Table 3. Interaction ratios in the clonogenic assay of drug and radiation combinations in HCT116 GFP cells cultured as spheroids for 7 days, then irradiated (6 Gy) 4–6 h after drug addition and 20 h later dissociated into single cells, transferred to 6-well plates and incubated for 10 days.

SIo/SIe	5-FU + 4 Gy	5-FU + 6 Gy	VLX600 + 4 Gy	VLX600 + 6 Gy
Highest conc.	0.57*	0.67	0.51*	0.30*
	0.61*	0.58	0.57	0.36*
	N/A	N/A	0.78	0.29*
	0.88	0.96	-	-
Lowest conc.	0.81	0.90	-	-

One-sample t-test. Mean interaction ratios (SI_o/SI_e) significantly different than 1.0 (two-tailed p-value <0.05) are shown with an asterisk. Mean interaction ratios (SI_o/SI_e) less than 1 indicates synergy and are shown in bold. Drug concentrations for 5-FU ranged 100–6.5 μM and for VLX600 6.5–1.5 μM with 2-fold dilution steps. Mean values are based on 2–3 independent experiments, with triplicate wells for each drug concentration. N/A, 5-FU at 25 μM not shown due to technical failure. -, not tested.