUISITION

A search of the PubMed, EMBASE, CENTRAL, and Scopus databases was conducted from inception to December 2021. Only studies in which patients underwent both PSMA-PET and CIM and imaging was referenced against histopathology or composite reference standards were included. Quality was assessed using the Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2) checklist and its extension for comparative reviews (QUADAS-C). Pairwise comparisons of the sensitivity and specificity of PSMA-PET versus CIM were performed by adding imaging modality as a covariate to bivariate mixed-effects meta-regression models. The likelihood ratio test was applied to determine whether statistically significant differences existed.

EVIDENCE SYNTHESIS

A total of 31 studies (2431 patients) were included. PSMA-PET/MRI was more sensitive than mpMRI for detection of extra-prostatic extension (78.7% versus 52.9%) and seminal vesicle invasion (66.7% versus 51.0%). For nodal staging, PSMA-PET was more sensitive and specific than mpMRI (73.7% versus 38.9%, 97.5% versus 82.6%) and CT (73.2% versus 38.5%, 97.8% versus 83.6%). For bone metastasis staging, PSMA-PET was more sensitive and specific than BS with or without single-photon emission computerised tomography (98.0% versus 73.0%, 96.2% versus 79.1%). A time interval between imaging modalities >1 month was identified as a source of heterogeneity across all nodal staging analyses.

CONCLUSIONS

Direct comparisons revealed that PSMA-PET significantly outperforms CIM, which suggests that PSMA-PET should be used as a first-line approach for the initial staging of PCa.

PATIENT SUMMARY

We reviewed direct comparisons of the ability of a scan method called PSMA-PET (prostate-specific membrane antigen positron emission tomography) and current imaging methods to detect the spread of prostate cancer outside the prostate gland. We found that PSMA-PET is more accurate for detection of the spread of prostate cancer to adjacent tissue, nearby lymph nodes, and bones.

Validation of 18F-rhPSMA-7 and 18F-rhPSMA-7.3 PET Imaging Results with Histopathology from Salvage Surgery in Patients with Biochemical Recurrence of Prostate Cancer

¹⁸F-rhPSMA-7, and its single diastereoisomer form, ¹⁸F-rhPSMA-7.3, are prostate-specific membrane antigen (PSMA)-targeting radiopharmaceuticals. Here, we investigated their accuracy for the assessment of lymph node (LN) metastases validated by histopathology. Methods: Data from 58 patients with biochemical recurrence of prostate cancer after radical prostatectomy receiving salvage surgery after PET imaging with ¹⁸F-rhPSMA-7 or ¹⁸F-rhPSMA-7.3 were retrospectively reviewed. Two nuclear medicine physicians reviewed all PET scans and morphologic imaging in consensus. Readers were masked from the results of histopathology. PET and morphologic imaging were correlated with histopathology from resected LNs. Results: In 75 of 150 resected regions in 54 of 58 patients, tumor lesions were present in histopathology. The template-based specificity of PET (¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 combined) and morphologic imaging was 93.3% and 100%, respectively. However, ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET detected metastases in 61 of 75 histopathologically proven metastatic LN fields (81.3%) whereas morphologic imaging was positive in only 9 of 75 (12.0%). The positive predictive value was 92.4% for ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET and 100% for morphologic imaging. ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET performance was significantly superior to morphologic imaging (difference in the areas under the receiver-operating-characteristic curves, 0.222; 95% CI, 0.147-0.298; P < 0.001). The mean size of PET-positive and histologically confirmed LN metastases was 6.3 ± 3.1 mm (range, 2-15 mm) compared with a mean size of 9.8 ± 2.5 mm (range, 7-15 mm) on morphologic imaging. Conclusion: ¹⁸F-rhPSMA-7 and ¹⁸F-rhPSMA-7.3 PET offer a high positive predictive value comparable to that reported for ⁶⁸Ga-PSMA-11 and represent a valuable tool for guiding salvage lymphadenectomy.

Introduction of a SiFA Moiety into the D-Glutamate Chain of DOTA-PP-F11N Results in Radiohybrid-Based CCK-2R-Targeted Compounds with Improved Pharmacokinetics In Vivo

In order to enable ¹⁸F- and ¹⁷⁷Lu-labelling within the same molecule, we introduced a silicon-based fluoride acceptor (SiFA) into the hexa-D-glutamate chain of DOTA-PP-F11N. In addition, minigastrin analogues with a prolonged as well as γ-linked D-glutamate chain were synthesised and evaluated. CCK-2R affinity (IC₅₀, AR42J cells) and lipophilicity (logD_7.4) were determined. Biodistribution studies at 24 h post-injection (p.i.) and µSPECT/CT imaging at 1, 4 and 24 h p.i. were carried out in AR42J tumour-bearing CB17-SCID mice. CCK-2R affinity of (R)-DOTAGA-rhCCK-1 to 18 was enhanced with increasing distance between the SiFA building block and the binding motif. Lipophilicity of [¹⁷⁷Lu]Lu-(R)-DOTAGA-rhCCK-1 to 18 was higher compared to that of [¹⁷⁷Lu]Lu-DOTA-PP-F11N and [¹⁷⁷Lu]Lu-CP04. The respective α- and γ-linked rhCCK derivatives revealing the highest CCK-2R affinity were further evaluated in vivo. In comparison with [¹⁷⁷Lu]Lu-DOTA-PP-F11N, [¹⁷⁷Lu-]Lu-(R)-DOTAGA-rhCCK-9 and -16 exhibited three- to eight-fold increased activity levels in the tumour at 24 h p.i. However, activity levels in the kidneys were elevated as well. We could show that the introduction of a lipophilic SiFA moiety into the hydrophilic backbone of [¹⁷⁷Lu]Lu-DOTA-PP-F11N led to a decelerated blood clearance and thus improved tumour retention. However, elevated kidney retention has to be addressed in future studies.

