Antifungals Flashcards
Key targets for antifungals
The fungal cell wall is rigid and essential for survival of the fungal cell; if this rigidity is lost, the cell dies
Fungal lipid envelope contains ergosterol, not cholesterol; key for drug targeting
Structure of Amphotericin B and envrionement it can work in
Ampho B is macrocyclic, amphoteric (meaning, soluble only at extremes of pH, which is key to clinical use limitations) with a lipid tail
Ampho B MOA
Ampho B works by forming complexes with ergosterol in cell membrane, causing membrane damage and leaking of cellular contents.
What causes resistance to Ampho B
Replacement of ergosterol in membrane with another lipid
Amphotericin B toxicities
Infusion reactions (give with acetaminophen and steroids)
Nephrotoxicity
hypokalemia, hypomagnesemia, and renal tubular acidosis (inability of kidney to hold onto bicarbonate) causing a bicarbonate deficiency, which is also called a non-anion-gap metabolic acidosis (NAGMA)
Anemia
MOA of flucytosine (5-fluorocytosine)
It is activated by fungi via enzymatic deamination to 5-FU, an antimetabolite drug also used in cancer chemotherapy.
The enzyme (cytosine deaminase) that converts prodrug to 5-FU is only found in fungi, making it specifically active against those species of cells only.
As an antimetabolite, 5-FU blocks action of thymidylate synthetase and halts production of fungal DNA.
It is the only antifungal drug that acts on DNA reproduction
flucytosine
Can flucytosine be taken orally
Yes, Absorbs well from GI tract, making it an orally effective systemic antifungal drug
Flucytosine toxicities
Toxicity of the drug is theorized to be due to conversion of drug to 5-FU by gut bacteria and thus looks like cancer chemotherapy toxicity on rapidly dividing cells; hair loss, bone marrow suppression, mucositis/enterocolitis.
When does flucytosine resistance occur
When used as monotherapy, so usually coupled with Ampho B
Azole antifungals MOA
Block Cyt p450-dependent production of ergosterol precursor in fungi.
Fungal cyt p450 enzymes are 100-1000x more sensitive to azoles than mammalian p450’s.
Administration of azole antifungals
These drugs are orally absorbable with good tissue penetration and lower toxicity than Ampho B
Can be topical or systemic
What is important about azole antifungals and drug interactions
They cause competitive inhibition of metabolism of other drugs as well as toxic levels of azole drug if cyt 450 inhibitors also given with these drugs.
drug toxicity will usually look like toxic effects of the second drug, ie bleeding with warfarin, toxicity of any p450-metabolized drug as would be seen in overdose, due to higher than desired serum levels.
Azole toxicities
Liver dysfunction, related to the effect on mammalian enzyme systems, and estrogenization, including gynecomastia, due to inhibition of estrogen breakdown by p450 enzymes.
Azole antifungal that inhibits sterol synthesis so used in cushing’s
Ketoconazole
What is important about voriconazole
Has a nonlinear metabolism, and is a teratogen so must take contraception
Causes QTc prolongation, arrythmia and sudden death risk esp with other QTc prolonging drugs, causes transient visual or auditory hallucinations, usually reserved for resistant infections
Itraconazole specific toxicities
causes worsening CHF in patients who already have heart failure as well as hepatic toxicity and death
Echinocandins MOA
Echinocandins are derived from products of fungal fermentation. They prevent synthesis of glucans, which are required for cross-linkage with chitin to maintain cell wall rigidity and structure. The enzyme inhibited = glucan synthetase, only found in fungi!
Best use for echinocandins
Only given IV and can’t penetrate CSF, so good for non-CSF invasive infections
Example of echinocandin
Caspofungin
Griseofulvin MOA
Inhibits microtubule formation by binding to alpha/beta tubulin in the mitotic spindle and microtubule-associated protein, disrupting the fungal mitotic spindle.
FUngi becomes multinucleated
Use of griseofulvin and why
This drug is very useful for dermatophyte infections because it deposits in precursors of keratin-producing cells, with almost no drug deposition in any other tissue. When these epidermal cells differentiate, the drug stays tightly bound to keratin. This drug is thus used only for infection of keratin-structure-producing tissue.
Instructions for taking griseofulvin
It is very lipophilic and should be taken with fatty foods (PB, ice cream) so it can be absorbed in the stomach
Griseofulvin toxicities
Serious side effects are rare, but can cause headaches, rashes, some estrogenic effects in children.
Induces Cyt p450 so makes warfarin less effective
Terbinafine MOA
Terbinafine inhibits squalene epoxidase, blocking ergosterol synthesis.
Bioavailability of Terbinafine
It has good bioavailability and is given orally. 99% of the drug is protein bound. Like griseofulvin, the drug accumulates in skin, nails, fat and thus is used only for dermatophyte infections. It is metabolized by CYP3A4 and can interact with multiple medications as a result.
Topical drugs from allyamines class
Naftifine, butenafine (Lotrimin Ultra)
MOA and use of nystatin
This drug is structurally similar to Ampho B, complexing with ergosterol in the membrane, but is not absorbed systemically from any mucosal surface, including the GI tract. Because it is not absorbed systemically, it cannot be used for invasive fungal infections.
Tavaborole use
topical treatment of onchomycosis
