Objectives
Among cases of invasive candidiasis in the United States, Candida glabrata is the second most common causative species. Resistance to the commonly used antifungal fluconazole is increasing in C. glabrata with reports of multidrug resistant isolates becoming a cause for concern. VT-1598 and VT-1161 are novel tetrazole CYP51 inhibitors that possess increased specificity for fungal CYP51 as compared to the human CYP450 enzymes. Previous studies have shown promising results against yeasts, endemics, and molds, including multiple species of Candida, often revealing greater potency than members of the azole class of antifungals. In this study we assessed the activity of VT-1598 and VT-1161 against a panel of azole-resistant C. glabrata clinical isolates, as well as strains deleted for the genes encoding the key azole-resistance determinants Pdr1, Cdr1, Pdh1, Snq2, and Upc2A. 

Methods
Seven unrelated fluconazole-resistant clinical isolates were used in this study. Also included was a matched pair of fluconazole susceptible-dose dependent and resistant clinical isolates and their respective mutants deleted for the major determinants of fluconazole resistance.  Susceptibility testing was performed according to standard CLSI methods to determine minimum inhibitory concentrations (MICs) for fluconazole, itraconazole, posaconazole, voriconazole, VT-1598, and VT-1161 (50% growth inhibition). 

Results
The seven fluconazole-resistant isolates (fluconazole MIC ≥ 64) exhibited MIC for VT-1598 between 1 and 2 mg/L and for VT-1161 between 0.5 and 2 mg/L as compared to 0.125 mg/L and 0.03 mg/L respectively for the fluconazole susceptible-dose dependent isolate. MIC for clinically approved azoles tested against the fluconazole-resistant isolates were also 1 to 2 mg/L compared to 0.25, 0.06, and ≤ 0.03 mg/L in the fluconazole susceptible-dose dependent isolate for itraconazole, posaconazole, and voriconazole respectively. Deletion of PDR1, CDR1, PDH1, and SNQ2 all increased susceptibility to VT-1598 and VT-1161 in fluconazole-resistant isolate SM3. Deletion of UPC2A resulted in increased susceptibility to VT-1598 and deletion of UPC2A and PDR1 resulted in increased susceptibility to VT-1161 in the SM3 background as well as the fluconazole susceptible-dose dependent isolate SM1.  

Conclusion
As with fluconazole, activating mutations in PDR1 leading to overexpression of CDR1, PDH1, and SNQ2 reduces susceptibility of C. glabrata to VT-1598 and VT-1161. Likewise, deletion of UPC2A results in increased susceptibility of C. glabrata to VT-1598 and VT-1161 suggesting a strategy for improving the activity of these novel antifungals against fluconazole resistant C. glabrata.

Contributions of the major &lt;i&gt;C. glabrata&lt;/i&gt; triazole resistance determinants to the novel investigational tetrazoles VT-1598 and VT-1161

Contributions of the major <i>C. glabrata</i> triazole resistance determinants to the novel investigational tetrazoles VT-1598 and VT-1161