18F-rhPSMA-7 PET for the Detection of Biochemical Recurrence of Prostate Cancer After Curative-Intent Radiation Therapy: A Bicentric Retrospective Study

This bicentric, retrospective analysis investigated the efficacy of PET/CT with a novel theranostic prostate-specific membrane antigen (PSMA)--targeting ligand, ¹⁸F-rhPSMA-7, in patients with biochemical recurrence (BCR) of prostate cancer after curative-intent primary radiotherapy. Methods: Datasets from patients with BCR of prostate cancer after external-beam radiation therapy or brachytherapy who underwent ¹⁸F-rhPSMA-7 PET/CT at either Technical University Munich or Ludwig-Maximilians-University Munich were retrospectively reviewed by experienced nuclear medicine physicians and radiologists at both centers. The median injected activity was 299 MBq (range, 204-420 MBq), and the median uptake time was 77 min (range, 46-120 min). All lesions suggestive of recurrent prostate cancer were noted. Detection rates were correlated with patients' prostate-specific antigen (PSA) level, primary Gleason score, and prior use of androgen-deprivation therapy (ADT). Results: Ninety-seven patients were included (65 at Technical University Munich and 32 at Ludwig-Maximilians-University Munich). The median prescan PSA was 4.19 ng/mL (range, 0.1-159 ng/mL). The primary Gleason score was ≤6 in 19 patients, 7 in 25, ≥8 in 33, and unknown in 20. Thirty patients received ADT in the 6 mo preceding PET/CT. ¹⁸F-rhPSMA-7 identified lesions in 91 of 97 (94%) patients. Detection rates stratified by PSA were 88% (22/25), 97% (30/31), 90% (19/21), and 100% (20/20) for a PSA of <2, 2-<5, 5-<10, and ≥10 ng/mL, respectively. Detection rates in the subgroup of patients not meeting the Phoenix criteria for BCR were 80% (4/5), 90% (9/10), 100% (4/4), and 83% (5/6) for a PSA of <0.5, 0.5-<1, 1-<1.5, and 1.5-2 ng/mL, respectively. There were no significant differences in detection rates between patients with and without prior ADT (100% vs. 91%, P = 0.173) or patients with a Gleason score of ≤7 and a Gleason score of ≥8 (98% vs. 91%, P = 0.316).¹⁸F-rhPSMA-7 revealed local recurrence in 80% (78/97); pelvic lymph node metastases in 38% (37/97); retroperitoneal and supradiaphragmatic lymph node metastases in 9% (9/97) and 4% (4/97), respectively; bone metastases in 27% (26/97); and visceral metastases in 3% (3/97). In the subgroup of patients with a PSA of <2 ng/mL above nadir, local recurrence occurred in 76% (19/25) and pelvic lymph node metastases in 36% (9/25). Conclusion:¹⁸F-rhPSMA-7 PET/CT demonstrates high detection rates in prostate cancer patients with BCR after primary radiation therapy, even at low PSA values. Its diagnostic efficacy is comparable to published data for other PSMA ligands.

The Role of PSMA PET/CT in the Primary Diagnosis and Follow-Up of Prostate Cancer-A Practical Clinical Review

Simple Summary

The combination of positron emission tomography (PET)-diagnostics with ligands binding to the prostate-specific membrane antigen (PSMA) has been a diagnostic milestone in the situation of biochemical recurrence of prostate cancer and is gaining importance in primary diagnostics, providing a highly specific and sensitive diagnostic method in various clinical situations. However, the clinical application of this method requires a comprehensive knowledge of its advantages and disadvantages, potential pitfalls and influencing factors. This review aims to provide a practical clinical review of the currently available background data on PSMA PET/CT, as well as the clinical implications. Although a large amount of data already exist, a thorough analysis is complicated by study heterogeneity, showing the need for future systematic and prospective research.

Abstract

The importance of PSMA PET/CT in both primary diagnostics and prostate cancer recurrence has grown steadily since its introduction more than a decade ago. Over the past years, a vast amount of data have been published on the diagnostic accuracy and the impact of PSMA PET/CT on patient management. Nevertheless, a large heterogeneity between studies has made reaching a consensus difficult; this review aims to provide a comprehensive clinical review of the available scientific literature, covering the currently known data on physiological and pathological PSMA expression, influencing factors, the differences and pitfalls of various tracers, as well as the clinical implications in initial TNM-staging and in the situation of biochemical recurrence. This review has the objective of providing a practical clinical overview of the advantages and disadvantages of the examination in various clinical situations and the body of knowledge available, as well as open questions still requiring further research.

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

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

18F-Fluciclovine versus PSMA PET Imaging in Primary Tumor Detection during Initial Staging of High-Risk Prostate Cancer: A Systematic Review and Meta-Analysis

Purpose Fluorine 18 (¹⁸F)-fluciclovine and prostate-specific membrane antigen (PSMA) tracers are commonly used for localizing biochemical recurrence of prostate cancer, but their accuracy in primary tumor detection in the initial staging of high-risk prostate cancer has not been established. Materials and Methods A systematic review was performed of the electronic databases for original studies published between 2012 and 2020. Included studies were those in which ¹⁸F-fluciclovine or PSMA PET was used for initial staging of patients with high-risk prostate cancer. The diagnostic performance data were collected for primary tumor with histopathologic results as reference standard. The Quality Assessment of Diagnostic Accuracy Studies-2 tool was used for quality appraisal. A random-effects model was used to summarize the effect sizes and to evaluate the difference between two groups. Results Overall, 28 studies met the eligibility criteria, and 17 were included in the meta-analysis (¹⁸F-fluciclovine = 4, PSMA = 13). Of these 17 studies, 12 (70%) were judged to have high risk of bias in one of the evaluated domains, and nine studies were deemed to have applicability concerns. The pooled sensitivity, specificity, and diagnostic odds ratio for ¹⁸F-fluciclovine versus PSMA were 85% (95% CI: 73%, 92%) versus 84% (95% CI: 77%, 89%) (P = .78), 77% (95% CI: 60%, 88%) versus 83% (95% CI: 76%, 89%) (P = .40), and 18.88 (95% CI: 5.01, 71.20) versus 29.37 (95% CI: 13.35, 64.60) (P = .57), respectively, with no significant difference in diagnostic test accuracy. Conclusion ¹⁸F-fluciclovine and PSMA PET demonstrated no statistically significant difference in diagnostic accuracy in primary tumor detection during initial staging of high-risk prostate cancer. Keywords: PET, Prostate, Molecular Imaging-Cancer, Staging Supplemental material is available for this article. © RSNA, 2022.

Preclinical biodistribution and dosimetry and human biodistribution comparing 18F-rhPSMA-7 and single isomer 18F-rhPSMA-7.3

Background

Radiohybrid prostate-specific membrane antigen (rhPSMA) ligands such as ¹⁸F-rhPSMA-7 are a new class of theranostic agents in clinical development for prostate cancer. We compared preclinical dosimetry and human biodistribution of ¹⁸F-rhPSMA-7 with that of single diastereoisomer form, ¹⁸F-rhPSMA-7.3.

