Echinocandins (caspofungin, micafungin, anidulafungin) are fungicidal against Candida but fungistatic against Aspergillus despite inhibiting the same enzyme. The reason for species-dependent fungicidal vs. fungistatic activity is:

• A Candida lacks cell wall chitin redundancy; Aspergillus upregulates compensatory chitin synthase (chs3) — the 'paradoxical effect' — allowing partial cell wall repair at high drug concentrations ✓
• B Echinocandins only inhibit beta-1,3-glucan synthase in the hyphal tips of Aspergillus but reach Candida blastoconidia uniformly
• C Aspergillus FKS (beta-1,3-glucan synthase) has lower drug affinity due to F639Y mutation in all Aspergillus spp.
• D Aspergillus produces beta-1,3-glucanase that degrades the glucan-drug complex outside the cell membrane

Explanation

Echinocandins inhibit FKS1/FKS2 (beta-1,3-glucan synthase), depleting beta-1,3-glucan from the fungal cell wall. In Candida, this leads to osmotic lysis and fungicidal killing. In Aspergillus, the fungistatic activity is partly explained by the 'paradoxical effect' (Eagle effect): at high echinocandin concentrations, Aspergillus upregulates compensatory chitin synthase (particularly Chs3), increasing cell wall chitin content which partially substitutes for depleted glucan, allowing hyphal viability. Additionally, echinocandins primarily affect actively growing hyphal tips in Aspergillus, limiting complete killing. This pharmacodynamic difference has important clinical implications in aspergillosis treatment where echinocandins are second-line.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.