177Lu-PSMA Radioligand Therapy Is Favorable as Third-Line Treatment of Patients with Metastatic Castration-Resistant Prostate Cancer. A Systematic Review and Network Meta-Analysis of Randomized Controlled Trials

PSMA-Targeted Radiopharmaceuticals for Prostate Cancer Diagnosis and Therapy

ABSTRACT

Prostate cancer (PCa) is the most common noncutaneous malignancy in men. Until recent years, accurate imaging of men with newly diagnosed PCa, or recurrent or low-volume metastatic disease, was limited. Further, therapeutic options for men with advanced, metastatic, castration-resistant disease were increasingly limited as a result of increasing numbers of systemic therapies being combined in the upfront metastatic setting. The advent of urea-based, small-molecule inhibitors of prostate-specific membrane antigen (PSMA) has partially addressed those shortcomings in diagnosis and therapy of PCa. On the diagnostic side, there are multiple pivotal phase III trials with several different agents having demonstrated utility in the initial staging setting, with generally modest sensitivity but very high specificity for determining otherwise-occult pelvic nodal involvement. That latter statistic drives the utility of the scan by allowing imaging interpreters to read with very high sensitivity while maintaining a robust specificity. Other pivotal phase III trials have demonstrated high detection efficiency in patients with biochemical failure, with high positive predictive value at the lesion level, opening up possible new avenues of therapy such as metastasis-directed therapy. Beyond the diagnostic aspects of PSMA-targeted radiotracers, the same urea-based chemical scaffolds can be altered to deliver therapeutic isotopes to PCa cells that express PSMA. To date, one such agent, when combined with best standard-of-care therapy, has demonstrated an ability to improve overall survival, progression-free survival, and freedom from skeletal events relative to best standard-of-care therapy alone in men with metastatic, castration-resistant PCa who are post chemotherapy. Within the current milieu, there are a number of important future directions including the use of artificial intelligence to better leverage diagnostic findings, further medicinal chemistry refinements to the urea-based structure that may allow improved tumor targeting and decreased toxicities, and the incorporation of new radionuclides that may better balance efficacy with toxicities than those nuclides that are available.

European Association of Nuclear Medicine Focus 5: Consensus on Molecular Imaging and Theranostics in Prostate Cancer

BACKGROUND

In prostate cancer (PCa), questions remain on indications for prostate-specific membrane antigen (PSMA) positron emission tomography (PET) imaging and PSMA radioligand therapy, integration of advanced imaging in nomogram-based decision-making, dosimetry, and development of new theranostic applications.

OBJECTIVE

We aimed to critically review developments in molecular hybrid imaging and systemic radioligand therapy, to reach a multidisciplinary consensus on the current state of the art in PCa.

DESIGN, SETTING, AND PARTICIPANTS

The results of a systematic literature search informed a two-round Delphi process with a panel of 28 PCa experts in medical or radiation oncology, urology, radiology, medical physics, and nuclear medicine. The results were discussed and ratified in a consensus meeting.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

Forty-eight statements were scored on a Likert agreement scale and six as ranking options. Agreement statements were analysed using the RAND appropriateness method. Ranking statements were analysed using weighted summed scores.

RESULTS AND LIMITATIONS

After two Delphi rounds, there was consensus on 42/48 (87.5%) of the statements. The expert panel recommends PSMA PET to be used for staging the majority of patients with unfavourable intermediate and high risk, and for restaging of suspected recurrent PCa. There was consensus that oligometastatic disease should be defined as up to five metastases, even using advanced imaging modalities. The group agreed that [177Lu]Lu-PSMA should not be administered only after progression to cabazitaxel and that [223Ra]RaCl2 remains a valid therapeutic option in bone-only metastatic castration-resistant PCa. Uncertainty remains on various topics, including the need for concordant findings on both [18F]FDG and PSMA PET prior to [177Lu]Lu-PSMA therapy.

CONCLUSIONS

There was a high proportion of agreement among a panel of experts on the use of molecular imaging and theranostics in PCa. Although consensus statements cannot replace high-certainty evidence, these can aid in the interpretation and dissemination of best practice from centres of excellence to the wider clinical community.

PATIENT SUMMARY

There are situations when dealing with prostate cancer (PCa) where both the doctors who diagnose and track the disease development and response to treatment, and those who give treatments are unsure about what the best course of action is. Examples include what methods they should use to obtain images of the cancer and what to do when the cancer has returned or spread. We reviewed published research studies and provided a summary to a panel of experts in imaging and treating PCa. We also used the research summary to develop a questionnaire whereby we asked the experts to state whether or not they agreed with a list of statements. We used these results to provide guidance to other health care professionals on how best to image men with PCa and what treatments to give, when, and in what order, based on the information the images provide.