Methods

Preclinical dosimetry was performed with SCID-mice sacrificed at multiple timepoints (10–300 min) post-injection of 25.6 ± 3.6 MBq ¹⁸F-rhPSMA-7 or 28.5 ± 4.8 MBq ¹⁸F-rhPSMA-7.3 (n = 3–6 mice per timepoint). Heart, lung, liver, spleen, pancreas, fat, stomach, small intestine, large intestine, kidney, muscle, bone, bladder, testicles, tail, and brain tissue were harvested, and urine and blood samples collected. Percentage of injected dose per gram was calculated. Absorbed doses were estimated with OLINDA/EXM 1.0.

¹⁸F-rhPSMA-7 (n = 47) and ¹⁸F-rhPSMA-7.3 (n = 33) PET/CT exams were used to estimate human biodistribution. Mean (range) injected activities were 324 (236–424) MBq versus 345 (235–420) MBq, and acquisition times were 84 (42–166) versus 76 (59–122) minutes for ¹⁸F-rhPSMA-7 versus ¹⁸F-rhPSMA-7.3, respectively. SUV_mean was determined for background (gluteal muscle), normal organs (salivary glands, blood pool, lung, liver, spleen, pancreas, duodenum, kidney, bladder, bone) and up to three representative tumour lesions. Qualitative analyses assessed image quality, non-specific blood pool activity, and background uptake in bone/marrow using 3/4-point scales.

Results

Preclinical dosimetry revealed that at 3.5 h and 1 h bladder voiding intervals, the extrapolated total effective doses were 26.6 and 12.2 µSv/MBq for ¹⁸F-rhPSMA-7 and 21.7 and 12.8 µSv/MBq for ¹⁸F-rhPSMA-7.3 respectively.

Human biodistribution of both agents was typical of other PSMA-ligands and broadly similar to each other; SUV_mean were 16.9 versus 16.2 (parotid gland), 19.6 versus 19.9 (submandibular gland), 2.0 versus 1.9 (blood pool, p < 0.005), 0.7 versus 0.7 (lungs), 7.0 versus 7.3 (liver), 9.1 versus 8.4 (spleen), 32.4 versus 35.7 (kidney), 2.5 versus 2.8 (pancreas), 10.9 versus 11.0 (duodenum), 1.1 versus 1.3 (bone) and 4.6 versus 2.0 (bladder; p < 0.001) for ¹⁸F-rhPSMA-7 versus ¹⁸F-rhPSMA-7.3, respectively. Tumour SUV_mean was higher for ¹⁸F-rhPSMA-7.3 (32.5 ± 42.7, n = 63 lesions) than for ¹⁸F-rhPSMA-7 (20.0 ± 20.2, n = 89 lesions).

Conclusions

Radiation dosimetry is favourable for both agents. Radiation exposure, assuming a 1 h voiding interval, is less than 5 mSv after injection of 370 MBq. ¹⁸F-rhPSMA-7.3 showed significantly lower bladder uptake, and a higher uptake trend in tumours compared with ¹⁸F-rhPSMA-7.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-021-00872-w.

Utility of 18F-rhPSMA-7.3 positron emission tomography for imaging of primary prostate cancer and pre-operative efficacy in N-staging of unfavorable intermediate to very high-risk patients validated by histopathology

¹⁸F-rhPSMA-7.3, the lead compound of a new class of radiohybrid prostate-specific membrane antigen (rhPSMA) ligands, is currently in phase III trials for prostate cancer (PCa) imaging. Here, we describe our experience in primary PCa staging. Methods: We retrospectively identified 279 patients with primary PCa who underwent ¹⁸F-rhPSMA-7.3 PET/CT (staging cohort). A subset of patients (83/279) subsequently underwent prostatectomy with lymph node (LN) dissection without prior treatment (efficacy cohort). Distribution of tumor lesions was determined for the staging cohort and stratified by National Comprehensive Cancer Network (NCCN) risk score. Involvement of pelvic LN was assessed retrospectively by 3 blinded independent central readers, and a majority rule was used for analysis. Standard surgical fields were rated on a five-point scale independently for PET and for morphological imaging. Results were compared to histopathological findings on a patient-, right vs. -left, and template-basis. Results: For the staging cohort ¹⁸F-rhPSMA-7.3 PET was positive in 275/279 (98.6%), 106/279 (38.0%), 46/279 (16.5%), 65/279 (23.3%) and 5/279 (1.8%) patients for local, pelvic nodal, extrapelvic nodal, metastatic bone, and visceral metastatic disease. In the efficacy cohort, LN metastases were present in 24/83 patients (29%), located in 48/420 (11%) resected templates and in 33/166 (19.9%) hemi-pelvic templates in histopathology. Based on majority vote results, the patient-level sensitivity, specificity and accuracy for pelvic nodal metastases were 66.7% (95%CI, 44.7-83.6%), 96.6% (95%CI, 87.3-99.4%) and 88.0% (95%CI, 78.5-93.8%) for ¹⁸F-rhPSMA-7.3 PET and 37.5% (95%CI, 19.6-59.2%), 91.5% (95%CI, 80.6-96.8%) and 75.9% (95%CI, 65.0-84.3%) for morphological imaging, respectively. ¹⁸F-rhPSMA-7.3 showed higher interobserver agreement than morphological imaging (patient-level Fleiss' κ=0.54; 95%CI, 0.47-0.62 vs. 0.24; 95%CI, 0.17-0.31). A mean standardized uptake value ratio of 6.6 (95%CI, 5.2-8.1) documented a high image contrast between local tumors and adjacent low urinary tracer retention. Conclusion: ¹⁸F-rhPSMA-7.3 PET offers superior diagnostic performance to morphological imaging for primary N-staging of newly diagnosed PCa, shows lower inter-reader variation, and offers good distinction between primary tumor and bladder background activity. With increasing NCCN risk group an increasing frequency of extra-prostatic tumor lesions was observed.

Synthesis and Preclinical Evaluation of [18F]SiFA-PSMA Inhibitors in a Prostate Cancer Model

Positron emission tomography (PET) imaging of prostate-specific membrane antigen (PSMA) with gallium-68 (⁶⁸Ga) and fluorine-18 (¹⁸F) radiotracers has aroused tremendous interest over the past few years. The use of organosilicon-[¹⁸F]fluoride acceptors (SiFA) conjugated to urea-based peptidomimetic PSMA inhibitors provides a "kit-like" multidose synthesis technology. Nine novel ¹⁸F-labeled SiFA-bearing PSMA inhibitors with different linker moieties were synthesized and analyzed for their in vitro binding against [¹²⁵I]I-TAAG-PSMA in LNCaP cells. IC₅₀ values ranged from 58-570 nM. Among all compounds, [¹⁸F]SiFA-Asp₂-PEG₃-PSMA (IC₅₀ = 125 nM) showed the highest tumor uptake in LNCaP tumors (SUV_60min 0.73). A substantial increase in molar activity (A_m) (from 7.5 ± 0.5 to 86 ± 3 GBq/μmol) led to a significant increase in LNCaP tumor uptake (SUV_60min 1.18; Δ 0.45 corresponding to +62%). In vivo blocking with DCFPyL resulted in -32% uptake after 60 min. The SiFA-isotopic exchange chemistry offers a method that is readily adaptable for a "kit-type" labeling procedure and clinical translation.

