Abstract
Candidiasis caused by Candida albicans is one of the most common microbial infections. Azoles, polyenes, allylamines, and echinocandins are classes of antifungals used for treating Candida infections. Standard drug doses often become ineffective due to the emergence of multidrug resistance (MDR). This leads to the use of higher drug doses for prolonged duration, resulting in severe toxicity (nephrotoxicity and liver damage) in humans. However, combination therapy using very low concentrations of two or more antifungal agents together, can lower such toxicity and limit evolution of drug resistance. Herein, 4–6 nm zinc oxide quantum dots (ZnO QDs) were synthesized and their in vitro antifungal activities were assessed against drug-susceptible (G1, F1, and GU4) and resistant (G5, F5, and GU5) isolates of C. albicans. In broth microdilution assay, ZnO QDs exhibited dose dependent growth inhibition between 0 – 200 µg/ml and almost 90% growth was inhibited in all Candida strains at 200 µg/ml of ZnO QDs. Synergy between ZnO QDs and antifungal drugs at sub-inhibitory concentrations of each was assessed by checkerboard analysis and expressed in terms of the fractional inhibitory concentration (FIC) index. ZnO QDs were used with two different classes of antifungals (azoles and polyenes) against Candida isolates: combination 1 (with fluconazole); combination 2 (with ketoconazole); combination 3 (with amphotericin B), and combination 4 (with nystatin). Results demonstrated that the potency of combinations of ZnO QDs with antifungal drugs even at very low concentrations of each was higher than their individual activities against the fungal isolates. The FIC index was found to be less than 0.5 for all combinations in the checkerboard assay, which confirmed synergism between sub-inhibitory concentrations of ZnO QDs (25 µg/ml) and individual antifungal drugs. Synergism was further confirmed by spot assay where cell viabilities of Candida strains were significantly reduced in all combinations, which was clearly evident from the disappearance of fungal cells on agar plates containing antifungal combinations. For safer clinical use, the in vitro cytotoxic activity of ZnO QDs was assessed against HeLa cell line and it was found that ZnO QDs were non-toxic at 25 µg/ml. Results suggested that the combination of ZnO QDs with drugs potentiate antimicrobial activity through multitargeted action. ZnO QDs could therefore offer a versatile alternative in combination therapy against MDR fungal pathogens, wherein lowering drug concentrations could reduce toxicity and their multitargeted action could limit evolution of fungal drug resistance.
Highlights
Recurrence of local and systemic fungal infections have posed a life-threatening public health problem globally for immunocompromised individuals, and treatment for fungal infections is estimated to cost around more than $7.2 billion (Benedict et al, 2019)
The corresponding selected area electron diffraction (SAED) pattern confirmed the crystalline nature of ZnO quantum dots” (QDs) and the obtained rings matched with the diffraction planes of the X-ray diffraction (XRD) spectra (Figure 2D)
Monotherapy against microbial infections often allow for the emergence of MDR in microbes with time and makes the standard drug doses ineffective leading to failure in treatment
Summary
Recurrence of local and systemic fungal infections have posed a life-threatening public health problem globally for immunocompromised individuals, and treatment for fungal infections is estimated to cost around more than $7.2 billion (Benedict et al, 2019). Individuals with cancer treatment, organ transplantation surgeries, congenital immunodeficiency syndromes, acquired immunodeficiency syndrome (AIDS), and indwelling medical devices are frequently infected by Candida, resulting in morbidity and mortality due to invasive candidiasis (Prasad et al, 2011; Pal et al, 2017; Tso et al, 2018; Xiao et al, 2019). Antifungal drugs such as triazoles (fluconazole, itraconazole, and voriconazole) constitute the first line of standard treatment for candidiasis but due to their fungistatic action, fungal strains acquire resistance in due course toward the standard doses of these drugs (Watt et al, 2013). Echinocandins have a few drawbacks such as poor oral availability, high cost, and administration by intravenous injection only; which limit their use as standard treatment for invasive candidiasis (Spampinato and Leonardi, 2013)
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