The Effect of the Echinocandin Analogue Caspofungin on Cell Wall Glucan Synthesis byCryptococcus neoformans

Abstract

The echinocandin derivative caspofungin (MK-0991, L-743,872) inhibits 1,3-beta-d-glucan synthesis and is active against several medically important fungi but is relatively ineffective against Cryptococcus neoformans. To investigate the mechanism of C. neoformans resistance, the prevalence of 1,3- and 1,6-beta-d-glucan linkages was determined in cells grown with and without caspofungin, using affinity-purified antisera and gold particle immunoelectron microscopy. Cryptococcal strains ATCC 24067 (serotype D) and MY2061 (serotype A) were studied. Growth at 4 microg/mL of caspofungin reduced both glucan linkages in both strains. However, growth at 2 microg/mL resulted in reduced 1,6-beta-d-glucan linkage only for MY2061. Inhibition of 1,6-beta-d-glucan synthesis may be an additional mechanism of action for pneumocandins. The relatively low efficacy of caspofungin against C. neoformans may result from reduced activity against C. neoformans glucan synthase or from yet undiscovered mechanisms of action operative in other fungal pathogens but not in C. neoformans.

Stress, Drugs, and Evolution: the Role of Cellular Signaling in Fungal Drug Resistance

“…Using an in vitro (1,3)-␤-glucan synthase activity assay, this possibility was ruled out: C. neoformans (1,3)-␤-glucan synthase activity was determined to be very sensitive to the echinocandins, with an apparent caspofungin K i of 0.17 Ϯ 0.02 (mean Ϯ standard deviation) M and cilofungin K i of 22 Ϯ 5.7 M (114). Consistent with these results, caspofungin has been shown to reduce both ␤-(1,3) and ␤-(1,6) glucan linkages in the C. neoformans cell wall (65). Other possible explanations for the resistance to echinocandins include rapid efflux from the cell, for example, by multidrug transporters, or the echinocandins could be degraded, either extracellularly or intracellularly.…”

Cell Wall β-(1,6)-Glucan of Saccharomyces cerevisiae

“…Although the biosynthetic pathways of both ␤-(1,3)-and ␤-(1,6)-glucans are differently regulated, data presented here and in other studies show that the synthesis of these two glucan polymers seem coordinated. Reduction of ␤-(1,3)-glucan either seen in different mutants or consecutively to the addition of ␤-(1,3)-glucan synthase inhibitors is associated to a diminishment of the ␤-(1,6)-glucan content of the cell wall (54,56,57). These results suggested that ␤-(1,6)-glucan synthesis anchored neosynthesized ␤-(1,6)-glucan to ␤-(1,3)-glucan.…”

Binding of Serum Mannan Binding Lectin to a Cell Integrity-DefectiveCryptococcus neoformans ccr4Δ Mutant

“…The cell integrity-defective ccr4⌬ mutant was found to be sensitive to caspofungin, an echinocandin antifungal to which C. neoformans exhibits intrinsic resistance (12). Cell integrity is currently the target of the majority of antifungal agents in use.…”

“…C. neoformans, however, exhibits a high level of intrinsic resistance to this class of drug (12). The cell integrity defects of the ccr4⌬ mutant led us to hypothesize that loss of CCR4 sensitized C. neoformans to the antifungal echinocandin caspofungin.…”