Change in total lesion PSMA (TLP) during [177Lu]Lu-PSMA-617 radioligand therapy predicts overall survival in patients with mCRPC: monocentric evaluation of a prospective registry.

PURPOSE: This study investigates imaging response of [177Lu]Lu-PSMA-617 radioligand therapy (RLT) based on the whole-body parameter total lesion PSMA (TLP), derived by PSMA-PET/CT and reflecting the total tumor burden, in patients with metastatic castration-resistant prostate cancer (mCRPC) enrolled in a prospective registry (NCT04833517). METHODS: A total of n = 102 mCRPC patients received a [68Ga]Ga-PSMA-11 PET/CT at baseline and after two cycles of PSMA-RLT, in which TLP was measured by using a semi-automated tumor segmentation. TLP was defined as the summed products of volume and uptake (∑ Volume × SUVmean) of all tumor lesions. The Kaplan-Meier method was used to determine the most appropriate ∆TLP thresholds for classification into partial remission (PR), stable disease (SD), and progressive disease (PD) regarding overall survival (OS). Furthermore, we analyzed criteria that are also frequently used in established response frameworks, such as the occurrence of new metastases as independent criterion (I) or in combination with change in tumor burden (II), and the change in PSA serum value (III). RESULTS: For the ∆TLP thresholds -30%/+30% (and also for higher thresholds, -40%/+40% or -50%/+50%), significant differences between all three response categories became apparent (PR/PD: p = 0.001; PR/SD: p = 0.001; SD/PD: p = 0.018). Including the development of new metastases as independent criterion of PD, there was no significant difference in OS between SD and PD (p = 0.455), neither when applied in combination with TLP (p = 0.191). Similarly, significant differentiation between SD and PD was not achieved by PSA serum value (p = 0.973). CONCLUSION: In the largest monocentric study to date, TLP is shown to be a qualified prognostic biomarker, applying ∆TLP thresholds of -30%/+30%. It significantly differentiated between PR, SD, and PD, whereas other response criteria did not differentiate SD vs. PD. Using TLP, the development of new metastases is not a required information for predicting OS.

Efficacy and safety of rechallenge [177Lu]Lu-PSMA-617 RLT after initial partial remission in patients with mCRPC: evaluation of a prospective registry (REALITY study).

AIM: Rechallenge of [177Lu]Lu-PSMA-617 radioligand therapy (RLT) was proposed for patients who initially responded to PSMA-RLT experiencing partial remission, but relapsed into progression after a certain period of remission. However, only limited data is available regarding this approach. In this study, we analyzed the efficacy and safety profile of one or more series of [177Lu]Lu-PSMA-617 RLT rechallenge in patients from a prospective registry (REALITY Study, NCT04833517) after they initially benefited from PSMA-RLT. METHODS: Forty-seven patients with metastatic castration-resistant prostate cancer (mCRPC) who had biochemical response to initial [177Lu]Lu-PSMA-617 RLT followed by disease progression received at least one (up to three) series of [177Lu]Lu-PSMA-617 RLT rechallenge. Biochemical response rates based on prostate-specific antigen (PSA) serum value, PSA-based progression-free survival (PFS) and overall survival (OS) were calculated. Adverse events of the treatment were assessed according to 'common terminology criteria for adverse events' (CTCAE). RESULTS: After one series of RLT rechallenge, a PSA decline of at least 50% was achieved in 27/47 patients (57.4%). The median PFS of all patients was 8.7 mo and the median OS was 22.7 mo, each calculated from the administration of the first rechallenge series. Patients who responded (PSA decline > 50%) to the rechallenge showed a median OS of 27.3 mo. Regarding PFS, a significant correlation (r = 0.4128, p = 0.0323) was found for these patients comparing initial and rechallenge RLT. Ten patients received a second and 3 patients received a third rechallenge series with 8/10 and 3/3 patients responding to repeated RLT rechallenge. No severe deterioration of adverse events rated by CTCAE criteria was observed. CONCLUSION: [177Lu]Lu-PSMA-617 RLT rechallenge is associated with significant PSA response and encouraging survival outcome as well as a very favourable safety profile and should therefore be considered as a straight-forward treatment option in mCRPC patients, who previously benefited from PSMA-RLT.

Detection efficacy of [89Zr]Zr-PSMA-617 PET/CT in [68Ga]Ga-PSMA-11 PET/CT-negative biochemical recurrence of prostate cancer.

