DFT Study of the Biological Activity of Fullerenol - Cisplatin Conjugate as an Antitumor Therapy Agent

In the era of increasing the effectiveness of treatment methods and drugs used in modern neuro-oncology, the targeted delivery of diagnostic and medicinal substances to the tumor is of great importance. The aim of the work is to study in silico optimal and rational approaches to the creation of nanocontainers for targeted drug delivery. Here we present the results of DFT simulation of the molecular and electronic structure, as well as possible mechanisms for the formation of water-soluble conjugate of the cytotoxic drug cisplatin (cis-[Pt(NH3)2Cl2]) and fullerenol (C₆₀(OH)₂₄). The calculations were performed using the DFT/CAM-B3LYP/cc-pvdz/LanL2DZ(Pt) level of theory. Polarizable Continuum Model (PCM) was used for solvent phase calculations. From the results of the calculation of structural parameters and electronic structure for the C₆₀(OH)₂₄ - cisplatin conjugate, was concluded that in the aqueous solution stable hydrogen bonds are formed between the molecules of fullerenol and cisplatin. Biological activity descriptors were calculated, which showed that the conjugate has a higher reactivity than the cisplatin molecule.

The paper presents the results of DFT simulation of the molecular and electronic structure of water-soluble cisplatin and fullerenol (C₆₀(OH)₂₄) as an adjuvant, as well as the results of studying the possible mechanisms of the biological activity of these complexes

In the era of increasing the effectiveness of treatment methods and drugs used in modern neuro-oncology, the targeted delivery of diagnostic and medicinal substances to the tumor is of great importance. The aim of the work is to study optimal and rational approaches to the creation of nanocontainers for targeted drug delivery cisplatin (CPt).

Water-soluble derivatives of fullerene are a promising material for use as a means of drug delivery [1]. The possibility of effective use of cisplatin conjugates and nanocarbon substances is described in [2-8].

In addition, a number of studies have shown the possibility of reducing the dosage of classical chemotherapy drugs by combining known cytostatic agents with substances that do not have a cytostatic effect themselves, which was accompanied by the activation of antitumor mechanisms, because the effects of tumor cell death were intensified [9,10].

For study of the structural and electronic characteristics of C₆₀(OH)₂₄-based conjugates, as well as possible mechanisms for the formation of the corresponding conjugates we started from molecules of the cytotoxic drug cisplatin (cis-[Pt(NH3)2Cl2]) (CPt) and of fullerenol as an adjuvant.

Calculations were carried out both for individual compounds and their conjugates in aqueous medium, which simulates situation in living cells. Polarizable continuum model (PCM, solvent is considered as continuous dielectric medium) was used for solvent phase calculations [11].

The calculations were performed using the DFT method (DFT/CAM-B3LYP/cc-pvdz/LanL2DZ(Pt) level of theory was used). Following discussions are based on this method if not noted otherwise. The ORCA 4.1.2 package [12] software was used to calculation. For visualization of quantum chemistry computations ChemCraft and Avogadro software ware used [13,14].

The results of the calculation of the structural parameters of CPt and the conjugate after full geometry optimization with the solvent are shown in figures 1,2.

Figure 1: The CPt molecule after calculation by DFT method in aqueous medium. Distances are given in Å.

Figure 2: –C₆₀(OH)₂₄-CPt conjugate after calculation by the DFT method in aqueous medium in two points of view.
(a) The complex with indicating hydrogen bonds (dashed lines)
(b) The complex with indicating Pt-N and Pt-Cl bonds. Distances are given in Å.

As shown from figures 1,2 C₆₀(OH)₂₄-CPt conjugate is formed as a result of hydrogen bonding between H atoms from NH3 groups of CPt and OH – groups of fullerenol. Moreover, characteristics of Pt-N and Pt-Cl bonds do not change.

Calculations of the electronic structure were also performed for the studied compounds. The localization of frontier Molecular Orbitals (FMO): Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) was studied. Figures 3,4 are show the results of calculating of localization of FMO of the C60OH24-CPt conjugate using the DFT method, in aqueous medium.

Figure 3: Localization of a) – HOMO, b) – LUMO of the C₆₀(OH)₂₄-CPt conjugate after DFT calculation in aqueous medium. Hydrogen bonds are indicating by dashed lines.

Figure 4: Molecular electrostatic potential diagram of C₆₀(OH)₂₄-CPt conjugate in different formats.

The localization of the frontier molecular orbitals HOMO and LUMO shows that the C₆₀(OH)₂₄ itself is not inert and participates in the manifestation of the reactivity of the conjugate, which is one explanation for the decrease in tumor cells survival upon exposure to these complexes. Thus, in the case of the formation of the C₆₀(OH)₂₄-CPt conjugate, fullerenol is not just an inert carrier of the biologically active substance CPt, but also increases the biological activity of the conjugate compared to pure CPt, due to the participation of the C₆₀(OH)₂₄ molecule in the primary act interactions of the conjugate upon approaching the protein target.

The combination of these factors explains the enhancement of the antitumor effect of cisplatin during its interaction with fullerenol.

