The Interactions of H2TMPyP, Analogues and Its Metal Complexes with DNA G-Quadruplexes-An Overview

Abstract

The evidence that telomerase is overexpressed in almost 90% of human cancers justifies the proposal of this enzyme as a potential target for anticancer drug design. The inhibition of telomerase by quadruplex stabilizing ligands is being considered a useful approach in anticancer drug design proposals. Several aromatic ligands, including porphyrins, were exploited for telomerase inhibition by adduct formation with G-Quadruplex (GQ). 5,10,15,20-Tetrakis(N-methyl-4-pyridinium)porphyrin (H₂TMPyP) is one of the most studied porphyrins in this field, and although reported as presenting high affinity to GQ, its poor selectivity for GQ over duplex structures is recognized. To increase the desired selectivity, porphyrin modifications either at the peripheral positions or at the inner core through the coordination with different metals have been handled. Herein, studies involving the interactions of TMPyP and analogs with different DNA sequences able to form GQ and duplex structures using different experimental conditions and approaches are reviewed. Some considerations concerning the structural diversity and recognition modes of G-quadruplexes will be presented first to facilitate the comprehension of the studies reviewed. Additionally, considering the diversity of experimental conditions reported, we decided to complement this review with a screening where the behavior of H₂TMPyP and of some of the reviewed metal complexes were evaluated under the same experimental conditions and using the same DNA sequences. In this comparison under unified conditions, we also evaluated, for the first time, the behavior of the Ag^II complex of H₂TMPyP. In general, all derivatives showed good affinity for GQ DNA structures with binding constants in the range of 10⁶-10⁷ M^-1 and ligand-GQ stoichiometric ratios of 3:1 and 4:1. A promising pattern of selectivity was also identified for the new Ag^II derivative.

Keywords: AgIITMPyP; G-quadruplexes; H2TMPyP; aromatic ligands; metalloporphyrins; porphyrins; selectivity; telomerase inhibition.

Figure 1

Telomerase function.

Figure 2

G-quartet and a possible G-quadruplex structure.

Figure 3

Cation positions between stacked quartets (A) or within the plane (B) in a G-quadruplex containing three quartets (for simplicity, backbones were omitted).

Figure 4

(A) Different conformations assumed by G-quadruplexes structures depending on strand orientation and (B) possible loop geometries.

Figure 5

Anti and syn conformation of glycosidic torsion angles and groove sizes of parallel and anti-parallel GQ structures, (figure reused from [18], with permission from John Wiley and Sons.

Figure 6

(A) Telomere end elongation by telomerase; (B) G-quadruplex-ligand adduct formation for indirect telomerase inhibition.

Figure 7

(A–D) Types of interactions ligand-G-quadruplexes. For simplicity, grooves are not represented.

Figure 8

Examples of polycyclic ligands reported to inhibit telomerase activity.

Figure 9

Structure of the first porphyrins and metalloporphyrins evaluated as GQ ligands.

Figure 10

Structure of the Ni^II and Mn^III porphyrins derivatives studied by Dixon et al. [71].

Figure 11

Structure of H₂TMPyP and analogs with bulky substituents (A); presence or absence of axial substituents in different metal ions (B) studied by Romera et al. [60].

Figure 12

Structures of 5,10,15,20-tetrakis(4-(N-methyl-pyridinium-2-yl)phenyl)-porphyrin (MTMPy₂PP) and 5,10,15,20-tetrakis(4-guanidinophenyl)porphyrin (TGP) derivatives, studied by Sabater et al. [73].

Figure 13

Pb-GQ structure with end-stacking of two Zn^IITMPyP molecules (pink ellipses).

Figure 14

Structures of the porphyrins referred in Table 1.

Figure 15

Structure and possible topology of the studied G-quadruplexes.

Figure 16

UV-Vis spectra (350–650 nm) obtained for Ag^IITMPyP and (A) tetramolecular GQ, (B) bimolecular GQ, (C) unimolecular GQ and (D) salmon sperm double-stranded DNA. DNA structures were prepared in 20 mM PBS buffer with 100 mM KCl.

Figure 17

(A) binding constants (K_b) (M⁻¹) and (B) red shift observed at the end of the UV-Vis titration of the MTMPyP ligands and the selected DNA sequences.

Figure 18

G4-FID assay performed for the in PBS at 25 °C with the Ag^IITMPyP using the unimolecular GQ (AG₃(T₂AG₃)₃) and double-stranded salmon sperm. A comparison with the results obtained with the H₂TMPyP and the Zn^IITMPyP is presented.