RATIONALE: In patients with biochemical recurrence of prostate cancer (BCR), preliminary data suggest that prostate-specific membrane antigen (PSMA) ligand radiotracers labeled with zirconium-89 (89Zr; half-life ~ 78.41 h), which allow imaging ≥ 24 h post-injection, detect suspicious lesions that are missed when using tracers incorporating short-lived radionuclides. MATERIALS AND METHODS: To confirm [89Zr]Zr-PSMA-617 positron emission tomography/computed tomography (PET/CT) detection efficacy regarding such lesions, and compare quality of 1-h, 24-h, and 48-h [89Zr]Zr-PSMA-617 scans, we retrospectively analyzed visual findings and PET variables reflecting lesional [89Zr]Zr-PSMA-617 uptake and lesion-to-background ratio. The cohort comprised 23 men with BCR post-prostatectomy, median (minimum-maximum) prostate-specific antigen (PSA) 0.54 (0.11-2.50) ng/mL, and negative [68Ga]Ga-PSMA-11 scans 40 ± 28 d earlier. Primary endpoints were percentages of patients with, and classifications of, suspicious lesions. RESULTS: Altogether, 18/23 patients (78%) had 36 suspicious lesions (minimum-maximum per patient: 1-4) on both 24-h and 48-h scans (n = 33 lesions) or only 48-h scans (n = 3 lesions). Only one lesion appeared on a 1-h scan. Lesions putatively represented local recurrence in 11 cases, and nodal or bone metastasis in 21 or 4 cases, respectively; 1/1 lesion was histologically confirmed as a nodal metastasis. In all 15 patients given radiotherapy based on [89Zr]Zr-PSMA-617 PET/CT, PSA values decreased after this treatment. Comparison of PET variables in 24-h vs 48-h scans suggested no clear superiority of either regarding radiotracer uptake, but improved lesion-to-background ratio at 48 h. CONCLUSIONS: In men with BCR and low PSA, [89Zr]Zr-PSMA-617 PET/CT seems effective in finding prostate malignancy not seen on [68Ga]Ga-PSMA-11 PET/CT. The higher detection rates and lesion-to-background ratios of 48-h scans versus 24-h scans suggest that imaging at the later time may be preferable. Prospective study of [89Zr]Zr-PSMA-617 PET/CT is warranted.

Radiometal Complexes as Pharmacokinetic Modifiers: A Potent 68Ga-Labeled Gastrin-Releasing Peptide Receptor Antagonist Based on the Macrocyclic Metal Chelator NODIA-Me.

The gastrin-releasing peptide receptor (GRPr) is overexpressed in various cancer types including prostate and breast carcinomas, making it an attractive target for molecular imaging and therapy. In this work, we designed a novel GRPr antagonistic probe comprising metal chelator NODIA-Me. This 1,4,7-triazacyclononane-based chelator forms positively charged metal complexes due to its neutral methylimidazole arms. Because a positive charge at the N-terminus of GRPr conjugates is responsible for high receptor affinity as exemplified by the current gold standard DOTA-RM2, we investigated if a positively charged radiometal complex can be used as a pharmacokinetic modifier to also produce high-affinity GRPr conjugates. In this respect, the bioconjugate NODIA-Me-Ahx-JMV594 was prepared by a combination of solid-phase peptide synthesis and solution-based reactions in a 94% yield. Radiolabeling provided the 68Ga-labeled conjugate in radiochemical yields of >95% and radiochemical purities of >98% with mean molar activities of Am ∼17 MBq nmol-1. The competitive GRPr affinity of the metal-free and 69/71Ga-labeled conjugate was determined to be IC50 = 0.41 ± 0.06 and 1.45 ± 0.06 nM, respectively. The metal-free GRPr antagonist DOTA-RM2 and its corresponding 69/71Ga complex had IC50 values of 1.42 ± 0.07 and 0.98 ± 0.19 nM, respectively. Small-animal PET imaging of mice bearing GRPr(+) PC-3 tumors revealed high radioactivity accumulation in the tumors and in the pancreas as an organ with high levels of GRPr expression. These findings were corroborated by the corresponding ex vivo biodistribution data, in which the tumors and the pancreas exhibited the highest radioactivity accumulation. By coinjection of an excess of NODIA-Me-Ahx-JMV594, uptake in the tumors and GRPr(+) organs was significantly reduced, confirming specific receptor-mediated uptake. The estrogen receptor-positive tumor of a female breast cancer patient was clearly visualized by PET imaging using 68Ga-labeled NODIA-Me-Ahx-JMV594. To summarize, the positive charge at the N-terminus of the conjugate induced by the Ga(NODIA-Me) complex resulted in high GRPr affinity comparable to that of the potent antagonist DOTA-RM2. The conjugate NODIA-Me-Ahx-JMV594 is a promising probe for imaging of GRPr tumors that warrants further evaluation in larger patient cohorts as well as in combination with other radiometals.

HNODThia: A Promising Chelator for the Development of 64Cu Radiopharmaceuticals.

Herein, we present the synthesis and coordination chemistry of copper(II) and zinc(II) complexes of two novel heterocyclic triazacyclononane (tacn)-based chelators (HNODThia and NODThia-AcNHEt). The chelator HNODThia was further derivatized to obtain a novel PSMA-based bioconjugate (NODThia-PSMA) and a bifunctional photoactivatable azamacrocyclic analogue, NODThia-PEG3-ArN3, for the development of copper-64 radiopharmaceuticals. 64Cu radiolabeling experiments were performed on the different metal-binding chelates, whereby quantitative radiochemical conversion (RCC) was obtained in less than 10 min at room temperature. The in vitro stability of NODThia-PSMA in human plasma was assessed by ligand-challenge and copper-exchange experiments. Next, we investigated the viability of the photoactivatable analog (NODThia-PEG3-ArN3) for the light-induced photoradiosynthesis of radiolabeled proteins. One-pot photoconjugation reactions to human serum albumin (HSA) as a model protein and the clinically relevant monoclonal antibody formulation MetMAb were performed. [64Cu]Cu-7-azepin-HSA and [64Cu]Cu-7-azepin-onartuzumab were prepared in less than 15 min by irradiation at 395 nm, with radiochemical purities (RCP) of >95% and radiochemical yields (RCYs) of 42.7 ± 5.3 and 49.6%, respectively. Together, the results obtained here open the way for the development of highly stable 64Cu-radiopharmaceuticals by using aza-heterocyclic tacn-based chelators, and the method can easily be extended to the development of 67Cu radiopharmaceuticals for future applications in molecularly targeted radio(immuno)therapy.

