Search

Simple Summary: Prostate-specific membrane antigen (PSMA) radioligands have proven effective to treat patients with metastatic castration-resistant prostate cancer. Targeted α-therapy using actinium-225 has been used for patients with end-stage disease who no longer responded to βࢤ-therapy through the use of 177Lu-based radioligand therapy (RLT). In this study, we investigated and compared the therapeutic efficacy of [225Ac]Ac-SibuDAB and [225Ac]Ac-PSMA-617 and assessed potential undesired side effects in the preclinical setting. Due to a dedicated albumin-binding entity integrated into [225Ac]Ac-SibuDAB, this radioligand showed an enhanced blood circulation time and, hence, increased tumor uptake but also higher retention in normal tissues. The therapeutic efficacy of [225Ac]Ac-SibuDAB was enhanced as compared to that of [225Ac]Ac-PSMA-617, yet, undesired side effects were in the same range for both radioligands. Our data suggest that [225Ac]Ac-SibuDAB could be a powerful alternative to [225Ac]Ac-PSMA-617, however, the safe therapeutic window should be carefully defined in clinical dose escalation studies. In the present study, SibuDAB, an albumin-binding PSMA ligand, was investigated in combination with actinium-225 and the data were compared with those of [225Ac]Ac-PSMA-617. In vitro, [225Ac]Ac-SibuDAB and [225Ac]Ac-PSMA-617 showed similar tumor cell uptake and PSMA-binding affinities as their 177Lu-labeled counterparts. The in vitro binding to serum albumin in mouse and human blood plasma, respectively, was 2.8-fold and 1.4-fold increased for [225Ac]Ac-SibuDAB as compared to [177Lu]Lu-SibuDAB. In vivo, this characteristic was reflected by the longer retention of [225Ac]Ac-SibuDAB in the blood than previously seen for [177Lu]Lu-SibuDAB. Similar to [225Ac]Ac-PSMA-617, [225Ac]Ac-SibuDAB was well tolerated at 30 kBq per mouse. Differences in blood cell counts were observed between treated mice and untreated controls, but no major variations were observed between values obtained for [225Ac]Ac-SibuDAB and [225Ac]Ac-PSMA-617. [225Ac]Ac-SibuDAB was considerably more effective to treat PSMA-positive tumor xenografts than [225Ac]Ac-PSMA-617. Only 5 kBq per mouse were sufficient to eradicate the tumors, whereas tumor regrowth was observed for mice treated with 5 kBq [225Ac]Ac-PSMA-617 and only one out of six mice survived until the end of the study. The enhanced therapeutic efficacy of [225Ac]Ac-SibuDAB as compared to that of [225Ac]Ac-PSMA-617 and reasonable safety data qualify this novel radioligand as a candidate for targeted α-therapy of prostate cancer. [ABSTRACT FROM AUTHOR]

[177Lu]Lu-Ibu-DAB-PSMA was previously characterized with moderate albumin-binding properties enabling high tumor accumulation but reasonably low retention in the blood. The aim of this study was to investigate [177Lu]Lu-Ibu-DAB-PSMA in preclinical in vivo experiments and compare its therapeutic efficacy and potential undesired side effects with those of [177Lu]Lu-PSMA-617 and the previously developed [177Lu]Lu-PSMA-ALB-56. BALB/c nude mice without tumors were investigated on Day 10 and 28 after injection of 10 MBq radioligand. It was revealed that most plasma parameters were in the same range for all groups of mice and histopathological examinations of healthy tissue did not show any alternations in treated mice as compared to untreated controls. Based on these results, a therapy study over twelve weeks was conducted with PC-3 PIP tumor-bearing mice for comparison of the radioligands's therapeutic efficacy up to an activity of 10 MBq (1 nmol) per mouse. In agreement with the increased mean absorbed tumor dose, [177Lu]Lu-Ibu-DAB-PSMA (~ 6.6 Gy/MBq) was more effective to inhibit tumor growth than [177Lu]Lu-PSMA-617 (~ 4.5 Gy/MBq) and only moderately less potent than [177Lu]Lu-PSMA-ALB-56 (~ 8.1 Gy/MBq). As a result, the survival of mice treated with 2 MBq of an albumin-binding radioligand was significantly increased (p < 0.05) compared to that of mice injected with [177Lu]Lu-PSMA-617 or untreated controls. The majority of mice treated with 5 MBq or 10 MBq [177Lu]Lu-Ibu-DAB-PSMA or [177Lu]Lu-PSMA-ALB-56 were still alive at study end. Hemograms of immunocompetent mice injected with 30 MBq [177Lu]Lu-Ibu-DAB-PSMA or 30 MBq [177Lu]Lu-PSMA-617 showed values in the same range as untreated controls. This was, however, not the case for mice treated with [177Lu]Lu-PSMA-ALB-56 which revealed a drop in lymphocytes and hemoglobin at Day 10 and Day 28 after injection. The data of this study demonstrated a significant therapeutic advantage of [177Lu]Lu-Ibu-DAB-PSMA over [177Lu]Lu-PSMA-617 and a more favorable safety profile as compared to that of [177Lu]Lu-PSMA-ALB-56. Based on these results, [177Lu]Lu-Ibu-DAB-PSMA may has the potential for a clinical translation. [ABSTRACT FROM AUTHOR]

