Interaction of azole compounds with DOPC and DOPC/Ergosterol bilayers by spin probe EPR spectroscopy: implications for antifungal activity

The influence of two antifungal azoles, the newly synthesized compound CPA18 and the commercial drug fluconazole (FLC), on the physical state of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and DOPC/ergosterol bilayers was investigated by spin probe electron paramagnetic resonance (EPR) spectroscopy with the aim of ascertaining if direct interactions with the plasma membrane are implied in the mechanism of action against Candida albicans. 5- and 16-Doxyl-stearic acids (5-DSA and 16-DSA, respectively) were employed to this purpose, and EPR spectra were acquired in the 0 to 40 °C temperature range. Spectral line shapes were analyzed within the theory for slow motion EPR using a microscopically ordered macroscopically disordered model to describe the DOPC multilamellar vesicles and an axially symmetric Brownian model for the spin probe motion. For CPA18 remarkable changes in the molecular organization and dynamics of the phospholipid bilayers were found in the region immediately below the polar head groups, probed by 5-DSA, whereas the bilayer core, probed by 16-DSA, was only slightly affected. On the other hand, no effects of FLC on DOPC bilayers were revealed in the regions examined. Small differences were observed between DOPC and DOPC/ergosterol systems. The direct interactions of CPA18 with model membranes here highlighted may contribute to the observed fungicidal properties against both fluconazole-sensitive and -resistant C. albicans strains.