The telomerase enzyme is a potential therapeutic target in many human cancers. A series of potent inhibitors has been designed by computer modeling, which exploit the unique structural features of quadruplex DNA. These 3,6,9-trisubstituted acridine inhibitors are predicted to interact selectively with the human DNA quadruplex structure, as a means of specifically inhibiting the action of human telomerase in extending the length of single-stranded telomeric DNA. The anilino substituent at the 9-position of the acridine chromophore is predicted to lie in a third groove of the quadruplex. Calculated relative binding energies predict enhanced selectivity compared with earlier 3,6-disubstituted compounds, as a result of this substituent. The ranking order of energies is in accord with equilibrium binding constants for quadruplex measured by surface plasmon resonance techniques, which also show reduced duplex binding compared with the disubstituted compounds. The 3,6,9-trisubstututed acridines have potent in vitro inhibitory activity against human telomerase, with EC50 values of up to 60 nM. T he telomeric ends of chromosomes consist of tandem repeats of simple guanine-rich DNA protein-associated motifs whose function is to protect the ends from unwanted DNA damage-repair, recombination, and end-fusions. In eukaryotics the repeat is TTAGGG, with telomere length varying between ca. 5 and 15 kb (1, 2). Cancer cells typically have short telomeres, whereas stem cell telomere length tends to be at the high end of this range. The terminal 150-200 bases at the 3Ј end of human telomeres form a single-stranded overhang, whose exact structure is not fully established, although loop-type arrangements have been suggested from electron microscope studies (3). Telomeres shorten in somatic cells on each round of replication, by 50-200 bases, as a consequence of the inability of DNA polymerase to fully replicate the ends (4). Once telomeres reach a critically short length, cells enter a senescent state and do not replicate further (5). By contrast, the short telomeres in tumor cells are stable in length, maintained by the action of a specialized DNA polymerase, the telomerase enzyme complex, which catalyses the synthesis of further telomere repeats (6). Telomerase is activated in 80-90% of human tumors and is undetectable in most normal somatic cells (7). This activation has been shown to be a key step in the immortalization process in human cells, leading to tumorigenesis (8). A small proportion of tumor cells have an alternative telomere maintenance pathway (ALT) which appears to be independent of telomerase and involves recombination events. Inhibition of telomerase by a dominant negative mutant (9, 10), or by synthetic oligonucleotides targeted to the RNA template (11), leads to telomere shortening, growth arrest and apoptosis for tumor cells in culture. Telomerase is thus a highly attractive target for selective anti-cancer therapy (12).We have focused on the rational discovery of small-molecule telomerase inhibitors with pharma...

SummaryAcquired resistance to cisplatin (cis-diamminedichloroplatinum (II)) has been generated in vitro in the 41M The 4lMcisR6 cells were not found to be cross-resistant to ouabain, a postulated specific inhibitor of sodium-potassium adenosine triphosphatase (Na+, K+-ATPase), suggesting that decreased cisplatin accumulation in these cells is probably not regulated by alterations in their Na+, K+-ATPase levels, and Na+ potential across the plasma membrane. Cellular accumulation of a novel class of platinum (IV) ammine/cyclohexylamine dicarboxylates, which exhibit enhanced cytotoxicity over cisplatin and completely circumvent resistance to cisplatin in the 41McisR line, was also examined. The data suggests that increased accumulation of these compounds, as a result of their enhanced lipophilicity, could account for the dramatic increase in their potency over cisplatin.

No abstract

The expression of the BCL-2 family proteins, BCL-2, BAX, BCLXLand BAK have been determined in a panel of 12 human ovarian carcinoma cell lines encompassing a wide range in sensitivity to cisplatin. Whereas BAX, BCLXLand BAK levels did not correlate with sensitivity, there was a statistically significant inverse correlation (r = –0.81;P = 0.002) between growth inhibition by cisplatin and BCL-2 levels. In sublines possessing acquired resistance to various platinum-based drugs or across a panel of human ovarian carcinoma xenografts, there was no consistent pattern of BCL-2 expression. Two relatively sensitive lines (A2780 and CH1) have been stably transfected with bcl-2 and bclXLrespectively and two relatively resistant lines (A2780cisR and SKOV-3) stably transfected with bax. Overexpression of BCL-2 in A2780 cells led to resistance to cisplatin compared to the vector control when assayed at 48 h post-drug incubation but a significant increase in sensitivity at 96 h. Relative rates of apoptosis at 48- and 96-h post-cisplatin exposure mirrored the growth inhibition. There was no significant difference in sensitivity of the pair of lines by clonogenic assay. No significant changes in chemosensitivity to a variety of DNA-damaging or tubulin-interactive agents were observed in the remaining transfected lines. Taken together, these results suggest that, in human ovarian carcinoma cells, high BCL-2 levels (either naturally occurring or through gene transfection) confers a trend towards sensitivity not resistance to platinum drugs. © 2000 Cancer Research Campaign