Table 1. Cytotoxic Activities of 2,4-Diaminopteridine Derivatives on HeLa Cancer Cells and Ect1/E6E7 Normal Cells with (4 Gy) or without (0 Gy) X-rays by MTT Assay.

	IC50 (μM)
	2a	2b	2c	2d	2e	2f	MTX
HeLa + 0 Gy	2.1 ± 0.2	>80	9.8 ± 0.3	32.6 ± 1.2	24.5 ± 1.2	4.6 ± 0.3	1.5 ± 0.2
HeLa + 4 Gy	0.64 ± 0.03	>80	6.3 ± 0.2	14.8 ± 0.4	14.6 ± 0.8	2.3 ± 0.2	0.82 ± 0.08
SERa	3.3 ± 0.2	NA	1.6 ± 0.1	2.20 ± 0.2	1.7 ± 0.1	2.0 ± 0.4	1.8 ± 0.3
Ect1/E6E7 + 0 Gy	6.9 ± 0.3	>80	20.0 ± 1.0	59.3 ± 1.6	40.1 ± 1.6	10.9 ± 1.8	2.9 ± 0.3
Ect1/E6E7 + 4 Gy	3.1 ± 0.2	>80	13.9 ± 1.0	38.1 ± 1.9	19.8 ± 1.2	5.3 ± 0.3	1.8 ± 0.2
SERb	2.2 ± 0.1	NA	1.4 ± 0.2	1.6 ± 0.1	2.0 ± 0.1	2.1 ± 0.2	1.6 ± 0.3
SI0c	3.3 ± 0.4	NA	2.0 ± 0.1	1.8 ± 0.1	1.6 ± 0.1	2.5 ± 0.3	1.9 ± 0.2
SI4d	4.8 ± 0.5	NA	2.2 ± 0.1	2.6 ± 0.1	1.4 ± 0.1	2.3 ± 0.4	2.2 ± 0.1

^aSensitivity enhancement ratio (SER) = IC₅₀(HeLa + 0 Gy)/IC₅₀(HeLa + 4 Gy).

^bSER = IC₅₀ (Ect1/E6E7 + 0 Gy)/IC₅₀ (Ect1/E6E7 + 4 Gy).

^cSafe index for 0 Gy (SI₀) = (Ect1/E6E7 + 0 Gy)/(HeLa + 0 Gy).

^dSafe index for 4 Gy (SI₄) = (Ect1/E6E7 + 4 Gy)/(HeLa + 4 Gy).