Phytolacca tetramera berries extracts and its main constituents as potentiators of antifungal drugs against Candida spp.

Abstract

Background: Extensive antifungal drug use has enhanced fungal resistance, resulting in persistent mycoses. Combining antifungal plant extracts/compounds with these drugs offers good alternatives to increase the activity of both partners, minimize side effects, and overcome drug resistance. In our previous study, Phytolacca tetramera berries extracts demonstrated activity Candida spp., correlating with the amount of the main constituent phytolaccoside B and its genin, phytolaccagenin. The extracts and phytolaccagenin altered the fungal plasma membrane by binding to ergosterol, whereas phytolaccoside B increased chitin synthase activity. However, the presence of triterpenoid saponins in Phytolacca spp. has been linked to acute toxicity in humans. Purpose: This study aimed to evaluate combinations of P. tetramera berries extracts, phytolaccoside B and phytolaccagenin, together with commercial antifungals [amphotericin B, fluconazole, itraconazole, posaconazole, and caspofungin] against Candida albicans and Candida glabrata, to find synergistic effects with multi-target actions, in which the doses of both partners are reduced, and therefore their toxicity. Additionally, we intended to explore their anti-virulence capacity, thereby hindering the development of drug-resistant strains. Methods: The effects of these combinations were evaluated using both the checkerboard and isobologram methods. Fractional Inhibitory Concentration Index and Dose Reduction Index were calculated to interpret the combination results. To confirm the multi-target effect, studies on mechanisms of action of synergistic mixtures were performed using ergosterol-binding and quantification assays. The ability to inhibit Candida virulence factors, including biofilm formation and eradication from inert surfaces, was also evaluated. Quantification of active markers was performed using a validated UHPLC-ESI-MS method. Results: Eight synergistic combinations of P. tetramera extracts or phytolaccagenin (but not phytolaccoside B) with itraconazole or posaconazole were obtained against C. albicans, including a resistant strain. These mixtures acted by binding to ergosterol, decreasing its whole content, and inhibiting Candida biofilm formation in 96-well microplates and feeding tubes in vitro, but were unable to eradicate preformed biofilms. Conclusions: This study demonstrated the synergistic and anti-virulence effects of P. tetramera berries extracts and phytolaccagenin with antifungal drugs against Candida spp., providing novel treatment avenues for fungal infections with reduced doses of both natural products and commercial antifungals, thereby mitigating potential human toxicity concerns.