The energies of (FMO) are also used as one of characteristics of biological activity of a molecule. The key characteristics of a molecule in FMO theory are difference between the energies of HOMO and LUMO (ΔE), global hardness and softness of the system (η and S) which are treated as descriptors.

Molecular dipole moment is a generalized measure of bond properties and charge density in a molecule. Essentially, it is an index of reactivity, which is very important for determining biological properties, especially those associated with interaction with the active sites of the enzyme. In addition, dipole moment and ΔE, are related to aqueous solubility [15,16]. Obtained values are shown in table 1.

We used one more descriptor for the analysis of C₆₀(OH)₂₄-CPt conjugate system – Molecular Electrostatic Potential (MEP) [17]. It allows evaluating electrostatic component of the energy of intermolecular interactions and is clearly presented in the form of color diagrams. The MEP chart can be used to predict reactivity and active sites for interactions. Negative electrostatic potential corresponds to proton attraction by total electron density in a molecule, that is, protonation of a molecule (shades of red), and positive electrostatic potential corresponds to proton repulsion by atoms (shades of blue). Potential increases in the following order: red < orange < yellow < green < blue. Calculated MEP distribution diagrams in different formats are shown in figure 4.

Calculation results show that regions with negative potential are located on oxygen atoms of hydroxyl group of C₆₀(OH)₂₄. Areas with positive potential are localized around amino groups of cisplatin. Thus, conjugation of adjuvant with cisplatin results in formation of molecular substrate in which types of electrostatic and hydrogen bond interactions are shared between cisplatin and fullerenol ligand.

In general, the following conclusions can be drawn from the obtained results. Quantum-chemical calculations of optimal geometry and electronic structure of molecules allow revealing one of possible reasons for the effect of adjuvant in combined chemotherapy of tumors, namely:

From the results of the calculation of structural parameters and electronic structure for the C₆₀(OH)₂₄ - cisplatin conjugate, was concluded that the Pt-N and Pt-Cl bond in the aqueous solution do not change during the formation of the conjugate, while stable hydrogen bonds are formed bonds between fullerenol and cisplatin molecules.

Adjuvants of C₆₀(OH)₂₄ series shape conjugates with cisplatin with non-covalent interactions between molecules due to hydrogen, van der Waals and electrostatic bonds. The resulting conjugate of two molecules acts as a single unit.

These conjugates form stable, non-covalently bound complexes. The analysis of the MO localization obtained after the calculation of the electronic structure are showed that , the contribution of the orbitals of C₆₀(OH)₂₄ atoms to the formation of the HOMO and LUMO, increases and determines the reactivity of the conjugate. In addition, the calculation results show that there is a high probability that this kit exists in the form of an ionic solution in an aqueous environment.

Thus, in the case of the formation of the C₆₀(OH)₂₄ -CPt conjugate, fullerenol is not just an inert carrier of the biologically active substance CPt, but also increases the biological activity of the conjugate compared to pure CPt, due to the participation of the C₆₀(OH)₂₄ molecule in the primary act interactions of the conjugate upon approaching the protein target. Biological activity descriptors were calculated, which showed that the conjugate has a higher reactivity than the cisplatin molecule.

The localization of the boundary orbitals HOMO and LUMO shows that the C₆₀(OH)₂₄ itself is not inert and participates in the manifestation of the reactivity of the conjugate, which is one explanation for the decrease in tumor cells survival upon exposure to these complexes.

Increase in dipole moment of conjugates in aqueous medium, i.e., polarity of their molecules, in comparison with individual cisplatin, also promotes activation of cytotoxic agent binding to the target, which enhances manifestation of adjuvant effect.

Diagrams of MEP distribution in conjugates demonstrate significant role of fullerenol ligand in distribution of electrostatic potential in the system.

All ORCA package computation were performed on Computer cluster of Institute for Nuclear Problems BSU.

Author contributions

““Conceptualization, A.P., V.P (V. Potkin). and A.S..; methodology, A.P., V.P. (V. Potkin) and V.K.; software, A.P., T.B., E.D.,V.P. (V. Pushkarchuk) and D.E..; validation, V.K. and S.K. (S. Kilin); formal analysis, A.N, S.K. (S. Kuten) and H.Z.; investigation, A.P., V.P. (V. Potkin) and A.S.; resources, S.K. (S. Kuten) and D.E.; data curation, A.P., V.P. (V. Potkin) and H.Z.; writing—original draft preparation, A.P., V.P. (V. Potkin) and V.K.; writing—review and editing, A.P., V.P. (V. Potkin). and V.K.; visualization, A.P., V.P. (V. Pushkarchuk); supervision, A.P., V.P. (V. Potkin) and V.K.; project administration, A.P., V.P. (V. Potkin), V.K. and S.K. (S. Kilin); funding acquisition, S.K. (S. Kilin), A.P. A.S. All authors have read and agreed to the published version of the manuscript.””

Funding

This research was funded by the State Research Programs “Convergence 2025”. A.S. were partially supported by the BRFR grant № Т22Mn-005

Informed consent statement

Not applicable.

Institutional review board statement

Not applicable.

Data availability statement

The data that support the findings of this study are available upon reasonable request from the authors.

Conflicts of interest

The authors declare no conflict of interest.

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