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

Prostate-specific Membrane Antigen: Diagnostics

Since its clinical introduction in May 2011, prostate-specific membrane antigen (PSMA)-PET/computed tomography has quickly gained worldwide recognition as a significant breakthrough in prostate cancer diagnostics. In the meantime, several new PSMA radioligands for PET imaging have been introduced into routine clinical practice. This article aims to introduce the most commonly used tracers and their key areas of application.

Significant reduction of activity retention in the kidneys via optimized linker sequences in radiohybrid-based minigastrin analogs

BACKGROUND

We recently introduced radiohybrid (rh)-based minigastrin analogs e.g., DOTA-rhCCK-18 (DOTA-D-Dap(p-SiFA)-(D-γ-Glu)8-Ala-Tyr-Gly-Trp-Nle-Asp-Phe-NH2), that revealed substantially increased activity retention in the tumor. However, one major drawback of these first generation rh-based cholecystokinin-2 receptor (CCK-2R) ligands is their elevated activity levels in the kidneys, especially at later time points (24 h p.i.). Therefore, this study aimed to reduce kidney retention with regard to a therapeutic use via substitution of negatively charged D-glutamic acid moieties by hydrophilic uncharged polyethylene glycol (PEG) linkers of various length ((PEG)4 to (PEG)11). Furthermore, the influence of differently charged silicon-based fluoride acceptor (SiFA)-moieties (p-SiFA: neutral, SiFA-ipa: negatively charged, and SiFAlin: positively charged) on in vitro properties of minigastrin analogs was evaluated. Out of all compounds evaluated in vitro, the two most promising minigastrin analogs were further investigated in vivo.

RESULTS

CCK-2R affinity of most compounds evaluated was found to be in a range of 8-20 nM (by means of apparent IC50), while ligands containing a SiFA-ipa moiety displayed elevated IC50 values. Lipophilicity was noticeably lower for compounds containing a D-γ-glutamate (D-γ-Glu) moiety next to the D-Dap(SiFA) unit as compared to their counterparts lacking the additional negative charge. Within this study, combining the most favorable CCK-2R affinity and lipophilicity, [177/natLu]Lu-DOTA-rhCCK-70 (DOTA-D-Dap(p-SiFA)-D-γ-Glu-(PEG)7-D-γ-Glu-(PEG)3-Trp-(N-Me)Nle-Asp-1-Nal-NH2; IC50: 12.6 ± 2.0 nM; logD7.4: - 1.67 ± 0.08) and [177/natLu]Lu-DOTA-rhCCK-91 (DOTA-D-Dap(SiFAlin)-D-γ-Glu-(PEG)4-D-γ-Glu-(PEG)3-Trp-(N-Me)Nle-Asp-1-Nal-NH2; IC50: 8.6 ± 0.7 nM; logD7.4 =  - 1.66 ± 0.07) were further evaluated in vivo. Biodistribution data of both compounds revealed significantly reduced (p < 0.0001) activity accumulation in the kidneys compared to [177Lu]Lu-DOTA-rhCCK-18 at 24 h p.i., leading to enhanced tumor-to-kidney ratios despite lower tumor uptake. However, overall tumor-to-background ratios of the novel compounds were lower than those of [177Lu]Lu-DOTA-rhCCK-18.

CONCLUSION

We could show that the reduction of negative charges within the linker section of radiohybrid-based minigastrin analogs led to decreased activity levels in the kidneys at 24 h p.i., while maintaining a good tumor uptake. Thus, favorable tumor-to-kidney ratios were accomplished in vivo. However, further optimization has to be done in order to improve tumor retention and general biodistribution profile.

[Salvage lymphadenectomy for recurrent prostate cancer]

Prostate-specific membrane antigen (PSMA)-based positron emission tomography (PET) imaging allows early detection of metastases in patients with biochemical recurrence. Salvage lymphadenectomy became a widely used method of metastasis-directed treatment. Retrospective analyses show that a low prostate-specific antigen (PSA) value and presence of no more than two affected lymph nodes within the pelvis are factors associated with a good outcome. In all, 40-80% of patients achieve a complete biochemical response with a mean time without biochemical recurrence of 8 months and a prolonged treatment-free interval. About 10% of patients with a complete biochemical response will live without recurrence after 10 years. The utilization of PSMA-radioguided surgery increases the likelihood of intraoperative detection of suspicious affected lymph nodes. Complications can mostly be avoided by prudent patient selection and surgical expertise.

