The reactivation of telomerase activity in most cancer cells supports the concept that telomerase is a relevant target in oncology, and telomerase inhibitors have been proposed as new potential anticancer agents. The telomeric G-rich single-stranded DNA can adopt an intramolecular G-quadruplex structure in vitro, which has been shown to inhibit telomerase activity. The C-rich sequence can also adopt a quadruplex (intercalated) structure (i-DNA). Two acridine derivatives were shown to increase the melting temperature of the G-quadruplex and the C-quadruplex at 1 µM dye concentration. The increase in Tm value of the G-quadruplex was associated with telomerase inhibition in vitro. The most active compound, ?BisA?, showed an IC₅₀ value of 0.75 µM in a standard TRAP assay.