Assessment of nonreleasing antifungal surface coatings bearing covalently attached pharmaceuticals

There are many reports of antimicrobial coatings bearing immobilized active agents on surfaces; however, strong analytical evidence is required to verify that the agents are indeed covalently attached to the surface. In the absence of such evidence, antimicrobial activity could result from a release of active agents. We report a detailed assessment of antifungal surface coatings prepared using covalent attachment chemistries, with the aim of establishing a set of instrumental and biological evidence required to convincingly demonstrate antimicrobial activity due to nonreleasing, surface active compounds and to exclude the alternate possibility of activity due to release. The strongest biological evidence initially supporting permanent antifungal activity was the demonstration of the ability to reuse samples in multiple, sequential pathogen challenges. However, additional supporting evidence from washing studies and instrumental analysis is also required to probe the possibility of gradual desorption of strongly physisorbed compounds versus covalently attached compounds. Potent antifungal surface coatings were prepared from approved pharmaceutical compounds from the echinocandin drug class (caspofungin, anidulafungin, and micafungin) and assessed by microbiological tests and instrumental methods. Carbonyl diimidazole linking chemistry enabled covalent attachment of caspofungin, anidulafungin, and micafungin to plasma polymer surfaces, with antifungal surface activity likely caused by molecular orientations that present the lipophilic tail toward interfacing fungal cells. This study demonstrates the instrumental and biological evidence required to convincingly ascertain activity due to nonreleasing, surface active compounds and summarize these as three criteria for assessing other reports on surface-immobilized antimicrobial compounds.