Novel Biological and Molecular Characterization in Radiopharmaceutical Preclinical Design

Abstract

In this study, the potential of a digital autoradiography system equipped with a super resolution screen has been evaluated to investigate the biodistribution of a 18F-PSMA inhibitor in a prostate cancer mouse model. Twelve double xenograft NOD/SCID mice (LNCAP and PC3 tumours) were divided into three groups according to post-injection time points of an 18F-PSMA inhibitor. Groups of 4 mice were used to evaluate the biodistribution of the radiopharmaceutical after 30-, 60- and 120-min post-injection. Data here reported demonstrated that the digital autoradiography system is suitable to analyse the biodistribution of an 18F-PSMA inhibitor in both whole small-animal bodies and in single organs. The exposure of both whole mouse bodies and organs on the super resolution screen surface allowed the radioactivity of the PSMA inhibitor distributed in the tissues to be detected and quantified. Data obtained by using a digital autoradiography system were in line with the values detected by the activity calibrator. In addition, the image obtained from the super resolution screen allowed a perfect overlap with the tumour images achieved under the optical microscope. In conclusion, biodistribution studies performed by the autoradiography system allow the microscopical modifications induced by therapeutic radiopharmaceuticals to be studied by comparing the molecular imaging and histopathological data at the sub-cellular level.

Keywords: digital autoradiography system; nuclear imaging; pre-clinical model; radiopharmaceutical.

Figure 1

Representative scheme of the biodistribution study with the digital autoradiographic system. After the radionuclide injection (a) mice are used for both whole-body imaging (b) and single organ analysis (c) after autoptic examination (d). The exposed super resolution storage phosphor screen is then scanned at 150 DPI (170 mm pixel size) (e) to create a digitized image for analysis (f). Simultaneously, excised organs can be used to perform histological analysis (g).

Figure 2

Measurement by activity calibrator. (A) Graph shows the radioactivity value detected by the activity calibrator in terms of MBq in LNCAP and PC3 tumours after 30, 60 and 90 min. (B) Graph displays the radioactivity value detected by the activity calibrator in terms of MBq in LNCAP, PC3, bowel, heart, liver and kidney tumours after 30, 60 and 90 min.

Figure 3

Evaluation of radioactivity detection by a digital autoradiography system and histological analysis. (A) Graph shows the radioactivity value detected by a digital autoradiography system in terms of DLU in LNCAP and PC3 tumours after 30, 60 and 90 min. (B) Graph displays the radioactivity value detected by a digital autoradiography system in terms of DLU in LNCAP, PC3, bowel, heart, liver and kidney tumours after 30, 60 and 90 min. (C) Whole body and excised organs. (D) Autoradiographic image shows 18F-PSMA inhibitor uptake in both the whole body and excised organs. (E) Morphological and immunohistochemical images of LNCAP tumours reveal an association among 18F-PSMA inhibitor uptake, mitosis and ki67 expression.

Figure 4

Immunohistochemical investigation of xenograft tumours. (A) LNCAP xenograft tumour mass characterized by numerous PSMA-positive cells. (B) No/rare PSMA-positive cells in PC3 tumour mass. (C,D) Images show numerous Ki67 positive cells in both LNCAP (C) and PC3 (D) xenografts. (E) Moderate expression of vimentin in a LNCAP xenograft tumour. (F) Image displays numerous vimentin-positive cells in a PC3 xenograft. Scale bar 100 µm for all images.

Figure 5

Electron microscopy investigation of xenograft tumours. (A,B) LNCAP xenograft tumour mass shows a heterogenic cell population characterized by well-differentiated prostate cancer cells and some mesenchymal-like cells (asterisks). (C,D) Images show a PC3 xenograft tumour characterized by numerous mesenchymal-like cells (asterisks). Scale bars (A) 5 µm, (B) 10 µm, (C) 5 µm, (D) 5 µm.