This is an antifungal drug-mechanism module structured as a biological pathway, not a specific disease. Its nodes are the successive biochemical steps of flucytosine activation and antimetabolite action (permease uptake -> cytosine deaminase activation to 5-FU -> anabolism to fluorinated nucleotides -> disruption of RNA and DNA synthesis), with an adaptive-resistance branch; the pyrimidine-antimetabolite drug class (flucytosine) that acts on the pathway is described in the node text rather than modelled as a separate node. Disorder entries reference individual nodes via conforms_to (e.g., "fungal_nucleic_acid_antimetabolite#Deamination to 5-Fluorouracil by Fungal Cytosine Deaminase"), and their flucytosine treatments point at the inhibited node via target_mechanisms (analogous to how cell-wall-active antibiotic treatments link to "bacterial_cell_wall_synthesis_inhibition#Peptidoglycan Cross-Linking by Penicillin-Binding Proteins"). Key conformance / treatment target: "Deamination to 5-Fluorouracil by Fungal Cytosine Deaminase" — the fungal-specific prodrug-activation step that gives selective toxicity, since mammalian cells lack cytosine deaminase. The resistance node captures the gating knowledge that distinguishes "flucytosine is used" from durable activity — rapid monotherapy resistance via FCY1/FCY2/FUR1 mutations that mandates combination therapy (classically with amphotericin B for cryptococcal meningitis). See projects/ANTIFUNGAL.md for the broader antifungal drug-fungus strategy.
Flucytosine Uptake by Fungal Cytosine Permease
trigger
The antimetabolite cascade begins when flucytosine (5-fluorocytosine, 5-FC), a synthetic cytosine analogue with no intrinsic antifungal capacity, is imported across the fungal plasma membrane by the cytosine (purine-cytosine) permease Fcy2. This permease-mediated entry is the committed first step of the pathway: the drug cannot be activated unless it first enters the fungal cell, which is why loss of the permease is one of the routes to acquired resistance. The drug class acting on this pathway is the pyrimidine antimetabolite antifungal flucytosine.
Downstream
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Deamination to 5-Fluorouracil by Fungal Cytosine Deaminase
Once inside the fungal cell, imported 5-FC is deaminated by fungal cytosine deaminase to the active antimetabolite 5-fluorouracil.
Deamination to 5-Fluorouracil by Fungal Cytosine Deaminase
therapeutic vulnerability
Inside the fungal cell, cytosine deaminase (Fcy1) deaminates 5-FC to 5-fluorouracil (5-FU), the committed activation step that turns an inert prodrug into an active antimetabolite. This step is the structural basis of flucytosine's selective toxicity: mammalian cells lack cytosine deaminase and therefore never carry out this conversion, so the fungal-specific deaminase is both the molecular drug target and the source of the therapeutic index. This is the canonical conformance / treatment target of the module — a conforming fungal-disease entry's flucytosine treatment links here via target_mechanisms. Because activation depends entirely on this enzyme, loss of the deaminase is also a principal route of acquired resistance.
Downstream
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Conversion to Fluorinated Nucleotides (5-FUTP and 5-FdUMP)
Activated 5-fluorouracil is anabolised to the fluorinated nucleotides 5-FUTP and 5-FdUMP that mediate the antimetabolite effect.
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Flucytosine Monotherapy Resistance
Loss-of-function mutation of the deaminase (or the upstream permease) abolishes activation, the principal route of acquired flucytosine resistance.
Conversion to Fluorinated Nucleotides (5-FUTP and 5-FdUMP)
effector
Activated 5-fluorouracil is channelled into the fungal nucleotide pool by uracil phosphoribosyltransferase (Fur1) and downstream kinases, generating two fluorinated nucleotide effectors. One arm produces 5-fluorouridine triphosphate (5-FUTP), the ribonucleotide that will be mis-incorporated into RNA. The other arm produces 5-fluorodeoxyuridine monophosphate (5-FdUMP), the deoxynucleotide that will inhibit thymidylate synthase. This anabolic step converts the activated antimetabolite into the actual nucleotide poisons; because the phosphoribosyltransferase (FUR1) is required here, its loss is a further route to resistance.
Downstream
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Disruption of Fungal RNA and DNA Synthesis
The fluorinated nucleotides 5-FUTP and 5-FdUMP poison fungal RNA-directed protein synthesis and DNA synthesis.
Disruption of Fungal RNA and DNA Synthesis
consequence
The two fluorinated nucleotide effectors converge to poison fungal nucleic-acid metabolism. 5-Fluorouridine triphosphate (5-FUTP) — unlike normal uridylate — is incorporated into fungal RNA in place of uracil, corrupting the transcript and inhibiting protein synthesis. In parallel, 5-fluorodeoxyuridine monophosphate (5-FdUMP) inhibits thymidylate synthase, the enzyme that supplies thymidine for DNA biosynthesis, thereby blocking DNA synthesis. This dual antimetabolite effect on both RNA and DNA is the antifungal consequence of engaging the upstream activation and anabolism steps — the fungistatic/ fungicidal endpoint of the module.
Flucytosine Monotherapy Resistance
adaptive escape
Flucytosine is notorious for the rapid emergence of resistance when used alone, because the entire antimetabolite cascade depends on an intact activation and incorporation pathway that a single mutation can disable. Clinically relevant resistance arises through loss-of-function alterations in FCY2 (the cytosine permease that imports the drug), FCY1 (the cytosine deaminase that converts 5-FC to 5-FU), or FUR1 (uracil phosphoribosyltransferase, required to channel 5-FU into the nucleotide pool). Any of these breaks the chain between drug entry and nucleic-acid poisoning, so monotherapy selects resistant subpopulations quickly. This node encodes the gating knowledge that distinguishes "flucytosine is used" from durable efficacy and is the mechanistic reason flucytosine is given in combination — classically with amphotericin B for severe systemic mycoses such as cryptococcal meningitis. Conforming entries can attach a flucytosine combination rationale to this node.