Early molecular imaging response assessment based on determination of total viable tumor burden in [68Ga]Ga-PSMA-11 PET/CT independently predicts overall survival in [177Lu]Lu-PSMA-617 radioligand therapy.

PURPOSE: In patients with metastatic castration-resistant prostate cancer (mCRPC) treated with prostate-specific membrane antigen-targeted radioligand therapy (PSMA-RLT), the predictive value of PSMA PET/CT-derived response is still under investigation. Early molecular imaging response based on total viable tumor burden and its association with overall survival (OS) was explored in this study. METHODS: Sixty-six mCRPC patients who received [177Lu]Lu-PSMA-617 RLT within a prospective patient registry (REALITY Study, NCT04833517) were analyzed. Patients received a [68Ga]Ga-PSMA-11 PET/CT scan before the first and after the second cycle of PSMA-RLT. Total lesion PSMA (TLP) was determined by semiautomatic whole-body tumor segmentation. Molecular imaging response was assessed by change in TLP and modified PERCIST criteria. Biochemical response was assessed using standard serum PSA and PCWG3 criteria. Both response assessment methods and additional baseline parameters were analyzed regarding their association with OS by univariate and multivariable analysis. RESULTS: By molecular imaging, 40/66 (60.6%) patients showed partial remission (PR), 19/66 (28.7%) stable disease (SD), and 7/66 (10.6%) progressive disease (PD). Biochemical response assessment revealed PR in 34/66 (51.5%) patients, SD in 20/66 (30.3%), and PD in 12/66 (18.2%). Response assessments were concordant in 49/66 (74.3%) cases. On univariate analysis, both molecular and biochemical response (p = 0.001 and 0.008, respectively) as well as two baseline characteristics (ALP and ECOG) were each significantly associated with OS. The median OS of patients showing molecular PR was 24.6 versus 10.7 months in the remaining patients (with SD or PD). On multivariable analysis molecular imaging response remained an independent predictor of OS (p = 0.002), eliminating biochemical response as insignificant (p = 0.515). CONCLUSION: The new whole-body molecular imaging-derived biomarker, early change of total lesion PSMA (TLP), independently predicts overall survival in [177Lu]Lu-PSMA-617 RLT in mCRPC, outperforming conventional PSA-based response assessment. TLP might be considered a more distinguished and advanced biomarker for monitoring PSMA-RLT over commonly used serum PSA.

177 Lu-PSMA-617 radioligand therapy of metastatic castration-resistant prostate cancer: Initial 254-patient results from a prospective registry (REALITY Study).

PURPOSE: Preliminary data from retrospective analyses and recent data from large randomized controlled trials suggest safety and efficacy of radioligand therapy (RLT) targeting prostate-specific membrane antigen (PSMA) in men with metastatic castration-resistant prostate cancer (mCRPC). Limited data on this modality have been published regarding large samples treated in everyday practice. METHODS: We analyzed prospectively collected registry data regarding lutetium-177 (177Lu)-PSMA-617 RLT of 254 consecutive men with mCRPC seen in everyday academic practice. Since 177Lu-PSMA-617 was experimental salvage treatment following failure of individually appropriate conventional therapies, patients were generally elderly and heavily pretreated (median age 70 years; prior taxanes 74.0%, 188/254), with late-end-stage disease (visceral metastasis in 32.7%, 83/254). Primary endpoints were response to RLT, defined by changes from baseline serum prostate-specific antigen (PSA) concentration, PSA progression-free survival (PSA-PFS), and overall survival (OS), estimated with Kaplan-Meier statistics, and caregiver-reported and patient-reported safety. Unless noted, median (minimum-maximum) values are given. RESULTS: Patients received 3 (1-13) 177Lu-PSMA-617 activities (6.5 [2.5-11.6] GBq/cycle) every 5.7 (3.0-11.0) weeks. Best response was ≥ 50% PSA reduction in 52.0% of patients (132/254). PSA-PFS was 5.5 (95% confidence interval [95%CI] 4.4-6.6) months and OS, 14.5 (95%CI 11.5-17.5) months. In multivariable Cox proportional-hazards modeling, response to the initial ≤ 2 RLT administrations was the strongest significant prognosticator related to OS (hazard ratio 3.7 [95%CI 2.5-5.5], p < 0.001). No RLT-related deaths or treatment discontinuations occurred; the most frequent RLT-related Grade 3/4 adverse events were anemia (18/254 patients, 7.1%), thrombocytopenia (11/254, 4.3%), and lymphopenia (7/254, 2.8%). RLT-related xerostomia, all grade 1/2, was noted in 53/254 (20.9%). CONCLUSIONS: In a large, prospectively observed "real-world" cohort with late-stage/end-stage mCRPC and conventional treatment failure, 177Lu-PSMA-617 RLT was effective, safe, and well-tolerated. Early biochemical disease control by such therapy was associated with better OS. Prospective study earlier in the disease course may be warranted.

[89Zr]Zr-PSMA-617 PET/CT in biochemical recurrence of prostate cancer: first clinical experience from a pilot study including biodistribution and dose estimates.

