G-quadruplex-binding and telomerase-inhibiting capacities of G-quadruplex ligands were examined under a cell nuclei-mimicking condition including excess double-stranded DNA (lambda 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 (K-a = 3.6 x 10(6) M-1) under a diluted condition without lambda DNA and PEG 200. Correspondingly, TMPyP4 inhibited telomerase activity under the diluted condition (IC50 = 1.6 mu M) but not under the cell nuclei-mimicking condition. In contrast, the K-a and IC50 values of an anionic copper phthalocyanine (Cu-APC) under the diluted (2.8 x 10(4) M-1 and 0.86 mu M) and the cell nuclei-mimicking (2.8 x 10(4) M-1 and 2.1 mu M) conditions were similar. In accordance with these results, 10 mu M TMPyP4 did not affect the proliferation of HeLa cells, while Cu-APC efficiently inhibited the proliferation (IC50 = 1.4 mu M). 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.