In vitro activity of fluconazole in combination with cyclosporine A or lactoferrin against Candida auris

Objective. Candida auris is an emergent fungus that causes a wide spectrum of mycoses, being the invasive candidiasis in intensive care settings one of the infections of greatest concern. Most Candida auris isolates are resistant to fluconazole and other current antifungal drugs. This fact makes the finding of new therapeutic strategies an important issue. The combination of fluconazole with different compounds, such as the calcineurin inhibitor cyclosporine A or the lactoferrin, with antimicrobial and antiinflammatory properties, has been effective against fluconazole-resistant Candida isolates.
The aim of this work was to evaluate the in vitro interaction between fluconazole and cyclosporin A or lactoferrin against clinical isolates of Candida auris.
Methods: We studied 21 Candida auris isolates obtained from different clinical samples (seven from blood culture, seven from oropharynx and seven from urine cultures). Candida parapsilosis ATCC 22019, Candida albicans ATCC 64550 and Candida albicans ATCC 64548 were used as quality controls. The minimum inhibitory concentration (MIC) was determined according to the European Committee on Antibiotic Susceptibility Testing (EUCAST) EDef 7.3.1 document. The interaction between fluconazole and cyclosporine A or lactoferrin was assessed by a checkerboard microdilution method. The final concentration of fluconazole ranged from 1 to 64 μg/ml, cyclosporine A concentrations ranged from 0.125 to 64 μg/ml, and lactoferrin concentrations from 0.031 to 32 μg/ml. The in vitro interaction of the drug combinations was interpreted in terms of the fractional inhibitory concentration index (FICI) as follows: FICI ≤ 0.5, synergistic; 0.5 < FICI ≤ 1, additive; 1 < FICI ≤ 4, indifferent and FICI > 4 antagonistic.
Results: The combination of fluconazole with lactoferrin significantly reduced the MIC of fluconazole against Candida auris isolates. The MIC of fluconazole decreased from > 64 μg/ml to 1-16 μg/ml against 20 of the 21 isolates included. The combination of fluconazole with cyclosporin A did not alter the MIC of fluconazole. The effect of the combination of fluconazole with lactoferrin against 16 of the isolates was interpreted as additive and against the other five isolates was indifferent. On the other side, the effect of the combination of fluconazole with cyclosporin A against all the isolates studied was indifferent. 
Conclusion: The combination of fluconazole with cyclosporin A or lactoferrin did not show synergistic effect against Candida auris isolates studied, although lactoferrin-fluconazole combination significantly reduced the fluconazole MICs against most of the isolates.
Funding: This study was partially financed by GIC15/78 IT-990-16 (Gobierno Vasco-Eusko Jaurlaritza), and SAF2017-86188-P (MINECO, Spain).