Matched-pair analysis of [177Lu]Lu-PSMA I&T and [177Lu]Lu-PSMA-617 in patients with metastatic castration-resistant prostate cancer

Background Labelled with lutetium-177, the urea-based small molecules PSMA I&T and PSMA-617 are the two agents most frequently used for radioligand therapy (RLT) in patients with advanced metastatic castration-resistant and prostate-specific membrane antigen (PSMA) expressing prostate cancer (mCRPC). In this matched-pair analysis, we aimed to compare the toxicity and efficacy of both agents for PSMA-directed RLT. Materials and methods A total of 110 mCRPC patients from two centres were accrued, 55 individuals treated with [177Lu]Lu-PSMA I&T, and a matched cohort of 55 patients treated with [177Lu]Lu-PSMA-617. Matching criteria included age at the first cycle, Gleason score, prostate-specific antigen (PSA) values, and previous taxane-based chemotherapy. Using common terminology criteria for adverse events (CTCAE v. 5.0), toxicity profiles were investigated (including bone marrow and renal toxicity). Overall survival (OS) between both groups was compared. Results Toxicity assessment revealed grade III anaemia in a single patient (1.8%) for [177Lu]Lu-PSMA I&T and five (9.1%) for [177Lu]Lu-PSMA-617. In addition, one (1.9%) grade III thrombopenia for [177Lu]Lu-PSMA-617 was recorded. Apart from that, no other grade III/IV toxicities were present. A median OS of 12 months for patients treated with [177Lu]Lu-PSMA I&T did not differ significantly when compared to patients treated with [177Lu]Lu-PSMA-617 (median OS, 13 months; P = 0.89). Conclusion In this matched-pair analysis of patients receiving one of the two agents most frequently applied for PSMA RLT, the rate of clinically relevant toxicities was low for both compounds. In addition, no relevant differences for OS were observed.


Introduction
As the second most frequent malignancy in men worldwide, the annual incidence of prostate cancer (PC) is 1.4 million and accounts for > 375,000 deaths [1]. The transition from an androgen-dependent to a castration-resistant phenotype poses challenges for treating clinicians, especially in widespread diseases. Recent years have witnessed the introduction of novel therapeutic regimens for metastatic castration-resistant prostate cancer (mCRPC), including androgen receptor signalling inhibitors [2,3] chemotherapeutics [4][5][6], and prostate-specific membrane antigen (PSMA)-directed radioligand therapies (RLT), which are typically carried out with small-molecule, urea-based agents linked to the ß-emitting radionuclide lutetium-177 [7]. In this regard, recent prospective trials have shown remarkable outcomes after PSMA RLT in patients with mCRPC [8][9][10][11]. For instance, the VISION trial reported a substantial prolongation of overall survival (OS) when compared to current standard-of-care therapy, along with an objective response rate in approximately half of the patients [12].

3
To date, PSMA-targeted RLT is mainly conducted with the two different agents, [ 177 Lu]Lu-PSMA I&T and [ 177 Lu]Lu-PSMA-617 [7]. As shown in Fig. 1, the chemical structures of the two labelled peptides for PSMAdirected RLT show significant differences. Only the ureabinding motif of the two peptides is identical, but the majority of the two molecules show no structural similarity. On closer inspection, even the chelators are different: while in [ 177 Lu]Lu-PSMA-617, the peptide is linked to a DOTA chelator [13], in [ 177 Lu]Lu-PSMA I&T, the related DOTAGA chelator is used [14]. Of note, a recent dosimetry study reported on comparable mean absorbed tumour doses for both compounds [15]. In a preclinical setting, [ 177 Lu]Lu-PSMA-617 seems to be superior when compared to [ 177 Lu]Lu-PSMA I&T [16]. Head-to-head comparisons in patients, however, are still lacking [7]. In this two-centre, matched-pair analysis, we compared the toxicity and efficacy of [ 177 Lu]Lu-PSMA I&T and [ 177 Lu] Lu-PSMA-617 for treatment of patients with mCRPC.

Patient cohort
This retrospective study consisted of 55 patients who were treated with [ 177 Lu]Lu-PSMA I&T for mCRPC at the University Hospital Würzburg (Germany) and received at least two cycles of RLT. The matched cohort included 55 patients with mCRPC treated with at least two cycles of [ 177 Lu]Lu-PSMA-617 at the University Hospital Bonn (Germany). Matching criteria included age at the first cycle, Gleason score (categories: 6-7, 8-10, or unknown, [17]), baseline PSA values (0-50 ng/mL, 50-200 ng/ml, 200-600 ng/ml, and > 600 ng/mL), and previous taxane-based chemotherapies (CTx). Approvals were waived by the local Ethics Committees of Bonn and Würzburg due to the retrospective nature of this analysis (20210422 04).

