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      The Future of PSMA-Targeted Radionuclide Therapy: An Overview of Recent Preclinical Research

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          Abstract

          Prostate specific membrane antigen (PSMA) has become a major focus point in the research and development of prostate cancer (PCa) imaging and therapeutic strategies using radiolabeled tracers. PSMA has shown to be an excellent target for PCa theranostics because of its high expression on the membrane of PCa cells and the increase in expression during disease progression. Therefore, numerous PSMA-targeting tracers have been developed and (pre)clinically studied with promising results. However, many of these PSMA-targeting tracers show uptake in healthy organs such as the salivary glands, causing radiotoxicity. Furthermore, not all patients respond to PSMA-targeted radionuclide therapy (TRT). This created the necessity of additional preclinical research studies in which existing tracers are reevaluated and new tracers are developed in order to improve PSMA-TRT by protecting the (PSMA-expressing) healthy organs and improving tumor uptake. In this review we will give an overview of the recent preclinical research projects regarding PCa-TRT using PSMA-specific radiotracers, which will give an indication of where the PSMA-TRT research movement is going and what we can expect in future clinical trials.

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          German Multicenter Study Investigating 177Lu-PSMA-617 Radioligand Therapy in Advanced Prostate Cancer Patients.

          (177)Lu-labeled PSMA-617 is a promising new therapeutic agent for radioligand therapy (RLT) of patients with metastatic castration-resistant prostate cancer (mCRPC). Initiated by the German Society of Nuclear Medicine, a retrospective multicenter data analysis was started in 2015 to evaluate efficacy and safety of (177)Lu-PSMA-617 in a large cohort of patients.
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            Targeted α-Therapy of Metastatic Castration-Resistant Prostate Cancer with225Ac-PSMA-617: Swimmer-Plot Analysis Suggests Efficacy Regarding Duration of Tumor Control

            The aim of this evaluation was to identify the first indicators of efficacy for 225Ac-labeled prostate-specific membrane antigen (PSMA)-617 therapy in a retrospectively analyzed group of patients. Methods: Forty patients with metastatic castration-resistant prostate cancer were selected for treatment with three 100 kBq/kg cycles of 225Ac-PSMA-617 at 2-mo intervals. Prostate-specific antigen (PSA) and blood cell count were measured every 4 wk. PSMA PET/CT or PSMA SPECT/CT were used for baseline staging and imaging follow-up at month 6. Follow-up included the duration of PSA response and radiologic progression-free survival at month 6. Patient histories were reviewed for the duration of previous treatment lines, and a swimmer plot was used to intraindividually compare the duration of tumor control by PSMA therapy versus prior treatment modalities. Results: Thirty-one of 40 patients were treated per protocol. Five patients discontinued treatment because of nonresponse, and 4 because of xerostomia. Of the 38 patients surviving at least 8 wk, 24 (63%) had a PSA decline of more than 50%, and 33 (87%) had a PSA response of any degree. The median duration of tumor control under 225Ac-PSMA-617 last-line therapy was 9.0 mo; 5 patients had an enduring response of more than 2 y. Because all patients had advanced disease, this result compares favorably with the tumor control rates associated with earlier-phase disease; the most common preceding first-, second-, third-, and fourth-line therapies were abiraterone (median duration 10.0 mo), docetaxel (6.5 mo), enzalutamide (6.5 mo), and cabazitaxel (6.0 mo), respectively. Conclusion: A positive response for surrogate parameters demonstrates remarkable antitumor activity for 225Ac-PSMA-617. Swimmer-plot analysis indicates a promising duration of tumor control, especially considering the unfavorable prognostic profile of the selected advanced-stage patients. Xerostomia was the main reason patients discontinued therapy or refused additional administrations and was in the same dimension as nonresponse; this finding indicates that further modifications of the treatment regimen with regard to side effects might be necessary to further enhance the therapeutic range.
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              Prostate specific membrane antigen expression in prostatic intraepithelial neoplasia and adenocarcinoma: a study of 184 cases.

