Organic Moiety on Sn(IV) Does Matter for In Vitro Mode of Action: n Bu3Sn(IV) Compounds with Carboxylato N-Functionalized 2-Quinolones Induce Anoikis-like Cell Death in A375 Cells

Abstract

Objectives: New tributyltin(IV) complexes containing the carboxylate ligands 3-(4-methyl-2-oxoquinolin-1(2H)-yl)propanoic acid (HL1) and 2-(4-methyl-2-oxoquinolin-1(2H)-yl)acetic acid (HL2) have been synthesized. Methods: Their structures have been determined by elemental microanalysis, FT-IR and multinuclear NMR (¹H, ¹³C and ¹¹⁹Sn) spectroscopy and X-ray diffraction study. A solution state NMR analysis reveals a four-coordinated tributyltin(IV) complex in non-polar solvents, while an X-Ray crystallographic analysis confirms a five-coordinated trigonal-bipyramidal geometry around the tin atom due to the formation of 1D chains. A theoretical structural analysis was performed by optimization employing B3LYP-D3BJ functional and 6-311++G(d,p)/def2-TZVP(Sn) basis sets for H, C, N, O/Sn, respectively. The interactions between tin(IV) and surrounding atoms were examined by QTAIM approach. The in vitro antiproliferative activity of the synthesized compounds was evaluated by MTT and CV assays versus MCF-7 (human breast adenocarcinoma), HCT116 (human colorectal carcinoma), A375 (human melanoma), 4T1 (mouse breast carcinoma), CT26 (mouse colon carcinoma) and B16 (mouse melanoma) tumor cell lines. Results: Both synthesized compounds (nBu₃SnL1 and nBu₃SnL2) exerted powerful micromolar IC₅₀ cytotoxicity values and demonstrated high selectivity toward malignant cells. Both experimental drugs affected cell adhesion and induced anchorage independent apoptosis, a favorable type of cell death with an essential role in cancer dissemination prevention. The BSA-binding affinity of the obtained organotin compounds was followed by spectrofluorometric titration and molecular docking simulations. Conclusions: The tributyltin(IV) compounds selectively induce anoikis-like cell death in A375 cells, also highlighting the importance of the organic moiety on the tin(IV) ion in the mechanism of action.

Keywords: 2-quinolones; BSA interactions; apoptosis; in vitro cytotoxicity; molecular docking; tributyltin(IV) compounds.

Scheme 1

Synthesis of the ligand precursors, HL1 and HL2 [39], and tributyltin(IV) compounds, nBu₃SnL1 and nBu₃SnL2. The numbering of atoms was used for NMR assignation and matched the one applied in the X-ray study of nBu₃SnL2.

Figure 1

ORTEP diagram (25% probability ellipsoids) of a selected part of the chain formed by nBu₃SnL2 in the solid state. All hydrogen atoms are omitted for clarity and, of disordered atoms, only one atomic position is shown. Symmetry codes: “A” = ½ − x, y + ½, ½ − z; “B” = ½ − x, −½ + y, ½ − z.

Figure 2

Optimized structures of organotin compounds at B3LYP-D3BJ/6-311++G(d,p)(H,C,N,O)/def2-TZVP(Sn) level of theory: (a) nBu₃SnL1, (b) nBu₃SnL2 (Hydrogen—white, carbon—gray, nitrogen—blue, oxygen—red, tin—dark green).

Figure 3

Experimental (black) and theoretical IR spectra (blue) of investigated compounds: (a) nBu₃SnL1, (b) nBu₃SnL2 (at B3LYP-D3BJ/6-311++G(d,p)(H,C,N,O)/def2-TZVP(Sn) level of theory).

Figure 4

Compounds nBu₃SnL1 and nBu₃SnL2 induced anchorage independent apoptosis in A375 cultures. A375 cells were exposed to an IC₅₀ dose of compounds nBu₃SnL1 and nBu3SnL2, and flow cytometry of Ann/PI-stained cells was performed after (a) 72 h treatment and (b) 48 h treatment.

Figure 5

PI staining on chamber slides after 48 h treatment of A375 cells with compounds nBu₃SnL1 and nBu₃SnL2 (IC₅₀ dose).

Figure 6

Compounds nBu₃SnL1 and nBu₃SnL2 affected the adhesion and ROS/RNS production. (a) A375 cell adhesion on Matrigel^® and (b) ROS/RNS production (* and ** are for p < 0.05, p < 0.01, respectively).

Figure 7

Fluorescence emission spectra of BSA for the titration with nBu₃SnL2 at (a) 27°, (b) 32°, (c) 37°, and (d) Van ’t Hoff plot for the binding process.

Figure 7

Fluorescence emission spectra of BSA for the titration with nBu₃SnL2 at (a) 27°, (b) 32°, (c) 37°, and (d) Van ’t Hoff plot for the binding process.

Figure 8

The most stable positions of (a) nBu₃SnL1 and (b) nBu₃SnL2 compounds within BSA structure.

Figure 9

The most important interactions between organotin(IV) compounds (a) nBu₃SnL1 and (b) nBu₃SnL2 and BSA in the active pockets close to Trp213. Different colors of the interactions represent the following: van der Waal interactions and carbon–hydrogen bonds—light green; conventional hydrogen bonds—dark green; π–alkyl—orange; π–π stacked—dark pink; alkyl and π–alkyl—light pink (Hydrogen—white, carbon—gray, nitrogen—blue, oxygen—red, tin—dark green).