Using advanced cell models for targeted radionuclide therapy evaluation: increased efficacy in 3D versus 2D

Preclinically, radiopharmaceuticals are currently mainly evaluated in two-dimensional (2D) cell models, which lack clinical features that are relevant for accurate evaluation of targeted radionuclide therapy (TRT) responses. The development of three-dimensional (3D) cell models in the last years offers opportunities to overcome at least parts of the limitations of 2D cell models, but such 3D cell models are currently only rarely used in nuclear medicine research. Moreover, the comparison between 2D and 3D cell models, aimed at demonstrating the potential added value of 3D cell models, is even more scarce. To fill this gap and promote the use of more clinically relevant 3D cell models in nuclear medicine research, we performed such a comparative study. For this, we developed and evaluated 3D cell models derived from the prostate-specific membrane antigen (PSMA)-expressing human cancer cell lines LNCaP and PC3-PIP, by culturing these cells in anti-adhesive round bottom plates (“bio-spheroids”) or by culturing LNCaP cells in Matrigel (MG) or Noviogel-P5K (NG) domes (”MG-spheroids” or “NG-spheroids”). Hereafter, PSMA expression levels and [111In]In-PSMA-I&T uptake were determined and compared between 3D and 2D cell models. Additionally, we assessed cell viability of 3D- versus 2D-cultured cells after external beam radiation therapy (EBRT) and PSMA-TRT using [177Lu]Lu-PSMA-I&T. No significant differences in viability were observed between bio-spheroids versus 2D cell models after EBRT and PSMA-TRT, neither in LNCaP nor in PC3-PIP cells. In contrast, LNCaP MG-spheroids had a significantly better response to PSMA-TRT in comparison to the 2D-cultured cells. This was despite a lower PSMA expression level and lower [111In]In-PSMA-I&T uptake in the MG-spheroids. Importantly, no significant difference in radiosensitivity was observed between these MG-spheroids and the 2D cell model. Despite lower PSMA expression levels and lower radiopharmaceutical uptake in LNCaP MG-spheroids, and albeit similar radiosensitivity, PSMA-TRT induced a stronger reduction in viability in the MG-spheroids in comparison to the 2D-cultured cells. In contrast, no differences were observed in PSMA-TRT efficacy between bio-spheroids and the 2D cell model. The aforementioned leads to the hypothesis that biological factors important for TRT, e.g. cross-radiation and radiopharmaceutical retention within the 3D cell structure, are better represented in MG-spheroids where cell-cell interactions are already formed prior to radiopharmaceutical incubation.