PURPOSE: Prostate-specific membrane antigen (PSMA)-targeted PET/CT has become increasingly important in the management of prostate cancer, especially in localization of biochemical recurrence (BCR). PSMA-targeted PET/CT imaging with long-lived radionuclides as 89Zr (T1/2 = 78.4 h) may improve diagnostics by allowing data acquisition on later time points. In this study, we present our first clinical experience including preliminary biodistribution and dosimetry data of [89Zr]Zr-PSMA-617 PET/CT in patients with BCR of prostate cancer. METHODS: Seven patients with BCR of prostate cancer who revealed no (n = 4) or undetermined (n = 3) findings on [68Ga]Ga-PSMA-11 PET/CT imaging were referred to [89Zr]Zr-PSMA-617 PET/CT. PET/CT imaging was performed 1 h, 24 h, 48 h, and 72 h post injection (p.i.) of 111 ± 11 MBq [89Zr]Zr-PSMA-617 (mean ± standard deviation). Normal organ distribution and dosimetry were determined. Lesions visually considered as suggestive of prostate cancer were quantitatively analyzed. RESULTS: Intense physiological uptake was observed in the salivary and lacrimal glands, liver, spleen, kidneys, intestine and urinary tract. The parotid gland received the highest absorbed dose (0.601 ± 0.185 mGy/MBq), followed by the kidneys (0.517 ± 0.125 mGy/MBq). The estimated overall effective dose for the administration of 111 MBq was 10.1 mSv (0.0913 ± 0.0118 mSv/MBq). In 6 patients, and in particular in 3 of 4 patients with negative [68Ga]Ga-PSMA-11 PET/CT, at least one prostate cancer lesion was detected in [89Zr]Zr-PSMA-617 PET/CT imaging at later time points. The majority of tumor lesions were first visible at 24 h p.i. with continuously increasing tumor-to-background ratio over time. All tumor lesions were detectable at 48 h and 72 h p.i. CONCLUSION: [89Zr]Zr-PSMA-617 PET/CT imaging is a promising new diagnostic tool with acceptable radiation exposure for patients with prostate cancer especially when [68Ga]Ga-PSMA-11 PET/CT imaging fails detecting recurrent disease. The long half-life of 89Zr enables late time point imaging (up to 72 h in our study) with increased tracer uptake in tumor lesions and higher tumor-to-background ratios allowing identification of lesions non-visible on [68Ga]Ga-PSMA-11 PET/CT imaging.

89Zr-labeled PSMA ligands for pharmacokinetic PET imaging and dosimetry of PSMA-617 and PSMA-I&T: a preclinical evaluation and first in man.

RATIONALE: Prolonged in vivo evaluation of PSMA tracers could improve tumor imaging and patient selection for 177Lu-PSMA-617 and 177Lu-PSMA-I&T. In this study, we present the radiolabeling method of PSMA-617 and PSMA-I&T with the long-lived positron emitter 89Zr to enable PET imaging up to 7 days post-injection. We compared the biodistribution of 89Zr-PSMA-617 and 89Zr-PSMA-I&T to those of 177Lu-PSMA-617 and 177Lu-PSMA-I&T, respectively, in a PSMA+ xenograft model. Moreover, we provide the first human 89Zr-PSMA-617 images. MATERIALS AND METHODS: PSMA ligands were labeled with 50-55 MBq [89Zr]ZrCl4 using a two-step labeling protocol. For biodistribution, BALB/c nude mice bearing PSMA+ and PSMA- xenografts received 0.6 µg (0.6-1 MBq) of 89Zr-PSMA-617, 89Zr-PSMA-I&T, 177Lu-PSMA-617, or 177Lu-PSMA-I&T intravenously. Ex vivo biodistribution and PET/SPECT imaging were performed up to 168 h post-injection. Dosimetry was performed from the biodistribution data. The patient received 90.5 MBq 89Zr-PSMA-617 followed by PET/CT imaging. RESULTS: 89Zr-labeled PSMA ligands showed a comparable ex vivo biodistribution to its respective 177Lu-labeled counterparts with high tumor accumulation in the PSMA+ xenografts. However, using a dose estimation model for 177Lu, absorbed radiation dose in bone and kidneys differed among the 177Lu-PSMA and 89Zr-PSMA tracers. 89Zr-PSMA-617 PET in the first human patient showed high contrast of PSMA expressing tissues up to 48 h post-injection. CONCLUSION: PSMA-617 and PSMA-I&T were successfully labeled with 89Zr and demonstrated high uptake in PSMA+ xenografts, which enabled PET up to 168 h post-injection. The biodistribution of 89Zr-PSMA-I&T and 89Zr-PSMA-617 resembled that of 177Lu-PSMA-I&T and 177Lu-PSMA-617, respectively. The first patient 89Zr-PSMA-617 PET images were of high quality warranting further clinical investigation.

Benefit of including CT urography in [68Ga]PSMA-11 PET/CT with low-dose CT: first results from a larger prostate cancer cohort analysis.

