Antimetabolites

Question 1

Describe antimetabolites

Answer 1

Drugs that interfere w formation of key biomolecules within the cell
Several classes

Question 2

What are three classes of antimetabolites

Answer 2

Folate antagonists
Pyrimidine antagonists
Purine antagonists

Question 3

Describe folic acid

Answer 3

Essential water-soluble B vitamin
Acquired through diet, body can’t synthesise it
Require 400mg per day

Question 4

Describe folates as coenzymes

Answer 4

Help in the synthesis of nucleotide precursors
One carbon donors
Only active in fully reduced form

Question 5

Describe the de novo synthesis reaction of dUMP to dTMP via folate

Answer 5

Catalysed by thymidylate synthase
Folate in tetra hydrofolate form donates carbon group -> dihydrofolate

Question 6

What is the fully reduced form of folate called

Answer 6

Tetrahydrofolate

Question 7

What is the role of dihydrofolate reductase

Answer 7

Reduced dihydrofolate -> tetrahydrofolate to make the folate fully reduced so it can be used again
Uses NADPH as co factor to reduce fol

Question 8

Describe folates involvement in purine synthesis

Answer 8

Co factor in de novo purine synthesis to form IMP -> GMP and AMP

Alongside GAR and AICAR transformylases

Question 9

Describe intracellular polyglutamation

Answer 9

Via folypolyglutamate synthetase
Adds glutamate residues to folate to make it larger and anionic = intracellular accumulation and retention

Enhances cofactor affinity in purine pathway

Question 10

Describe antifolates

Answer 10

Folate analogues
Tight binding inhibitor not substrate for dihydrofolate reductase

Question 11

What is methotrexate

Answer 11

Antifolate
Widely used in many tumour types
Used in non-malignant conditions

Question 12

Describe the mechanism of action for MTX

Answer 12

Glutamated by folypolyglutamate
Inhibits dihydrofolate reductase
= inhibits thymidylate synthesis
= build up of dUMP and dihydrofolate

= can’t generate dDTPs
= inhibits GART and AICART

Question 13

When is MTX a stronger inhibitor of thymidylate synthetase

Answer 13

When it has been glutamated

Question 14

Why is there better retention of MTX In tumour cells

Answer 14

High polyglutamation formation in malignant cells
= 80% MTX polyglutamated

Question 15

What are the biochemical consequences of MTX

Answer 15

Decreased dTTP and increased dUTP
= inappropriate incorporation of U into DNA
Inhibition of chain elongation
-> excision of U = strand breaks -> death

Question 16

How does MTX enter the cells

Answer 16

Via passive diffusion at high concs
= temp sensitive and energy dependent

Via reduced folate carrier

Question 17

How does folic acid enter the cell

Answer 17

Via the folate receptor

Question 18

How does serum folate enter the cell

Answer 18

Via folate receptors or the reduced folate carrier

Question 19

How can malignant cells affect the folate carrier to form resistance to MTX

Answer 19

Decrease expression = reduced amount of MTX into cell

Question 20

How can malignant cells reduced the amount of glutamate MTX to form resistance

Answer 20

Enhanced levels of gammaGH or decreased levels of FPGS enzyme activity

MTX not well retained and has less affinity for enzymes

Question 21

How does a malignant cells form resistance against MTX via DHFR

Answer 21

Specific mutation = decreased DHFR binding to MTX but still allow dihydrofolate bind

Question 22

How can malignant cells affect gene expression to form resistance to MTX

Answer 22

Amplification of DHFR at gene level
= more protein
= not enough MTX to sufficiently inhibit

Question 23

Describe the pharmacology of MTX

Answer 23

Oral/IV or intrathecal administration
1st phase half life = 3 hours
Final phase half life = 8-10 hours

Question 24

How is MTX excreted

Answer 24

Renal

Question 25

What toxicities are expected with MTX

Answer 25

Myelosuppression, mucositis and nephrotoxicity

Question 26

What does require LV rescue

Answer 26

Intermediate and high 100mg/m2 +

Question 27

What is the role of LV

Answer 27

Prevents/ rescues cells from toxic effects of MTX Can prevent toxicity to bone marrow and GI tract

Question 28

What is the mechanism of action for LV

Answer 28

Competitor for transport/ polyglutamation and targeted enzymes

Question 29

Is MTX inhibition reversible

Answer 29

Yes Can be out competed with FH2 or reduced folates

Question 30

What else can be used to rescue cells instead of LV

Answer 30

Thymidine

Question 31

Name 2 new antifolates

Answer 31

Pralatrexate Pemetexed

Question 32

Describe pralatrexate

Answer 32

10x greater affinity for RFC than MTX Higher polyglutamation Inhibits tumour cell growth at lower conc

Question 33

Describe Pemetrexed

Answer 33

Readily transported into cells via RFC Multitargeted antifolate, mainly TS but also DHFR and GART

Question 34

Describe purine and pyrimidine anti-metabolites

Answer 34

Analogues for bases and nucleosides = Inhibits DNA synthesis

Question 35

Name pyrimidine anti-metabolites

Answer 35

5-flurouracil Cytosine arabinoside

Question 36

Name purine anti-metabolites

Answer 36

Mercaptopurine Thiogunaine

Question 37

Describe 5-fluorouracil

Answer 37

Analogue of uracil with F atom on C5 Causes replication errors Requires intracellular metabolism to cytotoxic forms

Question 38

What can be used to inhibit thymidylate synthase

Answer 38

FdUMP In the presence of folate cofactor Competes with dUMP for catalytic site

Question 39

What is another mechanism that 5-FU uses

Answer 39

Incorporates into RNA Inhibits RNA processing Incorporated into DNA - strand breaks

Question 40

Describe the catabolism of 5-FU

Answer 40

Initial reduction by DPD Final breakdown product is FBAL

Question 41

What can low DPD activity cause

Answer 41

Increased exposure to active metabolites = results in severe or fatal toxicity

Question 42

What is advised prior to 5FU treatment

Answer 42

DPD status e.g enzyme activity and SNP presence

Question 43

Describe the pharmacology of 5FU

Answer 43

Poor oral bioavailability (25-45%) -> DPD inhibitor = 100% Saturable first pass metabolism High clearance Half life 10-20mins -> 5hrs