We have previously synthesised a number of novel head-to-head
bis-benzimidazole derivatives that are structurally related to the
fluorochrome,
Hoechst 33258, and which possess strong affinity for A:T sites in the minor groove of duplex
DNA. Initial studies revealed these compounds to exhibit potent antiproliferative activity against a range of ovarian cell lines and to inhibit transcription in an in vitro setting. In this study, we have examined their cellular behaviour in detail and have shown that two of these compounds (
ABA13 and
ABA833) potently inhibit the proliferation of a range of human tumour cell lines, and show some specificity towards
breast carcinoma cell lines. In most of the cell lines investigated,
ABA833 was the more potent of the two compounds. Flow cytometric analysis revealed that
ABA13 and
ABA833 (50-500 nM) induced an S phase block and increased the pre-G1 population in MCF-7 and MDA 468 human
breast cancer cells. An increase in the pre-G1 population of RKO colon
carcinoma cells was seen only at 500 nM with
ABA833, reflecting the reduced sensitivity of this cell line to the bis-
benzimidazoles in comparison to the
breast cancer cell lines. Mechanistic studies revealed that neither
ABA13 or
ABA833 act as
topoisomerase I (
topo I) or
topoisomerase II (
topo II)
poisons in plasmid or
kinetoplast DNA (
kDNA) relaxation assays, but both compounds do inhibit the catalytic activity of these
enzymes.
Drug uptake studies showed that reduced sensitivity of MCF-7adr and RKO cells compared with MCF-7 to both
ABA13 and
ABA833 correlated with a markedly reduced intracellular
drug accumulation.