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Despite many advances in the past years, the treatment of metastatic prostate cancer still remains challenging. In recent years, prostate-specific membrane antigen (PSMA) inhibitors were intensively studied to develop low-molecular-weight ligands for imaging prostate cancer lesions by PET or SPECT. However, the endoradiotherapeutic use of these compounds requires optimization with regard to the radionuclide-chelating agent and the linker moiety between chelator and pharmacophore, which influence the overall pharmacokinetic properties of the resulting radioligand. In an effort to realize both detection and optimal treatment of prostate cancer, a tailor-made novel naphthyl-containing DOTA-conjugated PSMA inhibitor has been developed. Methods: The peptidomimetic structure was synthesized by solid-phase peptide chemistry and characterized using reversedphase high-performance liquid chromatography and matrix-assisted laser desorption/ionization mass spectrometry. Subsequent 67/68 Ga and 177 Lu labeling resulted in radiochemical yields of greater than 97% or greater than 99%, respectively. Competitive binding and internalization experiments were performed using the PSMA-positive LNCaP cell line. The in vivo biodistribution and dynamic small-animal PET imaging studies were investigated in BALB/c nu/nu mice bearing LNCaP xenografts. Results: The chemically modified PSMA inhibitor PSMA-617 demonstrated high radiolytic stability for at least 72 h. A high inhibition potency (equilibrium dissociation constant [K i ] 5 2.34 ± 2.94 nM on LNCaP; K i 5 0.37 ± 0.21 nM enzymatically determined) and highly efficient internalization into LNCaP cells were demonstrated. The small-animal PET measurements showed high tumor-tobackground contrasts as early as 1 h after injection. Organ distribution revealed specific uptake in LNCaP tumors and in the kidneys 1 h after injection. With regard to therapeutic use, the compound exhibited a rapid clearance from the kidneys from 113.3 ± 24.4 at 1 h to 2.13 ± 1.36 percentage injected dose per gram at 24 h. The favorable pharmacokinetics of the molecule led to tumor-to-background ratios of 1,058 (tumor to blood) and 529 (tumor to muscle), respectively, 24 h after injection. Conclusion: The tailor-made DOTA-conjugated PSMA inhibitor PSMA-617 presented here is sustainably refined and advanced with respect to its tumor-targeting and pharmacokinetic properties by systematic chemical modification of the linker region. Therefore, this radiotracer is suitable for a first-in-human theranostic application and may help to improve the clinical management of prostate cancer in the future.

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In recent years, several radiotracers targeting the prostate-specific membrane antigen (PSMA) have been introduced. Some of them have had a high clinical impact on the treatment of patients with prostate cancer. However, the number of 18 F-labeled tracers addressing PSMA is still limited. Therefore, we aimed to develop a radiofluorinated molecule resembling the structure of therapeutic PSMA-617. Methods: The nonradioactive reference compound PSMA-1007 and the precursor were produced by solid-phase chemistry. The radioligand 18 F-PSMA-1007 was produced by a 2-step procedure with the prosthetic group 6-18 F-fluoronicotinic acid 2,3,5,6-tetrafluorophenyl ester. The binding affinity of the ligand for PSMA and its internalization properties were evaluated in vitro with PSMA-positive LNCaP (lymph node carcinoma of the prostate) cells. Further, organ distribution studies were performed with mice bearing LNCaP and PC-3 (prostate cancer cell line; PSMA-negative) tumors. Finally, small-animal PET imaging of an LNCaP tumorbearing mouse was performed. Results: The identified ligand had a binding affinity of 6.7 6 1.7 nM for PSMA and an exceptionally high internalization ratio (67% 6 13%) in vitro. In organ distribution studies, high and specific tumor uptake (8.0 6 2.4 percentage injected dose per gram) in LNCaP tumor-bearing mice was observed. In the small-animal PET experiments, LNCaP tumors were clearly visualized. Conclusion: The radiofluorinated PSMA ligand showed promising characteristics in its preclinical evaluation, and the feasibility of prostate cancer imaging was demonstrated by small-animal PET studies. Therefore, we recommend clinical transfer of the radioligand 18 F-PSMA-1007 for use as a diagnostic PET tracer in prestaging and monitoring of prostate cancer.

