Effect of Carbohydration on the Theranostic Tracer PSMA I&T.

To investigate the effect of carbohydrate moieties on the pharmacokinetic profile of prostate-specific membrane antigen (PSMA) inhibitors, carbohydrated derivatives of the established PSMA-targeted radiopharmaceutical PSMA I&T were developed and evaluated. As observed for the reference PSMA I&T, the natGa/natLu complexes of the respective galactose-, mannose-, and cellobiose-conjugated analogs showed high PSMA affinity. Carbohydration had almost no effect on the lipophilicity, whereas PSMA-mediated internalization was reduced. The specific binding toward human serum albumin (HSA) decreased from 78.6% for [natLu]PSMA I&T to 19.9% for the natLu-labeled cellobiose derivative. Compared to [68Ga]PSMA I&T, [68Ga]PSMA galactose displayed lower nonspecific tissue and kidney accumulation but also slightly lower tumor uptake in small-animal positron emission tomography (µPET) imaging. Biodistribution studies confirmed reduced unspecific uptake in nontarget tissue and decreased renal accumulation of the metabolically stable [68Ga]PSMA galactose derivative, resulting in overall improved tumor-to-tissue ratios. However, carbohydration has no significant beneficial in vivo effect on the targeting performance of PSMA I&T. Nevertheless, carbohydration expands the repertoire of feasible modifications within the linker area and might be a valuable tool for the future development of PSMA inhibitors with decreased kidney uptake.

Synthesis and in vitro and in vivo evaluation of urea-based PSMA inhibitors with increased lipophilicity.

BACKGROUND: Several radiolabeled prostate-specific membrane antigen (PSMA) inhibitors based on the lysine-urea-glutamate (KuE) motif as the pharmacophore proved to be suitable tools for PET/SPECT imaging of the PSMA expression in prostate cancer patients. PSMA I&T, a theranostic tracer developed in our group, was optimized through alteration of the peptidic structure in order to increase the affinity to PSMA and internalization in PSMA-expressing tumor cells. However, further structural modifications held promise to improve the pharmacokinetic profile. RESULTS: Among the investigated compounds 1-9, the PSMA inhibitors 5 and 6 showed the highest PSMA affinity (lowest IC50 values) after the introduction of a naphthylalanine modification. The affinity was up to three times higher compared to the reference PSMA I&T. Extended aromatic systems such as the biphenylalanine residue in 4 impaired the interaction with the lipophilic binding pocket of PSMA, resulting in a tenfold lower affinity. The IC50 of DOTAGA-conjugated 10 was slightly increased compared to the acetylated analog; however, efficient PSMA-mediated internalization and 80% plasma protein binding of 68Ga-10 resulted in effective tumor targeting and low uptake in non-target tissues of LNCaP tumor-bearing CD-1 nu/nu mice at 1 h p.i., as determined by small-animal PET imaging and biodistribution studies. For prolonged tumor retention, the plasma protein binding was increased by insertion of 4-iodo-D-phenylalanine resulting in 97% plasma protein binding and 16.1 ± 2.5% ID/g tumor uptake of 177Lu-11 at 24 h p.i. CONCLUSIONS: Higher lipophilicity of the novel PSMA ligands 10 and 11 proved to be beneficial in terms of affinity and internalization and resulted in higher tumor uptake compared to the parent compound. Additional combination with para-iodo-phenylalanine in the spacer of ligand 11 elevated the plasma protein binding and enabled sustained tumor accumulation over 24 h, increasing the tumor uptake almost fourfold compared to 177Lu-PSMA I&T. However, high renal uptake remains a drawback and further studies are necessary to elucidate the responsible mechanism behind it.

Dendritic poly-chelator frameworks for multimeric bioconjugation.

Starting from multifunctional triazacyclononane-triphosphinate chelator cores, dendritic molecules with the ability to bind metal ions within their framework were synthesized. A cooperative interaction of the chelator cages resulted in a markedly increased affinity towards 67/68GaIII. A hexameric PSMA inhibitor conjugate with high affinity (IC50 = 1.2 nM) and favorable in vivo PET imaging properties demonstrated practical applicability. The novel scaffolds are useful for synthesis of structurally well-defined multimodal imaging probes or theranostics.