BACKGROUND: Accuracy of [68Ga]PSMA-11 PET/CT may be hampered by ureter accumulation, mimicking lymph node metastases depending on localization and configuration. The benefit of CT urography for differentiation of lymph node metastasis from urinary tract activity was evaluated in a "PET/CT with low-dose CT" setting. METHODS: Retrospective analysis of PET/CT for primary staging, biochemical recurrence or local treatment planning in patients with prostate cancer. For CT urography (CTU), iodinated contrast agent was administered 10 minutes prior to image acquisition. All potential pathologic (peri)ureteral tracer uptake was assigned to excretory ureteral accumulation or pathological lesion. To assess additional provided benefit of CTU all foci were rated with an introduced scoring system (ranging from 0 pts: CTU not needed; up to 3 pts: no differentiation possible without CTU). Success of ureter contrasting was assessed by measurement of Hounsfield units. Besides benefit for reading urography-enhanced PET/CT, the possible impact on subsequent patient treatment was evaluated. RESULTS: A number of N.=247 patients were included in this study. By CT urography, it was possible to identify each ureter on low-dose CT, with its major part contrasted. In 120/247 (48.6%) patients, urography increased the diagnostic confidence while providing substantial support for interpretation in 60 (24.3%) cases. In 42 (17.0%) patients, urography was clinically relevant (up-/downstaging) with potential impact on subsequent patient care. In 30 of these 42 cases (12.1% of all), discrepant treatment would have resulted from a misdiagnosed tracer accumulation without urography. CONCLUSIONS: CT urography benefits the interpretation of [68Ga]-PSMA-11 PET/CT with low-dose CT and leads to discrepant patient treatment in a small but significant subset of patients (12% in our cohort). The implementation of CT urography into standard protocols of [68Ga]PSMA-11 PET/CT with low-dose CT is recommended.

Response and outcome of liver metastases in patients with metastatic castration-resistant prostate cancer (mCRPC) undergoing 177Lu-PSMA-617 radioligand therapy.

PURPOSE: Little is known about the efficacy of prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) against liver metastases of metastatic castration-resistant prostate cancer (mCRPC). We retrospectively analyzed efficacy-related outcomes of 177Lu-PSMA-617 RLT in this setting and potential predictors of those outcomes. METHODS: Twenty-eight consecutive mCRPC patients with liver metastases given 177Lu-PSMA-617 RLT were analyzed retrospectively. Their planned regimen was 4-6 cycles at 6 ± 2-week intervals; the mean activity/cycle was 6.5 ± 0.5 GBq. Hepatic response was determined by modified positron emission tomography response criteria in solid tumors; association of such response with overall survival (OS) was tested, as were relationships of the selected patient, disease, and treatment characteristics with hepatic progression-free survival (PFShep) and OS. Survival analyses used Kaplan-Meier curves, log-rank test at p < 0.05 significance, and Cox proportional-hazards modeling. RESULTS: Median (minimum-maximum) follow-up was 37.5 (2.3-50.6) months. In liver metastases, complete or partial response was observed in 6 patients (21%) each, and stable disease in 1 (4%), for hepatic disease control in 46%. Overall, median (95% confidence interval) PFShep was 5.7 (2.2-9.2) months, and OS, 11.7 (3.0-20.4) months. Patients with hepatic disease control did not reach the median OS, while those with hepatic progressive disease had median OS (95% confidence interval) of 6.4 (1.6-11.1) months. In multivariate analysis, hepatic disease control by 177Lu-PSMA-617 RLT was significantly independently associated with OS, as was a prostate-specific antigen decline of ≥ 50% after 2 RLT cycles, and good baseline performance status (Eastern Cooperative Oncology Group 0-1). Hepatic tumor burden (≤ 25% vs. > 25% of liver volume) had no apparent relationship with hepatic tumor response, PFShep, or OS. CONCLUSION: 177Lu-PSMA-617 RLT frequently controlled liver metastases, resulting in long PFShep and significantly improved OS. Hepatic tumor burden appeared to lack any relationship with treatment efficacy, supporting 177Lu-PSMA-617 RLT of late-stage/end-stage mCRPC with liver metastases.

New insights in the paradigm of upregulation of tumoral PSMA expression by androgen receptor blockade: Enzalutamide induces PSMA upregulation in castration-resistant prostate cancer even in patients having previously progressed on enzalutamide.

PURPOSE: There is preliminary evidence for prostate-specific membrane antigen (PSMA) upregulation effects of androgen receptor blockade in prostate cancer. In an attempt to find the best condition for PSMA radioligand therapy in metastatic castration-resistant prostate cancer (mCRPC) patients, we evaluated the effect of oral enzalutamide in patients, predominantly having previously progressed on enzalutamide treatment. METHODS: Ten patients with advanced mCRPC scheduled for PSMA radioligand therapy were examined with 68Ga-PSMA-11 PET/CT before and after a mean of 11.8 days of enzalutamide 160 mg/day. Imaging results were compared using total PSMA tumor burden quantification. We assessed whole-body total lesion PSMA (TLP), defined as SUVmean × tumor volume and calculated TLP-to-liver ratio (TLP-LR), TLP-to-parotid gland ratio (TLP-PR), and TLP-to-kidney ratio (TLP-KR). RESULTS: The mean (median) increase of TLP-LR, TLP-PR, and TLP-KR in the cohort was 49.3% (38.8%), 45.1% (23.5%), and 54.9% (37.6%), respectively. These increases were statistically significant (p = 0.002, p = 0.014, and p = 0.014), while PSA values did not change significantly (p = 0.846). Seven of the 10 patients had previously undergone enzalutamide treatment with eventual progression, formally classified as treatment failure. No side effects were noted in the short term. CONCLUSIONS: Our results suggest that enzalutamide could be considered as a PSMA radioligand treatment enhancing primer medication, which may increase PSMA expression by a dimension of 50% in mCRPC. The effect was shown even in patients having previously failed enzalutamide treatment for arrest of progression in the mCRPC setting. Our observation deserves evaluation in a prospective setting.

