Antifungal Activity of Lipophilic Bisphosphonates

We investigated the activity of a series of 75 lipophilic bisphosphonates against <i>Candida glabrata</i>. Thirty-six compounds had MIC < 1 μg/mL, 18 had MIC < 0.5 μg/mL, and 2 had MIC = 0.13 μg/mL, comparable to amphotericin B and caspofungin. The lipophilic bisphosphonates were ∼20-fold more potent against <i>C. glabrata</i> than the most potent hydrophilic bisphosphonate, zoledronate. The most active compounds were pyridinium bisphosphonates followed by imidazolium bisphosphonates, while aryl bisphosphonates were less active. Several compounds had selectivity index values against a human cell line in the 1000-3600 range, the most selective compounds being <i>para</i>-substituted pyridinium bisphosphonates. We also found similar activity against a caspofungin-resistant <i>FKS2</i> (F659S) mutant. Some combinations of lipophilic bisphosphonates had synergistic activity with FICI values in the ∼0.3-0.5 range, and similar synergies were observed with fluconazole, implicating ergosterol biosynthesis inhibition leading to compromised membrane structure and function. Cell growth inhibition was rescued by ascorbic acid, glutathione, and <i>N</i>-acetyl cysteine, indicating a ROS-based killing mechanism. There was also synergy with other antifungals but very strong antagonism with verapamil (FICI ∼4), which blocks calcium channels. Unlike hydrophilic bisphosphonates, which target farnesyl diphosphate synthase, lipophilic bisphosphonates also target squalene synthase, suggesting that the combination of multitargeting bisphosphonates is one origin of the synergistic interactions observed. Given that one of the lipophilic bisphosphonates studied here (BPH-1237) has been shown to have activity against many other human fungal pathogens, combinations with the compounds described here may be of interest as antifungal leads.