Value of 111 In-prostate-specific membrane antigen (PSMA)-radioguided surgery for salvage lymphadenectomy in recurrent prostate cancer: correlation with histopathology and clinical follow-up.

OBJECTIVES: To evaluate the use of 111 In-labelled prostate-specific membrane antigen (PSMA)-I&T-based radioguided surgery (111 In-PSMA-RGS) for salvage surgery in recurrent prostate cancer (PCa) using comparison of intra-operative gamma probe measurements with histopathological results of dissected specimens. In addition, to determine the success of 111 In-PSMA-RGS with regard to postoperative prostate-specific antigen (PSA) responses, PCa-specific treatment-free survival rates and postoperative complication rates. PATIENTS AND METHODS: A total of 31 consecutive patients with localized recurrent PCa undergoing salvage surgery with PSMA-targeted radioguided surgery using a 111 In-labelled PSMA ligand between April 2014 and July 2015 were retrospectively included in this study. The preoperative (interquartile range; range) median PSA level was 1.3 (0.57-2.53 ng/mL; 0.2-13.9 ng/mL). Results of ex vivo radioactivity rating (positive vs negative) of resected tissue specimens were compared with findings of postoperative histological analysis. Best PSA response without additional treatment was determined after 111 In-PSMA-RGS, and salvage-surgery-related postoperative complications and PCa-specific additional treatments were recorded. RESULTS: In 30/31 patients, 111 In-PSMA-RGS allowed intra-operative identification of metastatic lesions. In total, 145 surgical specimens were removed and 51 showed metastatic involvement at histological analysis. According to 111 In-PSMA-RGS ex vivo measurements, 48 specimens were correctly classified as metastatic and 87 as cancer-free, four were false-negative and six were false-positive compared with histological evaluation. Follow-up information was available for 30/31 patients. PSA declines of >50% and >90% were observed in 23/30 patients and in 16/30 patients, respectively. In 18/30 patients, a PSA decline to <0.2 ng/mL was observed. In 10/30 patients further PCa-specific treatment was given after a median (range) of 125 (48-454) days post-111 In-PSMA-RGS. The remaining 20 patients remained treatment-free at a median (range) follow-up of 337 (81-591) days. Of 30 patients, 10 presented with surgery-related complications (Clavien-Dindo grade 1, n = 6, Clavien-Dindo grade 3b, n = 4). CONCLUSION: 111 In-PSMA-RGS proved to be of high value for intra-operative detection of even small metastatic lesions in patients with PCa scheduled for salvage lymphadenectomy. It allows the exact localization and resection of metastatic tissue during 111 In-PSMA-RGS and is therefore anticipated to have a beneficial influence on further disease progression; however, identification of suitable patients on the basis of PSMA-positron-emission tomography imaging as well as clinical variables is essential for satisfactory results to be obtained.

Intrapatient Comparison of 111In-PSMA I&T SPECT/CT and Hybrid 68Ga-HBED-CC PSMA PET in Patients With Early Recurrent Prostate Cancer.

