Wk 14: Antimetabolites Flashcards
What is an antimetabolite?
- Structurally similar to essential metabolite but cannot take its place
- Analogue of natural substrate + changed biosterically
- Interfere w/ normal cellular function, affects nucleotide production
- Interacts with all rapidly dividing cells
Is folic acid (b9) active?
No - converted to dihydrofolic + tetrahydrofolic acid
What does lack of dietary folic acid lead to in pregnant women?
Neural tube defects in embryos
What is folic acid essential for in the body?
- Nucleotide biosynthesis
- DNA synthesis
- DNA repair
- Cofactor in bio reactions
- Prod healthy RBC
Is folic acid used in leukaemia patients?
No - decline in health
How does folic acid help in DNA synthesis?
Provides essential 1 carbon donors:
- Formyl donor: purine biosynthesis
- Methyl donor: pyrimidine biosynthesis
What does methotrexate do at low doses?
- Interferes w/ inflammatory actions of IL-1
- Stims adenosine release
- Apoptosis + death of activated T-lymphocytes
What does methotrexate do at high doses?
- Dihydrofolate reductase inhibitor
- DHFR - regenerates folic acid cofactors required for DNA synthesis
- Binds to DHFR upside down relative to dihydrofolate
- Depletes formyl + methyl 1 carbon donors
- Inhibits pyrimidine + purine biosynthesis
Other than inhibiting DHFR, what does methotrexate also inhibit?
Folate dependent enzymes:
- Accumulation of methotrexate polyglutamates
- Accumulation of dihydrofolate polyglutamate
What is used fo rescue therapy in methotrexate toxicity?
Folinic acid (N5-formyltetrahydrofolate)
- Readily converted to reduced folic acid derivatives
- Doesn’t require DHFR for conversion: unaffected by DHFR inhibition + allows purine/pyrimidine synthesis in presence of DHFR inhibition
What is 5-fluorouracil?
- Pro-drug
- In vivo: converted unto 5-FdUMP
What does 5-FdUMP do?
- Directly inhibits enzyme thymidylate synthase, tf inhibits DNA synthesis
- Resistance: down regulation of enzymes needed to add deoxyribose phosphate to form 5-FdUMP
What is the mechanism of 5-FU?
- Fl = electronegative
- C-F is polarised
- Base from thymidylate synthase cannot abstract proton from 5-FU
- 5-H in dUMP replaced by 5-F in 5-FdUMP
- Fδ- + base on TS repel each other
- C-F bond strong
- F+ not leaving group
- 5-FU covalently bonded to thymidylate synthase
- Cells unable to make dTMP = inhibits DNA synthesis
How does 5-FU cause apoptosis?
- Mimic pyrimidine nucleotide, producing abnormal variant: Unstable DNA, Vital cellular function interfered, Leads to apoptosis (S phase)
- Inhibits TS, depletes cell of dTTP, accumulation of dUMP, inc uracil misincorporation in DNA
How does 5-FU cause sustained inhibition?
- TS, FdUMP + active folate cofactor covalently bind
- Mechanism dependent on cofactor conc
- Boost cofactor conc + 5-FU activity using folinic acid
What is in tegafur-uracil?
- Tegafur: 5FU prodrug - converted by orate phosphoribosyltransferase, greater conversion in tumour cells
- Uracil: inhibits 5-FU breakdown, dihydropyrimidine dehydrogenase inhibitor
What is in tegafur-gimeracil-oteracil?
- Tegafur: 5FU prodrug
- Gimeracil: inhibits 5-FU metabolism by reversibly blocking dihydropyrimidine dehydrogenase
- Oteracil: remains in GI tract due to low permeability, red 5-FU prod by blocking metabolism of tegafur, lower 5-FU in GI = lower GI toxicity
How does resistance of 5-FU occur?
- Red uptake of 5-FU in cell
- Red expression of enzymes that activate prodrug
- Inc expression of thymidylate synthase
- More discriminating target enzymes
- Low folate cofactor levels
What is capecitabine?
- Oral prodrug of 5-FU
- More selective: Thymidine phosphorylase found in higher levels in tumour tissue
What is given following an overdose of fluropyrimidine treatment?
Uridine triacetate
What increases the risk of toxicity in patients receiving fluoropyrimidine treatment?
Inherited DPD deficiency
When would you not need DPYD screening when using fluoropyrimidine treatment?
- Topical fluorouracil cream: systemic abs low
- Flucytosine: severe fungal infections shouldn’t be delayed
What would you do if a patient is partially DPD deficient?
- Dose red
- Inc monitoring
How many DPYD variants are associated with DPD deficiency?
4
How do you screen for DPD deficiency?
- PCR + DNA analysis
- Phenotype: measure uracil in plasma, measure DPD/TS activity in mononuclear cells, measure dihydrouracil in blood, urine or saliva
What is raltitrexed?
- Folic acid analogue
- Blocks folate binding of thymidine synthetase by mimcing N5, N10 methylenetetrahydrofolate
- Inhibits thymidylate synthase activity
What is the mechanism of action of pemetrexed?
Inhibits:
- TS
- DHFR
- GARFT: catalyses formation of glycinamide ribonucleotide
What is the mechanism of action of cytarabine?
- OH inverted at C-2 compared to ribose, araC phosphorylated to araCTP, competes w/ dCTP for DNA
- Blocks strand elongation + repair mechanism
- Inhibits reduction of CDP to dCDP
- Orally inactivated: cytidine deaminase in intestinal mucosa
What is the mechanism of action of gemcitabine?
- Phosphorylation by deoxycytidine kinase = dFdCMP
- dFdCMP = ribonucleotide reductase inhibitor, inhibits DNA synthesis
- Additional phosphorylation = active metabolite dFdCTP
- dFdCTP inhibits DNA polymerase + chain elongation
- dFdCTP added to growing DNA causing strand termination + apoptosis
What is the mechanism of action of azacitidine?
Metabolised to triphosphate + incorporated into RNA/DNA
RNA
- Dissembly polyribosome
- Defective methylation + acceptor function of transfer RNA
- Inhibits protein prod
DNA
- Covalent binding w/ DNA methyltransferase preventing DNA synthesis
What is the mechanism of action of decitabine?
- Metabolised to triphosphate + incorporated into DNA
- Covalent binding w/ DNA methyltransferase preventing DNA synthesis
What is the mechanism of action of cladribine?
- Mimics adenosine inhibiting adenosine demaines
- Interfere w/ DNA processing
- Activated by lymphocytes
What is the mechanism of action of fludarabine?
- Interferes w/ ribonucleotide reductase + DNA polymerase
- Active against dividing + resting cells
- Ionised at physiological pH
What is the mechanism of action of lonsurf?
Trifluridine:
- Monophosphate: binds covalently to tyrosine 146 active site of TS, inhibiting the enzyme
- Triphosphate: incorporated in DNA
Tipiracil:
- Prevents degradation of trifluridine via TD
What is the mechanism of action of mercaptopurine?
- Biosteric change = aspartic acids doesn’t react to mercaptopurine as C isn’t electrophilic enough
- Competes w/ IMP, blocking AMP production
- Also converted to XMP, blocking GMP synthesis
- Mimics IMP + converted to thioGMP: unstable nucleic acids + interferes w/ replication