Can Negative Prostate-specific Membrane Antigen Positron Emission Tomography/Computed Tomography Avoid the Need for Pelvic Lymph Node Dissection in Newly Diagnosed Prostate Cancer Patients? A Systematic Review and Meta-analysis with Backup Histology as Reference Standard

CONTEXT

The role of positron emission tomography/computed tomography (PET/CT) with prostate-specific membrane antigen (PSMA) in the primary staging for patients with prostate cancer (PCa) is still debated.

OBJECTIVE

To analyze published studies reporting the accuracy of PSMA PET/CT for detecting lymph node invasion (LNI) at pelvic lymph node dissection (PLND).

EVIDENCE ACQUISITION

A search of PubMed/MEDLINE, Cochrane library's Central, EMBASE and Scopus databases, from inception to May 2021, was conducted. The primary outcome was to evaluate the sensitivity, specificity, positive (PPV) and negative (NPV) predictive values of PSMA PET/CT in detecting LNI on a per-patient level. As a secondary outcome, NPV of PET PSMA was tested on a per-node-level analysis. Detection rates were pooled using random-effect models. Preplanned subgroup analyses tested the diagnostic accuracy after stratification for the preoperative risk group. PPV and NPV variation over LNI prevalence was evaluated. Only studies including extended PLND (ePLND) as the reference standard test were included.

EVIDENCE SYNTHESIS

Twenty-seven studies, with a total of 2832 participants, were included in quantitative synthesis. The sensitivity, specificity, PPV, and NPV of PSMA PET/CT for LNI were, respectively, 58% (95% confidence interval [CI] 50-66%), 95% (95% CI 93-97%), 79% (95% CI 72-85%), and 87% (95% CI 84-89%), with overall moderate heterogeneity between studies. At bivariate analysis, the diagnostic accuracy of PSMA PET/CT estimated through summary receiver operating characteristic-derived area under the curve was 84% (95% CI 81-87%). On a per-node level, NPV of PET PSMA was 97% (95% CI 96-99%). At subgroup analyses, according to preoperative risk groups, sensitivity, specificity, PPV, and NPV were 51%, 93%, 73%, and 81%, respectively, in high-risk patients. Over the LNI prevalence range of 5-40%, PPV increased from 59% to 91%, while NPV decreased from 99% to 84%.

CONCLUSIONS

PSMA PET/CT scan provides promising accuracy in the field of primary nodal staging for PCa. The high NPV in men with a lower risk of LNI might be clinically useful to reduce the number of unnecessary PLND procedures performed. Conversely, in high-risk patients, negative PSMA PET/CT cannot replace staging ePLND.

PATIENT SUMMARY

In this systematic review and meta-analysis, we demonstrated that prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography (PET/CT) scan may optimize the primary nodal staging and surgical management of prostate cancer patients candidate to radical prostatectomy. The high negative predictive value in men with a lower risk of lymph node invasion might be clinically useful for reducing the number of useless pelvic lymph node dissection (PLND) procedures performed. Conversely, in high-risk patients, negative PSMA PET/CT cannot allow avoiding of PLND.

Matched-Pair Comparison of 68Ga-PSMA-11 and 18F-rhPSMA-7 PET/CT in Patients with Primary and Biochemical Recurrence of Prostate Cancer: Frequency of Non-Tumor-Related Uptake and Tumor Positivity

Radiohybrid prostate-specific membrane antigen (rhPSMA) ligands are a new class of prostate cancer theranostic agents. 18F-rhPSMA-7 offers the advantages of 18F labeling and low urinary excretion compared with 68Ga-PSMA-11. Here, we compare the frequency of non-tumor-related uptake and tumor positivity with 68Ga-PSMA-11 and 18F-rhPSMA-7 in patients with primary or recurrent prostate cancer. Methods: This retrospective matched-pair comparison matched 160 18F-rhPSMA-7 with 160 68Ga-PSMA-11 PET/CT studies for primary staging (n = 33) and biochemical recurrence (n = 127) according to clinical characteristics. Two nuclear medicine physicians reviewed all scans, first identifying all PET-positive lesions and then differentiating lesions suggestive of prostate cancer from those that were benign, on the basis of known pitfalls and ancillary information from CT. For each region, the SUVmax of the lesion with the highest PSMA ligand uptake was noted. Tumor positivity rates were determined, and SUVmax was compared separately for each tracer. Results:18F-rhPSMA-7 and 68Ga-PSMA-11 PET revealed 566 and 289 PSMA ligand-positive lesions, respectively. Of these, 379 and 100 lesions, equaling 67.0% and 34.6%, respectively, of all PSMA-positive lesions, were considered benign. The distribution of their etiology was similar (42%, 24%, and 25% with 18F-rhPSMA-7 vs. 32%, 24%, and 38% with 68Ga-PSMA-11 for ganglia, bone, and unspecific lymph nodes, respectively). All primary tumors were positive with both agents (n = 33 each), whereas slightly more metastatic lesions were observed with 68Ga-PSMA-11 in both disease stages (113 for 18F-rhPSMA-7 and 124 for 68Ga-PSMA-11). The SUVmax of 18F-rhPSMA-7 and 68Ga-PSMA-11 did not differ (P > 0.05) in local recurrence or primary prostate cancer; however, the tumor-to-bladder ratio was significantly higher with 18F-rhPSMA-7 (4.9 ± 5.3 vs. 2.2 ± 3.7, P = 0.02, for local recurrence; 9.8 ± 9.7 vs. 2.3 ± 2.6, P < 0.001, for primary prostate cancer). Conclusion: The tumor positivity rate was consistently high for 68Ga-PSMA-11 and 18F-rhPSMA-7. Both tracers revealed a considerable number of areas of uptake that were reliably identified as benign by trained physicians making use of corresponding morphologic imaging and known PSMA pitfalls. These were more frequent with 18F-rhPSMA-7. However, the matched-pair comparison could have introduced a source of bias. Adequate reader training can allow physicians to differentiate benign uptake from disease and be able to benefit from the logistical and clinical advantages of 18F-rhPSMA-7.

