Lecture 11 Pyrimidine and Polyenes Flashcards
Name the one example of a pyrimidine
5-fluorocytosine
By which enzyme is 5-flurocytosine taken up by fungi?
Cytosine permease
Explain the mode of action of 5-flyrocytosine
inhibits 2 of the following pathways:
5-fluorocytosine → Fluorouridine monophosphate (FUMP) → fluorouridine triphosphate (FUTP) → incorporation in RNA → disrupts translocation
5-fluorocytosine → fluorodeoxyuridine monophosphate (FdUMP) → inhibition of thymidylate synthetase → inhibition of DNA synthesis
Which fungi is 5-fluorocytosine active against?
Generally yeast: Cryptococcus neoformans, most candida, some dematiaceous (brown) molds
Which fungi is 5-fluorocytosine inactive against?
Candida krusei
Aspergillus spp.
Histoplasma capsulatum
Most molds
Why is 5-fluorocytosine used in combination with amphotericin B/fluconazole?
Risk of resistance developing quickly in monotherapy
What uses is 5-fluorocytosine licensed for?
Treatment of systemic fungal infections caused by candidosis, cryptococcosis and chromoblastomycosis (brown mold)
Main use with amphotericin B in cryptococcal meningitis
Describe the study that explains the efficacy of amphotericin B and fluorocytosine by Bennett et al, (1979)
27 patients treated with amphotericin B alone and 24 treated with amphotericin B + fluorocytosine
Mortality rate found to be similar
Combination therapy showed: more rapid CFS sterilization, a lower rate of relapse.
Describe the study that explains the efficacy of fluorocytosine monotherapy vs combined by Day et al, (2013)
Cryptococcal meningitis in HIV-AIDS
3 groups: Amp B only, Amp B + fluorocytosine, Amp B + fluconazole
Mortality at 2 weeks as follows:
Amp B 25%, Amp B + fluorocytosine 15%, Amp B + fluconazole 22%
Therefore combination therapies were more effective
Explain the absorption of 5-fluorocytosine
readily absorbed
wide distribution in tissues and body fluids e.g. almost the same levels in CFS as in the blood
Minimally absorbed by gut flora due to lack of deaminase
What is the half-life of 5-fluorocytosine?
3-6 hours
What is the normal dose of 5-fluorocytosine given?
3-4 doses a day
How is 5-fluorocytosine excreted?
mainly via urine
Therefore a good choice for UTIs as a monotherapy - though there is still a risk of resistance
What are the side effects associated with 5-fluorocytosine?
> 100mg/L for 2 weeks = risk of bone marrow suppression - leucopenia, thrombocytopenia, aplastic anemia
Rare: allergic reactions, liver toxicity
What drugs are known to interact with 5-fluorocytosine?
Brivudine (antiviral) - inhibits dihydropyrimidine dehydrogenase (DPD) which normally degrades fluorouracil = can lead to fluorouracil toxicity
Phenytoin - higher levels of phenytoin may occur
Which fungi are particularly vulnerable to developing resistance to 5-fluorocytosine?
Candida sp. and Cryptococcus
Explain the mechanism of action of resistance to 5-fluorocytosine
1) Most mutations occur to the uracil phosphoribosyltransferase (UPRT) → inhibition of protein synthesis.
2) Mutations also occur to the cytosine permease and cytosine deaminase →inhibition of thymidylate
synthetase ∴ disruption of DNA synthesis
What are the 2 examples of polyene antifungals?
Nystatin
Amphotericin B
Where is the source of polyene antifungals?
natural products of Streptomyces species
What is the mode of action of polyene antifungals?
damage to cells by increasing cell permeability = K+ release
Bind to ergosterol via the hydrophobic side of macrolide rings → distortion of membrane bilayers = leakage
Also theory of damage through auto-oxidation of amphotericin B but the exact mechanism is unknown
Explain the spectrum of activity for amphotericin B
Broad spectrum
Most yeasts/molds are sensitive
Visceral Leishmania
Exceptions: Aspergillus terreus, most Scedosporium, and Lomentospora are resistant
What was the problem associated with amphotericin B deoxycholate and how was this overcome?
