Mechanism of action: from enzyme to cell death
The basis of azole action is severe interference with the ergosterol synthesis cascade. Normally, this multi-step process begins with acetyl-CoA molecules, followed by the synthesis of squalene via mevalonic acid. Next, it converts into lanosterol, which is sequentially transformed into zymosterol, fecosterol, and finally, finished ergosterol.
Azoles act at the stage of lanosterol transformation. They bind tightly to the iron atom in the heme of the 14-α-demethylase enzyme (which belongs to the cytochrome P-450 system) and inactivate it. This triggers a chain of biochemical consequences detrimental to the fungus:
- Ergosterol synthesis is completely blocked.
- Toxic precursors—lanosterol and 14α-methylsterols—accumulate in excess inside the cell.
- The accumulated methylsterols physically disrupt the tight packing of phospholipid acyl chains in the membrane.
- The membrane becomes destabilized, leading to dysfunction of membrane-bound enzymes (including the electron transport chain). The result is the inevitable death of the fungal cell.
Classification and spectrum of activity
Based on the chemical structure of the heterocyclic ring, the entire group is divided into two major categories:
- Imidazoles. Contain a five-membered ring with two nitrogen atoms. This subgroup includes: ketoconazole, clotrimazole, miconazole, econazole, butoconazole, oxiconazole, sertaconazole, sulconazole.
- Triazoles. Contain a five-membered ring with three nitrogen atoms. A typical representative in diagrams is fluconazole.
These drugs possess a broad spectrum of activity and are available in both systemic and topical formulations. They are highly effective against candidiasis, aspergillosis, cryptococcosis, dermatomycoses, as well as severe endemic infections (blastomycosis, histoplasmosis, coccidioidomycosis, and paracoccidioidomycosis). An important clinical advantage of the group is that resistance in pathogenic fungi develops rarely.
Selectivity issues and drug interactions
Azoles do not possess absolute selectivity. Since their target is a heme-containing enzyme of the cytochrome P-450 system, they can inhibit similar isoenzymes in the human liver that are responsible for the metabolism of various xenobiotics.
A striking clinical example of a dangerous interaction is the combination with the immunosuppressant cyclosporine. It is prescribed to patients after kidney, liver, or heart transplantation to prevent graft rejection. Cyclosporine is metabolized by hepatic P-450 enzymes and excreted in the bile. If systemic azoles are co-administered to such a patient, the metabolism of the immunosuppressant sharply slows down. Its concentration in the blood will rise to toxic levels, which is fraught with severe nephrotoxicity and hepatotoxicity. In such cases, a mandatory reduction in cyclosporine dose and strict therapeutic monitoring are required.
Side effects of systemic therapy
The toxicity profile of azoles during systemic use is quite broad and affects several body systems at once:
- Endocrine system: Cytochrome P-450 enzymes participate in the synthesis of human steroid hormones. Azoles can block the 17,20-desmolase enzyme, which is necessary for the biosynthesis of testosterone and corticosteroids. This produces a pronounced anti-androgen effect: men experience gynecomastia, oligospermia, and impotence, while women experience menstrual cycle irregularities. This is why modern drugs are required to have high selectivity for fungal demethylase.
- Hepatobiliary system: Direct hepatotoxicity is characteristic.
- Central nervous system: General cerebral symptoms (drowsiness, dizziness, headache) and focal neurological manifestations (paresthesias, tremor, seizures) are possible.
- Hematopoietic system: There is a risk of developing hemolytic anemia and thrombocytopenia.
- Gastrointestinal tract and allergic reactions: Dyspeptic disorders (nausea, vomiting, anorexia) are frequently observed, as well as skin rash, urticaria, and drug fever.
Briefly: answers to topical questions
Which antifungal drugs belong to the triazole subgroup?
The triazole subgroup includes synthetic antifungal drugs containing a five-membered ring with three nitrogen atoms in their chemical structure. This group includes:
- Itraconazole
- Fluconazole
- Voriconazole
- Terconazole
- Posaconazole
- Ravuconazole (currently in clinical trials).
These drugs exhibit higher selectivity for fungal 14$\alpha$-demethylase compared to imidazoles and practically do not block the human 17,20-desmolase enzyme, thereby avoiding disruption of sex hormone and glucocorticoid synthesis.
The concentration of which drugs in the blood is increased by azoles due to hepatic cytochrome P-450 inhibition?
Due to the inhibition of hepatic cytochrome P-450, azoles increase the blood concentration of a number of medications by slowing down their metabolism. These drugs include:
- Cyclosporine — an increase in its concentration raises the risk of nephrotoxicity and hepatotoxicity.
- Tacrolimus — co-administration with voriconazole requires a reduction in its dose.
- Warfarin — ketoconazole and other azoles mentioned in the source slow down its oxidation, which enhances the anticoagulant effect and increases the risk of bleeding.
- Astemizole and terfenadine — inhibitors of their metabolism, including ketoconazole and itraconazole, can increase the risk of cardiac arrhythmias.
What are the pharmacokinetic features of fluconazole?
The pharmacokinetics of fluconazole is characterized by high bioavailability and the ability to penetrate various tissues and fluids.
- Absorption — the drug is well absorbed orally with a bioavailability of about 100%, and the process is independent of gastric juice pH.
- Distribution — being a hydrophilic compound, fluconazole freely penetrates all body fluids, including sputum, urine, saliva, and cerebrospinal fluid (CSF).
- Elimination — is carried out predominantly by the kidneys.
The drug can be administered both orally and intravenously.
How can one easily remember the structural difference between imidazoles and triazoles? The prefix "tri-" in triazoles directly indicates the number of nitrogen atoms in the five-membered heterocyclic ring (there are exactly three). Imidazoles have two, accordingly.
Class and exam questions
How does the mechanism of action of azoles differ from allylamines (e.g., terbinafine)?
Allylamines work at an earlier stage of the cascade—they inhibit the squalene epoxidase enzyme, causing ergosterol deficiency and the accumulation of toxic squalene. Azoles act later, blocking 14-demethylase and disrupting the conversion of lanosterol.
How do azoles interact with polyenes (amphotericin B)?
Amphotericin B does not disrupt synthesis at all; instead, it binds to pre-formed ergosterol molecules in the cell membrane, forming pores through which cell contents leak out. Azoles, on the other hand, deprive the cell of the ability to synthesize this ergosterol.
Why can azoles cause gynecomastia and impotence in men during treatment?
Due to insufficient selectivity, the drugs inhibit not only fungal enzymes but also human 17,20-desmolase. This leads to a drop in testosterone levels and the development of anti-androgen side effects.
What else to study on this topic
- Complete ergosterol biosynthesis cascade: from acetyl-CoA to the finished molecule
- Comparative pharmacokinetics of systemic and topical imidazole derivatives
- Mechanisms of antifungal hepatotoxicity and liver function monitoring
- Cyclosporine pharmacokinetics and dose adjustment rules in polypharmacy
- Features of antimycotic use in organ transplant patients
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