Prospective comparison of 18F-PSMA-1007 PET/CT, whole-body MRI and CT in primary nodal staging of unfavourable intermediate- and high-risk prostate cancer

PURPOSE

To prospectively compare 18F-prostate-specific membrane antigen (PSMA)-1007 positron emission tomography (PET)/CT, whole-body magnetic resonance imaging (WBMRI) including diffusion-weighted imaging (DWI) and standard computed tomography (CT), in primary nodal staging of prostate cancer (PCa).

METHODS

Men with newly diagnosed unfavourable intermediate- or high-risk PCa prospectively underwent 18F-PSMA-1007 PET/CT, WBMRI with DWI and contrast-enhanced CT within a median of 8 days. Six readers (two for each modality) independently reported pelvic lymph nodes as malignant, equivocal or benign while blinded to the other imaging modalities. Sensitivity, specificity and accuracy were reported according to optimistic (equivocal lesions interpreted as benign) and pessimistic (equivocal lesions interpreted as malignant) analyses. The reference standard diagnosis was based on multidisciplinary consensus meetings where available histopathology, clinical and follow-up data were used.

RESULTS

Seventy-nine patients completed all the imaging modalities, except for one case of interrupted WBMRI. Thirty-one (39%) patients had pelvic lymph node metastases, which were detected in 27/31 (87%), 14/31 (45%) and 8/31 (26%) patients by 18F-PSMA-1007 PET/CT, WBMRI with DWI and CT, respectively (optimistic analysis). In 8/31 (26%) patients, only 18F-PSMA-1007 PET/CT detected malignant lymph nodes, while the other two imaging modalities were reported as negative. At the patient level, sensitivity and specificity values for 18F-PSMA-1007 PET/CT, WBMRI with DWI and CT in optimistic analysis were 0.87 (95%CI 0.71-0.95) and 0.98 (95%CI 0.89-1.00), 0.37 (95%CI 0.22-0.55) and 0.98 (95%CI 0.89-1.00) and 0.26 (95%CI 0.14-0.43) and 1.00 (95%CI 0.93-1.00), respectively.

CONCLUSION

18F-PSMA-1007 PET/CT showed significantly greater sensitivity in nodal staging of primary PCa than did WBMRI with DWI or CT, while maintaining high specificity.

CLINICAL TRIAL REGISTRATION

Clinicaltrials.gov ID: NCT03537391.

Comparative Preclinical Biodistribution, Dosimetry, and Endoradiotherapy in Metastatic Castration-Resistant Prostate Cancer Using 19F/177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T

Radiohybrid prostate-specific membrane antigen (rhPSMA) ligands are applicable as radiochemical twins for both diagnostic PET imaging and endoradiotherapy. On the basis of preliminary data as a diagnostic ligand, the isomer rhPSMA-7.3 is a promising candidate for potential endoradiotherapy. The aim of this preclinical evaluation was to assess the biodistribution, dosimetry, and therapeutic efficacy of 19F/177Lu-rhPSMA-7.3 in comparison to the established therapeutic agent 177Lu-PSMA I&T (imaging and therapy). Methods: The biodistribution of 19F/177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T was determined in LNCaP tumor-bearing severe combined immunodeficiency (SCID) mice after sacrifice at defined time points up to 7 d (n = 5). Organs and tumors were dissected, percentage injected dose per gram (%ID/g) was determined, and dosimetry was calculated using OLINDA/EXM, version 1.0. The therapeutic efficacy of a single 30-MBq dose of 19F/177Lu-rhPSMA-7.3 (n = 7) was compared with that of 177Lu-PSMA I&T in treatment groups (n = 7) and control groups (n = 6-7) using C4-2 tumor-bearing SCID mice by evaluating tumor growth and survival over 6 wk after treatment. Results: The biodistribution of 19F/177Lu-rhPSMA-7.3 revealed fast blood clearance (0.63 %ID/g at 1 h after injection), and the highest activity uptake was in the spleen and kidneys, particularly in the first hour (33.25 %ID/g and 207.6 %ID/g, respectively, at 1 h after injection), indicating a renal excretion pathway. Compared with 177Lu-PSMA I&T, 19F/177Lu-rhPSMA-7.3 exhibited an initial (1 h) 2.6-fold higher tumor uptake in LNCaP xenografts and a longer retention (4.5 %ID/g vs. 0.9 %ID/g at 168 h). The tumor dose of 19F/177Lu-rhPSMA-7.3 was substantially higher (e.g., 7.47 vs. 1.96 µGy/MBq at 200 mm3) than that of 177Lu-PSMA I&T. In most organs, absorbed doses were higher for 177Lu-PSMA I&T. A significantly greater tumor size reduction was shown for a single dose of 19F/177Lu-rhPSMA-7.3 than for 177Lu-PSMA I&T at the end of the experiment (P = 0.0167). At the predefined termination of the experiment at 6 wk, 7 of 7 and 3 of 7 mice were still alive in the 19F/177Lu-rhPSMA-7.3 and 177Lu-PSMA I&T groups, respectively, compared with the respective control groups, with 0 of 7 and 0 of 6 mice. Conclusion: Compared with 177Lu-PSMA I&T, 19F/177Lu-rhPSMA-7.3 can be considered a suitable candidate for clinical translation because it has similar clearance kinetics and a similar radiation dose to healthy organs but superior tumor uptake and retention. Preliminary treatment experiments showed a favorable antitumor response.

Recent Advances in the Clinical Translation of Silicon Fluoride Acceptor (SiFA) 18F-Radiopharmaceuticals

The incorporation of silicon fluoride acceptor (SiFA) moieties into a variety of molecules, such as peptides, proteins and biologically relevant small molecules, has improved the generation of ¹⁸F-radiopharmaceuticals for medical imaging. The efficient isotopic exchange radiofluorination process, in combination with the enhanced [¹⁸F]SiFA in vivo stability, make it a suitable strategy for fluorine-18 incorporation. This review will highlight the clinical applicability of [¹⁸F]SiFA-labeled compounds and discuss the significant radiotracers currently in clinical use.

