Antifungal Pharmacokinetics and Toxicity

Antifungal pharmacokinetics describes how the body absorbs, distributes, metabolises, and eliminates antifungal drugs, while toxicity describes the harm these agents can cause the host. The two are tightly linked: the same eukaryotic similarities that make fungi hard to target selectively also make several antifungals prone to host toxicity and to wide variability in drug exposure.

Definition

Antifungal pharmacokinetics is the quantitative study of the time course of an antifungal drug's absorption, distribution, metabolism, and excretion in the body; antifungal toxicity refers to the adverse effects these agents exert on host tissues, which together constrain how the drugs can be used.

Scope

This entry covers how route of administration, oral bioavailability, tissue distribution, metabolism, and elimination differ across the antifungal classes, why therapeutic drug monitoring is used for some agents, and the characteristic toxicities and drug interactions of each class. It is a reference description of antifungal pharmacology and explicitly not dosing or prescribing guidance.

Core questions

• How does the route of administration shape antifungal exposure?
• Why is oral absorption variable for some triazoles?
• Which agents require therapeutic drug monitoring and why?
• What characteristic toxicities and interactions define each class?

Key concepts

• Routes of administration (intravenous, oral, topical)
• Oral bioavailability and food/pH effects
• Tissue and central-nervous-system penetration
• Hepatic metabolism and cytochrome-P450 interactions
• Therapeutic drug monitoring
• Pharmacokinetic/pharmacodynamic (PK/PD) indices
• Nephrotoxicity and hepatotoxicity
• Lipid formulations to reduce toxicity

Mechanisms

Antifungal classes differ sharply in their pharmacology, as summarised by Lewis (2011). Amphotericin B is given intravenously, distributes widely, and is best known for dose-related nephrotoxicity and infusion reactions, toxicities that lipid formulations were designed to lessen. The triazoles vary in oral bioavailability — some depend on gastric acidity or food, and exposure can be erratic — and are metabolised by, and inhibit, hepatic cytochrome-P450 enzymes, generating clinically important drug interactions and a rationale for therapeutic drug monitoring with several agents (Sheehan et al., 1999). Flucytosine is renally cleared and penetrates well into tissues including the central nervous system but can cause marrow suppression at high exposures. The echinocandins are given intravenously, have limited oral absorption, are not major cytochrome-P450 substrates, and are generally well tolerated (Denning, 2003). Linking drug exposure to effect, pharmacokinetic/pharmacodynamic indices describe whether activity tracks with peak concentration, total exposure, or time above a threshold for each class.

Clinical relevance

Pharmacokinetic differences explain why some antifungals are reserved for intravenous use, why drug-level monitoring is employed for certain triazoles, and why toxicity and interactions weigh heavily in how these drugs are studied and managed (Pappas et al., 2009). This entry describes the pharmacology of the classes at a conceptual level; it provides no dosing, monitoring thresholds, or individualised treatment advice.

History

Antifungal pharmacology matured as the classes accumulated: amphotericin B's toxicity drove decades of work on safer lipid formulations and infusion practices, the triazoles introduced the central themes of variable oral absorption and cytochrome-P450 interactions, and the echinocandins added a class with a comparatively simple, favourable profile. The synthesis of these strands into a coherent pharmacology is reflected in reviews such as Lewis (2011).

Debates

When is therapeutic drug monitoring of triazoles warranted?

Because triazole exposure can vary widely between patients and both under- and over-exposure carry consequences, the value, targets, and routine use of therapeutic drug monitoring for agents such as voriconazole and itraconazole remain debated.

Key figures

• Russell Lewis
• David Andes
• David Denning
• Dorothy Sheehan

Seminal works

• lewis-2011
• sheehan-1999

Frequently asked questions

Why is amphotericin B given intravenously rather than by mouth?

Amphotericin B is very poorly absorbed from the gastrointestinal tract, so oral administration does not achieve the systemic drug levels needed to treat invasive infection. For systemic disease it is given intravenously, while oral forms serve only local effect in the gut.

Why do some antifungals need therapeutic drug monitoring?

Several triazoles have variable absorption and metabolism, so the drug exposure achieved can differ widely between individuals. Measuring drug levels helps relate that exposure to the wide between-patient variability, which is why monitoring is discussed for agents such as voriconazole and itraconazole. This is a general concept, not dosing advice.