Echinocandins

Echinocandins are a new class of antifungal drugs that inhibit the synthesis of β-glucan in the fungal cell wall via noncompetitive inhibition of the enzyme 1,3-β glucan synthase. The class has been termed the 'penicillin of antifungals,' along with the related papulacandins, as their mechanism of action resembles that of penicillin in bacteria. β-glucans are carbohydrate polymers that are cross-linked with other fungal cell wall components, equivalent to bacterial peptidoglycan. Caspofungin, micafungin, and anidulafungin are semisynthetic echinocandin derivatives with clinical use due to their solubility, antifungal spectrum, and pharmacokinetic properties. Echinocandins are a new class of antifungal drugs that inhibit the synthesis of β-glucan in the fungal cell wall via noncompetitive inhibition of the enzyme 1,3-β glucan synthase. The class has been termed the 'penicillin of antifungals,' along with the related papulacandins, as their mechanism of action resembles that of penicillin in bacteria. β-glucans are carbohydrate polymers that are cross-linked with other fungal cell wall components, equivalent to bacterial peptidoglycan. Caspofungin, micafungin, and anidulafungin are semisynthetic echinocandin derivatives with clinical use due to their solubility, antifungal spectrum, and pharmacokinetic properties. Drugs and drug candidates in this class are fungicidal against some yeasts (most species of Candida, but not against Cryptococcus, Trichosporon, and Rhodotorula). Echinocandins also have displayed activity against Candida biofilms, especially in synergistic activity with amphotericin B and additive activity with fluconazole. Echinocandins are fungistatic against some molds (Aspergillus, but not Fusarium and Rhizopus), and modestly or minimally active against dimorphic fungi (Blastomyces and Histoplasma). These have some activity against the spores of the fungus Pneumocystis jirovecii, formerly known as Pneumocystis carinii. Caspofungin is used in the treatment of febrile neutropenia and as salvage therapy for the treatment of invasive aspergillosis. Micafungin is used as prophylaxis against Candida infections in hematopoietic stem cell transplantation patients. All three agents are well tolerated, with the most common adverse effects being fever, rash, nausea, and phlebitis at the infusion site. They can also cause a histamine-like reaction (flushing) when infused too rapidly. Toxicity is uncommon. Its use has been associated with elevated aminotransferases and alkaline phosphatase levels. The present-day clinically used echinocandins are semisynthetic pneumocandins, which are chemically lipopeptide in nature, consisting of large cyclic (hexa)peptoid. Caspofungin, micafungin, and anidulafungin are similar cyclic hexapeptide antibiotics linked to long modified N-linked acyle fatty acid chains. The chains act as anchors on the fungal cell membrane to help facilitate antifungal activity. Due to their limited oral bioavailability, echinocandins are administered through intravenous infusion. Echinocandins noncompetitively inhibit beta-1,3-D-glucan synthase enzyme complex in susceptible fungi to disturb fungal cell glucan synthesis. Beta-glucan destruction prevents resistance against osmotic forces, which leads to cell lysis. They have fungistatic activity against Aspergillus species. and fungicidal activity against most Candida spp., including strains that are fluconazole-resistant. In vitro and mouse models show echinocandins may also enhance host immune responses by exposing highly antigenic beta-glucan epitopes that can accelerate host cellular recognition and inflammatory responses. Echinocandin resistance is rare. However, case studies have shown some resistance in C. albicans, C. glabrata, C. lusitaniae, C. tropicalis, and C. parapsilosis. Resistance patterns include alterations in the glucan synthase (Fks1-Fks2 complex), overexpression of efflux pumps, strengthening of cell wall by increased chitin production, and upregulation of stress-response pathways. Due to the large molecular weight of echinocandins, they have poor oral bioavailability and are administered by intravenous infusion. In addition, their large structures limit penetration into cerebrospinal fluid, urine, and eyes. In plasma, echinocandins have a high affinity to serum proteins. Echinocandins do not have primary interactions with CYP450 or P-glycoprotein pumps. Caspofungin has triphasic nonlinear pharmacokinetics, while micafungin (hepatically metabolized by arylsulfatase, catechol O-methyltransferase, and hydroxylation) and anidulafungin (degraded spontaneously in the system and excreted mostly as a metabolite in the urine) have linear elimination. Younger patients exhibit a faster rate of elimination of micafungin and caspofungin. Caspofungin has some interference with ciclosporin metabolism, and micafungin has some interference with sirolimus (rapamycin), but anidulafungin needs no dose adjustments when given with ciclosporin, tacrolimus, or voriconazole.