PURPOSE: The aim of this study was to evaluate the detection efficiency of In-PSMA-I&T SPECT/CT in comparison to hybrid Ga-PSMA HBED-CC PET in patients with early recurrent prostate cancer. METHODS: Twenty-two patients (mean age, 68.2 ± 6.8 years; range, 52-76 years) with rising prostate-specific antigen (PSA; median, 1.03 ng/mL; range, 0.2-7.2ng/mL) and known positive lesions in hybrid Ga-PSMA HBED-CC PET scheduled for salvage surgery were included. Whole-body scintigraphy and SPECT/CT were performed 4 hours after application of 147.0 ± 24.8 MBq (range, 90-183 MBq) In-PSMA I&T. Images were evaluated for suspected lesions, and conspicuity of all lesions was rated using a 4-point-scale (0 = not seen, 1 = retrospectively seen in knowledge of Ga-PSMA HBED-CC PET, 2 = low signal, 3 = high signal). Tumor-to-background ratios were determined for SPECT and PET and compared. Tumor-to-background ratio of SPECT was correlated with lesion size as well as patients' Gleason score and PSA level. RESULTS: In-PSMA I&T SPECT/CT detected 14 of 29 PET-positive lesions (48.3%) with no additional lesions identified with In-PSMA I&T SPECT/CT. There was a significant weak to moderate correlation of PSA level with tumor-to-background ratio of In-PSMA I&T SPECT/CT (correlation coefficient r = 0.6406; 95% confidence interval, 0.1667-0.8741; P = 0.0136). There was no significant difference (P > 0.05), but a weak trend toward a higher detectability in In-PSMA I&T SPECT/CT regarding lesion size and initial PSA level. CONCLUSIONS: In a preselected collective of recurrent prostate cancer patients with low PSA values, In-PSMA I&T SPECT/CT showed lower detection rates than hybrid Ga-HBED-CC PSMA PET. However, In-PSMA I&T SPECT/CT showed a patient based detection rate of 59%, making it a potentially valuable imaging tool where PET is not available apart from its proven value as a PSMA-targeted probe for radioguided surgery.

177Lu-Labeled Prostate-Specific Membrane Antigen Radioligand Therapy of Metastatic Castration-Resistant Prostate Cancer: Safety and Efficacy.

UNLABELLED: The objective of this study was to analyze the safety and efficacy of the (177)Lu-labeled DOTAGA-based prostate-specific membrane antigen (PSMA) ligand (177)Lu-DOTAGA-(I-y)fk(Sub-KuE) ((177)Lu-PSMA) in patients with metastatic castration-resistant prostate cancer (mCRPC). METHODS: Fifty-six mCRPC patients underwent PSMA radioligand therapy (RLT) with (177)Lu-PSMA. (68)Ga-PSMA-(N,N'-bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid) ((68)Ga-PSMA) PET/CT was used for patient selection and follow-up after PSMA RLT. Hematologic status, renal function, and serum prostate-specific antigen levels were documented before and after therapy. Dosimetry was performed in 30 patients. RESULTS: (177)Lu-PSMA demonstrated high absorbed tumor doses (median, 3.3 mGy/MBq) compared with the levels in normal organs. Parotid glands received higher doses (1.3 mGy/MBq) than kidneys (0.8 mGy/MBq). All patients tolerated the therapy without any acute adverse effects. Except for mild reversible xerostomia in 2 patients, no long-term side effects were observed. There was a small but statistically significant reduction in erythrocyte and leukocyte counts; only the reduction in erythrocyte counts decreased slightly below the reference range. No thrombocytopenia occurred. The severity of pain was significantly reduced in 2 of 6 patients (33.3%). A decrease in prostate-specific antigen levels was noted in 45 of 56 patients (80.4%). Of 25 patients monitored for at least 6 mo after 2 or more PSMA RLT cycles, a molecular response evaluation ((68)Ga-PSMA PET/CT) revealed partial remission in 14, stable disease in 2, and progressive disease in 9 patients. Contrast-enhanced CT revealed partial remission in 5, stable disease in 13, and progressive disease in 7 patients. The median progression-free survival was 13.7 mo, and the median overall survival was not reached during follow-up for 28 mo. CONCLUSION: PSMA RLT with (177)Lu-PSMA is feasible, safe, and effective in end-stage progressive mCRPC with appropriate selection and follow-up of patients by (68)Ga-PSMA PET/CT through application of the concept of theranostics.

First 18F-Labeled Pentixafor-Based Imaging Agent for PET Imaging of CXCR4 Expression In Vivo.

