5A: Antifungals

Question 1

Antifungal targets

Answer 1

Ideally unique fungal targets not present in mammalian cells, & usefully therapeutic with minimal mammalian damage:
- Cell wall & membrane:
\+ Ergosterol synthesis
\+ Beta-glucan synthesis
\+ Pore formation
- DNA synthesis

Resistance is seen less in a more complex organism & less population exposure to antifungals

Question 2

Pathogenic fungi

Answer 2

Mostly environmental fungi that are inhaled, ingested or have traumatic implantation

Yeast or mould:

• Yeast – single cells that multiple by budding or fission e.g. candida & cryptococci
• Mould produce conidida or spores e.g. aspergillus & mucor

Some yeasts are commensal - live in us

Question 3

Cell wall or membrane active agents

Answer 3

• Rigid cell wall made of glucan, mannoproteins & chitin

- Cell membrane with ergosterol as key component

Question 4

Polyenes (Nystatin, amphotericin B & natamycin)

Answer 4

• Closed macrolide lactone ring – 1 side of the ring is a rigid lipophilic chain (lots of double bonds) & on the opposite side are lots of hydroxide groups - AMPHIPATHIC
• Very effective against wide range of fungi (all except Scedosporium & Fusarium spp.) but high toxicity
• Polyenes bind to sterol (ergosterol) in cell membrane causing disruption & cell death
• 8 molecules bind in an annulus linked hydrophobically creating a pore with hydroxide residues in facing
• Oxidative damage to cell is also postulated for some species of fungus – the binding kicks off a cascade of oxidation
• All polyenes are poorly stable, poorly bioavailable & toxic (cholesterol vs ergosterol)
• Nystatin & natamycin not formulated for systemic use
• Nystatin – intra-oral/topical use & capsules or tablets have no absorption used for gut “decontamination” only
• Natamycin – topical eye formulation for fungal keratitis

Question 5

Amphotericin B

Answer 5

• Derived from streptomyces nadosus
• Conventional formulation is a micellar suspension called AmB deoxycholate – precipitates at pH < 4.2
• Newer lipid formulation AmBisome is encased in liposomes changing the distribution & toxicity of the drug
  + Increased liver, spleen, lung but reduced renal concentrations with liposomal
  + L-AmB has non-linear kinetics with increased doses
• Different PK, different dosing = errors
  o Deoxycholate Cmax 1-2 mg/L vs liposomal 80 mg/L
• PK target Cmax:MIC 2 – 4 AmBD vs 10 L-AmB

ADRs:

• Infusion reactions
• Renal tubular damage including K+, Mg2+, PO4- wasting
• Pulmonary reactions with neutrophil infiltrate

Question 6

Triazoles (Fluconazole, itraconazole, voriconazole, posaconazole, isavuconazole - oral miconazole)

Answer 6

• Inhibit fungal cytochrome P450 enzymes (ERG11) that converts lanosterol to ergosterol leading to disruption of cell membrane function
• Resistance is seen usually after mutation in binding sites but can be increased expression of target or efflux pumps
• All are metabolised via human CYP450 isoenzymes so need to monitor for interactions
• All have similar PK targets – AUC:MIC
  + Candida 25 – 100
  + Aspergillus 10 (more fungicidal)

Question 7

1. Fluconazole

Answer 7

• Active against most candida & cryptococcus species – also many dermatophytes
• AUC:MIC ratio of 25 – 100 depending on the clinical scenario
  + 100 mg fluconazole gives approx. 100 AUC (dose = AUC)
  + Dose = MIC x 25 – 100
• Highly bioavailable
• Hepatic metabolism but renally excreted (50-80% of original dose in urine)
• ADRs include hepatotoxicity, hair loss & reduced appetite

Question 8

2. Itraconazole

Answer 8

• Broader spectrum treatment – also active against Aspergillus species & mucor
• Poorly absorbed orally (30%) but increased with an acidic environment e.g. with food or cola
  + Capsule formulation achieves lower concentration compared to liquid (not bioequivalent)
  + Demonstrates non-linear kinetics with increasing doses
• Has excellent tissue concentrations as well distributed but extensively metabolised into non-urinary active metabolites
• Marked inter-patient variability
• TDM trough > 1 mg/L or > 0.5 mg/L for prophylaxis
• ADRs often seen at >5 mg/L & include hepatotoxicity, nausea ++, rashes

Question 9

3. Voriconazole

Answer 9

• Highly active against Aspergillus, Fusarium & Scedosporium species – doesn’t provide cover against mucor
• Highly bioavailable (96%) with saturable 1st pass metabolism
  + Long half-life requires a loading dose for Css
  + Designed to overcome absorption issues
• Also have high inter-patient variability up to 100-fold
  o CYP2C9 metabolism (genetic polymorphism)
• Trough:MIC 2 – 5 = AUC:MIC 30 – 100
• ADRs include hepatotoxicity, rash & vision changes (include colour changes, spots & visual hallucinations – usually transient)

Question 10

4. Posaconazole

Answer 10

• Broader spectrum coverage than voriconazole for mucor but less for scedosporium & mucor
• Difficult formulation into oral – original form was liquid which needed high fat meals (increase AUC-4X) to absorb
• Acidic environment decreases absorption
• Saturable absorption at 800 mg
• Now formulated as MR tablet
• TDM routine aiming for trough >1 mg/L or >0,7 mg/L for prophylaxis
• ADRs limited with liquid (low levels) now similar to voriconazole

Question 11

Echinocandin (Caspofungin, micafungin, anidulafungin)

Answer 11

• Semi-synthetic lipopeptides that inhibit beta-D-glucan synthase (FKS subunit) stopping production of beta-D-glucan
• Increased permeability of cell wall leads to cell lysis & death
• All 3 are basically the same with minimal PK differences
• PKPD target is Cmax:MIC or AUC:MIC equally – suggests optimising dosing to once daily for best effect
• Effective against candida & aspergillus (branching tips)
  + Cmax:MIC 1 & 10 respectively
• ADRs – generally well tolerated but some minor LFT changes

Question 12

Terbinafine

Answer 12

• Allylamine group – inhibit squalene epoxide; an enzyme in the ergosterol synthesis pathway, this leads to cell death from toxic accumulation of precursors rather than cell membrane deficiency
• Active against moulds & dermatophytes with some activity (synergy) against other species
• Good bioavailability & tissue concentrations
• Relatively toxic with gastrointestinal ADRs, rashes, taste disturbances, and cases of liver failure

Question 13

Intracellular - flucytosine

Answer 13

• Synthetic fluorinated analogue of cytosine (anti-metabolite)
• Taken up by fungal cells by cytosine permeases & then converted to 5-fluououracil by cytosine deaminase
• 5-FU is then incorporated into fungal RNA blocking DNA synthesis by inhibiting thymidylate synthetase inhibition
• Active against candida & cryptococcus – only clinically used in cryptococcal disease & urinary candidiasis
• High bioavailability, well distributed & renally excreted (unchanged 99%)
• PKPD target T>MIC (!) 40% & no PAE
  + Very variable PK with renal toxicity & myelo-suppression seen with increased concentrations
  + Need to do TDL peaks & troughs