The pentacyclic acridinium methosulfate
salt RHPS4 induces the 3'single-stranded
guanine-rich telomeric overhang to fold into a G-quadruplex structure. Stabilisation of the latter is incompatible with an attachment of
telomerase to the telomere and thus G-quadruplex
ligands can effectively inhibit both the catalytic and capping functions of
telomerase. In this study, we examined mechanisms underlying telomere uncapping by RHPS4 in uterus
carcinoma cells (UXF1138L) with short telomeres and compared the susceptibility of bulk and clonogenic
cancer cells to the G-quadruplex
ligand. We show that treatment of UXF1138L cells with RHPS4 leads to the displacement of the
telomerase catalytic subunit (hTERT) from the nucleus, induction of telomere-initiated DNA-damage signalling and chromosome fusions. We further report that RHPS4 is more potent against
cancer cells that grow as colonies in soft
agar than cells growing as monolayers. Human cord blood and HEK293T embryonic kidney cell colony forming units, however, were more resistant to RHPS4. RHPS4-treated UXF1138L xenografts had a decreased clonogenicity, showed loss of nuclear hTERT expression and an induction of mitotic abnormalities compared with controls. Although single-agent RHPS4 had limited in vivo efficacy, a combination of RHPS4 with the mitotic spindle
poison Taxol caused tumour remissions and further enhancement of telomere dysfunction.