PSMA-targeted radioligand therapy in prostate cancer: current status and future prospects

INTRODUCTION

The prostate-specific membrane antigen (PSMA) targeted radioligand therapy (PRLT) for the treatment of metastatic castration-resistant prostate cancer (mCRPC) patients has generated significant interest among the oncologic community, with several publications documenting good response rates and survival benefits with low toxicity profiles.

AREAS COVERED

Indications, patient preparation, dose administration, post-treatment imaging, dosimetry, and side effect profiles of 177Lu-PSMA-617 are discussed in this article. We also discuss results from prospective studies, major retrospective studies, meta-analyses, clinical trials, and mentioned major ongoing clinical trials on PRLT. We have also portrayed our own experiences and future perspectives on PRLT.

EXPERT OPINION

For PRLT, PSMA-617 and PSMA-I&T molecules have revolutionized the theranostic approach in the management of advanced prostate cancer, with solid backing from several published articles showing favorable outcomes and an excellent safety profile of 177Lu-PSMA-617. Improvement in quality of life and survival was seen in the majority of mCRPC patients after 177Lu-PSMA-617 PRLT. Patients with good performance status, asymptomatic, only lymph node metastases, high PSMA expressing lesions, and no discordant FDG avid lesions have a longer survival after 177Lu-PSMA-617 PRLT than patients with poor performance status, symptomatic, hepatic, brain, and skeletal metastases, discordant PSMA, and FDG-avid lesions. Docetaxel and cabazitaxel are approved treatments for mCRPC patients. 177Lu-PSMA-617 is approved as a third-line systemic treatment for mCRPC patients with failure to respond to androgen receptor pathway inhibitors and docetaxel therapy. PRLT is a safe and effective alternative to cabazitaxel (third-line systemic treatment), but it has a higher cost. 177Lu-PSMA-617 could be a more efficient therapeutic option for mCRPC patients as first-line or combined therapy, and it may be a useful therapeutic option for the treatment of metastatic hormone-sensitive prostate cancer (mHSPC) patients. Several clinical studies and clinical trials on PRLT are currently underway. In the future, the results of these trials will be helpful in evolving treatment strategies for prostate cancer patients.

177 Lu-rhPSMA-10.1 Induces Tumor Response in a Patient With mCRPC After PSMA-Directed Radioligand Therapy With 177 Lu-PSMA-I&T

ABSTRACT

177 Lu-rhPSMA-10.1 is a novel PSMA-targeting radiopharmaceutical that has been optimized in terms of pharmacological and pharmacokinetic properties and may be therefore advantageous in treatment of metastatic castrate-resistant prostate cancer. In this image, we present the case of an 86-year-old man with metastastic castrate-resistant prostate cancer undergoing 177 Lu-PSMA-I&T treatment. After initial partial response to radioligand therapy, another 2 treatment cycles resulted in a rising serum PSA level that could be correlated with increasingly PSMA-positive as well as a new bone lesion. Consequently, the patient was changed to 177 Lu-rhPSMA-10.1 treatment on a compassionate use basis achieving a renewed tumor response.

A Retrospective Comparative Study of Sodium Fluoride Na18F-PET/CT and 68Ga-PSMA-11 PET/CT in the Bone Metastases of Prostate Cancer Using a Volumetric 3-D Radiomic Analysis