Safety, Biodistribution, and Radiation Dosimetry of 18F-rhPSMA-7.3 in Healthy Adult Volunteers

This first-in-humans study investigated the safety, biodistribution, and radiation dosimetry of a novel ¹⁸F-labeled radiohybrid prostate-specific membrane antigen (rhPSMA) PET imaging agent, ¹⁸F-rhPSMA-7.3. Methods: Six healthy volunteers (3 men, 3 women) underwent multiple whole-body PET acquisitions at scheduled time points up to 248 min after the administration of ¹⁸F-rhPSMA-7.3 (mean activity, 220; range, 210–228 MBq). PET scans were conducted in 3 separate sessions, and subjects were encouraged to void between sessions. Blood and urine samples were collected for up to 4 h after injection to assess metabolite-corrected radioactivity in whole blood, plasma, and urine. Quantitative measurements of ¹⁸F radioactivity in volumes of interest over target organs were determined directly from the PET images at 8 time points, and normalized time–activity concentration curves were generated. These normalized cumulated activities were then inputted into the OLINDA/EXM package to calculate the internal radiation dosimetry and the subjects’ effective dose. Results:¹⁸F-rhPSMA-7.3 was well tolerated. One adverse event (mild headache, not requiring medication) was considered possibly related to ¹⁸F-rhPSMA-7.3. The calculated effective dose was 0.0141 mSv/MBq when using a 3.5-h voiding interval. The organs with the highest mean absorbed dose per unit of administered radioactivity were the adrenals (0.1835 mSv/MBq), the kidneys (0.1722 mSv/MBq), the submandibular glands (0.1479 mSv), and the parotid glands (0.1137 mSv/MBq). At the end of the first scanning session (mean time, 111 min after injection), an average of 7.2% (range, 4.4%–9.0%) of the injected radioactivity of ¹⁸F-rhPSMA-7.3 was excreted into urine. Conclusion: The safety, biodistribution, and internal radiation dosimetry of ¹⁸F-rhPSMA-7.3 are considered favorable for PET imaging.

Kinetic analysis and optimisation of 18F-rhPSMA-7.3 PET imaging of prostate cancer

Purpose

This phase 1 open-label study evaluated the uptake kinetics of a novel theranostic PET radiopharmaceutical, ¹⁸F-rhPSMA-7.3, to optimise its use for imaging of prostate cancer.

Methods

Nine men, three with high-risk localised prostate cancer, three with treatment-naïve hormone-sensitive metastatic disease and three with castration-resistant metastatic disease, underwent dynamic 45-min PET scanning of a target area immediately post-injection of 300 MBq ¹⁸F-rhPSMA-7.3, followed by two whole-body PET/CT scans acquired from 60 and 90 min post-injection. Volumes of interest (VoIs) corresponding to prostate cancer lesions and reference tissues were recorded. Standardised uptake values (SUV) and lesion-to-reference ratios were calculated for 3 time frames: 35–45, 60–88 and 90–118 min. Net influx rates (K_i) were calculated using Patlak plots.

Results

Altogether, 44 lesions from the target area were identified. Optimal visual lesion detection started 60 min post-injection. The ¹⁸F-rhPSMA-7.3 signal from prostate cancer lesions increased over time, while reference tissue signals remained stable or decreased. The mean (SD) SUV (g/mL) at the 3 time frames were 8.4 (5.6), 10.1 (7) and 10.6 (7.5), respectively, for prostate lesions, 11.2 (4.3), 13 (4.8) and 14 (5.2) for lymph node metastases, and 4.6 (2.6), 5.7 (3.1) and 6.4 (3.5) for bone metastases. The mean (SD) lesion-to-reference ratio increases from the earliest to the 2 later time frames were 40% (10) and 59% (9), respectively, for the prostate, 65% (27) and 125% (47) for metastatic lymph nodes and 25% (19) and 32% (30) for bone lesions. Patlak plots from lesion VoIs signified almost irreversible uptake kinetics. K_i, SUV and lesion-to-reference ratio estimates showed good agreement.

Conclusion

¹⁸F-rhPSMA-7.3 uptake in prostate cancer lesions was high. Lesion-to-background ratios increased over time, with optimal visual detection starting from 60 min post-injection. Thus, ¹⁸F-rhPSMA-7.3 emerges as a very promising PET radiopharmaceutical for diagnostic imaging of prostate cancer.

Trial Registration

NCT03995888 (24 June 2019).

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05346-8.

Detection efficacy of 18F-rhPSMA-7.3 PET/CT and impact on patient management in patients with biochemical recurrence of prostate cancer after radical prostatectomy and prior to potential salvage treatment

Purpose: Radiohybrid prostate-specific membrane antigen (rhPSMA) ligands are a new class of 18F-labeled PSMA-targeting agents. 18F-rhPSMA-7.3 is a lead compound which is currently under investigation in two multicenter phase III trials for PET-imaging. Here, we report the first retrospective data on its detection efficacy and potential impact on clinical management in a homogeneous cohort of patients with biochemical recurrence after radical prostatectomy, and prior to any salvage therapy. Methods: 242 patients (median [range] PSA, 0.60 [0.2-60.8] ng/mL) who underwent 18F-rhPSMA-7.3 PET/CT were retrospectively selected from the institutions' database. Images were re-read by an experienced nuclear medicine physician. Lesion detection rates were stratified by PSA. Further, potential management before and after PET was assessed by an interdisciplinary simulated tumor board and categorized (major vs. minor vs. no therapeutic change). The distribution of management change identified in each PSA subgroup was determined. Results: In total, 176/242 (72.7%) patients showed PSMA-ligand positive findings. 18F-rhPSMA-7.3 detection rates were 61.8% (63/102), 67.9% (38/56), 81.1% (30/37) and 95.7% (45/47) for PSA-levels of 0.2-<0.5 ng/mL, 0.5-<1 ng/mL, 1-<2 ng/mL and ≥2 ng/mL, respectively. 18F-rhPSMA-7.3 PET/CT revealed local recurrence, pelvic lymph node metastases, retroperitoneal lymph nodes metastases, supradiaphragmatic lymph nodes, bone metastases, and visceral metastases in 48.8% (n = 118), 28.9% (n = 70), 6.6% (n = 16), 1.2% (n = 3), 13.2% (n = 32) and 1.2% (n = 3) of patients, respectively. Notably, bone lesions were identified in 8.8% of patients (9/102) with PSA <0.5 ng/mL. Results from the interdisciplinary simulated tumor board indicated change of therapeutic management in 153/242 patients (63.2%) with 54/242 (22.3%) considered major and 99/242 (40.9%) minor, respectively. 18F-rhPSMA-7.3 PET/CT did not prompt any therapeutic changes in 64/242 patients (26.4%). Conclusion: 18F-rhPSMA-7.3 PET offers high detection efficacy in patients with biochemical recurrence after radical prostatectomy, and prior to potential salvage therapy, and results in a potential change in treatment plans in nearly 2/3 of patients. Keywords: Biochemical recurrence; hybrid imaging; positron emission tomography; prostate cancer; prostate-specific membrane antigen.

