Lecture 11 Pyrimidine and Polyenes Flashcards
Name the one example of a pyrimidine
5-flurocytosine
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-flurocytosine → Flurouridine monophosphate (FUMP) → flurouridine triphosphate (FUTP) → incorporation in RNA → disrupts translocation
5-flurocytosine → flurodeoxyuridine monophosphate → inhibition of thymidylate synthetase → inhibition of DNA synthesis
Which fungi is 5-flurocytosine active against?
Generally yeast: cryptococcus neoformans, most candida, some dematiaceous (brown) moulds
Which fungi is 5-flurocytosine inactive against?
Candida krusei
Aspergillus spp.
Histoplama capulatum
Most moulds
Why is 5-flurocytosine used in combination with amphotericin B/fluconozole?
Risk of resistance developing quickly in monotherapy
What uses is 5-flurocytosine licenced for?
Treatment of systemic fungal infections caused by candidosis, cryptococcosis and chromoblastomycosis (brown mould)
Main use with amphoterinin B in cryptococcal meningitis
Describe the study that explains the efficacy of amphotericin B and flurocytosine by Bennett et al, 1979
27 patients treated with either amphotericin B alone and 24 treated with amphotericin B + flurocytosine
Mortality rate found to be similar
Combination therapy showed: more rapid CFS sterilisation, lower rate of relapse
Describe the study that explains the efficacy of flurocytosine monotherapy vs combined by Day et al, 2013)
Cyptococcal meningitis in HIV-AIDS
3 groups: Amp B only, Amp B + flurocytosine, Amp B + fluconozole
Mortality at 2 weeks as follows:
Amp B only 25/99, Amp B + flurocytosine 15/100, Amp B + fluconozole 22/99
Therefore combination therapies were more effective
Explain the absorption of 5-flurocytosine
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-flurocytosine?
3-6 hours
What is the normal dose of 5-flurocytosine given?
3-4 doses a day
How is 5-flurocytosine excreted?
mainly via urine
Therefore good choice for UTIs as a monotherapy - though there is still a risk of resistance
What are the side effects associated with 5-flurocytosine?
> 100mg/L for 2 weeks = risk of bone marrow supression - leucopenia, thrombocytopenia, aplastic anaemia
Rare: allergic reactions, liver toxicity
What drugs are known to interact with 5-flurocytosine?
Brivudine (antiviral) - inhibits dihydropyrimidine dehydrogenase which normally degrades flurouracil = can lead to fluoruracil toxicity
Phenytoin - higher levels of phenytoin may occur
Which fungi are particularly vulnerable to developing resistance to 5-flurocytosine?
can develop quickly for candida and cryptococcus
Explain the mechanism of action of resistance to 5-flurocytosine
Most mutations occur to the uracil phosphoribosyltransferase - therefore probably the more important pathway during drug activation
Most mutations also occur to the cytosine permease and cytosine deaminase (see slide for pathway)
What are the 2 examples of polyene antifugals?
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 hydrophobic side of macrolide rings = distortion of membrane bilayers = pore for leakage to occur
Also theory of damage through auto-oxidation of amphotericin B though the exact mechanism is unknown
Explain the spectrum of activity for amphotericin B
Broad spectrum
Most yeasts/moulds are sensitive
The parasite Leishmania
Exceptions: Aspergillus terreus, most scedosporium and lomentospora are resistant
What was the problem associated with amphotericin B deoxylate and how was this overcome?
Deoxylate was associated with infusion related kidney toxicity - now rarely used
Ambisome produced - a lipid based formulation that has less side effects (see slide for structure of molecule)
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
Occassional 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 empirical treatment?
Neuropenia
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 a amphotericin B for persistant febrile neutropenia despite antibacterial therapy - 50% had leukaemia
Amphotericin B deoxylate = 49.4% success
Ambisome 50.1% success
Therefore no difference in success but less side effects with Ambisome
Explain the study that compares Ambisome and other agents for empirical therapy
Ambisome vs virconazole:
Ambisome = 30.6% success
Virconazole = 26% success
Ambisome vs caspofungin:
Ambisome = 33.7%
Caspofungin = 33.9%
Explain the study that looks into the use of amphotericin as a prophylactic drug
355 acute lymphocytic leukaemia 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 lunch and kidney, low in heart and brain
What is the half life of abisome?
initially 24h
later up to 15 days
How is abisome eliminated?
43% in bile
21% in urine
Does abisome 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 deoxylate?
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 effects of muscle relaxants
Flurocytosine: nephrotoxicity may reduce flurocytosine 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 high minimum inhibitory concentrations
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
Due to the main target of the drug being 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 invoved in ergosterol synthesis = increase in minimum inhibitory concentration
Increased catalase activity may reduce oxidative damage caused
