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Objectives (1) To evaluate the antifungal activity of P-27530 against a broad range of yeast and filamentous fungi, by minimum inhibitory concentration (MIC) assay. (2) To evaluate the natural resistance frequency and the evolution to resistance of P-27530 in T. rubrum, compared to amorolfine, terbinafine, itraconazole and ciclopirox. Methods (1) Test isolates included 5 clinical isolates each (3 suscep- tible and 2 resistant strains where possible) of yeasts (Candida albi- cans, C. glabrata, C. parapsislosis, C. krusei, C. lusitaniae, C. tropicalis and Cryptococcus neoformans), moulds (Fusarium solani, Aspergillus fla- vus, A. fumigatus, A. niger, A. terreus, Pseudallescheria boydii, Scopulari- opsis brevicaulis, Scedosporium spp., S. prolificans, Cunninghamella bertholletiae, Rizopus oryzae, Mucor plumbeus, M. fragilis) and dermato- phytes (Trichophyton rubrum, T. mentagrophytes, Epidermophyton flocco- sum, Microsporum canis). MIC testing was performed according to the CLSI M27-A3 and M38-A2 standards for the susceptibility testing of yeasts and filamentous fungi, respectively. (2) The frequency of spon- taneous resistance of 3 T. rubrum strains was evaluated by counting the number of colonies obtained following incubation of agar plates inoculated with a total of about 109 CFU on agar plates containing drug concentrations higher than the minimal fungicidal concentra- tion; the evolution of resistance was evaluated in 2 T. rubrum strains serially propagated for 10 transfers on agar plates containing sub- inhibitory drug concentrations. After the 5th and the 10th transfer, colonies were transferred on plates containing inhibitory drug con- centrations and assessed for growth following incubation. Results (1) The in vitro susceptibility of the tested strains to P- 27530 was as follows: Yeasts P-27530 MIC range of 0.06–1 lg/ml was recorded, following 24 hours incubation, for all yeasts but sev- eral C. glabrata isolates which were read at 48 hours (MIC range of 0.06–2 lg/ml). P-27530 performed similarly against both Candida susceptible strains and strains resistant to caspofungin, fluconazole, miconazole, and voriconazole. Slightly reduced activity was observed against Cryptococcus neoformans with a MIC range of 2– 4 lg/ml. Moulds P-27530 performed similarly against the suscepti- ble strains and the strains with elevated MICs to amphotericin B, caspofungin, itraconazole, and miconazole with a MIC range of 0.06–2 lg/ml. Dermatophytes P-27530 demonstrated similar activ- ity against the isolates (range of 0.25–2 lg/ml) and performed simi- larly (within one dilution) against both susceptible and resistant (terbinafine and fluconazole) T. rubrum and T. mentagrophytes strains. (2) no spontaneous resistant mutant to P-27530 and CPX was isolated; resistance frequency was 10\u7f7 for itraconazole and 10\u7f9 for terbinafine and amorolfine. All mutants collected con- firmed the loss of susceptibility. P-27530 and CPX did not induce any resistance; frequency was 10\u7f5 for itraconazole and 10\u7f7 for terbinafine and amorolfine. In both experiments itraconazole resis- tant mutants showed increased resistance also to amorolfine and terbinafine. Conclusions P-27530 shows a good antifungal activity in vitro against a broad spectrum of yeasts, moulds and dermatophytes, with- out evidencing any cross-resistance potential. Moreover, the lack of resistant mutant isolation in the in vitro assays suggests a low pro-pensity of T. rubrum to develop resistance against P-27530