Prostate-Specific Membrane Antigen PET for Assessment of Primary and Recurrent Prostate Cancer with Histopathology as Reference Standard: A Systematic Review and Meta-Analysis

Prostate-specific membrane antigen PET is a promising diagnostic tool in prostate cancer. The gold standard for the detection of prostate tumor and lymph node metastases is histopathology. The aim of the present review was to investigate accuracy measures of ⁶⁸Ga/¹⁸F-labeled prostate-specific membrane antigen PET tracers in primary and recurrent prostate cancer with systematic sector-based histopathology as the reference standard. A systematic literature search was performed and 34 studies were included. Overall, prostate-specific membrane antigen PET showed high specificity, but variable sensitivity to localize known prostate cancer and detect pelvic lymph node metastases.

Automated synthesis of [18F]Ga-rhPSMA-7/ -7.3: results, quality control and experience from more than 200 routine productions

Introduction

The radiohybrid (rh) prostate-specific membrane antigen (PSMA)-targeted ligand [¹⁸F]Ga-rhPSMA-7 has previously been clinically assessed and demonstrated promising results for PET-imaging of prostate cancer. The ligand is present as a mixture of four stereoisomers ([¹⁸F]Ga-rhPSMA-7.1, − 7.2, − 7.3 and − 7.4) and after a preclinical isomer selection process, [¹⁸F]Ga-rhPSMA-7.3 has entered formal clinical trials. Here we report on the establishment of a fully automated production process for large-scale production of [¹⁸F]Ga-rhPSMA-7/ -7.3 under GMP conditions (EudraLex).

Methods

[¹⁸F]Fluoride in highly enriched [¹⁸O]H₂O was retained on a strong anion exchange cartridge, rinsed with anhydrous acetonitrile and subsequently eluted with a solution of [K⁺ ⊂ 2.2.2]OH⁻ in anhydrous acetonitrile into a reactor containing Ga-rhPSMA ligand and oxalic acid in DMSO. ¹⁸F-for-¹⁹F isotopic exchange at the Silicon-Fluoride Acceptor (SiFA) was performed at room temperature, followed by dilution with buffer and cartridge-based purification. Optimum process parameters were determined on the laboratory scale and thereafter implemented into an automated synthesis. Data for radiochemical yield (RCY), purity and quality control were analyzed for 243 clinical productions (160 for [¹⁸F]Ga-rhPSMA-7; 83 for [¹⁸F]Ga-rhPSMA-7.3).

Results

The automated production of [¹⁸F]Ga-rhPSMA-7 and the single isomer [¹⁸F]Ga-rhPSMA-7.3 is completed in approx. 16 min with an average RCY of 49.2 ± 8.6% and an excellent reliability of 98.8%. Based on the different starting activities (range: 31–130 GBq, 89 ± 14 GBq) an average molar activity of 291 ± 62 GBq/μmol (range: 50–450 GBq/μmol) was reached for labeling of 150 nmol (231 μg) precursor. Radiochemical purity, as measured by radio-high performance liquid chromatography and radio-thin layer chromatography, was 99.9 ± 0.2% and 97.8 ± 1.0%, respectively.

Conclusion

This investigation demonstrates that ¹⁸F-for-¹⁹F isotopic exchange is well suited for the fast, efficient and reliable automated routine production of ¹⁸F-labeled PSMA-targeted ligands. Due to its simplicity, speed and robustness the development of further SiFA-based radiopharmaceuticals is highly promising and can be of far-reaching importance for future theranostic concepts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-021-00120-5.

Preclinical comparison of four [18F, natGa]rhPSMA-7 isomers: influence of the stereoconfiguration on pharmacokinetics

Introduction

Radiohybrid (rh) ligands, a novel class of prostate-specific membrane antigen (PSMA)-targeted radiopharmaceuticals, can be labeled either with [¹⁸F]fluorine via isotopic exchange or with radiometals (such as [⁶⁸Ga]Gallium, [¹⁷⁷Lu]Lutetium, [²²⁵Ac]Actinium). Among these, [¹⁸F, ^natGa]rhPSMA-7 has recently entered clinical assessment.

Aim

Since [¹⁸F, ^natGa]rhPSMA-7 is composed of four stereoisomers ([¹⁸F, ^natGa]rhPSMA-7.1, -7.2, -7.3 and -7.4), we initiated a preclinical selection process to identify the isomer with the most favorable pharmacokinetics for further clinical investigation.

Methods

A synthetic protocol for enantiopure [¹⁹F, ^natGa]rhPSMA-7 isomers has been developed. The comparative evaluation of the four isomers comprised human serum albumin binding, lipophilicity, IC₅₀, internalization and classical biodistribution studies and competition experiments in LNCaP tumor-bearing CB-17 SCID mice. In addition, a radio high-performance liquid chromatography-based method was developed allowing quantitative, intraindividual comparison of [¹⁸F, ^natGa]rhPSMA-7.1 to -7.4 in LNCaP tumor-bearing mice.

Results

Cell studies revealed high PSMA affinity and internalization for [^18/19F, ^natGa]rhPSMA-7.2, -7.3 and -7.4, whereas [^18/19F, ^natGa]rhPSMA-7.1 showed approximately twofold lower values. Although the biodistribution profile obtained was typical of PSMA inhibitors, it did not allow for selection of a lead candidate for clinical studies. Thus, an intraindividual comparison of all four isomers in LNCaP tumor-bearing mice was carried out by injection of a diastereomeric mixture, followed by analysis of the differential uptake and excretion pattern of each isomer. Based on its high tumor accumulation and low uptake in blood, liver and kidneys, [¹⁸F, ^natGa]rhPSMA-7.3 was identified as the preferred isomer and transferred into clinical studies.

Conclusion

[¹⁸F, ^natGa]rhPSMA-7.3 has been selected as a lead compound for clinical development of a [¹⁸F]rhPSMA-based candidate. The intraindividual differential uptake and excretion analysis in vivo allowed for an accurate comparison and assessment of radiopharmaceuticals.