2023 FDA TIDES (Peptides and Oligonucleotides) Harvest

A total of nine TIDES (pepTIDES and oligonucleoTIDES) were approved by the FDA during 2023. The four approved oligonucleotides are indicated for various types of disorders, including amyotrophic lateral sclerosis, geographic atrophy, primary hyperoxaluria type 1, and polyneuropathy of hereditary transthyretin-mediated amyloidosis. All oligonucleotides show chemically modified structures to enhance their stability and therapeutic effectiveness as antisense or aptamer oligomers. Some of them demonstrate various types of conjugation to driving ligands. The approved peptides comprise various structures, including linear, cyclic, and lipopeptides, and have diverse applications. Interestingly, the FDA has granted its first orphan drug designation for a peptide-based drug as a highly selective chemokine antagonist. Furthermore, Rett syndrome has found its first-ever core symptoms treatment, which is also peptide-based. Here, we analyze the TIDES approved in 2023 on the basis of their chemical structure, medical target, mode of action, administration route, and common adverse effects.

An Intrapatient Dosimetry Comparison of 177Lu-rhPSMA-10.1 and 177Lu-PSMA-I&T in Patients with Metastatic Castration-Resistant Prostate Cancer

As the use of radioligand therapy moves earlier in the prostate cancer timeline, minimizing the absorbed dose to normal organs while maintaining high tumor radiation doses becomes more clinically important because of the longer life expectancy of patients. We performed an intrapatient comparison of pretherapeutic dosimetry with the novel radiohybrid prostate-specific membrane antigen-targeting radiopharmaceutical 177Lu-rhPSMA-10.1, along with 177Lu-PSMA-I&T, in patients with metastatic castration-resistant prostate cancer. Methods: Four consecutive patients with advanced histologically proven metastatic castration-resistant prostate cancer who were scheduled for radioligand therapy were evaluated. Before undergoing therapy, each patient received 1.06 ± 0.05 GBq of 177Lu-rhPSMA-10.1 and 1.09 ± 0.02 GBq of 177Lu-PSMA-I&T at least 7 d apart. For dosimetric assessment, whole-body planar scintigraphy was performed after 5 min, 4 h, 1 d, 2 d, and 7 d. In addition, SPECT/CT images were acquired over the thorax and the abdomen, 4 h, 1 d, 2 d, and 7 d after injection. Dosimetry of the whole body and salivary glands was based on the evaluation of the counts in whole-body planar imaging. Dosimetry of the kidneys, liver, spleen, bone marrow, and tumor lesions (≤4 per patient) was based on the activity in volumes drawn on SPECT/CT images. Doses were calculated using OLINDA/EXM version 1.0. The therapeutic index (TI), or ratio between mean dose of the metastases and mean dose of the kidneys, was calculated for each patient. Results: We found the dose to the kidneys to be higher with 177Lu-rhPSMA-10.1 than with 177Lu-PSMA-I&T (0.68 ± 0.30 vs. 0.46 ± 0.10 mGy/MBq); however, 177Lu-rhPSMA-10.1 delivered an average of a 3.3 times (range, 1.2-8.3 times) higher absorbed radiation dose to individual tumor lesions. Consequently, intraindividual comparison revealed a 1.1-3.1 times higher TI for 177Lu-rhPSMA-10.1 than for 177Lu-PSMA-I&T in all evaluated patients. The effective whole-body dose was 0.038 ± 0.008 mSv/MBq for 177Lu-rhPSMA-10.1 and 0.022 ± 0.005 mSv/MBq for 177Lu-PSMA-I&T. Conclusion: Using 177Lu-rhPSMA-10.1 can significantly increase the tumor-absorbed dose and improve the TI compared with 177Lu-PSMA-I&T. An improved TI gives the flexibility to maximize tumor-absorbed doses up to a predefined renal dose limit or, in earlier disease, to reduce the radiation exposure to the kidney while still achieving an effective tumor dose. The function of at-risk organs such as the kidneys is being assessed in a prospective clinical trial.

