Antimetabolites: Purines, Pyrimidines, and Folates

Antimetabolites

Most antimetabolites act as cancer chemotherapeutic agents by interacting with nucleic acid biosynthesis. Key drugs in this category include purines, pyrimidines, folates, and related compounds.

Antimetabolites can affect enzymes through:

  • Enzyme inhibition at active sites.

  • Allosteric sites.

  • Related sites.

Purine and pyrimidine antimetabolites can incorporate into nucleic acids and polymers like DNA and RNA. Antifolates target cofactor sites of enzymes involved in nucleic acid base biosynthesis.

Pyrimidine Drugs (Uracil Derivatives)

5-Fluorouracil (5-FU)

5-Fluorouracil (5-FU) is a pyrimidine derivative designed to inhibit the conversion of uridine to thymidine. Thymidine biosynthesis involves methylation at the 5-position of uridine's pyrimidine ring.

Replacing the hydrogen at the 5-position of uracil with fluorine creates an antimetabolite drug. This leads to a stable covalent ternary complex of 5-FU, thymidylate synthase (TS), and a tetrahydrofolate cofactor.

Metabolic activation (anabolism) of 5-FU, responsible for anticancer effects, accounts for no more than 20% of the administered drug amount in most patients.

Catabolic Inactivation of 5-FU

Approximately 80% of the 5-FU dose is consumed through catabolic inactivation via normal uracil pathways. Dihydropyrimidine dehydrogenase (DPD) is the major enzyme in pyrimidine catabolism, and 5-FU is a substrate for it.

Uracil is also a substrate for DPD and has been co-administered with 5-FU and its prodrugs to saturate DPD and preserve active drug species.

Variations in DPD activity among patients significantly affect 5-FU bioavailability.

Bioavailability and Prodrugs

Low bioavailability of 5-FU, due to DPD's catabolic efficiency, has led to:

  • Unique dosing routes and schedules.

  • Development of 5-FU prodrugs.

Thymidylate Synthase (TS)

TS facilitates the reductive methylation of deoxyuridine monophosphate (dUMP) by 5,10-methylenetetrahydrofolate to produce dTMP and dihydrofolate.

Because thymine is unique to DNA, the TS enzyme system is vital for replication and cell division.

The tetrahydrofolate cofactor donates both the one-carbon unit and the hydride in this system.

Biosynthesis of Thymidine

The normal biosynthesis of thymidine involves methylation of the 5-position of the pyrimidine ring of uridine, converting dUMP to dTMP. N5,N10{N^{5},N^{10}}-methylene THF is used as a cofactor and DHF (dihydrofolate) is produced. Serine hydroxymethyltransferase converts serine to glycine, using THF as a cofactor. Dihydrofolate reductase (DHFR) uses NADPH + H+H^{+} to convert DHF to THF, producing NADP+.

Mechanism of Thymidylate Synthase

The process begins with a nucleophilic attack by a cysteine residue's sulfhydryl group at the 6-position of dUMP.

The resulting enolate adds to the methylene of 5,10-CH2CH_2-THF, possibly activated via a reactive N-5-iminium ion.

The iminium ion likely forms at N-5 after 5,10-CH2CH_2-THF binds to TS, because N-5 is more basic than N-10 (the better leaving group).

Proton loss at the 5-position of dUMP and folate elimination generate an exocyclic methylene uracil species.

The final step involves hydride transfer from THF and elimination to yield the enzyme, DHF, and dTMP.

Prodrugs of 5-FU

Chemical modifications of 5-FU to prevent catabolism have led to prodrug forms that convert to 5-FU in vivo through metabolic and/or chemical transformations.

  • Capecitabine: A carbamate derivative of 5-deoxy-5-fluorocytidine, converted to 5-FU via several activation steps.

  • Tegafur: Slowly converts to 5-FU, requiring high doses to achieve therapeutic plasma concentrations.

  • Esters of N-hydroxymethyl tegafur: Show greater anticancer activity than tegafur.

Metabolic Activation of Capecitabine to 5-FU

  1. Carbamate hydrolysis (decarbamylation).

  2. Deamination.

  3. Hydrolysis of the sugar moiety to yield 5-FU.

Examples of Pyrimidine Drugs

  • 5-Fluorouracil

  • Floxuridine

  • Capecitabine

  • Tegafur

Mechanism of Action of 5-Fluorouracil

5-Fluorouracil is activated by conversion to 5-fluoro-2-deoxyuridylic acid, a potent inhibitor of thymidylate synthetase. TS converts 2-deoxyuridylic acid to thymidylic acid.

