6″-Disubstituted Amphiphilic Kanamycins

molecules Article Antifungal Activities of 4”,6”-Disubstituted Amphiphilic Kanamycins 1, 1, 2 2 Madher N. Alfindee y , Yagya P. Subedi y , Michelle M. Grilley , Jon Y. Takemoto and Cheng-Wei T. Chang 1,* 1 Department of Chemistry and Biochemistry, Utah State University, 0300 Old Main Hill, Logan, UT 84322-0300, USA; [email protected] (M.N.A.); [email protected] (Y.P.S.) 2 Department of Biology, Utah State University, 5305 Old Main Hill, Logan, UT 84322-5305, USA; [email protected] (M.M.G.); [email protected] (J.Y.T.) * Correspondence: [email protected] These authors contributed equally to this work. y Received: 16 April 2019; Accepted: 14 May 2019; Published: 16 May 2019 Abstract: Amphiphilic kanamycins derived from the classic antibiotic kanamycin have attracted interest due to their novel bioactivities beyond inhibition of bacteria. In this study, the recently described 4”,6”-diaryl amphiphilic kanamycins reported as inhibitors of connexin were examined for their antifungal activities. Nearly all 4”,6”-diaryl amphiphilic kanamycins tested had antifungal activities comparable to those of 4”,6”-dialkyl amphiphilic kanamycins, reported previously against several fungal strains. The minimal growth inhibitory concentrations (MICs) correlated with the degree of amphiphilicity (cLogD) of the di-substituted amphiphilic kanamycins. Using the fluorogenic dyes, SYTOXTM Green and propidium iodide, the most active compounds at the corresponding MICs or at 2 MICs caused biphasic dye fluorescence increases over time with intact cells. Further × lowering the concentrations to half MICs caused first-order dye fluorescence increases. Interestingly, 4 MIC or 8 MIC levels resulted in fluorescence suppression that did not correlate with the MIC and × × plasma membrane permeabilization. The results show that 4”,6”-diaryl amphiphilic kanamycins are antifungal and that amphiphilicity parameter cLogD is useful for the design of the most membrane-active versions. A cautionary limitation of fluorescence suppression was revealed when using fluorogenic dyes to measure cell-permeation mechanisms with these antifungals at high concentrations. Finally, 4”,6”-diaryl amphiphilic kanamycins elevate the production of cellular reactive oxygen species as other reported amphiphilic kanamycins. Keywords: amphiphilic kanamycin; Cryptococcus neoformans; antifungal; kinetic membrane permeabilization; SYTOXTM green; propidium iodide 1. Introduction Although commonly unappreciated, fungal diseases cause tremendous economic loss and health impacts globally. Annually, more than a billion people acquire fungal infections. The death rate from fungal diseases is equal to that of tuberculosis and malaria combined, with approximately 10% of those deaths due to cryptococcal meningitis [1,2]. Immunosuppressed individuals, such as those infected with HIV or undergoing treatment for cancer or organ transplants, are especially susceptible to invasive fungal infections. Current treatment options for fungal infections include polyene-based compounds (e.g., amphotericin B) [3–5], cytosine-based compounds (e.g., flucytosine) [5,6], and azole-based compounds (e.g., itraconazole and fluconazole) [5,7,8]. However, as exemplified in the recent outbreak of Candida auris [9], drug resistance is a growing problem. Further research to develop new and effective treatments for fungal diseases is urgent. Molecules 2019, 24, 1882; doi:10.3390/molecules24101882 www.mdpi.com/journal/molecules Molecules 2019, 24, 1882 2 of 12 Molecules 2019, 24, x FOR PEER REVIEW 2 of 13 Molecules 2019, 24, x FOR PEER REVIEW 2 of 13 Kanamycin belongs to a class of antibacterial compounds known as aminoglycosides that are Kanamycin belongs to a class of antibacterial compounds known as aminoglycosides that are activeKanamycin against both belongs Gram-negative to a class (G of) antibacterial and Gram-positive compounds (G+) bacteria, known albeitas aminoglycosides its clinical use is that limited are active against both Gram-negative −(G−) and Gram-positive (G+) bacteria, albeit its clinical use is dueactive to against the emergence both Gram of bacterial-negative resistance (G−) and (FigureGram-1positive)[ 10]. To(G+ overcome) bacteria, thealbeit problem its clinical of bacterial use is limited due to the emergence of bacterial resistance (Figure 1) [10]. To overcome the problem of resistance,limited due extensive to the emergence research has of beenbacterial devoted resistance to structural (Figure modifications 1) [10]. To overcome of aminoglycosides the problem that of bacterial resistance, extensive research has been devoted to structural modifications of leadbacterial to the resistance, discovery ofextensive amphiphilic research aminoglycosides has been [ 11devoted,12]. In contrast to structural