Antifungal Structure–Activity Relationship Studies of Broad-Spectrum Phenothiazines

Fungal infections remain a critical unmet medical need with millions of infections occurring annually. With only three classes of antifungal drugs available, drug resistance and modest activity toward some fungi represent threats to human health. To address this, optimization of the antifungal properties of approved drugs with appropriate pharmacokinetic properties represents an attractive strategy. Here, we have shown that the antifungal activity of phenothiazine-based CWHM-974 extends to include fluconazole-resistant Candida albicans, Candida auris, and Cryptococcus glabrata, filamentous molds such as Aspergillus fumigatus, Fusarium spp., and Rhizopus arrhizus, endemic human fungal pathogens Histoplasma capsulatum, Blastomyces dermatitidis, and Coccidioides spp. Thus, phenothiazines (PTZs) have consistent antifungal activity toward a broad range of medically relevant fungi, including organisms that range from difficult to nearly impossible to treat with current drugs. Unfortunately, CWHM-974 did not exhibit in vivo efficacy in either Cryptococcus neoformans or C. albicans mouse infection models, necessitating an effort to optimize the scaffold further. Toward this end, synthesis and minimum inhibitory concentration (MIC) values are reported for 15 novel PTZ analogs to extend structure–activity relationships (SARs). Six analogs were identified as 2- to 4-fold more potent. Azaphenothiazines (aza-PTZs) were tolerated and resulted in potent antifungals with moderate reduction in lipophilicity and more facile chemical synthesis. One analog displayed modest selectivity improvement against the serotonin 5HT_2c receptor versus CWHM-974, but its overall selectivity profile versus a panel of other serotonin and dopamine receptors did not improve. Overall, the broad-spectrum antifungal activity and reduced neuroreceptor affinity of PTZ-based analogs encourages continued optimization to develop a novel antifungal therapeutic drug.