BackgroundThe targeting of the prostate-specific membrane antigen (PSMA) is of particular interest for radiotheragnostic purposes of prostate cancer. Radiolabeled PSMA-617, a 1,4,7,10-tetraazacyclododecane-N,N′,N′′,N′′′-tetraacetic acid (DOTA)-functionalized PSMA ligand, revealed favorable kinetics with high tumor uptake, enabling its successful application for PET imaging (68Ga) and radionuclide therapy (177Lu) in the clinics. In this study, PSMA-617 was labeled with cyclotron-produced 44Sc (T 1/2 = 4.04 h) and investigated preclinically for its use as a diagnostic match to 177Lu-PSMA-617.Results 44Sc was produced at the research cyclotron at PSI by irradiation of enriched 44Ca targets, followed by chromatographic separation. 44Sc-PSMA-617 was prepared under standard labeling conditions at elevated temperature resulting in a radiochemical purity of >97% at a specific activity of up to 10 MBq/nmol. 44Sc-PSMA-617 was evaluated in vitro and compared to the 177Lu- and 68Ga-labeled match, as well as 68Ga-PSMA-11 using PSMA-positive PC-3 PIP and PSMA-negative PC-3 flu prostate cancer cells. In these experiments it revealed similar in vitro properties to that of 177Lu- and 68Ga-labeled PSMA-617. Moreover, 44Sc-PSMA-617 bound specifically to PSMA-expressing PC-3 PIP tumor cells, while unspecific binding to PC-3 flu cells was not observed. The radioligands were investigated with regard to their in vivo properties in PC-3 PIP/flu tumor-bearing mice. 44Sc-PSMA-617 showed high tumor uptake and a fast renal excretion. The overall tissue distribution of 44Sc-PSMA-617 resembled that of 177Lu-PSMA-617 most closely, while the 68Ga-labeled ligands, in particular 68Ga-PSMA-11, showed different distribution kinetics. 44Sc-PSMA-617 enabled distinct visualization of PC-3 PIP tumor xenografts shortly after injection, with increasing tumor-to-background contrast over time while unspecific uptake in the PC-3 flu tumors was not observed.ConclusionsThe in vitro characteristics and in vivo kinetics of 44Sc-PSMA-617 were more similar to 177Lu-PSMA-617 than to 68Ga-PSMA-617 and 68Ga-PSMA-11. Due to the almost four-fold longer half-life of 44Sc as compared to 68Ga, a centralized production of 44Sc-PSMA-617 and transport to satellite PET centers would be feasible. These features make 44Sc-PSMA-617 particularly appealing for clinical application.Electronic supplementary materialThe online version of this article (doi:10.1186/s13550-017-0257-4) contains supplementary material, which is available to authorized users.

The treatment of metastatic castration-resistant prostate cancer (mCRPC) remains challenging with current treatment options. The development of more effective therapies is, therefore, urgently needed. Targeted radionuclide therapy with prostate-specific membrane antigen (PSMA)-targeting ligands has revealed promising clinical results. In an effort to optimize this concept, it was the aim of this study to design and investigate PSMA ligands comprising different types of albumin binders. PSMA-ALB-53 and PSMA-ALB-56 were designed by combining the glutamate-urea-based PSMA-binding entity, a DOTA chelator and an albumin binder based on the 4-( p-iodophenyl)-moiety or p-(tolyl)-moiety. The compounds were labeled with Lu (50 MBq/nmol) resulting in radioligands of high radiochemical purity (≥98%). Both radioligands were stable (≥98%) over 24 h in the presence of l-ascorbic acid. The uptake into PSMA-positive PC-3 PIP tumor cells in vitro was in the same range (54-58%) for both radioligands; however,Lu-PSMA-ALB-53 showed a 15-fold enhanced binding to human plasma proteins. Biodistribution studies performed in PC-3 PIP/flu tumor-bearing mice revealed high tumor uptake of Lu-PSMA-ALB-53 andLu-PSMA-ALB-56, respectively, demonstrated by equal areas under the curves (AUCs) for both radioligands. The increased retention of Lu-PSMA-ALB-53 in the blood resulted in almost 5-fold lower tumor-to-blood AUC ratios when compared toLu-PSMA-ALB-56. Kidney clearance of Lu-PSMA-ALB-56 was faster, and hence, the tumor-to-kidney AUC ratio was 3-fold higher than in the case ofLu-PSMA-ALB-53. Due to the more favorable tissue distribution profile, Lu-PSMA-ALB-56 was selected for a preclinical therapy study in PC-3 PIP tumor-bearing mice. The tumor growth delay after application ofLu-PSMA-ALB-56 and Lu-PSMA-617 applied at the same activities (2 or 5 MBq per mouse) revealed better antitumor effects in the case ofLu-PSMA-ALB-56. As a consequence, the survival of mice treated with Lu-PSMA-ALB-56 was prolonged when compared to the mice, which received the same activity ofLu-PSMA-617. Our results demonstrated the superiority of Lu-PSMA-ALB-56 overLu-PSMA-ALB-53 indicating that the p-(tolyl)-moiety was more suited as an albumin binder to optimize the tissue distribution profile. Lu-PSMA-ALB-56 was more effective to treat tumors thanLu-PSMA-617 resulting in complete tumor remission in four out of six mice. This promising results warrant further investigations to assess the potential for clinical application of Lu-PSMA-ALB-56.

