Discovery of a Small-Molecule Inhibitor of β-1,6-Glucan Synthesis

Abstract

It is possible that antifungal drugs with novel modes of action will provide favorable options to treat fungal infections. In the course of our screening for antifungal compounds acting on the cell wall, a pyridobenzimidazole derivative with unique activities, named D75-4590, was discovered. During treatment of Saccharomyces cerevisiae with D75-4590, (i) incorporation of [ 14 C]glucose into the ␤-1,6-glucan component was selectively reduced, (ii) proteins released from the cell had lost the ␤-1,6-glucan moiety, and (iii) cells tended to clump, resulting in impaired cell growth. Genetic analysis of a D75-4590-resistant mutant of S. cerevisiae indicated that its primary target was Kre6p, which is considered to be one of the ␤-1,6-glucan synthases. These results strongly suggest that D75-4590 is a specific inhibitor of ␤-1,6-glucan synthesis. D75-4590 showed potent activities against various Candida species. It inhibited hyphal elongation of C. albicans as well. KRE6 is conserved in various fungi, but no homologue has been found in mammalian cells. These lines of evidence indicate that D75-4590 is a promising lead compound for novel antifungal drugs. To our knowledge, this is the first report of a ␤-1,6-glucan inhibitor.

Architecture and Biosynthesis of the Saccharomyces cerevisiae Cell Wall

“…Because cells treated with this compound showed lowered incorporation of radiolabeled Glc into a b1,6-glucan fraction, and because a resistant mutant had an amino acid substitution in Kre6, it was proposed that the compound is an inhibitor of b1,6-glucan synthesis and that Kre6 is its likely target (Kitamura et al 2009). …”

“…Kre6 has been implicated in the mode of action of a pyridobenzimidazole derivative identified in a screen for inhibitors of cell wall incorporation of a reporter GPI-CWP (Kitamura et al 2009). Because cells treated with this compound showed lowered incorporation of radiolabeled Glc into a b1,6-glucan fraction, and because a resistant mutant had an amino acid substitution in Kre6, it was proposed that the compound is an inhibitor of b1,6-glucan synthesis and that Kre6 is its likely target (Kitamura et al 2009).…”

In Vitro Antifungal Activities of D11-2040, a .BETA.-1,6-Glucan Inhibitor, with or without Currently Available Antifungal Drugs

“…9) It is expected to be promising lead for novel antifungal drugs because b-1,6-glucan is thought to be a fungi-specific and an essential component. 10) Besides, Kre6p, which is the predicted primary target of D75-4590, has been found in various fungi, including Candida species, Cryptococcus neoformans and Aspergillus species, but not in humans.…”

Problems and hopes in the development of drugs targeting the fungal cell wall

“…However, many of the azoles are fungistatic, and when the drug is removed from the medium, the fungus grows again. Similarly, caspofungin does not kill Aspergillus in vitro and a paradoxical effect is seen where the inhibitory effect is reduced when the amount of drug is increased [31]. Despite this, these drugs have been used successfully in the clinic for many years.…”