In vivo quantification of CXCR4 expression using [68Ga]pentixafor for positron emission tomography (PET) imaging has gained significant clinical interest as CXCR4 plays a fundamental role in oncology and possesses potential prognostic value when overexpressed. To combine the excellent CXCR4-targeting properties of pentixafor-based tracers with the favorable radionuclide properties of 18F for high-resolution PET imaging, we developed an Al18F-labeled 1,4,7-triazacyclononane-triacetic acid (NOTA) analog of pentixather. Al18F-labeling of NOTA-pentixather was performed in aqueous dimethyl sulfoxide (DMSO) at pH = 4 (105°C, 15 minutes). CXCR4 affinities were determined in competitive binding assays, and both biodistribution and small-animal PET studies were performed in Daudi lymphoma-bearing mice. Under non-optimized conditions, [18F]AlF-NOTA-pentixather was obtained in radiochemical yields of 45.5% ± 13.3% and specific activities of up to 24.8 GBq/µmol. Compared with [natGa]pentixafor, [natF]AlF-NOTA-pentixather showed 1.4-fold higher CXCR4 affinity. [18F]AlF-NOTA-pentixather displayed high and CXCR4-specific in vivo uptake in Daudi xenografts (13.9% ± 0.8% injected dose per gram [ID/g] at 1 hour post injection [p.i.]). Because of its enhanced lipophilicity (logP = -1.4), [18F]AlF-NOTA-pentixather showed increased accumulation in the gall bladder and intestines. However, tumor/background ratios of 7.0 ± 1.2, 2.0 ± 0.3, 2.2 ± 0.4, 16.5 ± 6.5, and 29.2 ± 4 for blood, liver, small intestine, gut, and muscle, respectively, allowed for high-contrast visualization of Daudi tumors using PET (1 hour p.i.). The relatively straightforward radiosynthesis and efficient CXCR4 targeting of [18F]AlF-NOTA-pentixather demonstrate the successful implementation of 18F-complexation chemistry and pentixather-based CXCR4 targeting. Upon pharmacokinetic optimization, this class of tracers holds great promise for future application in humans.

[(111)In]PSMA-I&T: expanding the spectrum of PSMA-I&T applications towards SPECT and radioguided surgery.

BACKGROUND: The relevance of prostate-specific membrane antigen (PSMA) targeting in the clinical management of prostate cancer (PCa) is continually increasing, entailing the development of PSMA-targeted molecular probes. Recently, a first PSMA-targeted theranostic concept has been successfully implemented by [(68)Ga/(177)Lu]PSMA-I&T. To further exploit the excellent PSMA-targeting characteristics and in vivo performance of the PSMA-I&T platform, [(111)In]PSMA-I&T was evaluated as a complementary probe for radioguided surgery and SPECT imaging. FINDINGS: Compared to [(68)Ga/(177)Lu]PSMA-I&T, [(111)In]PSMA-I&T showed unchangedly high PSMA-affinity and enhanced internalization into PSMA-expressing LNCaP PCa cells. Biodistribution studies in LNCaP xenograft-bearing mice (1 h p.i.) revealed slightly reduced background accumulation of [(111)In]PSMA-I&T compared to [(177)Lu]PSMA-I&T and identical tumor uptake of both compounds, leading to increased tumor/background ratios for [(111)In]PSMA-I&T. An exemplary patient with metastatic PCa underwent preoperative [(68)Ga]HBED-CC-PSMA PET/CT (1 h p.i.) and [(111)In]PSMA-I&T SPECT/CT (4 h p.i.), followed by prostatectomy and radioguided extended pelvic lymphadenectomy (24 h p.i.). In [(111)In]PSMA-I&T SPECT/CT, the previously identified PCa lesions ([(68)Ga]HBED-CC-PSMA PET/CT) showed high tracer accumulation and were also detectable using planar scintigraphy. The intraoperative use of a hand-held gamma probe allowed detection and resection of all [(111)In]PSMA-I&T-accumulating lesions. The presence of PSMA-positive tumor tissue in the resected specimens was confirmed histopathologically and via [(111)In]PSMA-I&T autoradiography. CONCLUSIONS: [(111)In]PSMA-I&T shows efficient PSMA targeting in vitro and in vivo, combined with low background accumulation. In an exemplary PCa patient, [(111)In]PSMA-I&T was successfully applied for preoperative SPECT/CT visualization and radioguided resection of PSMA-positive lesions, hinting towards a high value of [(111)In]PSMA-I&T as a complementary tool to [(68)Ga/(177)Lu]PSMA-I&T in the clinical management of prostate cancer.