TABLE 1.

IC₅₀ values, relative resistance, and reversal of resistance of HEK-293 parental cells and HEK-ABCG2 cells towards curcumin, gA, ouabain, and mitoxantrone

Compound	IC50		p value of IC50 from HEK-ABCG2 cells relative to IC50 from control cellsa	Relative resistanceb	Sensitization/reversal of resistancec
	HEK-293 control	HEK-ABCG2
Curcumin aloned	5.0 ± 0.6 μm	6.0 ± 0.9 μm	0.2	1.2 ± 0.2-fold
Curcumin + 10 μm FTC	5.1 ± 0.6 μm	6.3 ± 0.7 μm	0.1	1.2 ± 0.2-fold	1.0 ± 0.2-fold
Gramicidin alone	100 ± 22 nm	138 ± 16 nm	0.1	1.4 ± 0.3-fold
Gramicidin + 2 μm curcumin	96 ± 10 nm	26 ± 4 nm	1E-3	0.3 ± 0.1-fold	5.0 ± 1.5-fold
Gramicidin + 2 μm curcumin + 10 μm FTC	102 ± 16 nm	96 ± 15 nm	0.7	1.0 ± 0.2-fold	1.4 ± 0.5-fold
Ouabain alone	14 ± 2 nm	14 ± 3 nm	1.0	1.0 ± 0.3-fold
Ouabain + 2 μm curcumin	14 ± 2 nm	6 ± 1 nm	1E-2	0.4 ± 0.1-fold	2.5 ± 0.9-fold
Ouabain + 2 μm curcumin + 10 μm FTC	15 ± 3 nm	16 ± 3 nm	0.7	1.1 ± 0.3-fold	1.0 ± 0.2-fold
Mitoxantrone alone	5.6 ± 0.8 nm	206 ± 12 nm	1E-4	37 ± 5.6-fold
Mitoxantrone + 2 μm curcimun	5.1 ± 0.9 nm	32 ± 3 nm	4E-3	6.2 ± 1.2-fold	6.0 ± 2-fold
Mitoxantrone + 35 nm gramicidin	5.2 ± 0.6 nm	170 ± 4 nm	1E-4	33 ± 4.6-fold	1.1 ± 0.2-fold
Mitoxantrone + 7.5 nm ouabain	5.3 ± 0.8 nm	187 ± 7 nm	1E-4	35 ± 5.5-fold	1.0 ± 0.2-fold

^a To determine p values, we used a two-tailed Student's t test based on log(IC₅₀) values. We considered p values ≤ 0.05 as statistically significant.

^b Expressed as the ratio between the IC₅₀ value of ABCG2-expressing cells and the IC₅₀ value of control cells. This ratio is the inverse of the selectivity ratio, which is defined as IC₅₀(parental cells)/IC₅₀(MDR cells) (18).

^c Expressed as the ratio between the relative resistance value of ABCG2-expressing cells in the absence of the second compound and the relative resistance in its presence. In the case of experiments with gA or ouabain, ABCG2-expressing cells are more sensitive to the combined treatment with curcumin than the parental cells (i.e. sensitization). In the case of experiments with mitoxantrone, the ABCG2-expressing cells are less resistant to combined treatment with curcumin or gA (i.e. reversal of resistance).

^d Chearwae et al. (26) reported previously that curcumin is presumably not a substrate of ABCG2 transporters. Based on the slight curcumin resistance of ABCG2-expressing HEK cells compared to the parental HEK cells, it appears, however, also possible that the high permeability of curcumin through membranes makes it difficult to detect whether it is transported by ABCG2 transporters or not. In that sense, curcumin may actually be a substrate of ABCG2 transporters, but because of its fast passive back diffusion through membranes, it may appear not to be a substrate. We note that in general, net transport observed in transport assays is the sum of passive and active transport processes.