Deoxycholate was associated with infusion-related kidney toxicity - now rarely used
Ambisome produced - a lipid-based formulation that has fewer side effects
In what form is nystatin given?
topical
not orally absorbed
too toxic for IV
What is Amphotericin B indicated for?
Empirical treatment for suspected fungal infections in immunocompromised patients
Treatment of a wide range of fungal infections including candidosis, aspergillosis, zygomycosis, and cryptococcosis
Treatment of visceral leishmaniasis
Occasional topical use e.g. eyes, mouth, ears
What is Nystatin indicated for?
treatment of oral or vaginal candidosis
What signs/symptoms would a patient present with that would prompt a clinician to give amphotericin B as an empirical treatment?
Neutropenia
Pyrexia
Explain the evidence for the use of amphotericin B as empirical therapy
early studies comparing amphotericin and placebo
Improvement in survival from fungal infections
No overall increase in survival from empirical therapy
Many problems regarding side effects seen
Explain the study that compares Ambisome and amphotericin deoxycholate by Walsh et al, 1999
687 patients on either Ambisome or an amphotericin B for persistent febrile neutropenia despite antibacterial therapy - 50% had leukemia
Amphotericin B deoxycholate = 49.4% success
Ambisome 50.1% success
Therefore no difference in success but fewer side effects with Ambisome
Explain the study that compares Ambisome and other agents for empirical therapy
Ambisome vs virconazole:
Ambisome = 31% success
Virconazole = 26% success
Ambisome vs caspofungin:
Ambisome = 34%
Caspofungin = 34%
Explain the study that looks into the use of amphotericin as a prophylactic drug
355 acute lymphocytic leukemia patients Ambisome vs placebo Rates of invasive fungal disease (change not significant): Placebo = 45% Ambisome = 48%
Therefore no evidence for efficacy as prophylaxis
Explain the tissue distribution of Ambisome
95% protein bound
High accumulation in liver and spleen, medium in lung and kidney, low in heart and brain
What is the half-life of ambisome?
initially 24h
later up to 15 days
How is ambisome eliminated?
43% in bile
21% in urine
Does ambisome need drug monitoring?
No
What are the adverse effects of ambisome use?
Acute infusion-related toxicity: fever, chills and rigors, nausea, vomiting, headaches, hypotension
Nephrotoxicity: raised serum creatinine levels, hypokalemia
Which side effects have significantly reduced from the use of ambisome compared to amphotericin B deoxycholate?
Fever, chills and rigors, vomiting, hypotension
raised creatinine levels
What drugs does Amphotericin B interact with?
Other nephrotoxic agents: ciclosporin, aminoglycosides, antibiotics, some anti-neoplastic agents
Corticosteroids, diuretics: increases risk of hypokalaemia
Skeletal muscle relaxants - effects of hypokalaemia may heighten the effects of muscle relaxants
Fluorocytosine: nephrotoxicity may reduce fluorocytosine clearance and result in high serum levels = bone marrow suppression
Name species that are resistant to Amphotericin B
Candida krusei and C. glabrata may have a high minimum inhibitory concentration
C. lusitaniae can develop resistance in vitro
Aspergillus terreus intrinsically resistant
Scedosporium, Lomentospora, and Fusarium are often resistant
Explain why there is not a simple mutation-resistance relationship
Because the main target of the drug is ergosterol which is not a protein.
Explain the mechanisms of action of resistance to Amphotericin B
C. lusitaniae - reduced ergosterol content of membranes (Peyron et al, 2002)
C. glabrata - mutation in the ERG2 gene coding for an isomerase involved in ergosterol synthesis = increase in minimum inhibitory concentration
Increased catalase activity may reduce oxidative damage
