Inhibiting ABCG2 could potentially enhance the efficacy of hypericin-mediated photodynamic therapy in spheroidal cell models of colorectal cancer

•3D spheroids are more resistant to Hypericin-PDT than 2D cell models.•ABCG2 is upregulated in 3D spheroids as compared to 2D cell models.•Inhibiting ABCG2 could potentially improve response to Hypericin-PDT. Photodynamic Therapy (PDT) is an attractive modality for treating solid cancers. This study evaluates the efficacy of Hypericin-PDT as a cytotoxic therapy in colorectal cancer (CRC), using 2D cell cultures and 3D multicellular tumour spheroids. Spheroids were generated through forced-floating and agitation-based techniques. 2D and spheroid models of HT29 and HCT116 CRC cells were incubated with Hypericin (0–200 nM) for 16 h. Cultures were irradiated with light (1 J/cm2) and cytotoxicity assessed using Propidium Iodide fluorescence. Expression of ABCG2 protein was assessed by immunoassays in 2D and spheroid cultures. The effect of ABCG2 inhibition, using 10 μM Ko143, on cytotoxicity following Hypericin-PDT was evaluated. Hypericin-PDT produced a significant reduction in HT29 (p