Abstract
The increase in bacterial diseases and their antibiotic resistance has contributed immensely to the rise in global morbidity and mortality. Despite efforts made to develop therapeutic drugs to control infectious bacterial growth and resistance, studies have shown that most FDA-approved drugs become vulnerable to bacterial resistance over some time. Producing less toxic and potent antibacterial agents capable of inhibiting a wide range of bacterial strains is warranted. In this study, we have synthesized coumarin-bearing isoxazole, isoxazoline, and triazole hybrids and aimed for their encapsulation of Gold (Au) and Silver (Ag) nanoparticles (NPs). Both organic scaffolds and the encapsulated frameworks were tested for their bacterial activities against twelve strains (six Gram-positive and six Gram-negative).
The coumarin scaffolds were synthesized from 7-hydroxycoumarin through Williamson etherification. Functionalization of coumarin with isoxazole and isoxazoline was carried out through the hypervalent-iodine-assisted cycloaddition in the presence of various aryl-aldoximes. Conversion of the pendant alkyne to triazoles was carried out through click-chemistry with azides. Proton and carbon nuclear magnetic resonance. (1H-NMR, 13C-NMR), and Fourier transform infrared (FT-IR) were used to characterize the obtained organic scaffolds.
The Au and Ag NPs were prepared through a revised Turkevich and reduced citrate method. Both nano-metals were confirmed using a laser beam to visualize their colloidal nature in the media. UV-Vis, FT-IR, SEM-EDX, TEM, and XRD were used to characterize the prepared nanoparticles. Encapsulation was carried out using the protocols by Nemanashi et al., and Suvarna et al. The encapsulated nanoparticles were then assayed for their bacterial activity in comparison to their non-encapsulated forms through Broth microdilution using 96-well plates to culture the bacteria with obtained MIC values ranging from 15.625 to 500 μg/mL.
Furthermore, we investigated the cytotoxicity of the prepared frameworks that showed significant anticancer activity against MCF-7 and MDA-MB-231 cells. Doxorubicin was used as the positive control and untreated cells as the negative control in this study. The results revealed that tested synthesized coumarin-bearing isoxazole hybrids had significant anti-proliferative activity. Moreover, coumarin-bearing isoxazole [3m, 3g, and 3h,] and isoxazoline hybrids [4g and 4e] exhibited potent antiproliferative activity against most of the tested cancer cell lines with an IC50 range of 25.73 ± 0.41 - 54.5 ± 0.27 μg/mL. Compounds 3m, 3g, 3h, 4g, and 4e were non-toxic to Vero cells with IC50 values > 100 μg/mL as compared to Doxorubicin with significantly low IC50 values of 0.40 ± 0.07, 0.92 ± 1.33, and 0.78 ± 0.34 against Vero, MCF-7 and MDA-MB-231 cells, respectively.