drugs affecting nucleotide metabolism

Question 1

allopurinol

Answer 1

hypoxanthine (nucleotide) analog used to treat gout
nucleotide analog
oxidized by hepatic xanthine oxidase to alloxanthine (a xanthine analog)
alloxanthine binds to the enzyme xanthine oxidase and inhibits it irreversible - prevents hypoxanthine and xanthine from being oxidized to uric acid
hypoxanthine and xanthine that accumulates as a result are salvaged by HGPRT to IMP and XMP - increased nucleotide pools inhibit the rate-limiting enzymes in purine biosynthesis (PRPP synthetase and PRPP-amino transferase)

Question 2

antifolates

Answer 2

compounds that interfere with the formation of tetrahydrofolate often by interfering with dihydrofolate reductase = essential coenzyme for purine and pyrimidine synthesis
eg. methotrexate, aminopterin, trimethoprim, sulfanilamide
antagonists of folic acid pathway

Question 3

methotrexate

Answer 3

used in cancer chemotherapy and in chronic inflammatory illnesses (arthritis, psoriasis, crohn’s)
transported into cells as folic acid - polyglutanylated like folic acid too
binds with high affinity to dihydrofolate reductase (DHFR) which is the enzyme that converts folic acid to tetrahydrofolic acid in mammals (and bacteria)
cells accumulate dihydrofolate form of folic acid - depleted of C1-tetrahydrofolate needed for purine and pyrimidine biosynthesis
since rapidly dividing cells need high levels of purine and pyrimidine, rapidly dividing cells highly susceptible
but kills all dividing cells, so treat in high dose for brief time and then follow with rescue with a reduced form of folic acid (drug leucovorin) - method commonly used for treatment of osteogenic sarcoma

Question 4

sulfanilamide

Answer 4

antibacterial
analog of p-aminobenzoic acid (PABA)
PABA = intermediate of folic acid synthesis in many bacteria
bacteria require de novo folic acid synthesis so inhibits enzyme that dihydropteroate synthetase = enzyme that converts PABA to folic acid in bacteria

Question 5

trimethoprim

Answer 5

folate analog
binds to DHFR of some microorganisms much more tightly than it binds to mammalian DHFR
used to treat infections due to parasites and in combination with sulfanilamide to treat bacterial infections, esp. strep, staph, etc.

Question 6

fluorouracil

Answer 6

used in cancer chemotherapy
pyrimidine analog - structure similar to uracil except fluorinated
transported into cells, salvaged, and phosphorylated as uracil
has no cytotoxic activity until converted to F-UMP by orotic acid-phosphoribosyl transferase
then converted to F-UDP -> F-dUDP -> F-dUMP
f-dUMP forms ternary complex with thymidylate synthase and tetrahydrofolate => covalent binding of f-dUMP => inactivation of thymidylate synthase = “suicide inhibitor”
depletes cellular thymidine pools (dTTP) => inhibition of DNA synthesis

Question 7

mercaptopurine

Answer 7

anti-tumor drug
nucleotide analog class of drugs
tumor cells form 6-mercaptopurine rebonucleotide
enzyme HGPRT (purine salvage drug) converts 6-MP to nucleotide form using PRPP
nucleotide form (6-mercaptopurine ribonucleotide monophosphate) is potent inhibitor of amidotransferase in de novo purine biosynthesis - specifically inhibits synthesis of GMP from IMP
nucleotide accumulates in cell, serves as negative effector of PRPP-amidotransferase

Question 8

cytosine arabinoside

Answer 8

aka Ara-C
nucleotide analog
treats cancer
deoxycytidine kinase (part of salvage pathways) converts it to Ara-CMP
then conveted to di and tri-phosphorylated forms => ara-CTP
ara-CTP gets incorporated into polymerizing DNA strands - stops further DNA synthesis
toxicity determined by level of deoxycytidine kinase activity in a given cell type - some types of leukemic cells have high levels of activity - allows for selective cell killing

Question 9

acycloguanosine (Acyclovir)

Answer 9

treatment for herpesvirus (HSV) infection
activated by HSV-thymidine kinase encoded by HSV genome - converts it to monophosphate form
host cellular thymidine kinase can’t use drug as substrate so the acycloguanosine monophosphate is phosphorylated by cellular enzymes to di- and tri-phosphate forms
acycloguanosine triphosphate is a substrate for HSV-specific DNA polymerase - incorporated into growing viral DNA strand - causes termination

Question 10

azidothymidine (AZT) aka zidovudine

Answer 10

treatment for HIV
AZT is phosphorylated by cellular kinases to AZT-triphosphate
AZT-triphosphate inhibits HIV-DNA polymerase (an RNA-dependent polymerase) - blocks HIV replication
selective for HIV infected cells because affinity of DNA polymerase from HIV is at least 100-fold greater than the affinity to human DNA polymerase
combined with other drugs (nonnucleoside reverse transcriptase inhibitors (NNRTIs) and HIV protease inhibitors and fusion inhibitors)

Question 11

mechanisms of resistance to methotrexate

Answer 11

kills most tumor cells, but selects for the survival of resistant cells

resistance can be due to:

1: decreased influx of the drug - due to deceased transcription of or mutation in folate transporter
2: increased DHFR activity or amplification of DHFR gene
3: decreased binding of methotrexate to DHFR - point mutations in the DHFR gene
4: decreased polyglutamate formation => decreased folylpolyglutamate synthase activty

Question 12

drug types and the drugs in those categories

Answer 12

antifolates:
	- sulfanilamide
	- methotrexate
	- trimethoprim
nucleotide analogs:
	- fluorouracil
	- mercaptopurine
	- allopurinol
	- cytosine arabinoside (ara-C)
	- acycloguanosine (acyclovir)
	- azidothymadine (AZT)