Antifungals

• The presentation provides an extensive overview of drugs used to treat fungal infections. Fungal infections can be tricky to deal with because fungi are similar to human cells in many ways, making it challenging to find treatments that target the fungus without harming human cells. The presentation focuses on classifications, mechanisms of action, spectrum of activity, therapeutic uses, adverse effects, and drug interactions, which help us understand how these drugs work and why they are important in medicine.

Classifications of Antifungal Drugs

• Systemic Antifungal Drugs: These drugs are meant for treating systemic fungal infections, which affect the entire body rather than just the skin or a localized area. Examples include:
    - Amphotericin B: This drug is commonly used for severe fungal infections.
    - Caspofungin: Often used in cases where other treatments have failed or when the infection is particularly dangerous.
    - Flucytosine: This is usually combined with another medication for a stronger effect.
    - Itraconazole: An alternative that is effective against various fungi.

• Superficial or Topical Antifungal Drugs: These are used for infections that are only on the surface of the skin. Examples include:
    - Nystatin: Often used to treat candidiasis, commonly known as yeast infections.
    - Miconazole: Applied mostly for skin infections.
    - Terbinafine: Often used for nail and skin infections.

It’s important to know that systemic antifungal drugs are not suitable for superficial infections because of two main reasons:

• They might cause significant side effects if misused.

• Using them for superficial infections could lead to drug resistance, meaning they might not work when a serious infection occurs later on.

Fungal Characteristics

• Fungal Cells: Fungi are eukaryotic organisms, which means their cells have a nucleus. This similarity to human cells complicates the design of antifungal drugs because treatments need to target fungi without damaging human cells. Understanding the unique features of fungi is crucial.
    - Unique fungal structures that are targets for antifungal treatment include:
    - Cell Wall: This structure is made of beta glucans, and the enzyme that helps build it is called beta glucan synthase.
    - Cell Membrane: It contains ergosterol, which is essential for the function of fungal membranes, similar to cholesterol in human membranes but distinctly different, allowing for targeted treatments.

Mechanisms of Action

Systemic Antifungal Drugs

• Amphotericin B:
    - Mechanism: It binds to ergosterol in the fungal cell membrane, creating holes that cause the cell to leak and eventually die.
    - Consequences: This disruption leads to cell death because fungi cannot maintain their internal environment without a stable membrane.
    - Spectrum: It has a broad spectrum of activity, meaning it can treat various types of fungal infections, even beyond fungi, like Leishmaniasis (a disease caused by parasites).
    - Administration: Because it doesn't absorb orally, it must be given through an intravenous (IV) injection in a medical setting.
    - Major Side Effect: One of the major risks is nephrotoxicity, which can harm the kidneys and may not be reversible. To reduce this risk, lipid formulations of the drug are available.

• Caspofungin (Echinocandins):
    - Mechanism: It prevents fungi from forming their cell walls by inhibiting beta glucan synthase, weakening their structure.
    - Use: This is often used when treating serious systemic infections or when other treatments are not effective or too risky.
    - Administration: Also given IV because it doesn't absorb well through the digestive tract.

• Flucytosine:
    - Nature: This is a prodrug, meaning it gets activated by a specific fungal enzyme to become effective.
    - Mechanism: It blocks DNA synthesis in fungi by inhibiting thymidylate synthase, a key enzyme in DNA formation.
    - Combination Therapy: It works well with Amphotericin B; together, they are more effective than either drug alone.
    - Side Effects: It can lead to enterocolitis (inflammation in the intestines) and may suppress bone marrow, which can affect blood cell production.

• Itraconazole:
    - Class: This drug is in the azole category, which all work by inhibiting the synthesis of ergosterol.
    - Effect on Drug Metabolism: It can inhibit certain important liver enzymes, which means it may interfere with the breakdown of other medications, leading to potential side effects or drug interactions.
    - Spectrum: Like Amphotericin B, it treats many types of fungal infections.

Superficial Antifungal Drugs

• Nystatin:
    - Mechanism: Works similarly to Amphotericin B by binding to ergosterol and forming pores.
    - Use: Best known for treating Candidiasis, it’s mainly used in an oral suspension form so it can stay local in the mouth or gut.
    - Administration: Since it does not get absorbed into the body, it has fewer side effects compared to systemic antifungals.

• Miconazole:
    - Mechanism: Functions like Itraconazole but is typically used topically for localized infections.

• Terbinafine:
    - Mechanism: Targets a unique step in ergosterol synthesis, which helps prevent resistance issues that can happen with azoles.
    - Pharmacokinetics: This drug accumulates in skin and nail tissues, allowing for effective treatment of infections in those areas.
    - Use: Administered orally for skin and nail infections, making it effective for treating deeper infections.

Conclusion

• Understanding the mechanisms, classifications, and potential adverse effects of antifungal drugs is crucial for effective therapy against fungal infections. The speaker emphasized that knowing these details is important, and parts of this overview may be included in upcoming exams, so pay close attention to the specific mechanisms and purposes of each drug discussed.