Synthesis and Evaluation of 68Ga- and 177Lu-Labeled (R)- vs (S)-DOTAGA Prostate-Specific Membrane Antigen-Targeting Derivatives

Prostate-specific membrane antigen (PSMA) is overexpressed in prostate cancer cells and therefore is an attractive target for prostate cancer diagnosis and radionuclide therapy. Recently, published results from clinical studies using a new PSMA-targeting PET imaging agent, [⁶⁸Ga]Ga-PSMA-093 ([⁶⁸Ga]Ga-HBED-CC-O-carboxymethyl-Tyr-CO-NH-Glu), support the development of this agent for the diagnosis of prostate cancer. In this study, the HBED-CC chelating group in PSMA-093 was replaced by stereoselective (R)- or (S)-DOTAGA. This chelating group serves not only for chelating ⁶⁸Ga but is also amendable for complexing other radioactive metals for radionuclide therapy. The corresponding optically pure (R)- and (S)-[⁶⁸Ga/¹⁷⁷Lu]-DOTAGA derivatives, (R)-[⁶⁸Ga/¹⁷⁷Lu]-13 and (S)-[⁶⁸Ga/¹⁷⁷Lu]-13, were successfully prepared. Comparison of radiolabeling, binding affinity, cell uptake, and biodistribution between the two isomers was performed. Radiolabeling of (R)-[¹⁷⁷Lu]Lu-13 and (S)-[¹⁷⁷Lu]Lu-13 at 50 °C suggested that rates of complex formation were time-dependent and the formation of (S)-[¹⁷⁷Lu]Lu-13 was distinctly faster. The rates of complex formation for the corresponding ⁶⁸Ga agents were comparable between structural isomers. The ^natGa and ^natLu equivalents showed high binding PSMA affinity (IC₅₀ = 24-111 nM), comparable to that of the parent agent, [^natGa]Ga-PSMA-093 (IC₅₀ = 34.0 nM). Results of cell uptake and biodistribution studies in PSMA-expressing PC3-PIP tumor-bearing mice appeared to show no difference between the labeled (R)- and (S)-isomers. This is the first time that a pair of [⁶⁸Ga/¹⁷⁷Lu]-(R)- and (S)-DOTAGA isomers of PSMA agents were evaluated. Results of biological studies between the isomers showed no noticeable difference; however, the distinctions on the rate of Lu complex formation should be considered in the development of new ¹⁷⁷Lu-DOTAGA-based radionuclide therapy agents in the future.

Diagnostic Accuracy of 18F-PSMA-1007 PET/CT Imaging for Lymph Node Staging of Prostate Carcinoma in Primary and Biochemical Recurrence

Prostate-specific membrane antigen (PSMA)-ligand PET/CT is performed on patients with prostate cancer to stage the disease initially or to identify sites of recurrence after definitive therapy. On the basis of clinical results, ¹⁸F-PSMA-1007 is a promising PSMA PET tracer, but detailed histologic confirmation has been lacking. Methods: Ninety-six patients with prostate cancer underwent ¹⁸F-PSMA-1007 PET/CT followed by either radical prostatectomy with lymphadenectomy or salvage lymphadenectomy. The histologic findings of PSMA PET-positive nodes were analyzed retrospectively. A lesion-based and patient-based analysis was performed comparing all positive lesions and only lesions larger than 3 mm on histopathology. Results: Of the patients, 90.6% received ¹⁸F-PSMA-1007 PET/CT for staging before the primary treatment, whereas 9.4% underwent imaging for biochemical recurrence. In 34.4% of the cohort, positive lymph nodes were present on imaging. In total, 1,746 lymph nodes were dissected in 96 patients. ¹⁸F-PSMA-1007 PET had a lesion-based sensitivity of 81.7%, a specificity of 99.6%, a positive predictive value of 92.4%, and a negative predictive value of 98.9% for detecting positive lymph nodes larger than 3 mm. In the analysis of all malignant nodes regardless of size, the overall sensitivity, specificity, positive predictive value, and negative predictive value on lesion-based analysis were 71.2%, 99.5%, 91.3%, and 97.9%, respectively. The patient-based analysis showed a sensitivity of 85.9% and a specificity of 99.5% for lymph nodes larger than 3 mm. Conclusion: ¹⁸F-PSMA-1007 PET/CT reliably detects malignant lymph nodes and has an exceptional specificity of more than 99% for nodal metastases.

Radiohybrid Ligands: A Novel Tracer Concept Exemplified by 18F- or 68Ga-Labeled rhPSMA Inhibitors

When we critically assess the reason for the current dominance of 68Ga-labeled peptides and peptide-like ligands in radiopharmacy and nuclear medicine, we have to conclude that the major advantage of such radiopharmaceuticals is the apparent lack of suitable 18F-labeling technologies with proven clinical relevance. To prepare and to subsequently perform a clinical proof-of-concept study on the general suitability of silicon-fluoride-acceptor (SiFA)-conjugated radiopharmaceuticals, we developed inhibitors of the prostate-specific membrane antigen (PSMA) that are labeled by isotopic exchange (IE). To compensate for the pronounced lipophilicity of the SiFA unit, we used metal chelates, conjugated in close proximity to SiFA. Six different radiohybrid PSMA ligands (rhPSMA ligands) were evaluated and compared with the commonly used 18F-PSMA inhibitors 18F-DCFPyL and 18F-PSMA-1007. Methods: All inhibitors were synthesized by solid-phase peptide synthesis. Human serum albumin binding was measured by affinity high-performance liquid chromatography, whereas the lipophilicity of each tracer was determined by the n-octanol/buffer method. In vitro studies (IC50, internalization) were performed on LNCaP cells. Biodistribution studies were conducted on LNCaP tumor-bearing male CB-17 SCID mice. Results: On the laboratory scale (starting activities, 0.2-9.0 GBq), labeling of 18F-rhPSMA-5 to -10 by IE was completed in < 20 min (radiochemical yields, 58% ± 9%; radiochemical purity, >97%) with molar activities of 12-60 GBq/μmol. All rhPSMAs showed low nanomolar affinity and high internalization by PSMA-expressing cells when compared with the reference radiopharmaceuticals, medium-to-low lipophilicity, and high human serum albumin binding. Biodistribution studies in LNCaP tumor-bearing mice revealed high tumor uptake, sufficiently fast clearance kinetics from blood, low hepatobiliary excretion, fast renal excretion, and very low uptake of 18F activity in bone. Conclusion: The novel 18F-rhPSMA radiopharmaceuticals developed under the radiohybrid concept show equal or better targeting characteristics than the established 18F-PSMA tracers 18F-DCFPyL and 18F-PSMA-1007. The unparalleled simplicity of production, the possibility to produce the identical 68Ga-labeled 19F-68Ga-rhPSMA tracers, and the possibility to extend this concept to true theranostic radiohybrid radiopharmaceuticals, such as F-Lu-rhPSMA, are unique features of these radiopharmaceuticals.