Since prostate-specific membrane antigen (PSMA) is up-regulated in nearly all stages of prostate cancer (PCa), PSMA can be considered as a viable diagnostic biomarker and treatment target in PCa. This project is focused on the development and evaluation of a series of compounds directed against PSMA. The modifications to the linker are designed to improve the binding potential and pharmacokinetics for theranostic application. In addition, the results help to further elucidate the structure-activity relationships (SAR) of the resulting PSMA inhibitors. Both in vitro and in vivo experiments of 18 synthesized PSMA inhibitor variants showed that systematic chemical modification of the linker has a significant impact on the tumor-targeting and pharmacokinetic properties. This approach can lead to an improved management of patients suffering from recurrent prostate cancer by the use of one radiolabeling precursor, which can be radiolabeled either with (68)Ga for diagnosis or with (177)Lu or (225)Ac for therapy.

The prostate-specific membrane antigen (PSMA) has emerged as an attractive prostate cancer associated target for radiotheragnostic application using PSMA-specific radioligands. The aim of this study was to design new PSMA ligands modified with an albumin-binding moiety in order to optimize their tissue distribution profile. The compounds were prepared by conjugation of a urea-based PSMA-binding entity, a DOTA chelator, and 4-( p-iodophenyl)butyric acid using multistep solid phase synthesis. The three ligands (PSMA-ALB-02, PSMA-ALB-05, and PSMA-ALB-07) were designed with varying linker entities. Radiolabeling with Lu was performed at a specific activity of up to 50 MBq/nmol resulting in radioligands of>98% radiochemical purity and high stability. In vitro investigations revealed high binding of all three PSMA radioligands to mouse (>64%) and human plasma proteins (>94%). Uptake and internalization into PSMA-positive PC-3 PIP tumor cells was equally high for all radioligands. Negligible accumulation was found in PSMA-negative PC-3 flu cells, indicating PSMA-specific binding of all radioligands. Biodistribution and imaging studies performed in PC-3 PIP/flu tumor-bearing mice showed enhanced blood circulation of the new radioligands when compared to the clinically employed Lu-PSMA-617. The PC-3 PIP tumor uptake of all three radioligands was very high (76.4 ± 2.5% IA/g, 79.4 ± 11.1% IA/g, and 84.6 ± 14.2% IA/g, respectively) at 24 h post injection (p.i.) resulting in tumor-to-blood ratios of ∼176, ∼48, and ∼107, respectively, whereas uptake into PC-3 flu tumors was negligible. Kidney uptake at 24 h p.i. was lowest forLu-PSMA-ALB-02 (10.7 ± 0.92% IA/g), while Lu-PSMA-ALB-05 andLu-PSMA-ALB-07 showed higher renal retention (23.9 ± 4.02% IA/g and 51.9 ± 6.34% IA/g, respectively). Tumor-to-background ratios calculated from values of the area under the curve (AUC) of time-dependent biodistribution data were in favor of Lu-PSMA-ALB-02 (tumor-to-blood, 46; tumor-to-kidney, 5.9) when compared toLu-PSMA-ALB-05 (17 and 3.7, respectively) and Lu-PSMA-ALB-07 (39 and 2.1, respectively). The high accumulation of the radioligands in PC-3 PIP tumors was visualized on SPECT/CT images demonstrating increasing tumor-to-kidney ratios over time. Taking all of the characteristics into account,Lu-PSMA-ALB-02 emerged as the most promising candidate. The applied concept may be attractive for future clinical translation potentially enabling more potent and convenient prostate cancer radionuclide therapy.