RESUMO
In recent years, nuclear medicine has gained great interest, partly due to the success story of [177Lu]Lu-PSMA-617 (PluvictoTM). Still, in-depth preclinical characterization of radiopharmaceuticals mainly happens at centers that allow working with radioactive material. To support the development of novel radiopharmaceuticals, alternative non-radioactive characterization assays are highly desirable. The aim of this study was to demonstrate that inductively coupled plasma mass spectrometry (ICP-MS) associated with a chromatographic system can serve as a surrogate for the classical high-performance liquid chromatography (HPLC)-radiodetector combination for preclinical in vitro characterization of non-radioactive metal-labeled analogs of radiopharmaceuticals. In this proof-of-concept study, we demonstrate the applicability of HPLC-ICP-MS by assessing the stability of 175Lu- and natGa-labeled prostate-specific membrane antigen (PSMA)-targeting peptidomimetics, single domain antibody (sdAb) conjugates, and monoclonal antibody (mAb) conjugates. 175Lu-labeled DOTAGA-conjugated and natGa-labeled NODAGA-conjugated sdAbs and mAbs showed the highest stability with >90% still intact after 24 h. The peptidomime-tics [175Lu]Lu-PSMA-617 and [natGa]Ga-PSMA-11 showed identical in vitro serum stability as it was reported for their corresponding radioligands with >99% intact species after 24 h incubation in mouse serum, demonstrating the reliability of the method. Hence, the established HPLC-ICP-MS methods can support the development of novel radiopharmaceuticals in a classical pharmaceutical setting.
RESUMO
This study addresses the question whether inductively coupled plasma mass spectrometry (ICP-MS) can be used as a method for the in vitro and in vivo characterization of non-radioactive metal conjugates to predict the properties of analogous radiopharmaceuticals. In a "proof-of-concept" study, the prostate-specific membrane antigen (PSMA)-targeting [175Lu]Lu-PSMA-617 and [159Tb]Tb-PSMA-617 were compared with their respective radiolabeled analogues, [177Lu]Lu-PSMA-617 (PLUVICTO, Novartis) and [161Tb]Tb-PSMA-617. ICP-MS and conventional γ-counting of the cell samples revealed almost identical results (<6% absolute difference between the two technologies) for the in vitro uptake and internalization of the (radio)metal conjugates, irrespective of the employed methodology. In vivo, an equal uptake in PSMA-positive PC-3 PIP tumor xenografts was determined 1 h after the injection of [175Lu]Lu-/[177Lu]Lu-PSMA-617 (41 ± 6% ID/g and 44 ± 12% IA/g, respectively) and [159Tb]Tb-/[161Tb]Tb-PSMA-617 (44 ± 5% ID/g and 44 ± 5% IA/g, respectively). It was further revealed that it is crucial to use the same ratios of the (radio)metal-labeled and unlabeled ligands for both methodologies to obtain equal data in organs in which receptor saturation was reached such as the kidneys (12 ± 2% ID/g vs 10 ± 1% IA/g, 1 h after injection). The data of this study demonstrate that the use of high-sensitivity ICP-MS allows reliable and predictive quantification of compounds labeled with stable metal isotopes in cell and tissue samples obtained in preclinical studies. It can, hence, be employed as a valid alternative to the state-of-the-art γ-counting methodology to detect radioactive ligands.
