Isavuconazole-Amphotericin B and Isavuconazole-Caspofungin In Vitro Synergic Activity Against Invasive Pulmonary Aspergillosis Molds Isolates

Background/Objectives: Invasive pulmonary aspergillosis (IPA) represents a critical respiratory condition mainly caused by Aspergillus fumigatus and other ubiquitous species such as Aspergillus flavus, Aspergillus niger, and Aspergillus terreus. IPA clinical management has been complicated by diagnostic challenges and therapeutic difficulties due to antifungal intrinsic or secondary resistance episodes. Despite this assumption, few scientific data have been reported about possible antifungal drug combinations. Herein, we propose an experimental evaluation using isavuconazole/amphotericin B and isavuconazole/caspofungin in vitro synergy assays to investigate their combined activity on Aspergillus spp. IPA clinical isolates. Methods: We globally analyzed 55 Aspergillus spp. isolates, practicing the gradient test methods with single and combined antifungal drugs through the MIC Strip test (Liofilchem, Roseto degli Abruzzi, Italy). The collected MIC values were interpreted according to the EUCAST guidelines and classified as synergy, additivity, indifference, and antagonism cases through a FIC index calculation. A statistical analysis on species’ correlation with particular synergy testing results was finally provided. Results: Despite an interesting activity against A. fumigatus, isavuconazole/amphotericin B did not report statistical significance, obtaining a consistent antagonism percentage (43.6%). On the other hand, isavuconazole/caspofungin showed a promising in vitro synergic activity, except for A. flavus isolates. Conclusions: Synergy testing demonstrated a significant species-specific trend. Future studies should be focused on Aspergillus spp. isolates and antifungal in vitro synergy testing, aiming to discourage or recommend any specific antifungal combinations, depending on the isolated species.