225Ac-PSMA-617/177Lu-PSMA-617 tandem therapy of metastatic castration-resistant prostate cancer: pilot experience.

PURPOSE: Up to 30% of patients with prostate-specific membrane antigen (PSMA)-positive metastatic castration-resistant prostate cancer (mCRPC) never respond or develop resistance to 177Lu-labeled PSMA-targeted radioligand monotherapy. Single-agent PSMA-targeted radioligand therapy (PRLT) with the alpha-emitter 225Ac showed promise against mCRPC but may cause severe and/or persistent xerostomia, which may substantially impair patients' quality-of-life. We hypothesized that when 177Lu-PSMA ligand alone is ineffective, tandem therapy with low-activity 225Ac-PSMA ligand plus full activity of the beta emitter may enhance efficacy while minimizing xerostomia severity. METHODS: We retrospectively analyzed pilot experience with 1 course of 225Ac-PSMA-617/177Lu-PSMA-617 tandem therapy in our first 20 patients with mCRPC receiving this intervention after insufficiently responding to 177Lu-PSMA-617 monotherapy. This cohort had late-stage/end-stage disease with high baseline prostate-specific antigen (PSA) concentration (median 215 ng/mL), heavy pre-treatment (abiraterone and/or enzalutamide, and 177Lu-PRLT [median cumulative activity, 26.3 GBq] in 20/20 patients, 100%; docetaxel and/or cabazitaxel in 13/20 patients, 65%), and frequent Eastern Cooperative Oncology Group performance status of 2 (8/20 patients, 40%). RESULTS: Median (minimum-maximum) administered activities were 225Ac-PSMA-617, 5.3 (1.5-7.9) MBq, and 177Lu-PSMA-617, 6.9 (5.0-11.6) GBq. Significant responders to tandem therapy received 177Lu-PSMA-617 monotherapy as maintenance (median [minimum-maximum]: 1 [0-5] cycle). After a median (minimum-maximum) 22 (14-63) weeks' follow-up, 13/20 patients (65%) had as best biochemical response a PSA decline > 50%. Median (95% confidence interval) progression-free survival was 19 (12-26) weeks, and overall survival was 48 (4-92) weeks post-tandem therapy administration. Xerostomia was reported as grade 1 (very mild) in 8/20 patients (40%), grade 2 (mild) in 5/20 (25%), and grade 3/4 in 0/20. CONCLUSIONS: Our results suggest that a single course of tandem therapy with low-activity 225Ac-PSMA-617/full-activity 177Lu-PSMA-617 may safely enhance response to PRLT in men with late-stage/end-stage mCRPC while minimizing xerostomia severity. Formal study of this combination is warranted.

Methylthiazolyl Tacn Ligands for Copper Complexation and Their Bifunctional Chelating Agent Derivatives for Bioconjugation and Copper-64 Radiolabeling: An Example with Bombesin.

We present here the synthesis of two new bifunctionalized azachelators, no2th-EtBzNCS and Hno2th1tha, as bioconjugable analogues of two previously described di- and trimethylthiazolyl 1,4,7-triazacyclononane (tacn) ligands, no2th and no3th, for potential uses in copper-64 (64Cu) positron emission tomography imaging. The first one bears an isothiocyanate group on the remaining free nitrogen atom of the tacn framework, while the second one presents an additional carboxylic function on one of the three heterocyclic pendants. Their syntheses required regiospecific N-functionalization of the macrocycles. In order to investigate their suitability for in vivo applications, a complete study of their copper(II) chelation was performed. The acid-base properties of the ligands and their thermodynamic stability constants with copper(II) and zinc(II) cations were determined using potentiometric techniques. Structural studies were conducted in both solution and the solid state, consolidated by theoretical calculations. The kinetic inertness in an acidic medium of both copper(II) complexes was determined by spectrophotometry, while cyclic voltammetry experiments were performed to evaluate the stability at the copper(I) redox state. UV-vis, NMR (of the zinc complexes), electron paramagnetic resonance spectroscopy, and density functional theory studies showed excellent agreement between the solution structures of the complexes and their crystallographic data. These investigations unambiguously prove that these bifunctional derivatives display similar coordination properties as their no2th and no3th counterparts, opening the door to targeted bioapplications. The no2th-EtBzNCS and Hno2th1tha ligands were then conjugated to a bombesin antagonist peptide for targeting the gastrin-releasing peptide receptor (GRPr). To highlight the potential of the two chelators for radiopharmaceutical development, the 64Cu-radiolabeling properties, in vitro stability, and binding affinity to GRPr of the corresponding bioconjugates were determined. Altogether, the results of this work warrant the further development of 64Cu-based radiopharmaceuticals comprising our novel bifunctional chelators.

Performance of 111In-labelled PSMA ligand in patients with nodal metastatic prostate cancer: correlation between tracer uptake and histopathology from lymphadenectomy.

