Size-dependent resistance of human tumour spheroids to photodynamic treatment

Spheroids derived from the human colon adenocarcinoma cell line, WiDr, were exposed to 10 micrograms ml-1 Photofrin II and irradiated with light (700 nm, 50 mW cm-2). Compared with exponentially growing monolayer cultures, cells in spheroids of 100, 250 and 500 microns diameter were respectively 1.8, 2.5 and 22-fold less sensitive. The small resistance of plateau-phase cultures (1.3-fold) was insufficient to account for this marked spheroid size-dependent resistance. For monolayer cultures and for spheroids of 100 and 250 microns diameter, the results were the same whether irradiations were carried out pre- or post-trypsinisation. However, there was a difference for the largest spheroid size: when irradiations were carried out pre-trypsinisation, spheroids were more resistant than when irradiations were given post-trypsinisation. Drug extraction studies showed that there was no difference in the average drug uptake between cultures of exponentially growing or plateau-phase cells, and 100 microns diameter spheroids while 250 and 500 microns diameter spheroids took up proportionally 0.5 and 0.4 as much drug. Cell contact effects, drug heterogeneity between cells, hypoxia and problems in drug penetration are suggested as possible reasons for the resistance of large spheroids to photodynamic treatment.