              Prostate specific membrane antigen (PSM) is a membrane-bound antigen that is highly specific for benign and malignant prostate epithelial cells. Its expression in high grade prostatic intraepithelial neoplasia (PIN) has not been compared with that in prostate carcinoma. The authors performed an immunohistochemical study of representative sections from 184 radical prostatectomies from previously untreated patients with pathologic stage T2N0M0 adenocarcinoma treated at the Mayo Clinic between 1987 and 1991. Affinity-purified monoclonal antibody 7E11-5.3 directed against PSM was employed at a concentration of 20 microg/mL overnight. For comparison, serial sections in each case were stained with prostate specific antigen (PSA). Staining for all antibodies was performed using the streptavidin-biotin method. For each case, the percentage of immunoreactive cells in benign epithelium, PIN, and adenocarcinoma was estimated in increments of 10%. Cox proportional hazards models were used to identify the risk of carcinoma recurrence according to the number of immunoreactive PIN or cancer cells for PSM and PSA; the date of radical prostatectomy was used as the starting time, and serum PSA (biochemical) failure or clinical failure was the event. PSA biochemical failure was defined as serum PSA > 0.2 ng/mL at least 30 days after surgery. Intense cytoplasmic immunoreactivity for PSM was observed in the benign and neoplastic epithelial cells in all cases (100% of cases staining). The number of cells staining was lower in benign epithelium and PIN than in adenocarcinoma (69.5+/-17.3% [range, 20-90%] vs. 77.9+/-13.2% [range, 30-100%] vs. 80.2+/-13.7% [range, 30-100%], respectively). With rare exceptions, basal cells were negative, and there was no immunoreactivity of the prostate stroma, urothelium, or vasculature. Adenocarcinoma gave the most intense and extensive staining, and the highest grades of adenocarcinoma (Gleason primary patterns 4 and 5) showed staining in virtually every cell; there was greater heterogeneity of staining in lower grades of adenocarcinoma. By contrast, PSA immunoreactivity was more intense and extensive in benign epithelium than in PIN and adenocarcinoma. The number of immunoreactive PIN or cancer cells for PSM and PSA was not predictive of PSA biochemical or clinical failure as defined in this study. PSM was expressed in all cases of prostate adenocarcinoma, with the greatest extent and intensity observed in the highest grades. The expression increased incrementally from benign epithelium to high grade PIN or adenocarcinoma. Conversely, PSA showed the greatest staining in benign epithelium, with decreased expression incrementally from benign epithelium to high grade PIN or adenocarcinoma. Expression of PSM is clinically useful for the identification of prostate epithelium, particularly PIN or adenocarcinoma, and its expression is regulated independent of PSA. The number of PSM immunoreactive cells was not predictive of recurrence, most likely because of the presence of abundant immunoreactivity in most cases, or because of differential expression in primary and metastatic disease.
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                Author and article information

                Journal
                Pharmaceutics
                Pharmaceutics
                pharmaceutics
                Pharmaceutics
                MDPI
                1999-4923
                29 October 2019
                November 2019
                : 11
                : 11
                : 560
                Affiliations
                [1 ]Dept. of Radiology and Nuclear Medicine, Erasmus MC, 3015 GD Rotterdam, The Netherlands
                [2 ]Dept. of Experimental Urology, Erasmus MC, 3015 GD Rotterdam, The Netherlands
                [3 ]Dept. of Molecular Genetics, Erasmus MC, 3015 GD Rotterdam, The Netherlands
                [4 ]Oncode Institute, Erasmus MC, 3015 GD Rotterdam, The Netherlands
                Author notes
                [* ]Correspondence: e.ruigrok@ 123456erasmusmc.nl ; Tel.: +31-10-704-4971
                [†]

                Authors equally contributed to the work.

                Author information
                https://orcid.org/0000-0003-0324-4804
                Article
                pharmaceutics-11-00560
                10.3390/pharmaceutics11110560
                6921028
                31671763
                b2788073-d33e-44e4-9b07-aaebddd62f18
                © 2019 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 27 September 2019
                : 24 October 2019
                Categories
                Review

                prostate specific membrane antigen,prostate cancer,targeted radionuclide therapy

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