PURPOSE: Intraoperative identification of lymph node (LN) metastases (LNM) detected on preoperative PSMA PET/CT may be facilitated by PSMA radioguided surgery with the use of a gamma probe. We evaluated the uptake of 111In-labelled PSMA ligand DKFZ-617 (referred to as 111In-PSMA-617) in unaffected LN and LNM at the level of single LN. METHODS: Six patients with prostate cancer (PCa) with suspicion of LNM on preoperative PSMA PET/CT underwent 111In-PSMA-617-guided lymphadenectomy (LA; four salvage LA and two primary LA). 111In-PSMA-617 (109 ± 5 MBq). was injected Intravenously 48 h prior to surgery Template LAs were performed in small subregions: common, external, obturator and internal iliac vessels, and presacral and retroperitoneal subregions (n = 4). Samples from each subregion were isolated aiming at the level of single LN. Uptake was measured ex situ using a germanium detector. Receiver operating characteristic (ROC) analysis was performed based on 111In-PSMA-617 uptake expressed as standardized uptake values normalized to lean body mass (SUL). RESULTS: Overall 310 LN (mean 52 ± 19.7) were removed from 74 subregions (mean 12 ± 3.7). Of the 310 LN, 35 turned out to be LNM on histopathology. Separation of the samples from all subregions resulted in 318 single specimens: 182 PCa-negative LN samples with 275 LN, 35 single LNM samples, 3 non-nodal PCa tissue samples and 98 fibrofatty tissue samples. The median SULs of nonaffected LN (0.16) and affected LN (13.2) were significantly different (p < 0.0001). Based on 38 tumour-containing and 182 tumour-free specimens, ROC analysis revealed an area under the curve of 0.976 (95% CI 0.95-1.00, p < 0.0001). Using a SUL cut-off value of 1.136, sensitivity, specificity, positive predictive value, negative predictive value and accuracy in discriminating affected from nonaffected LN were 92.1% (35/38), 98.9% (180/182), 94.6% (35/37), 98.4% (180/183) and 97.7% (215/220), respectively. CONCLUSION: Ex situ analysis at the level of single LN showed that 111In-PSMA-617 had excellent ability to discriminate between affected and nonaffected LN in our patients with PCa. This tracer characteristic is a prerequisite for in vivo real-time measurements during surgery.

Optimized synthesis and indium complex formation with the bifunctional chelator NODIA-Me.

The bifunctional chelator NODIA-Me holds promise for radiopharmaceutical development. NODIA-Me is based on the macrocycle TACN (1,4,7-triazacyclononane) and incorporates two additional methylimidazole arms for metal chelation and an acetic acid residue for bioconjugation. The original two step synthesis was less than optimal due to low yields and the requirement of semi-preparative RP-HPLC purifications. Here, the overall yield for the preparation of NODIA-Me was improved two- to five-fold via two synthetic routes using different protection/deprotection techniques. This way, it was possible (1) to prepare of NODIA-Me on multi-gram scale and (2) to avoid time-consuming HPLC purifications. Inspired by recent results with nat/68Ga3+, preliminary studies on the radiolabeling properties and complex formation of NODIA-Me with nat/111In3+ were performed. Quantitative radiochemical yields were achieved at ambient temperature providing molar activities of ∼30 MBq nmol-1, which could be increased to ∼240 MBq nmol-1 at 95 °C. At r.t., pH 5.5 was optimal for 111In-labeling, but quantitative yields were also achieved in the pH range from 5.5 to 8.2, when the reaction temperature was increased. Stability tests of 111In complexes in vitro revealed high kinetic stabilities in serum and ligand challenge experiments, which is a consequence of the formation of rigid 1 : 1 indium chelates as shown by NMR studies in solution. In summary, the new synthetic routes afford the BFC NODIA-Me in high yields and on large scale. Further, 111In complexation experiments broaden the scope of our chelating system for radiopharmaceutical applications.

Radioimmunotherapy of solid tumors: Approaches on the verge of clinical application.

While radioimmunotherapy (RIT) for the treatment of hematological malignancies such as indolent B-cell lymphoma has proven quite successful, clinical results of RIT in solid tumors have only been moderate in the past. The reasons were manifold and can be mostly attributed to the different biological properties of solid tumors vs hematological cancers. Furthermore, the slow clearance of the radiolabelled antibody prevents the use of radiation doses necessary to achieve clinical responses. The long biological half-life of radioimmunoconjugates results in high background levels and is the main reason for radiation related toxicities. In recent years, researchers and clinicians have developed solutions for the successful application of RIT for the treatment of solid tumors. These include compartmental route of administration, neoadjuvant therapies, and pretargeting approaches. In this review, recent developments in RIT for the treatment of solid tumors that address these restrictions as well as future perspectives will be highlighted from a clinical perspective.

Radiochemistry and Preclinical PET Imaging of 68Ga-Desferrioxamine Radiotracers Targeting Prostate-Specific Membrane Antigen.

Radiotracers incorporating the urea-based Glu-NH-C(O)-NH-Lys group have gained prominence due to their role in targeting prostate-specific membrane antigen (PSMA)-a clinical biomarker of prostate cancer. Here, the synthesis, radiolabeling, and in vitro and in vivo characterization of two 68Ga-radiolabeled Glu-NH-C(O)-NH-Lys radiotracers conjugated to the desferrioxamine B (DFO) chelate were evaluated. Two linker groups based on amide bond and thiourea coupling chemistries were employed to develop 68Ga-DFO-Nsucc-PSMA (68Ga-4) and 68Ga-DFO- pNCS-Bn-PSMA (68Ga-7), respectively. Radiosynthesis proceeded quantitatively at room temperature with high radiochemical yields, chemical/radiochemical purities, and specific activities. Pharmacokinetic profiles of 68Ga-4 and 68Ga-7 were assessed using positron-emission tomography (PET) in mice bearing subcutaneous LNCaP tumors. Data were compared to the current clinical benchmark radiotracer 68Ga-HBED-CC-PSMA (68Ga-1) (HBED = N,N'-Bis(2-hydroxy-5-(ethylene-beta-carboxy)benzyl)ethylenediamine N,N'-diacetic acid). Results indicated that the target binding affinity, protein association, blood pool and background organ clearance properties, and uptake in PSMA-positive lesions are strongly dependent on the nature of the chelate, the linker, and the spacer groups. Protein dissociation constants ( Kd values) were found to be predictive of pharmacokinetics in vivo. Compared to 68Ga-1, 68Ga-4 and 68Ga-7 resulted in decreased tumor uptake but enhanced blood pool clearance and reduced residence time in the kidney. The study highlights the importance of maximizing protein binding affinity during radiotracer optimization.

