Table 1. G4 stabilization and telomerase inhibition by ethidium derivatives.

Compound	ΔT1/2 G4 (FRET)a	G4 (gel)b	IC50 telomerasec	IC50 Taqd
	(˚C)	(% retarded band)	(nM)	(nM)
Ethidium	0.6	3	183/286	>10 000
1	10.7	21	18	1200
2	9.7	31	19	560
3	7.9	34	97	9600
4	9.6	36	47	1080

The compounds used in this study are shown in Figure 1C. The stabilization (in °C) was determined from fluorescence emission measurements of the F21T oligonucleotide for 1 µm dye, and determined at a normalized fluorescence intensity of 0.5 (λ_exc = 470 nm; λ_emi = 515 nm). The concentration that gave 50% inhibition of telomerase by TRAP assay is given in nM.

^aDetermination of ΔT_1/2 at 1 µM dye concentration, using the FRET method.

^bObtained at 10 µM dye concentration. See Figure 4 for an illustration of ethidium derivative-induced formation of intermolecular quadruplexes.

^cTRAP assay (average of two to three independent experiments).

^dTaq polymerase (PCR inhibition) assay.