to antibacterial modifications kanamycin of aminoglycosides that lead to the discovery of amphiphilic aminoglycosides [11,12]. In contrast to thataminoglycoside is inactive againsts that lead fungi, to somethe discovery of the amphiphilic of amphiphilic kanamycins aminoglycosides (AKs) were [11,12 found]. In to contrast be active to antibacterial kanamycin that is inactive against fungi, some of the amphiphilic kanamycins (AKs) againstantibacterial a wide kanamycin range of fungal that is strains inactive [13, 14against] and concomitantlyfungi, some of not the active amphiphilic against bacteria.kanamycins Two (A ofKs) the were found to be active against a wide range of fungal strains [13,14] and concomitantly not active antifungalwere found AKs to be produced active against good specific a wide antifungalrange of fungal activity: strainsFG08 [13,14, which] and contains concomitantly an octyl group not active (C8) against bacteria. Two of the antifungal AKs produced good specific antifungal activity: FG08, which chainagainst attached bacteria at. Two the 4”of positionthe antifungal of kanamycin AKs produced analog good via an specific ether linkageantifungal [15 ]activity and K20: FG08, which, which has contains an octyl group (C8) chain attached at the 4″ position of kanamycin analog via an ether ancontains octanesulfonyl an octyl groupgroup at(C8) the chain 6” position attached (Figure at the1)[ 164″]. position of kanamycin analog via an ether linkage [15] and K20, which has an octanesulfonyl group at the 6″ position (Figure 1) [16]. linkage [15] and K20, which has an octanesulfonyl group at the 6″ position (Figure 1) [16]. Figure 1. StructureStructure of aminoglycosides and selected kanamycins ( (AKs).AKs). Figure 1. Structure of aminoglycosides and selected kanamycins (AKs). AKs are known to show their antimicrobial activity by increasing the membrane permeability AKs are known to show their antimicrobial activity by increasing the membrane permeability of microorganisms [17 [17––2121]].. Fluorogenic Fluorogenic dyes, dyes, such such as as SYTOX SYTOXTM TMgreengreen and and propidium propidium iodide iodide (PI), (PI), are of microorganisms [17–21]. Fluorogenic dyes, such as SYTOXTM green and propidium iodide (PI), are commonlyare commonly employed employed for the for study the study of membrane of membrane permeabilization permeabilization (Figure (Figure 2). SYTOX2). SYTOXTM greenTM isgreen non- commonly employed for the study of membrane permeabilization (Figure 2). SYTOXTM green is non- fluorescentis non-fluorescent and cannot and cannot penetrate penetrate the plasma the plasma membrane membrane of ofintact intact organisms. organisms. However, However, in in the fluorescent and cannot penetrate the plasma membrane of intact organisms. However, in the presence ofof agentsagents thatthat compromise compromise membrane membrane integrity, integrity, SYTOX SYTOXTMTMgreen green enters enters the the cytoplasm, cytoplasm, binds binds to presence of agents that compromise membrane integrity, SYTOXTM green enters the cytoplasm, binds tonucleic nucleic acids, acids and, and emits emits fluorescence. fluorescence. Propidium Propidium iodide iodide(PI) (PI) hashas similarsimilar propertiesproperties and is widely to nucleic acids, and emits fluorescence. Propidium iodide (PI) has similar properties and is widely used for evaluating membrane permeabilization of substances in fungi and bacteria. We We recently used for evaluating membrane permeabilization of substances in fungi and bacteria. We recently reported the synthesis of 4 4”,6”-diaryl″,6″-diaryl AK AKss (compounds (compounds 11––88)) as as connexin connexin inhibitor inhibitorss [22,23 [22,23]] (Figure (Figure 3).). reported the synthesis of 4″,6″-diaryl AKs (compounds 1–8) as connexin inhibitors [22,23] (Figure 3). Since these are structurally similar to the antifungal 4”,6”-dialkyl4″,6″-dialkyl AKs (compounds 9–13), we decided decided Since these are structurally similar to the antifungal 4″,6″-dialkyl AKs (compounds 9–13), we decided to investigate their antifungal activit activitiesies and to conduct mode of action studies of both groups of to investigate their antifungal activities and to conduct mode of action studies of both groups of compounds using fluorogenicfluorogenic dyes.dyes. compounds using fluorogenic dyes. Figure 2. Structures of SYTOXTM green and propidium iodide (PI). FigureFigure 2.2. StructuresStructures ofof SYTOXSYTOXTMTM green and pro propidiumpidium iodide ( (PI).PI). Molecules 2019, 24, 1882 3 of 12 Molecules 2019, 24, x FOR PEER REVIEW 3 of 13 FigureFigure 3.3. StructureStructure ofof selectedselected AKs.AKs. 2.2. ResultsResults andand DiscussionDiscussion 2.1. Fungal Growth Inhibition by 4”,6”-Disubstituted AKs 2.1.

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