In Vitro Assays Predictive of Telomerase Inhibitory Effect of G‑Quadruplex Ligands in Cell Nuclei

G-quadruplex-binding and telomerase-inhibiting capacities of G-quadruplex ligands were examined under a cell nuclei-mimicking condition including excess double-stranded DNA (λ DNA) and molecular crowding cosolute (PEG 200). Under the cell nuclei-mimicking condition, a cationic porphyrin (TMPyP4) did not bind to the G-quadruplex despite the high affinity (<i>K</i>_a = 3.6 × 10⁶ M^–1) under a diluted condition without λ DNA and PEG 200. Correspondingly, TMPyP4 inhibited telomerase activity under the diluted condition (IC₅₀ = 1.6 μM) but not under the cell nuclei-mimicking condition. In contrast, the <i>K</i>_a and IC₅₀ values of an anionic copper phthalocyanine (Cu-APC) under the diluted (2.8 × 10⁴ M^–1 and 0.86 μM) and the cell nuclei-mimicking (2.8 × 10⁴ M^–1 and 2.1 μM) conditions were similar. In accordance with these results, 10 μM TMPyP4 did not affect the proliferation of HeLa cells, while Cu-APC efficiently inhibited the proliferation (IC₅₀ = 1.4 μM). These results show that the cell nuclei-mimicking condition is effective to predict capacities of G-quadruplex ligands in the cell. In addition, the antiproliferative effect of Cu-APC on normal cells was smaller than that on HeLa cells, indicating that the cell nuclei-mimicking condition is also useful to predict side effects of ligands.