Treatment protocol
Standardized institutional protocols for the execution of RLT were applied. In-house labelling was carried out for  [18,19]. Identical treatment protocols at both study sites included administration of approximately 6.0 GBq of the PSMA ligand every 8 weeks with up to a maximum of 8 cycles, depending on the response to treatment [18,19].

Toxicity and efficacy assessment
Standard blood collection included prostate-specific antigen (PSA) values, leukocytes, haemoglobin, platelets, creatinine, lactate dehydrogenase, aspartate aminotransferase, gamma-glutamyltransferase, and alkaline phosphatase. By comparing with baseline parameters, toxicity was assessed at the last available cycle. Changes in blood cell counts and creatinine levels were also graded according to the Common Terminology Criteria for Adverse Events (CTCAE v. 5.0) [20]. Efficacy was assessed by comparing OS between both cohorts.

Statistical analysis
Statistical analyses were performed using GraphPad Prism version 9.3.0 (GraphPad Software, San Diego, CA, USA). Descriptive data (including OS) are presented as median and range in parentheses. Since not all parameters were normally distributed (Shapiro-Wilk test), comparisons between the two cohorts were performed using Wilcoxon signed-rank test. For comparison of dichotomous variables, Fisher's exact test was applied. OS was analysed using Kaplan-Meier curves and log-rank comparison. A P-value less than 0.05 was considered statistically significant.  Fig. 4).

Discussion
In this matched-pair analysis, we investigated the two most commonly applied urea-based, small-molecule inhibitors of PSMA, namely PSMA-617 and PSMA I&T. We carefully matched patients based on age, Gleason score, and PSA levels. Pre-therapeutic CTx has also been suggested to have a relevant impact on outcome [21] and, thus, we also matched according to this previous treatment regimen. After [12], and pain reduction in patients with disease progression after conventional treatment [9]. As the second most commonly used RLT agent [7], [ 177 Lu]Lu-PSMA I&T has been utilized in retrospective studies demonstrating beneficial outcomes in heavily pretreated mCRPC patients [22]. Results of a currently recruiting, a prospective clinical trial investigating the performance of this agent as a last-line option will be announced in due course [23]. Despite the encouraging results for both radiopharmaceuticals, head-to-head comparisons are still scarce [7]. In this matched-pair analysis focusing on outcome and toxicity, respective  [25], which also reported promising safety profiles for both agents. As such, the treating physician can have certainty that both [ 177 Lu] For PSMA RLT, the structures of the two labelled peptides show relevant differences (Fig. 1) [13,14]. Furthermore, murine studies performing head-to-head comparisons  [16]. In light of these preclinical findings and the structural variances among both agents, the similar efficacy and toxicity of the two radiopharmaceuticals were not necessarily expected.
This study has limitations. Striving for an accurate matching, and despite including a total of 110 mCRPC patients, the sample size was rather small. The grouping of patients by original Gleason score categories (i.e. Gleason 6-7 and Gleason 8-10) allowed for adequate matching [17]. Nonetheless, future studies are needed to compare both agents in an even more controlled environment, preferably in a prospective setting. In this regard, such efforts may then adjust for further important variables, including dosimetry aspects, biodistribution, radiotracer preparation, and logistics. Moreover, compared to the VISION trial, which reported on an OS of 15.3 months for [ 177 Lu]Lu-PSMA-617 after a median of five cycles of RLT [12], our patients underwent only three cycles of treatment, thereby explaining our shorter OS of only 13 months for this agent. As such, we did not follow current treatment standards as defined by the VISION trial, and future studies should also include patients with a higher number of cycles [12].

Conclusions
Investigating the most commonly used agents for PSMAtargeted RLT, [ 177 Lu]Lu-PSMA I&T and [ 177 Lu]Lu-PSMA-617, this matched-pair analysis demonstrates that toxicity profiles of both compounds are comparable, with very low rates of clinically relevant toxicities. In addition, both radiopharmaceuticals are similarly effective in patients with mCRPC.   Data availability Detailed information about the image analysis or the overall survivals of the subjects presented in this study is available on reasonable request from the corresponding author.

Declarations
Institutional Review Board statement The study was conducted according to the guidelines of the Declaration of Helsinki. Ethical review and approval were waived for this study by the local Ethics Committee due to the retrospective character of the study (# 20210422 04).

Consent to participate
All procedures have been conducted as part of clinical routine care. Informed consent has been obtained from all subjects.
Conflict of interest R.A.B. is a consultant for Bayer Healthcare (Leverkusen, Germany) and Eisai GmbH (Frankfurt, Germany). R.A.B. has a noncommercial research agreement and is on the speakers' list of Mediso Medical Imaging (Budapest, Hungary). M.E. is a consultant for Bayer Healthcare (Leverkusen, Germany), Eisai GmbH (Frankfurt, Germany), IPSEN, and Novartis.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.