Recent Progress of Targeted G-Quadruplex-Preferred Ligands Toward Cancer Therapy.

A G-quadruplex (G4) is a well-known nucleic acid secondary structure comprising guanine-rich sequences, and has profound implications for various pharmacological and biological events, including cancers. Therefore, ligands interacting with G4s have attracted great attention as potential anticancer therapies or in molecular probe applications. To date, a large variety of DNA/RNA G4 ligands have been developed by a number of laboratories. As protein-targeting drugs face similar situations, G-quadruplex-interacting drugs displayed low selectivity to the targeted G-quadruplex structure. This low selectivity could cause unexpected effects that are usually reasons to halt the drug development process. In this review, we address the recent research on synthetic G4 DNA-interacting ligands that allow targeting of selected G4s as an approach toward the discovery of highly effective anticancer drugs.

Molecular Characteristics of DNA-Alkylating PI Polyamides Targeting RUNX Transcription Factors.

The runt-related transcription factor (RUNX) family has been associated with cancer development. The binding of RUNX family members to specific DNA sequences is hypothesized to promote the expression of downstream genes and cause cancer proliferation. On the basis of this proposed mechanism of cancer growth, we developed conjugate 1, which inhibits the binding of RUNX to its target DNA. Conjugate 1 is a DNA-alkylating pyrrole-imidazole (PI) polyamide conjugate containing chlorambucil as an anticancer agent. Conjugate 1 was reported to have a marked anticancer effect in mouse models of acute myeloid leukemia. Although the effectiveness of 1 has been demonstrated in vivo, the detailed mechanism by which it alkylates DNA is unknown. Here, we chemically elucidated the molecular characteristics of conjugate 1 to confirm its potential as a RUNX-inhibiting drug. We also generated an alternative conjugate 2, which targets the same DNA sequence, by replacing one pyrrole with ß-alanine. Comparison of the characteristics of conjugates 1 and 2 suggested that reaction selectivity and binding affinity to the RUNX-binding sequence were improved by the introduction of ß-alanine. These findings indicate the possibility of DNA-alkylating PI polyamides as candidates for cancer chemotherapeutics.

Simultaneous Binding of Hybrid Molecules Constructed with Dual DNA-Binding Components to a G-Quadruplex and Its Proximal Duplex.

A G-quadruplex (quadruplex) is a nucleic acid secondary structure adopted by guanine-rich sequences and is considered to be relevant to various pharmacological and biological contexts. Although a number of researchers have endeavored to discover and develop quadruplex-interactive molecules, poor ligand designability originating from topological similarity of the skeleton of diverse quadruplexes has remained a bottleneck for gaining specificity for individual quadruplexes. This work reports on hybrid molecules that were constructed with dual DNA-binding components, a cyclic imidazole/lysine polyamide (cIKP), and a hairpin pyrrole/imidazole polyamide (hPIP), with the aim toward specific quadruplex targeting by reading out the local duplex DNA sequence adjacent to designated quadruplexes in the genome. By means of circular dichroism (CD), fluorescence resonance energy transfer (FRET), surface plasmon resonance (SPR), and NMR techniques, we showed the dual and simultaneous recognition of the respective segment via hybrid molecules, and the synergistic and mutual effect of each binding component that was appropriately linked on higher binding affinity and modest sequence specificity. Monitoring quadruplex and duplex imino protons of the quadruplex/duplex motif titrated with hybrid molecules clearly revealed distinct features of the binding of hybrid molecules to the respective segments upon their simultaneous recognition. A series of the systematic and detailed binding assays described here showed that the concept of simultaneous recognition of quadruplex and its proximal duplex by hybrid molecules constructed with the dual DNA-binding components may provide a new strategy for ligand design, enabling targeting of a large variety of designated quadruplexes at specific genome locations.

G-Quadruplex Induction by the Hairpin Pyrrole-Imidazole Polyamide Dimer.

The G-quadruplex (G4) is one type of higher-order structure of nucleic acids and is thought to play important roles in various biological events such as regulation of transcription and inhibition of DNA replication. Pyrrole-imidazole polyamides (PIPs) are programmable small molecules that can sequence-specifically bind with high affinity to the minor groove of double-stranded DNA (dsDNA). Herein, we designed head-to-head hairpin PIP dimers and their target dsDNA in a model G4-forming sequence. Using an electrophoresis mobility shift assay and transcription arrest assay, we found that PIP dimers could induce the structural change to G4 DNA from dsDNA through the recognition by one PIP dimer molecule of two duplex-binding sites flanking both ends of the G4-forming sequence. This induction ability was dependent on linker length. This is the first study to induce G4 formation using PIPs, which are known to be dsDNA binders. The results reported here suggest that selective G4 induction in native sequences may be achieved with PIP dimers by applying the same design strategy.