MRI and PSMA PET/CT of Biochemical Recurrence of Prostate Cancer

Prostate cancer may recur several years after definitive treatment, such as prostatectomy or radiation therapy. A rise in serum prostate-specific antigen (PSA) level is the first sign of disease recurrence, and this is termed biochemical recurrence. Patients with biochemical recurrence have worse survival outcomes. Radiologic localization of recurrent disease helps in directing patient management, which may vary from active surveillance to salvage radiation therapy, androgen-deprivation therapy, or other forms of systemic and local therapy. The likelihood of detecting the site of recurrence increases with higher serum PSA level. MRI provides optimal diagnostic performance for evaluation of the prostatectomy bed. Prostate-specific membrane antigen (PSMA) PET radiotracers currently approved by the U.S. Food and Drug Administration demonstrate physiologic urinary excretion, which can obscure recurrence at the vesicourethral junction. However, MRI and PSMA PET/CT have comparable diagnostic performance for evaluation of local recurrence after external-beam radiation therapy or brachytherapy. PSMA PET/CT outperforms MRI in identifying recurrence involving the lymph nodes and bones. Caveats for use of both PSMA PET/CT and MRI do exist and may cause false-positive or false-negative results. Hence, these techniques have complementary roles and should be interpreted in conjunction with each other, taking the patient history and results of any additional prior imaging studies into account. Novel PSMA agents at various stages of investigation are being developed, and preliminary data show promising results; these agents may revolutionize the landscape of prostate cancer recurrence imaging in the future. ©RSNA, 2023 Quiz questions for this article are available through the Online Learning Center. See the invited commentary by Turkbey in this issue. The slide presentation from the RSNA Annual Meeting is available for this article.

Flotufolastat F 18: Diagnostic First Approval

Flotufolastat F 18 (POSLUMA®) is an 18F-labelled radiohybrid (rh) prostate-specific membrane antigen (PSMA)-targeted imaging agent being developed by Blue Earth Diagnostics, a subsidiary of Bracco Imaging, for prostate cancer imaging. In May 2023, flotufolastat F 18 received its first approval in the USA as a radioactive diagnostic agent for positron emission tomography (PET) of PSMA positive lesions in men with prostate cancer with suspected metastasis who are candidates for initial definitive therapy or with suspected recurrence based on elevated serum prostate-specific antigen (PSA) level. This article summarizes the milestones in the development of flotufolastat F 18 leading to this first approval.

Engineering the MoS2 /MXene Heterostructure for Precise and Noninvasive Diagnosis of Prostate Cancer with Clinical Specimens

High-throughput metabolic fingerprinting has been deemed one of the most promising strategies for addressing the high false positive rate of prostate cancer (PCa) diagnosis in the prostate-specific antigen (PSA) gray zone. However, the current metabolic fingerprinting remains challenging in achieving high-precision metabolite detection in complex biological samples (e.g., serum and urine). Herein, a novel self-assembly MoS₂ /MXene heterostructure nanocomposite with a tailored doping ratio of 10% is presented as a matrix for laser desorption ionization mass spectrometry analysis in clinical biosamples. Notably, owing to the two-dimensional architecture and doping effect, MoS₂ /MXene demonstrates favorable laser desorption ionization performance with low adsorption energy, which is evidenced by efficient urinary metabolic fingerprinting with an enhanced area under curve (AUC) diagnosis capability of 0.959 relative to that of serum metabolic fingerprinting (AUC = 0.902) for the diagnosis of PCa in the PSA gray zone. Thus, this MoS₂ /MXene heterostructure is anticipated to offer a novel strategy to precisely and noninvasively diagnose PCa in the PSA gray zone.

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.

Assessing the accuracy of [18F]PSMA-1007 PET/CT for primary staging of lymph node metastases in intermediate- and high-risk prostate cancer patients

Background

[¹⁸F]PSMA-1007 is a promising tracer for integrated positron emission tomography and computed tomography (PET/CT).

Objective

Our aim was to assess the diagnostic accuracy of [¹⁸F]PSMA-1007 PET/CT for primary staging of lymph node metastasis before robotic-assisted laparoscopy (RALP) with extended lymph node dissection (ePLND).

Design, Setting and Participants

The study was a retrospective cohort in a tertiary referral center. Men with prostate cancer that underwent surgical treatment for intermediate- or high-risk prostate cancer between May 2019 and August 2021 were included.

Interventions

[¹⁸F]PSMA-1007 PET/CT for initial staging followed by RALP and ePLND.

