Multicenter Collaborative Study of the Interaction of Antifungal Combinations against Candida Spp. by Loewe Additivity and Bliss Independence-Based Response Surface Analysis

Abstract

Combination antifungal therapy is widely used but not well understood. We analyzed the spectrophotometric readings from a multicenter study conducted by the New York State Department of Health to further characterize the in vitro interactions of the major classes of antifungal agents against Candida spp. Loewe additivity-based fractional inhibitory concentration index (FICi) analysis and Bliss independence-based response surface (BIRS) analysis were used to analyze two-drug inter- and intraclass combinations of triazoles (AZO) (voriconazole, posaconazole), echinocandins (ECH) (caspofungin, micafungin, anidulafungin), and a polyene (amphotericin B) against Candida albicans, C. parapsilosis, and C. glabrata. Although mean FIC indices did not differ statistically significantly from the additivity range of 0.5−4, indicating no significant pharmacodynamic interactions for all of the strain−combinations tested, BIRS analysis showed that significant pharmacodynamic interactions with the sum of percentages of interactions determined with this analysis were strongly associated with the FIC indices (Χ2 646, p < 0.0001). Using a narrower additivity range of 1−2 FIC index analysis, statistically significant pharmacodynamic interactions were also found with FICi and were in agreement with those found with BIRS analysis. All ECH+AB combinations were found to be synergistic against all Candida strains except C. glabrata. For the AZO+AB combinations, synergy was found mostly with the POS+AB combination. All AZO+ECH combinations except POS+CAS were synergistic against all Candida strains although with variable magnitude; significant antagonism was found for the POS+MIF combination against C. albicans. The AZO+AZO combination was additive for all strains except for a C. parapsilosis strain for which antagonism was also observed. The ECH+ECH combinations were synergistic for all Candida strains except C. glabrata for which they were additive; no antagonism was found.

Keywords: Bliss independence; Candida; Loewe additivity; antagonism; antifungal drugs; pharmacodynamic interactions; synergy.

Figure 1

Schematic representation of Fractional inhibitory concentration (FIC) index analysis (A) and Bliss independence response surface analysis (B) of the caspofungin+amphotericin B combination against the C. albicans #92 strain tested in center 2. (A) The numbers in the cells are percentages of growth. ∑FICmin (green dot) was 0.625, and ∑FICmax (red dot) was 1.06. (B). The numbers in the cells are percentages of Bliss interactions. Green-colored cells represent synergistic interactions, whereas red-colored cells represent antagonistic interactions. Note that FIC index analysis captures pharmacodynamic interaction at concentrations near MIC, whereas Bliss interaction analysis captures interactions at the entire range of concentrations. Interactions at lower concentrations could be captured with FIC index using MIC-1, MIC-2, and MIC-3 corresponding to 25%, 50%, and 75% growth, shown by different shades of blue.

Figure 2

Spearman’s correlation analysis between ∑FIC_min and Bliss antagonistic interactions (left graph) and ∑FIC_max and Bliss antagonistic sum interactions (right graph).