A comparison of adriamycin and mAMSA. II. Studies with V79 and human tumour multicellular spheroids

Drug and radiation resistance in spheroids: cell contact and kinetics

Cells from multicellular spheroids are often more resistant than monolayers to drugs and radiation. While explanations for resistance can be based on differences in cell cycle distribution, inability of the drug to penetrate the spheroid, or the presence of hypoxic cells, these mechanisms do not adequately explain resistance to all agents. Small spheroids (containing about 25-50 cells) exposed to ionizing radiation, hyperthermia, photodynamic therapy, or topoisomerase II inhibitors, are more resistant to killing than monolayers; the close three-dimensional contact in spheroids has been implicated in this resistance. Proposed mechanisms for the 'contact effect' include gap junctional 'reciprocity', cell shape mediated changes in (repair-related) gene expression, and alterations in chromatin packaging which influence DNA repair. The consequences of the contact effect are especially important for multifraction exposures. Another form of resistance can be demonstrated during repetitive treatments; 'regrowth resistance' reflects the capacity of spheroid cells to proliferate more efficiently to compensate for cell killing.

Flow cytometric analysis of intracellular hematoporphyrin derivative in human tumor cells and multicellular spheroids

Flow cytometry (FCM) has been used to investigate the intracellular fluorescence of hematoporphyrin derivative (HPD) in monolayer and spheroid cultures of WiDr cells. For exponentially-growing monolayer cultures mean cellular fluorescence was directly proportion to the external HPD levels in the range 5-100 micrograms ml-1 (r = 0.99). Heterogeneity of cellular fluorescence was quantified by determining the ratio of the fluorescence value below which were observed values for 98% of the cell population compared to the fluorescence value for 2%. In exponentially-growing cultures, decreasing levels of HPD in the medium led to an increase in the 98:2% ratio, i.e. an increase in heterogeneity of intracellular drug levels. The growth of cells as multicellular spheroids confers a spheroid-size-dependent resistance to photodynamic treatment. With increasing spheroid size (100, 250, 500, 750 and 1000 microns diam.) there was a decrease in mean intracellular HPD levels and a large linear increase in the 98:2% ratio (r = 0.94).

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.