Bone is the most common metastatic site in prostate cancer (PCa). 68Ga-PSMA-11 (or gozetotide) and sodium fluoride-18 (Na18F) are rather new radiopharmaceuticals for assessing PCa-associated bone metastases. Gozetotide uptake reflects cell membrane enzyme activity and the sodium fluoride uptake measures bone mineralization in advanced PCa. Here, we aim to characterize this difference and possibly provide a new method for patient selection in targeted therapies. Methods: The study consisted of 14 patients with advanced PCa (M group > 5 lesions), who had had routine PET/CT both with PSMA and NaF over consecutive days, and 12 PCa patients with no skeletal metastases (N). The bone regions in CT were used to coregister the two PET/CT scans. The whole skeleton volume(s) of interest (VOIs) were defined using the CT component of PET (HU > 150); similarly, the sclerotic/dense bone was defined as HU > 600. Additional VOIs were defined for PET, with pathological threshold values for PSMA (SUV > 3.0) and NaF (SUV > 10). Besides the pathological bone volumes measured with each technique (CT, NaF, and PSMA-PET) and their contemporaneous combinations, overlapping VOIs with the CT-based skeletal and sclerotic volumes were also recorded. Additionally, thresholds of 4.0, 6.0, and 10.0 were tested for SUVPSMA. Results: In group M, the skeletal VOI volumes were 8.77 ± 1.80 L, and the sclerotic bone volumes were 1.32 ± 0.50 L; in contrast, in group N, they were 8.73 ± 1.43 L (skeletal) and 1.23 ± 0.28 L (sclerosis). The total enzyme activity for PSMA was 2.21 ± 5.15 in the M group and 0.078 ± 0.053 in the N group (p < 0.0002). The total bone demineralization activity for NaF varied from 4.31 ± 6.17 in the M group and 0.24 ± 0.56 in group N (p < 0.0002). The pathological PSMA volume represented 0.44−132% of the sclerotic bone volume in group M and 0.55−2.3% in group N. The pathological NaF volume in those patients with multiple metastases represented 0.27−68% of the sclerotic bone volume, and in the control group, only 0.00−6.5% of the sclerotic bone volume (p < 0.0003). Conclusions: These results confirm our earlier findings that CT alone does not suit the evaluation of the extent of active skeletal metastases in PCa. PSMA and NaF images give complementary information about the extent of the active skeletal disease, which has a clinical impact and may change its management. The PSMA and NaF absolute volumes could be used for planning targeted therapies. A cut-off value 3.0 for SUVPSMA given here is the best correlation in the presentation of active metastatic skeletal disease.

Advances in 177Lu-PSMA and 225Ac-PSMA Radionuclide Therapy for Metastatic Castration-Resistant Prostate Cancer

For patients with metastatic castration-resistant prostate cancer (mCRPC), the survival benefit of classic treatment options with chemotherapy and drugs targeting androgen signaling is limited. Therefore, beta and alpha radionuclide therapy (RNT) have emerged as novel treatment options for patients with mCRPC. Radioligands target the prostate-specific membrane antigen (PSMA) epitopes, which are upregulated up to a thousand times more in prostate cancer cells compared to the cells in normal tissues. For this reason, PSMA is an excellent target for both imaging and therapy. Over the past years, many studies have investigated the treatment effects of lutetium-177 labeled PSMA (177Lu-PSMA) and actinium-225 labeled PSMA (225Ac-PSMA) RNT in patients with mCRPC. While promising results have been achieved, this field is still in development. In this review, we have summarized and discussed the clinical data of 177Lu-PSMA and 225Ac-PSMA RNT in patients with mCRPC.

Decisions and dilemmas in non-metastatic castration-resistant prostate cancer management

Non-metastatic castration-resistant prostate cancer (nmCRPC) indicates a condition characterized by the progression of the prostate-specific antigen without radiographic evidence of distant metastasis on conventional imaging during androgen deprivation therapy (ADT). Recently, 3 phase III trials have shown that the addition of next-generation androgen-receptor inhibitors (ARIs) apalutamide, darolutamide, and enzalutamide to ADT allows patients with high-risk nmCRPC to delay the appearance of metastasis and to obtain long-term clinical benefits. However, the lack of head-to head comparison makes it difficult to choose one among these agents. We reviewed the literature and explained the rationale of the possible therapeutic choices. In any case, the availability of novel ARIs means that patients with nmCRPC have now a new effective treatment option that provides them a renewed hope.

Comparison of [18F]PSMA-1007 with [68Ga]Ga-PSMA-11 PET/CT in Restaging of Prostate Cancer Patients with PSA Relapse

