Recognition and Stabilization of Quadruplex DNA by a Potent New Telomerase Inhibitor: NMR Studies of the 2:1 Complex of a Pentacyclic Methylacridinium Cation with d(TTAGGGT)(4) We thank the EPSRC of the UK and AstraZeneca for financial support to E.G. M.F.G.S. and R.A.H. are supported by the Cancer Research Campaign of the UK.

Abstract

Telomerase is a cellular ribonucleoprotein, consisting of a reverse transcriptase and an endogenous RNA template, that maintains telomere length at the ends of eukaryotic chromosomes.[1] Telomeres consist of double-stranded DNA composed of tandem repeats of guanine-rich sequences (5 TTAGGG in humans) with a single-stranded 3 -end overhang necessary to ensure complete chromosomal DNA replication, while protecting the chromosome ends from fusion and degradation.[1, 2] The telomeres shorten with each round of cell division, due to incomplete lagging-strand replication, limiting the number of replicative cycles before the cell enters a state of senescence.[3] Telomere length in immortalized cells (e.g. cancer cells) is tightly regulated by the enzyme telomerase whose reverse transcriptase activity allows the addition of 5 -TTAGGG repeats to maintain telomere length through an indefinite number of cell divisions.[4] While telomerase activity is absent in human somatic cells it is detected in the majority of human tumor-derived cell lines. Consequently, telomerase has become a high-profile target for anticancer drug design.[5] The discovery that the G-rich telomeric repeats are able to assemble into novel four-stranded quadruplex structures, consisting of guanine tetrads stabilized by monovalent cations (Na and K ) (Figure 1),[6] has focused attention for structurespecific drug design. Since telomerase reverse transcriptase activity depends on a single-stranded DNA primer,[4] small molecules that bind and stabilize the folded quadruplex form of the primer are potential inhibitors of telomerase function. A number of quadruplex-specific ligands have been reported with the common feature of an extended aromatic ring system capable of interacting through extensive stacking with G-tetrads.[5] However, there has been a paucity of detailed structural data available on the drug ±DNA complexes due to intractable NMR spectra arising from extensive drug-induced line broadening.[7] The exception is a dicationic perylenetetracarboxylic diimide derivative (PIPER), which forms either a sandwich complex bound between the blunt ends of a quadruplex dimer formed from d(TTAGGG)4, or N