Sm. Gowan et al., Potent inhibition of telomerase by small-molecule pentacyclic acridines capable of interacting with G-quadruplexes, MOLEC PHARM, 60(5), 2001, pp. 981-988
A novel pentacyclic acridine, 3,11-difluoro-6,8,13-trimethyl-8H-quino[4,3,2
-kl]acridinium methosulfate (RHPS4), has been identified as a potent inhibi
tor of telomerase in the cell-free telomeric repeat amplification protocol
(TRAP). Modeling and biophysical studies suggest that RHPS4 inhibits telome
rase through stabilization of four-stranded G-quadruplex structures formed
by single-stranded telomeric DNA. In contrast to G-quadruplex interactive t
elomerase inhibitors described previously, RHPS4 inhibited telomerase at su
bmicromolar levels (50% inhibition in the TRAP assay at 0.33 +/- 0.13 muM).
Moreover, RHPS4 exhibited a wide differential between this potent inhibiti
on of telomerase and acute cellular cytotoxicity (mean IC50 value of 7.02 m
uM in 4-day growth inhibition assay). RHPS4, when added to 21 NT breast can
cer cells at nonacute cytotoxic concentrations (200 nM) every 3 to 4 days,
induced a marked cessation in cell growth after 15 days. Similar effects we
re observed using another cell line possessing relatively short telomeres,:
A431 human vulval carcinoma cells, but not in a human ovarian carcinoma ce
ll line (SKOV-3) possessing relatively long telomeres. In 21NT cells, growt
h cessation was accompanied by an increase in cells in the G(2)/M phase of
the cell cycle,: a reduction in cellular telomerase activity, and a lower e
xpression of the hTERT gene. These effects occurred in the absence of a det
ectable reduction in telomere length as measured by slot blotting. RHPS4 al
so induced a cessation of growth of GM847 cells that maintain telomeres by
a nontelomerase alternative mechanism for lengthening telomeres (ALT) after
15 days. RHPS4 represents a promising G-quadruplex interactive small molec
ule that is a potent cell-free inhibitor of human telomerase and induces gr
owth inhibitory effects in human tumor cell lines after prolonged (2-week)
exposure to nonacute cytotoxic drug concentrations.