Lecture 4 - Antimetabolites

Question 1

DNA replication and transcription require pools of

Answer 1

nucleotide building blocks
anti-metabolites block the production of one or more of these building blocks

Question 2

Anti-metabolites target what phase of the cell cycle

Answer 2

DNA replication

Question 3

Antimetabolites

Answer 3

are agents targeted at the biosynthesis or utilization of DNA and RNA precursors
target itermediary metabolic pathways involved in synthesis of essential molecules in proliferating cells
all drugs exhibit toxicities to rapidly proliferating cells - myelosuppression is often dose-limiting toxicity

Question 4

Antimetabolites are typically analogs of

Answer 4

naturally occurring metabolites
most of these drugs function as enzyme inhibitors: susbtrate mimics or cofactor mimics

Question 5

Most antimetabolite drugs require

Answer 5

biotransformation to nucleotide analog (sugar + phosphate) to generate active species
most drugs are nucleobases/nuclesosides that are then converted to nucleotides, which have a phosphate and can’t get into cells

Question 6

Purines

Answer 6

adenine and guanine

Question 7

Pyrimidine analogs

Answer 7

interfere with pyrimidine nucleotide synthesis - 5-FU and capecitabine
inhibition of DNA polymerase - Ara-C and gemcitabine
all can be incorporated into DNA and RNA and interfere with DNA/RNA function and DNA replication

Question 8

Central compound of pyrimidine synthesis

Answer 8

UTP

Question 9

Uridine analogs

Answer 9

primarily inhibit thymidine synthesis from uracil
block this step, no thymine, no DNA

Question 10

5-fluorouracil (5-FU)

Answer 10

fluorinated uracil analog
converted to flourodeoxyuridine monophosphate (FdUMP), this gets stuck in cells because it’s phosphorylated
in the presence of folates, thymidylate synthase synthesizes thymidine monophosphate (TMP) from deoxy uridine monophosphate (dUMP); folate acts as methyl donor and the H on dUMP is replaced with methyl on TMP

Question 11

FdUMP-enzyme-folate ternary complex

Answer 11

FdUMP mimics dUMP and binds in active site of thymidylate synthase –> forms ternary complex with enzyme and tetrahydrofolate –> rxn can’t go to completion, thymidylate synthase trapped in ternary complex –> TMP can’t be produced –> inhibition of DNA synthesis
toxicity can be rescued with thymidine treatment

Question 12

5-FU additional mechanisms

Answer 12

5-FU also converted to F-UMP and then to F-UTP
F-UTP is incorporated into RNA and DNA, flourine interferes with RNA processing and function, interferes with polyadenylation of mRNA, affecting RNA stability

Question 13

5-FU resistance

Answer 13

downregulation of activating enzymes that convert 5-FU to fd-UMP
upregulation of thymidylate synthase
~5% of pop has gene polymorphisms that result in deficiency of DPD which breaks down 5-FU, polymorphisms predict increased toxicity with 5-FU

Question 14

5-FU drug rescue

Answer 14

thymidine
rescues patient from overdose

Question 15

5-FU drug synergy

Answer 15

leucovorate
folate cofactor that is converted to tetrahydrofolate, higher tetrahydrofolate levels increases efficacy of 5-FU by increasing amount of covalent ternary complex with thymidylate synthase

Question 16

Capecitabine

Answer 16

orally active prodrug of 5-FU

Question 17

Cytosine analogs

Answer 17

primarily inhibit DNA synthesis

Question 18

Cytarabine

Answer 18

converted to Ara-CTP which is a competitive inhibitor of DNA polymerase alpha, also incorporated into DNA and inhibits further DNA synthesis

Question 19

Cytidine deaminase converts cytarabine to

Answer 19

non-toxic uracil arabinoside
decreased levels of cytidine deaminase in CNA makes Ara-C highly toxic in leukemia + lymphoma due to bone marrow suppression

Question 20

Multiple resistance mechanisms of cytarabine

Answer 20

downregulation of activating enzymes, upregulation of cytidine deaminase, and downregulation of transporter to move drug into cells

Question 21

Gemcitabine

Answer 21

greater affinity for activating enzyme
greater potency than cytarabine

Question 22

Tetrahydrouridine

Answer 22

cytidine deaminase inhibitor
cytarabine is inactivated by cytidine deaminase
tetrahydrouridine is given with cytarabine to increase efficacy and decrease resistance

Question 23

Purine biosynthesis from

Answer 23

6-mercaptopurine

Question 24

6-mercaptopurine

Answer 24

inhibits multiple enzymes in the de novo purine biosynthesis pathway, blocks synthesis of purine nucleotides
resistance - loss of HGPRT, the activating enzyme

Question 25

6-MP inactivated by

Answer 25

thiopurine methyl transferase (TPMT)
TPMT polymorphisms in ~10% of children
loss of function mutants increase risk of hematologic toxicity

Question 26

6-MP drug interaction with

Answer 26

allopurinol
allopurinol is a xanthine oxidase inhibitor, this enzyme breaks down 6-MP
at risk of 6-MP toxicity

Question 27

6-thioguanine

Answer 27

allopurinol does not block the breakdown of 6-TG

Question 28

Folic acid

Answer 28

dihydrofolate that is reduced to tetrahydrofolate by dihydrofolate reductase (DHFR)

Question 29

DHFR inhibition

Answer 29

reduces FH4 pools, folate pools accumulate as inactive dihydrofolate –> inhibition of thymidine monophosphate (TMP) synthesis; also inhibits RNA and protein synthesis and purine adn pyrimidine base synthesis

Question 30

Compounds that bind to and inhibit DHFR are

Answer 30

folate mimics - methotrexate, pralatrexate, pemetrexed

Question 31

Overdose rescue

Answer 31

leucovorin
used to rescue normal tissues from methotrexate toxicity
converted to tetrahydrofolate intracellularly and increases in this reverese toxic effects of DHFR inhibition

Question 32

Leucovorin used two different ways

Answer 32

to enhance the activity of 5-FU and capecitabine and to rescue the effects of methotrexate