The Strength of Synergistic Interaction between Posaconazole and Caspofungin Depends on the Underlying Azole Resistance Mechanism of Aspergillus fumigatus

Abstract

bThe majority of azole resistance mechanisms in Aspergillus fumigatus correspond to mutations in the cyp51A gene. As azoles are less effective against infections caused by multiply azole-resistant A. fumigatus isolates, new therapeutic options are warranted for treating these infections. We therefore investigated the in vitro combination of posaconazole (POSA) and caspofungin (CAS) against 20 wild-type and resistant A. fumigatus isolates with 10 different resistance mechanisms. Fungal growth was assessed with the XTT [2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide inner salt] method. Pharmacodynamic interactions were assessed with the fractional inhibitory concentration (FIC) index (FICi) on the basis of 10% (FICi-0), 25% (FICi-1), or 53 0% (FICi-2) growth, and FICs were correlated with POSA and CAS concentrations. Synergy and antagonism were concluded when the FICi values were statistically significantly (t test, P < 0.05) lower than 1 and higher than 1.25, respectively. Significant synergy was found for all isolates with mean FICi-0 values ranging from 0.28 to 0.75 (median, 0.46). Stronger synergistic interactions were found with FICi-1 (median, 0.18; range, 0.07 to 0.47) and FICi-2 (0.31; 0.07 to 0.6). The FICi-2 values of isolates with tandem-repeat-containing mutations or codon M220 were lower than those seen with the other isolates (P < 0.01). FIC-2 values were inversely correlated with POSA MICs (r s ‫؍‬ ؊0.52, P ‫؍‬ 0.0006) and linearly with the ratio of drug concentrations in combination over the MIC of POSA (r s ‫؍‬ 0.76, P < 0.0001) and CAS (r s ‫؍‬ 0.52, P ‫؍‬ 0.0004). The synergistic effect of the combination of POSA and CAS (POSA/CAS) against A. fumigatus isolates depended on the underlying azole resistance mechanism. Moreover, the drug combination synergy was found to be increased against isolates with elevated POSA MICs compared to wild-type isolates.T he opportunistic fungal pathogen Aspergillus fumigatus has been associated with several life-threatening infections in immunocompromised patients. Although azoles are the mainstay of antifungal therapy, treatment of aspergillosis is a difficult task which is further complicated by the lack of therapeutic efficacy in infections due to multiply azole-resistant A. fumigatus (1-3).Azoles are inhibitors of the 14-␣ sterol demethylase in A. fumigatus, which is a product of cyp51A and cyp51B. Since the discovery of these orthologs to the Candida albicans egr11 gene, a number of single nucleotide polymorphisms (SNPs) have been found, several of which have been associated with elevated azole MICs in vitro, corresponding to treatment failure in vivo (1, 4-7). It is believed that SNPs may develop through azole treatment or through exposure to azole fungicides in the environment (8-10). Treatment-induced SNPs in cyp51A are mainly allocated at codon 38, 54, or 220, while fungicide-induced SNPs are mostly located at codon 98, usually combined with tandem repeats within the promoter (8-10). Very recently, a new environmental azole resist...

Echinocandins for the Treatment of Invasive Aspergillosis: from Laboratory to Bedside

“…These observations are supported by a murine model of pulmonary IA in which the combination of voriconazole and anidulafungin was synergistic against an azole-susceptible A. fumigatus isolate but only additive against the azole-resistant isolate harboring the TR 34 /L98H mutation, with higher doses required for the latter (75). The combination of posaconazole and caspofungin was synergistic in most cases (76). Interestingly, the synergism was also present against azole-resistant A. fumigatus isolates and was more pronounced for those harboring the tandem repeat of the promoter region or the M220 mutation in cyp51A compared with other mutations of resistance.…”

In Vitro Combination of Isavuconazole with Echinocandins against Azole-Susceptible and -Resistant Aspergillus spp

“…Because antifungal combinations are difficult to test against filamentous fungi, we used two methods, based on different principles (a checkerboard microdilution broth technique and an agar diffusion technique), and we chose to interpret the results with complete inhibition endpoints. We also used two reading methods and showed, as reported previously (26,(37)(38)(39)(40), that spectrophotometric reading is a good alternative to visual reading for MIC determinations with the EUCAST-based technique.…”

A Cationic Polymer That Shows High Antifungal Activity against Diverse Human Pathogens

“…The MM-TM polymer displays strong synergistic activity with azole drugs against C. albicans and A. fumigatus, even against some azole-resistant strains. The latter finding is especially promising, as many studies are turning to combination approaches to stem the rise of resistance to conventional antifungal drugs and preserve activity of the limited number of therapeutic antifungal drugs available thus far (29,30). The decreased MIC 100 values (Յ1 M and Յ3 g/ml) of MM-TM against select strains of Candida, Cryptococcus, and Aspergillus in combination with either azoles or AMB highlight the potential of MM-TM in the combination format.…”

“…To probe the relationships between these mutations and synergy, we evaluated MIC 100 values for MM-TM with either itraconazole or posaconazole against a panel of strains with different combinations of mutations similar to those of strains F11628 and F16216 (see Table S3 in the supplemental material). There was no clear pattern between types of mutations in azole-resistant strains and synergism between an azole and the MM-TM polymer (Tables S4 and S5), which may reflect the considerable heterogeneity among A. fumigatus strains (29,33). These results indicate that whether synergy is observed for MM-TM/azole combinations is dependent on unknown properties of the strains.…”

“…This finding contrasts with the results for the itraconazole-resistant strain F16216, for which no synergy between the azole and the polymer was detected. This finding was somewhat surprising because the F11628 and F11626 strains have mutations in the same gene, cyp51A, which encodes a 14␣-sterol demethylase that is involved in ergosterol biosynthesis (Table 2) (29,31,32). To probe the relationships between these mutations and synergy, we evaluated MIC 100 values for MM-TM with either itraconazole or posaconazole against a panel of strains with different combinations of mutations similar to those of strains F11628 and F16216 (see Table S3 in the supplemental material).…”