Enhanced antifungal efficacy of azole drugs through combination with usnic acid
- 1Department of Clinical Medicine, Yangtze University
- 2Department of Dermatology, Hubei Provincial Clinical Research Center for Diagnosis and Therapeutics of Pathogenic Fungal Infection, Jingzhou Hospital Affiliated to Yangtze University
Abstract
Introduction: Drug-resistant fungal infections remain a major clinical challenge, and antifungal adjuvants offer a promising strategy to restore azole efficacy. This study investigated whether usnic acid (UA), which lacks intrinsic antifungal activity, enhances the activity of itraconazole (ITR), voriconazole (VOR), posaconazole (POS), isavuconazole (ISA), and fluconazole (FLU) against clinically relevant fungal pathogens.
Methods: The synergistic effects of UA combined with each azole were evaluated against clinical isolates of Aspergillus spp. ( n = 64), Candida spp. ( n = 24), Exophiala dermatitidis ( n = 14), and Cryptococcus neoformans ( n = 8) using CLSI-standardized broth microdilution and checkerboard assays. According to their clinical use, ISA was tested against molds, whereas FLU was tested against Candida spp. and C. neoformans . Interactions were classified by the fractional inhibitory concentration index as synergy (FICI ≤ 0.5), indifference (0.5 < FICI ≤ 4.0), or antagonism (FICI > 4.0). Rates were reported with 95% Wilson confidence intervals and compared using Fisher–Freeman–Halton and pairwise Fisher exact tests with Benjamini–Hochberg correction.
Results: UA alone showed no antifungal activity. The overall synergy rates were 67.27% (74/110; 95% CI 58.05–75.33%) for UA+POS, 11.82% (13/110; 95% CI 7.04–19.18%) for UA+ITR, 9.09% (2/22; 95% CI 2.53–27.81%) for UA+FLU, 5.45% (6/110; 95% CI 2.52–11.39%) for UA+VOR, and 0% (0/78; 95% CI 0–4.62%) for UA+ISA. UA+POS showed a significantly higher synergy rate than each other combination after multiple-comparison correction ( P < 0.001). UA+ITR caused no antagonism, whereas UA+POS caused antagonism in only 0.91% (1/110) of isolates. UA+VOR and UA+FLU showed marked antagonism against C. neoformans (100%, 8/8 and 87.5%, 7/8, respectively). UA+POS produced broad synergy against A. terreus and E. dermatitidis regardless of susceptibility, whereas activity against A. fumigatus was mainly restricted to susceptible isolates. Mechanistic assays showed increased reactive oxygen species levels in five of seven synergistic strains and efflux pump inhibition in four, with strains 382 and PY1b exhibiting both mechanisms.
Discussion: These findings identify UA+POS as the most promising adjuvant combination, while the species- and resistance-dependent effects suggest that its activity requires careful contextual interpretation. The dual involvement of reactive oxygen species accumulation and efflux pump inhibition supports a mechanistically diverse mode of synergy. However, these results are limited to in vitro testing, and further mechanistic and in vivo studies are required to evaluate the therapeutic potential and safety of UA+POS.
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BibTeX
@article{tang2026enhanced,
title = {{Enhanced antifungal efficacy of azole drugs through combination with usnic acid}},
author = {Menghua Tang and Xiaoman Yang and Lu Ge and Shuyang Xie and Yixin Zeng and Tian Chen and Heng Zhang and Yi Sun},
journal = {Frontiers in Microbiology},
year = {2026},
month = oct,
doi = {10.3389/fmicb.2026.1864975},
url = {https://www.frontiersin.org/articles/10.3389/fmicb.2026.1864975/full},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Enhanced antifungal efficacy of azole drugs through combination with usnic acid
AU - Menghua Tang
AU - Xiaoman Yang
AU - Lu Ge
AU - Shuyang Xie
AU - Yixin Zeng
AU - Tian Chen
AU - Heng Zhang
AU - Yi Sun
PY - 2026
DA - 2026/10/19
JO - Frontiers in Microbiology
DO - 10.3389/fmicb.2026.1864975
UR - https://www.frontiersin.org/articles/10.3389/fmicb.2026.1864975/full
AB - Introduction: Drug-resistant fungal infections remain a major clinical challenge, and antifungal adjuvants offer a promising strategy to restore azole efficacy. This study investigated whether usnic acid (UA), which lacks intrinsic antifungal activity, enhances the activity of itraconazole (ITR), voriconazole (VOR), posaconazole (POS), isavuconazole (ISA), and fluconazole (FLU) against clinically relevant fungal pathogens. Methods: The synergistic effects of UA combined with each azole were evaluated against clinical isolates of Aspergillus spp. ( n = 64), Candida spp. ( n = 24), Exophiala dermatitidis ( n = 14), and Cryptococcus neoformans ( n = 8) using CLSI-standardized broth microdilution and checkerboard assays. According to their clinical use, ISA was tested against molds, whereas FLU was tested against Candida spp. and C. neoformans . Interactions were classified by the fractional inhibitory concentration index as synergy (FICI ≤ 0.5), indifference (0.5 < FICI ≤ 4.0), or antagonism (FICI > 4.0). Rates were reported with 95% Wilson confidence intervals and compared using Fisher–Freeman–Halton and pairwise Fisher exact tests with Benjamini–Hochberg correction. Results: UA alone showed no antifungal activity. The overall synergy rates were 67.27% (74/110; 95% CI 58.05–75.33%) for UA+POS, 11.82% (13/110; 95% CI 7.04–19.18%) for UA+ITR, 9.09% (2/22; 95% CI 2.53–27.81%) for UA+FLU, 5.45% (6/110; 95% CI 2.52–11.39%) for UA+VOR, and 0% (0/78; 95% CI 0–4.62%) for UA+ISA. UA+POS showed a significantly higher synergy rate than each other combination after multiple-comparison correction ( P < 0.001). UA+ITR caused no antagonism, whereas UA+POS caused antagonism in only 0.91% (1/110) of isolates. UA+VOR and UA+FLU showed marked antagonism against C. neoformans (100%, 8/8 and 87.5%, 7/8, respectively). UA+POS produced broad synergy against A. terreus and E. dermatitidis regardless of susceptibility, whereas activity against A. fumigatus was mainly restricted to susceptible isolates. Mechanistic assays showed increased reactive oxygen species levels in five of seven synergistic strains and efflux pump inhibition in four, with strains 382 and PY1b exhibiting both mechanisms. Discussion: These findings identify UA+POS as the most promising adjuvant combination, while the species- and resistance-dependent effects suggest that its activity requires careful contextual interpretation. The dual involvement of reactive oxygen species accumulation and efflux pump inhibition supports a mechanistically diverse mode of synergy. However, these results are limited to in vitro testing, and further mechanistic and in vivo studies are required to evaluate the therapeutic potential and safety of UA+POS.
SN - 1664-302X
ER -