On the use of Europium (Eu) for designing new metal-based anticancer drugs

Highlights

• Europium oxide binds strongly and cooperatively to double-stranded DNA.

• The binding mode and the physical chemistry of the interaction were depicted using single molecule force spectroscopy.

• The study suggest that rare earth elements are promising to the rational design of new metal-based anticancer drugs.

Abstract

Europium oxide (Eu₂O₃) was used to evaluate the affinity of this rare earth element for interacting with double-stranded (ds) DNA molecules. To perform the study, we used single molecule force spectroscopy with optical tweezers and gel electrophoresis assays. Force spectroscopy experiments show that Eu₂O₃ presents a strong interaction with dsDNA, and the binding is independent on the ionic strength used in the surrounding environment. Among the main characteristics of the interaction, Eu₂O₃ tends to bind in a cooperative way, forming bound clusters of $\sim$ 3 molecules, and presents a high equilibrium association binding constant on the order of 10⁵ M⁻¹. In addition, gel electrophoresis confirm the weak electrostatic character of the interaction and explicit show that Eu₂O₃ does not interfere on drug intercalation into the double-helix. Such results demonstrate the potential of europium for interacting with nucleic acids and strongly suggest that this rare earth element may be considered for the design of new metal-based anticancer drugs in the future.

Introduction

The rational design of new drugs for chemotherapies and other kinds of treatments for human diseases is an important topic in pharmacology and related fields. In particular, metal-based compounds constitute a class of drugs largely employed in current chemotherapies [1,2], specially the platinum-based compound cisplatin and its derivatives [[3], [4], [5], [6]]. On the other hand, advances on drug development based on metals and semimetals such as Ruthenium, Osmium, Iridium, Iron, Cooper, Gold, Vanadium, Arsenic and others were recently reported [1]. Nevertheless, studies using rare earth elements are sparse, which opens up a huge range of possibilities for the design of new promising drugs using these elements.

Europium (Eu) is the most reactive rare earth metal, and a previous study performed with a particular Eu-based compound have achieved promising results concerning the development of Eu-based anticancer drugs [7]. Such compound, designed linking various aromatic rings to the Eu atom, interacts with DNA via hydrophobic groove binding with an equilibrium association binding constant on the order of 10⁵ M⁻¹, also exhibiting antimicrobial activity [7]. Nevertheless, since the aromatic rings alone can interact with DNA via hydrogen bonds, hydrophobic and π-π stacking interactions [8,9], it is difficult to discriminate the real effect of the Eu element in such compound. In addition, europium affinity of single-stranded DNA was also recently demonstrated [10].

In the present study, we investigate the effects of europium on double-stranded DNA using the simplest stable compound containing such element: the europium oxide Eu₂O₃. Pure europium cannot be used here because the element is very reactive and would rapidly oxidize in the sample chamber. To perform the study, we used force spectroscopy with optical tweezers and gel electrophoresis assays. Single molecule force spectroscopy is today recognized as the state-of-the-art technique for studies concerning the interactions between nucleic acids and ligands such as drugs and proteins, being able to provide information on the binding modes and on the physical chemistry of the interaction [[11], [12], [13]]. Gel electrophoresis, on the other hand, is a classic technique which allows one to observe the behavior of a large ensemble of nucleic acids molecules, thus complementing the single molecule analysis obtained from force spectroscopy. Our results show that Eu₂O₃ binds cooperatively to the double-helix with a high equilibrium association constant (on the order of 10⁵ M⁻¹), i. e., close to that found for many commercial anticancer drugs. Thus, the purpose of the present study is to show the potential of europium for interacting with nucleic acids and to motivate the development of new Eu-based anticancer drugs for chemotherapies.