In the inhibition reaction:

  • The sulfhydryl group of TS adds to the 6-position of the fluorouracil moiety via conjugate addition.

  • The carbon at the 5-position binds to the methylene group of 5,10-Methylene tetrahydrofolate after forming the N-5-iminium ion.

Inhibition of Thymidylate Synthetase by 5-Fluorouracil

In the normal process, dihydrofolate is eliminated from the ternary complex, regenerating the active enzyme and thymidine. The loss of the proton at the 5-position of uracil creates an exocyclic methylene uracil species.

The 5-fluorine is stable to elimination, resulting in a terminal product with the enzyme, cofactor, and substrate covalently bonded, forming a stable complex.

Cytarabine and Gemcitabine (Cytosine Derivatives)

Pyrimidine analogs based on the cytosine structure have been developed as antimetabolites for cancer therapy. Modification of the ribose or deoxyribose moiety has produced drugs like cytarabine (ara-C) and gemcitabine.

Cytosine arabinoside (ara-C or cytarabine) uses arabinose instead of ribose, differing only by the epimeric hydroxyl group at the 2-position of the pentose sugar.

Its mechanism may involve slowing DNA chain elongation via DNA polymerase or causing cellular inefficiencies in DNA processing/repair after incorporation.

Gemcitabine

Gemcitabine results from fluorination at the 2`-position of the sugar moiety. After conversion to diphosphate and triphosphate metabolites, it:

  • Inhibits ribonucleotide reductase.

  • Competes with 2-deoxycytidine triphosphate for incorporation into DNA.

Gemcitabine's action is likely similar to ara-C, altering the rate of DNA incorporation and DNA processing/repair.

Azapyrimidine and Deazapyrimidine Nucleosides

Modifications of the pyrimidine ring have been explored for anticancer drugs based on antimetabolite theory. These analogs have one more or one less nitrogen in the heterocyclic ring.

5-Azacytidine is a drug in this category.

5-Azacytidine

This drug's mode of action is complex, including reversible inhibition of DNA methyltransferase. This lack of methylated DNA activates tumor suppressor genes.

In certain tumor systems, it incorporates into nucleic acids, potentially causing misreading or processing errors.

  1. What is the primary mechanism of action for most antimetabolites?

    • A) Protein synthesis inhibition

    • B) Nucleic acid biosynthesis interaction

    • C) Cell division promotion

    • D) Enzyme activation

    • Correct Answer: B

  2. Which of the following is NOT classified as an antimetabolite?

    • A) 5-Fluorouracil

    • B) Methotrexate

    • C) Doxorubicin

    • D) Capecitabine

    • Correct Answer: C

  3. The incorporation of purine and pyrimidine antimetabolites directly into which biomolecules affects their function?

    • A) Proteins

    • B) Lipids

    • C) Nucleic acids

    • D) Carbohydrates

    • Correct Answer: C

  4. What enzyme is primarily responsible for the catabolism of 5-Fluorouracil in humans?

    • A) Dihydropyrimidine dehydrogenase

    • B) Thymidylate synthase

    • C) Dihydrofolate reductase

    • D) Ribosome

    • Correct Answer: A

  5. The method by which 5-Fluorouracil achieves its anticancer effects accounts for approximately what percentage of the administered drug?

    • A) 5%

    • B) 20%

    • C) 50%

    • D) 80%

    • Correct Answer: B

  6. Which drug forms a stable covalent complex with thymidylate synthase?

    • A) Cytarabine

    • B) 5-Fluorouracil

    • C) Gemcitabine

    • D) Vincristine

    • Correct Answer: B

  7. In the inhibition of thymidylate synthase, what compound acts as a cofactor donating a one-carbon unit?

    • A) THF

    • B) NADPH

    • C) Dihydrofolate

    • D) Serine

    • Correct Answer: A

  8. What is the end product of thymidylate synthase's reduction of dUMP?

    • A) dATP

    • B) dCMP

    • C) dTMP

    • D) dGMP

    • Correct Answer: C

  9. Which pyrimidine drug is a prodrug that converts to 5-FU through several activation steps?

    • A) Tegafur

    • B) Floxuridine

    • C) Capecitabine

    • D) Cytarabine

    • Correct Answer: C

  10. What type of reaction does 5-Fluorouracil undergo to form an enzyme-substrate covalent bond with thymidylate synthase?

