Inhibition of DNA Biosynthesis

folic acid (folate)

essential vitamin (B9)

plays crucial role in biosynthesis of several DNA bases

important in pregnancy and periods of rapid growth

biosynthetic pathway and role of folic acid

dietary folic acid is reduced by dihydrofolate reductase into dihydrofolic acid

reduces again by DHFR into tetrahydrofolic acid

formylation by hydroxymethyl transferase into 10-formyl-THF

10-formyl-THF

key biosynthetic intermediate for introduction of 1 carbon unit in several DNA bases

were folate agonists or antagonists researched for treating cancer

antagonists

methotrexate

antagonist of folic acid

can have serious side effects

competitively inhibits enzymes that use folic acid, competes for DHFR active site

5-fluorouracil

antagonist of folic acid

1962

treats breast, colorectal and other cancers and used topically on skin for several skin cancers

5-fluorouracil mode of action

interferes with biosynthetic route to make thymidine

irreversible inhibitor of 10-formyl-THF thymidylate synthase

interrupts action to block biosynthesis of thymidine monophosphate for DNA replication

cancer cells undergo ‘thymineless cell death’

thymidylate synthase mode of action

catalytic nucleophile in a Michael reaction with 10-formyl THF, providing the 1 carbon electrophile

loss of a proton regenerates the enzyme catalyst

(addition then elimination)

5-fluorouracil inhibition mechanism

is structurally similar to uracil part of deoxyuridine, so is also attacked by the enzyme

after the addition, the fluorine cannot be removed by a base, so thymidylate synthase is irreversibly inhibited

thymidine biosynthesis and DNA production is reduced

using ADEPT to selectively deliver 5-fluorouracil

development of prodrugs with 5-fluorouracil attached to a B-lactam unit

used in conjunction with antibodies to a tumour antigen linked to a B-lactamase enzyme