The favorable decay characteristics of 161 Tb attracted the interest of clinicians in using this novel radionuclide for radioligand therapy (RLT). 161 Tb decays with a similar half-life to 177 Lu, but beyond the emission of β - -particles and γ-rays, 161 Tb also emits conversion and Auger electrons, which may be particularly effective to eliminate micrometastases. The aim of this study was to compare the dosimetry and therapeutic efficacy of 161 Tb and 177 Lu in tumor-bearing mice using SibuDAB and PSMA-I&T, which differ in their blood residence time and tumor uptake. Methods: [ 161 Tb]Tb-SibuDAB and [ 161 Tb]Tb-PSMA-I&T were evaluated in vitro and investigated in biodistribution, imaging, and therapy studies using PC-3 PIP tumor-bearing mice. The 177 Lu-labeled counterparts served for dose calculations and comparison of therapeutic efficacy. The tolerability of RLT in mice was monitored on the basis of body mass, blood plasma parameters, blood cell counts, and the histology of relevant organs and tissues. Results: The prostate-specific membrane antigen (PSMA)-targeting radioligands, irrespective of whether labeled with 161 Tb or 177 Lu, showed similar in vitro data and comparable tissue distribution profiles. As a result of the albumin-binding properties, [ 161 Tb]Tb/[ 177 Lu]Lu-SibuDAB had an enhanced blood residence time and higher tumor uptake (62%-69% injected activity per gram at 24 h after injection) than [ 161 Tb]Tb/[ 177 Lu]Lu-PSMA-I&T (30%-35% injected activity per gram at 24 h after injection). [ 161 Tb]Tb-SibuDAB inhibited tumor growth more effectively than [ 161 Tb]Tb-PSMA-I&T, as can be ascribed to its 4-fold increased absorbed tumor dose. At any of the applied activities, the 161 Tb-based radioligands were therapeutically more effective than their 177 Lu-labeled counterparts, as agreed with the approximately 40% increased tumor dose of 161 Tb compared with that of 177 Lu. Under the given experimental conditions, no obvious adverse events were observed. Conclusion: The data of this study indicate the promising potential of 161 Tb in combination with SibuDAB for RLT of prostate cancer. Future clinical studies using 161 Tb-based RLT will shed light on a potential clinical benefit of 161 Tb over 177 Lu., (© 2023 by the Society of Nuclear Medicine and Molecular Imaging.)

[ 177 Lu]Lu-Ibu-DAB-PSMA, a radioligand modified with ibuprofen as the albumin binder, showed higher accumulation in PSMA-positive tumors of mice than the clinically used [ 177 Lu]Lu-PSMA-617 but lower retention in non-targeted tissues than previously developed albumin-binding PSMA radioligands. The aim of this study was to investigate whether the stereochemistry of the incorporated ibuprofen affects the radioligand's in vitro and in vivo properties and to select the more favorable radioligand for further development. For this purpose, SibuDAB and RibuDAB containing ( S )- and ( R )-ibuprofen, respectively, were synthesized and labeled with lutetium-177. In vitro , the two isomers had similar properties; however, [ 177 Lu]Lu-SibuDAB showed increased binding to mouse and human plasma proteins (91 ± 1 and 88 ± 2%, respectively) compared to [ 177 Lu]Lu-RibuDAB (75 ± 2 and 79 ± 2%, respectively). In vivo , [ 177 Lu]Lu-SibuDAB was metabolically more stable than [ 177 Lu]Lu-RibuDAB with ∼90 vs ∼67% intact radioligand detected in the blood at 4 h post injection (p.i.). In line with the lower albumin-binding affinity, the blood clearance of [ 177 Lu]Lu-RibuDAB in mice was considerably faster [27% of injected activity (% IA), 1 h p.i.] than for [ 177 Lu]Lu-SibuDAB (50% IA, 1 h p.i.). Time-dependent biodistribution studies performed in tumor-bearing athymic nude mice showed high PSMA-specific tumor uptake for both isomers. A twofold increased area under the curve (AUC 0→8d ) of the blood retention was determined for [ 177 Lu]Lu-SibuDAB as compared to [ 177 Lu]Lu-RibuDAB, whereas the kidney AUC 0→8d value of [ 177 Lu]Lu-SibuDAB was only half as high as for [ 177 Lu]Lu-RibuDAB. As a result, a more favorable tumor-to-kidney AUC 0→8d ratio was obtained for [ 177 Lu]Lu-SibuDAB, which was also visualized on SPECT/CT images. Based on its improved kidney clearance and higher metabolic stability, [ 177 Lu]Lu-SibuDAB was selected as the more favorable radioligand. Therapy studies performed with [ 177 Lu]Lu-SibuDAB (5 MBq/mouse) demonstrated the anticipated therapeutic superiority over the current gold-standard [ 177 Lu]Lu-PSMA-617 (5 MBq/mouse). The significantly increased survival time of mice treated with [ 177 Lu]Lu-SibuDAB as compared to those injected with [ 177 Lu]Lu-PSMA-617 justifies further development of this novel radioligand toward clinical application.