Diagnosis of recurrent prostate cancer with PET/CT imaging using the gastrin-releasing peptide receptor antagonist 68Ga-RM2: Preliminary results in patients with negative or inconclusive [18F]Fluoroethylcholine-PET/CT.

PURPOSE/BACKGROUND: [18F]fluoroethylcholine (18FECH) has been shown to be a valuable PET-tracer in recurrent prostate cancer (PCa), but still has limited accuracy. RM2 is a gastrin-releasing peptide receptor (GRPr) antagonist that binds to GRPr on PCa cells. Recent studies suggest that GRPr imaging with PET/CT is a promising technique for staging and restaging of PCa. We explore the value of GRPr-PET using the 68Ga-labeled GRPr antagonist RM2 in a selected population of patients with biochemically recurrent PCa and a negative/inconclusive 18FECH-PET/CT. MATERIAL AND METHODS: In this retrospective study 16 men with biochemical PCa relapse and negative (n = 14) or inconclusive (n = 2) 18FECH-PET/CT underwent whole-body 68Ga-RM2-PET/CT. Mean time from 18FECH-PET/CT to 68Ga-RM2-PET/CT was 6.1 ± 6.8 months. Primary therapies in these patients were radical prostatectomy (n = 13; 81.3%) or radiotherapy (n = 3; 18.7%). 14/16 patients (87.5%) had already undergone salvage therapies because of biochemical relapse prior to 68Ga-RM2-PET/CT imaging. Mean ± SD PSA at 68Ga-RM2-PET/CT was 19.4 ± 53.5 ng/ml (range 1.06-226.4 ng/ml). RESULTS: 68Ga-RM2-PET/CT showed at least one region with focal pathological uptake in 10/16 patients (62.5%), being suggestive of local relapse (n = 4), lymph node metastases (LNM; n = 4), bone metastases (n = 1) and lung metastasis with hilar LNM (n = 1). Seven of ten positive 68Ga-RM2 scans were positively confirmed by surgical resection and histology of the lesions (n = 2), by response to site-directed therapies (n = 2) or by further imaging (n = 3). Patients with a positive 68Ga-RM2-scan showed a significantly higher median PSA (6.8 ng/ml, IQR 10.2 ng/ml) value than those with a negative scan (1.5 ng/ml, IQR 3.1 ng/ml; p = 0.016). Gleason scores or concomitant antihormonal therapy had no apparent impact on the detection of recurrent disease. CONCLUSION: Even in this highly selected population of patients with known biochemical recurrence but negative or inconclusive 18FECH-PET/CT, a 68Ga-RM2-PET/CT was helpful to localize PCa recurrence in the majority of the cases. Thus, 68Ga-RM2-PET/CT deserves further investigation as a promising imaging modality for imaging PCa recurrence.

Gallium Complexation, Stability, and Bioconjugation of 1,4,7-Triazacyclononane Derived Chelators with Azaheterocyclic Arms.

We have recently introduced a 1,4,7-triazacyclononane (TACN) based chelating system with additional five-membered azaheterocyclic substituents for complexation of radioactive Cu2+ ions. In this work, we investigated the complexation properties of these novel chelators with Ga3+. In labeling experiments, we could show that the penta- and hexadentate imidazole derivatives NODIA-Me 4 and NOTI-Me 1 can be labeled with 68Ga in specific activities up to ∼30 MBq nmol-1, while the corresponding thiazole derivative NOTThia 2 did not label satisfactorily under identical conditions. NMR studies on the Ga complexes of 1 and the model compound NODIA-Me-NH-Me 5 revealed formation of rigid 1:1 chelates with a slow macrocyclic interconversion and inert Ga-N bonds to the methylimidazole residues on the NMR time scale. The TACN-derived bifunctional chelator NODIA-Me was furthermore conjugated to a prostate-specific membrane antigen (PSMA) targeting moiety to give the corresponding bioconjugate NODIA-Me-PSMA 7. Serum stability and ligand challenge experiments of 68Ga-7 confirmed formation of a stable complex for up to 4 h. The remaining coordination site of five-coordinate Ga complexes was found to be occupied by monodentate ligands including hydroxide and chloride anions depending on the conditions. According to density functional theory calculations, coordination of monodentate ligands as well as of the amide group for the bioconjugated ligand are energetically plausible. Finally, the labeled bioconjugate 68Ga-7 exhibited rapid renal clearance in biodistribution studies performed by small animal PET imaging with no indication of transchelation/demetalation in vivo. Altogether, our results provide strong evidence for a stable Ga complexation of our novel TACN-based chelators bearing imidazole arms. Despite the formation of two complexes incorporating different monodentate ligands in vitro, the imidazole type ligands show promise as chelating agents for the future development of gallium based radiopharmaceuticals.