This study aimed to compare the diagnostic performance of [18F]PSMA-1007 positron emission tomography/computed tomography (PET/CT) (18F-PSMA) and [68Ga]Ga-PSMA-11 PET/CT (68Ga-PSMA) by identifying prostate-specific antigen (PSA) threshold levels for optimal detecting recurrent prostate cancer (PC) and to compare both methods. Retrospectively, the study included 264 patients. The performances of 18F-PSMA and 68Ga-PSMA in relation to the pre-scan PSA were assessed by receiver operating characteristic (ROC) curve. 18F-PSMA showed PC-lesions in 87.5% (112/128 patients), while 68Ga-PSMA identified them in 88.9% (121/136). For 18F-PSMA biochemical recurrent (BCR) patients treated with radical prostatectomy (78/128, patient group: F-RP), a PSA of 1.08 ng/mL was found to be the optimal cut-off level for predicting positive and negative scans (AUC = 0.821; 95%, CI: 0.710−0.932), while for prostatectomized 68Ga-PSMA BCR-patients (89/136, patient group: Ga-RP), the cut-off was 1.84 ng/mL (AUC = 0.588; 95%, CI: 0.410−0.766). In patients with PSA < 1.08 ng/mL (F-RP) 76.3% and <1.84 ng/mL (Ga-RP) 78.6% scans were positive, whereas patients with PSA ≥ 1.08 ng/mL (F-RP) or 1.84 ng/mL (Ga-RP) had positive scan results in 100% and 91.5% (p < 0.001/p = 0.085). The identified PSA thresholds for PSMA-mappable PC lesions in BCR-patients (RP) showed a better separation for 18F-PSMA with regard to the distinguishing of positive and negative PC-lesions compared to 68Ga-PSMA. However, the two PSMA PET/CT tracers gave similar overall findings.

Efficacy and Safety of 225Ac-PSMA-617-Targeted Alpha Therapy in Metastatic Castration-Resistant Prostate Cancer: A Systematic Review and Meta-Analysis

Objective

To conduct a meta-analysis of the efficacy and safety of ²²⁵Ac-PSMA-617 in the treatment of metastatic castration-resistant prostate cancer based on existing clinical evidence.

Methods

Search for retrospective studies about ²²⁵Ac-PSMA-617 in the treatment of metastatic castration-resistant prostate cancer from establishment to July 2021 in PubMed and EMBASE. The primary endpoint was ²²⁵Ac-PSMA-617 biochemical response evaluation criteria after treatment [any prostate specific antigen (PSA) decrease and PSA decrease >50% from baseline] to evaluate the treatment effect. Secondary endpoints included assessment of overall survival (OS), progression-free survival (PFS), molecular response, and toxicity for all studies. Two researchers conducted literature screening, data extraction and quality evaluation according to the inclusion and exclusion criteria. Use stata16.0 software for analysis, fixed-effects model for data merging and forest plots for display.

Results

A total of 6 retrospective studies, namely, 201 patients, were included in the final analysis. The pooled proportions of patients with decreased PSA and PSA decreased by more than 50% were 87.0% (95% confidence interval, 0.820 to 0.920) and 66.1% (95% confidence interval, 0.596 to 0.726), respectively. The pooled proportions of OS and PFS were 12.5 months (95%CI: 6.2–18.8 months) and 9.1 months (95%CI: 2.6–15.7 months). The patients showing molecular responses were 54% (95% confidence interval: 25–84%). In all studies, the most common side effect of ²²⁵Ac-PSMA-617 TAT was xerostomia, with any degree of xerostomia occurring in 77.1% (155 out of 201), and grade III only accounted for 3.0%. The second was 30.3% (61 out of 201) anemia of any degree, and grade III accounts for 7.5% (15 out of 201). Grade III leukopenia and thrombocytopenia were 4.5% (9 out of 201) and 5.5% (11 out of 201), respectively. Only 6 (3.0%) of 201 patients had Grade III nephrotoxicity.

Conclusion

²²⁵Ac-PSMA-617 is an effective and safe treatment option for mCRPC patients, and the toxicity caused by it is relatively low. However, future randomized controlled trials and prospective trials are required in the future to judge the therapeutic effects and survival benefits compared with existing clinical treatments.

Systematic Review Registration

PROSPERO: CRD42021281967.

PSMA targeting in metastatic castration-resistant prostate cancer: where are we and where are we going?

Prostate-specific membrane antigen (PSMA) is highly expressed on the membrane of most prostate cancer cells and to a lesser extent in normal tissues. Many vectors targeting this protein have been created over the past decade and numerous clinical studies have positively demonstrated the tolerance and efficacy of radiolabeled prostate-specific membrane antigen ligands for PSMA radioligand therapy (PRLT). Preliminary results are encouraging that PRLT will become an important addition to the current therapeutic options in a number of settings. Improvement in radiopharmaceutical targeting and combination with other oncological agents are under investigation to further improve its therapeutic efficacy. These encouraging results have led to the development of other therapies using PSMA as a target, such as PSMA-targeted chimeric antigen receptor T-cells, PSMA-targeted antibody drug conjugates, and PSMA-targeted bi-specific T-cell-directed therapy. This narrative review details the current state and advancements in prostate-specific membrane antigen targeting in prostate cancer treatment.