    • A) Elimination

    • B) Addition

    • C) Hydrolysis

    • D) Transamination

    • Correct Answer: B

  11. Which drug shows greater anticancer activity than tegafur?

    • A) Capecitabine

    • B) Floxuridine

    • C) 5-Fluorouracil

    • D) Esters of N-hydroxymethyl tegafur

    • Correct Answer: D

  12. The stabilization of which complex prevents normal enzyme activity in the presence of 5-Fluorouracil?

    • A) TS and 5,10-methylenetetrahydrofolate

    • B) TS and dihydrofolate

    • C) TS and dUMP

    • D) TS and dTMP

    • Correct Answer: A

  13. What alteration in the sugar moiety distinguishes gemcitabine from cytarabine?

    • A) Fluorination at 2'-position

    • B) Addition of hydroxyl group

    • C) Substitution of ribose for deoxyribose

    • D) Modification of the base

    • Correct Answer: A

  14. What role does dihydrofolate reductase play in nucleic acid biosynthesis?

    • A) Converts DHF to THF

    • B) Forms dTMP

    • C) Inhibits ribonucleotide reductase

    • D) Reduces dUMP

    • Correct Answer: A

  15. The mechanism of action of 5-Azacytidine includes reversible inhibition of which enzyme?

    • A) Dihydropyrimidine dehydrogenase

    • B) DNA methyltransferase

    • C) Dihydrofolate reductase

    • D) Thymidylate synthase

    • Correct Answer: B

  16. Which of the following modifications affects cytarabine’s mechanism of action?

    • A) The base structure

    • B) The position of fluorine

    • C) The presence of arabinose

    • D) The hydroxyl group at 2-position

    • Correct Answer: C

  17. What is a primary consequence of 5-FU's metabolic activation?

    • A) Toxic metabolite formation

    • B) Increased direct cytotoxicity

    • C) Enhanced therapeutic window

    • D) Prodrug advancement

    • Correct Answer: B

  18. Which of the following drugs is a cytosine derivative developed as an antimetabolite?

    • A) 5-Fluorouracil

    • B) Gemcitabine

    • C) Vincristine

    • D) Methotrexate

    • Correct Answer: B

  19. Improved bioavailability of 5-FU can be attributed to what factor?

    • A) Reduced DPD activity

    • B) Lower doses

    • C) Increased metabolic conversion

    • D) High solubility

    • Correct Answer: A

  20. Which step in the biosynthetic pathway produces dihydrofolate?

    • A) Hydrolysis of THF

    • B) Methylation of dUMP

    • C) Reduction of DHF by DHFR

    • D) Interaction of serine and THF

    • Correct Answer: B

  21. What is the result of the interaction between nucleophilic residues and dUMP in the mechanism of thymidylate synthase?

    • A) Formation of stable bonds

    • B) Enzyme degradation

    • C) Reversible inhibition

    • D) Exocyclic methylene formation

    • Correct Answer: D

  22. The enzyme thymidylate synthase is vital for the synthesis of which nucleoside?

    • A) dCMP

    • B) dAMP

    • C) dTMP

    • D) dGMP

    • Correct Answer: C

  23. Why is capecitabine administered as a prodrug?

    • A) To improve absorption

    • B) To enhance stability

    • C) To reduce toxicity

    • D) All of the above

    • Correct Answer: D

  24. What is dapagliflozin's mechanism of action?

    • A) Inhibition of sodium-glucose transport

    • B) Activation of insulin receptors

    • C) Blocking potassium channels

    • D) Stimulation of glucagon secretion

    • Correct Answer: A

  25. Which class of drugs can incorporate into DNA and induce errors during replication?

    • A) Antimetabolites

    • B) Antibiotics

    • C) Alkylating agents

    • D) Hormonal agents

    • Correct Answer: A

  26. How does gemcitabine interfere with ribonucleotide reductase?

    • A) By acting as a substrate

    • B) By directly binding to the enzyme

    • C) By mimicking nucleotides

    • D) By inducing its degradation

    • Correct Answer: C

  27. The binding of which ion forms an N-5-iminium ion crucial for the activity of thymidylate synthase?

    • A) Na+

    • B) K+

    • C) H+

    • D) Mg2+

    • Correct Answer: C

  28. In the metabolic activation of 5-FU, the reaction that introduces a fluorine atom occurs at which position of the uracil moiety?

    • A) 2-position

    • B) 5-position

    • C) 1-position

    • D) 3-position

    • Correct Answer: B

  29. What side effects are commonly associated with thymidylate synthase inhibition by antimetabolites?

    • A) Hematologic toxicities

    • B) Neurological disorders

    • C) Cardiotoxicity

    • D) Renal dysfunction

    • Correct Answer: A

  30. Cytarabine's mechanism of action primarily affects which biological process?

    • A) Protein synthesis

    • B) DNA replication

    • C) RNA splicing

    • D) Lipid metabolism

    • Correct Answer: B