A New Gold(III) Complex, TGS 703, Shows Potent Anti-Inflammatory Activity in Colitis via the Enzymatic and Non-Enzymatic Antioxidant System-An In Vitro, In Silico, and In Vivo Study

Abstract

Inflammatory bowel diseases (IBD) and their main representatives, Crohn's disease and ulcerative colitis, are worldwide health-care problems with constantly increasing frequency and still not fully understood pathogenesis. IBD treatment involves drugs such as corticosteroids, derivatives of 5-aminosalicylic acid, thiopurines, and others, with the goal to achieve and maintain remission of the disease. Nowadays, as our knowledge about IBD is continually growing, more specific and effective therapies at the molecular level are wanted. In our study, we tested novel gold complexes and their potential effect on inflammation and IBD in vitro, in silico, and in vivo. A series of new gold(III) complexes (TGS 404, 512, 701, 702, and 703) were designed and screened in the in vitro inflammation studies. In silico modeling was used to study the gold complexes' structure vs. their activity and stability. Dextran sulphate sodium (DSS)-induced mouse model of colitis was employed to characterize the anti-inflammatory activity in vivo. Lipopolysaccharide (LPS)-stimulated RAW264.7 cell experiments proved the anti-inflammatory potential of all tested complexes. Selected on the bases of in vitro and in silico analyses, TGS 703 significantly alleviated inflammation in the DSS-induced mouse model of colitis, which was confirmed by a statistically significant decrease in the macro- and microscopic score of inflammation. The mechanism of action of TGS 703 was linked to the enzymatic and non-enzymatic antioxidant systems. TGS 703 and other gold(III) complexes present anti-inflammatory potential and may be applied therapeutically in the treatment of IBD.

Keywords: anti-inflammatory; gold; inflammatory bowel disease.

Figure 1

Neutral red uptake test (A,B) and Griess test (C,D) for RAW264.7 macrophages treated with TGS compounds. ## p < 0.01, ### p < 0.001 vs. control; *** p < 0.001 vs. 1 µg/mL lipopolysaccharide (LPS).

Figure 1

Neutral red uptake test (A,B) and Griess test (C,D) for RAW264.7 macrophages treated with TGS compounds. ## p < 0.01, ### p < 0.001 vs. control; *** p < 0.001 vs. 1 µg/mL lipopolysaccharide (LPS).

Figure 1

Neutral red uptake test (A,B) and Griess test (C,D) for RAW264.7 macrophages treated with TGS compounds. ## p < 0.01, ### p < 0.001 vs. control; *** p < 0.001 vs. 1 µg/mL lipopolysaccharide (LPS).

Figure 2

The structure of the most stable gold(III) tetracyanide isomer, Au(CN)4⁻. The values indicate the N-C (1.164 Å) and C-Au (2.106 Å) lengths. Atom coloring: Au—gold, C—grey, N—blue.

Figure 3

The structure of the most stable gold(III) tricyanide dimethyl sulfide complex isomer, [Au(CN)₃DMSO] κ-O. The value indicates the Au-O (2.077 Å) length. Atom coloring: Au—gold, C—grey, N—blue, O—red, S—yellow, and H—white. It should be noted that the substitution of one CN⁻ ligand by DMSO is both energetically and thermodynamically not favorable.

Figure 4

The structure of the most stable gold(III) tetracyanide dimethyl sulfide complex isomer, [Au(CN)₄DMSO]⁻ κ-S. Atom coloring: Au—gold, C—grey, N—blue, S—yellow, O—red, and H—white. The pink dashed line represents the close contact between Au and S atoms. It should be noted that while extending the coordination number of Au(III) above four, the sulfur atom forms an energetically more favorable interaction with the central ion, instead of the oxygen. This is in opposition to the complex when the CN⁻ ion was substituted by DMSO, as presented in Figure 3.

Figure 5

The structure of the most stable gold(III) tetracyanide: dimethyl sulfide (1:2) complex isomer, [Au(CN)₄DMSO₂]⁻ κ-S, S. Atom coloring: Au—gold, C—grey, N—blue, S—yellow, O—red, and H—white. The pink dashed line represents close contact between Au and S atoms. Although the stabilizing interactions between the additional ligands, DMSO molecules, and gold(III) tetracyanide are being formed, the presented complex is not stable due to the entropic penalty resulting in the positive value of free enthalpy of formation. The results of the calculations indicate the preference of the κ-S, S isomer. However, unexpectedly, the κ-S, O isomer was found to be the least stable one. Similarly, as in the case of mono DMSO complexes, the formation of the studied molecules is energetically favorable due to the formation of the intermolecular interactions between the positively charged Au(III) and DMSO, which is the Lewis base. However, due to the decrease of entropy resulting from such complexation, the formation of both mono and di DMSO complexes is not preferable thermodynamically due to the positive ΔG of such reactions.

Figure 6

The structure of the gold(III) tricyanide aquo complex, [Au(CN)₃H₂O]. The value indicates the Au-O (2.123 Å) length. Atom coloring: Au—gold, C—grey, O—red, N—blue, and H—white. It should be noted that although the complex with water serving as ligand is stable, the substitution of CN⁻ ligand by water is both energetically and thermodynamically not favorable. This explains why the dissociation of gold(III) tetracyanide in water environment is not observed.

Figure 7

The structures of the gold(III) tetracyanide aquo (right) and diaquo (left) complexes, [Au(CN)₄H₂O]⁻ and [Au(CN)₄(H₂O)₂]⁻. Atom coloring: Au—gold, C—grey, N—blue, O—red, and H—white. Despite the intermolecular electrostatic interactions occurring between the positively charged Au atom of Au(CN)₄⁻ and oxygen atoms of water, the decrease of entropy destabilizes those aquo complexes.

Figure 8

The effect of TGS 703 on colonic inflammation in a dextran sulphate sodium (DSS)-induced model of colitis: macroscopic score (A), stool score (B), myeloperoxidase activity (D), and microscopic score (E) for control, DSS-only treated mice, and mice with DSS-induced colitis treated with TGS 703 in two doses: 1.68 μg/kg (TGS 703 0.1A) and 16.8 μg/kg (TGS 703 1A). * p < 0.05, ** p < 0.01 and *** p < 0.001 as compared with the control mice; # p < 0.05, ## p < 0.01, ### p < 0.001 as compared with DSS-treated animals. (C) Presents changes in the weight of mice during the DSS-induced animal model of colitis.

Figure 9

Representative photos of hematoxylin and eosin staining of colon samples. Scale bar = 100 μm.

Figure 10

The influence of DSS colitis and TGS 703 treatment on the antioxidant profile in the mouse colon: heme oxygenase-1 (A), catalase (B), glutathione (C), glutathione disulfide (D), glutathione peroxidase (E), cyclooxygenase-1 (F), and cyclooxygenase-2 (G) for control, DSS-only treated mice, and mice with DSS-induced colitis treated with TGS 703 in two doses: 1.68 μg/kg (TGS 701 0.1A) and 16.8 μg/kg (TGS 703 1A). * p < 0.05, ** p < 0.01, *** p < 0.001 as compared with the control mice; # p < 0.05, ## p < 0.01, ### p < 0.001 as compared with DSS-treated animals.