Antimetabolites

An antimetabolite is a drug which disrupts or inhibits essential metabolic processes within cells. Anticancer antimetabolites exploit the need for dividing cells to have a constant supply of deoxynucleic acids required for DNA synthesis. Cells can get their supply of nucleic acids by salvaging them from within the body or by synthesing them de novo Growing cells synthesise deoxythymidine monophosphate (dTMP), required for DNA synthesis, from deoxyuridine monophosphate (dUTP). The catalytic cycle involves three enzymes

• serine hydroxymethyltransferase (SHMT) - catalyses the reaction of serine with tetrahydrofolate to form glycine and N⁵,N¹⁰-methylene-tetrahydrofolate
• thymidylate synthase (TS) - catalyses the reaction of dUTP and N⁵,N¹⁰-methylene-tetrahydrofolate to form dTMP and dihydrofolate
• dihydrofolate reductase (DHFR) - catalyses the reduction of dihydrofolate to tetrahydrofolate using the convesion of NADPH to NADP⁺

Thymidylate Synthase Inhibitors

Thymidylate synthase is the target of a number of anticancer drugs. Inhibition of the enzyme prevents the synthesis of thymidylate which is required for DNA synthesis. Cells die a 'thymineless death'. TS inhibitors have been based on both the substrate (uridine monophosphate) and the cofactor (N⁵,N¹⁰-methylene-tetrahydrofolate).

5-Fluorouracil

5-Fluorouracil (5-FU) is activated by metabolism; (i) to the ribonucleoside (base + sugar); (ii) by phosphorylation; then (iii) by modification to the deoxy- form. FUTP may become incorporated into RNA which is cytotoxic. Also, dFUTP may become incorporated into DNA which is cytotoxic.

5-Fluorouridinemonophosphate is a substrate-based irreversible inhibitor of TS. The electron withdrawing 5-fluoro group prevents the breakdown of the ternary complex between FUMP, N⁵,N¹⁰-methylene-tetrahydrofolate, and the enzyme.

Antifolates

The antifolate class of drugs e.g. CB3717 and tomudex mimic the activated N⁵,N¹⁰-methylene-tetrahydrofolate cofactor and so are competitive inhibitors of TS.

Tomudex is actively transported into cells using the reduced folate carrier (RFC). The drug is a substrate for the enzyme folyl polyglutamate synthase (FPGS) which builds up long polyglutamate chains from the glutamic acid side-chain. The polygluatmated metabolites are up to 100 times more potent inhibitors of TS than the parent drug.