Transfectant mosaic spheroids: a new model for evaluation of tumour cell killing in targeted radiotherapy and experimental gene therapy

Background: We describe an in vitro tumour model for targeted radiotherapy and gene therapy that incorporates cell population heterogeneity. Materials and methods: Transfectant mosaic spheroids (TMS) and transfected mosaic monolayers (TMM) are composed of two cell populations derived from a single cell line. The cells of one population were transfected with the noradrenaline transporter gene (NAT), allowing active uptake of a radiolabelled targeting agent meta-[¹³¹I]iodobenzylguanidine ([¹³¹I]MIBG); the other population of cells was derived from the same parent line and transfected with a marker gene - green fluorescent protein (GFP). After treatment With [¹³¹I]MIBG, cell kill was determined in TMM by clonogenic assay and in TMS by clonogenic assay and spheroid growth delay. Results: We have used the TMS model to assess the 'radiological bystander effect' (radiation cross-fire) conferred by the β-emitting radiopharmaceutical [¹³¹I] MIBG whose cellular uptake is facilitated by the transfected gene encoding NAT. We show that cell killing by [¹³¹I]MIBG in both TMS and TMM cultures increased in direct proportion to the fraction of NAT-transfected cells and that the degree of cell killing against fraction transfected was greater in TMS, suggestive of a greater bystander effect in the three-dimensional culture system. Conclusions: TMS provide a useful model for assessment of the effectiveness of targeted radiotherapy in combination with gene therapy when less than 100% of the target cell population is expressing the NAT transgene. Further, this novel model offers the unique opportunity to investigate radiation-induced bystander effects and their contribution to cell cytotoxicity in radiotherapy and other gene therapy applications.