Outcome measurements and statistical analyses

Sensitivity and specificity were calculated both for the entire cohort and for patients with lymph node metastasis ≥ 3 mm. Positive (PPV) and negative (NPV) predictive values were calculated.

Results and limitations

Among 104 patients included in the analyses, 26 patients had lymph node metastasis based on pathology reporting and metastases were ≥ 3 mm in size in 13 of the cases (50%). In the entire cohort, the sensitivity and specificity of [¹⁸F]PSMA-1007 were 26.9% (95% confidence interval (CI); 11.6–47.8) and 96.2% (95% CI; 89.2–99.2), respectively. The sensitivity and specificity of [¹⁸F]PSMA-1007 to detect a lymph node metastasis ≥ 3 mm on PET/CT were 53.8% (95% CI; 25.1–80.8) and 96.7% (95% CI; 90.7–99.3), respectively. PPV was 70% and NPV 93.6%.

Conclusions

In primary staging of intermediate- and high-risk prostate cancer, [¹⁸F]PSMA-1007 PET/CT is highly specific for prediction of lymph node metastases, but the sensitivity for detection of metastases smaller than 3 mm is limited. Based on our results, [¹⁸F]PSMA-1007 PET/CT cannot completely replace ePLND.

Patient summary

This study investigated the use of an imaging method based on a prostate antigen-specific radiopharmaceutical tracer to detect lymph node prostate cancer metastasis. We found that it is unreliable to discover small metastasis.

Predictors of 18F-DCFPyL PET/CT Positivity in Patients with Biochemical Recurrence of Prostate Cancer After Local Therapy

Our objective was to investigate the factors predicting scan positivity and disease location in patients with biochemical recurrence (BCR) of prostate cancer (PCa) after primary local therapy using prostate-specific membrane antigen-targeted ¹⁸F-DCFPyL PET/CT. Methods: This was a 2-institution study including 245 BCR PCa patients after primary local therapy and negative results on conventional imaging. The patients underwent ¹⁸F-DCFPyL PET/CT. We tested for correlations of lesion detection rate and disease location with tumor characteristics, time from initial therapy, prostate-specific antigen (PSA) level, and PSA doubling time (PSAdt). Multivariate logistic regression analyses were used to determine predictors of a positive scan. Regression-based coefficients were used to develop nomograms predicting scan positivity and extrapelvic disease. Results: Overall, 79.2% (194/245) of patients had a positive ¹⁸F-DCFPyL PET/CT result, with detection rates of 48.2% (27/56), 74.3% (26/35), 84% (37/44), 96.7% (59/61), and 91.8% (45/49) for PSAs of <0.5, 0.5 to <1.0, 1.0 to <2.0, 2.0 to <5.0, and ≥5.0 ng/mL, respectively. Patients with lesions confined to the pelvis had lower PSAs than those with distant sites (1.6 ± 3.5 vs. 3.0 ± 6.3 ng/mL, P < 0.001). In patients treated with prostatectomy (n = 195), 24.1% (47/195) had a negative scan result, 46.1% (90/195) showed intrapelvic disease, and 29.7% (58/195) showed extrapelvic disease. In the postradiation subgroup (n = 50), ¹⁸F-DCFPyL PET/CT was always negative at a PSA lower than 1.0 ng/mL and extrapelvic disease was seen only when PSA was greater than 2.0 ng/mL. At multivariate analysis, PSA and PSAdt were independent predictive factors of scan positivity and the presence of extrapelvic disease in postsurgical patients, with area under the curve of 78% and 76%, respectively. PSA and PSAdt were independent predictors of the presence of extrapelvic disease in the postradiation cohort, with area under the curve of 85%. Time from treatment to scan was significantly longer for prostatectomy-bed-only recurrences than for those with bone or visceral disease (6.2 ± 6.4 vs. 2.4 ± 1.3 y, P < 0.001). Conclusion:¹⁸F-DCFPyL PET/CT offers high detection rates in BCR PCa patients. PSA and PSAdt are able to predict scan positivity and disease location. Furthermore, the presence of bone or visceral lesions is associated with shorter intervals from treatment than are prostate-bed-only recurrences. These tools might guide clinicians to select the most suitable candidates for ¹⁸F-DCFPyL PET/CT imaging.

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.

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.