Antimetabolites (V)

Nitrogenous bases found in the DNA andRNA:​

Pyrimidines​

Purines​

Pyrimidines:​

• Cytosine (C)​

• Thymine (T)​

• Uracil (only in RNA) (U)

Purines:​

• Adenine (A)​

• Guanine (G)

Nucleosides

The sugar ribose or 2’-deoxyribose (indicated by d), attached to the nitrogen base.

​The names of purine nucleosides end in osine:

Adenosine and Guanosine.

The names of pyrimidine nucleosides end in idine:​

Uridine, Thymidine, and Cytidine.

Nucleotides

Adding one or more phosphates to the sugar portion of a nucleoside results in a nucleotide.

Some representative nucleotide names are:

AMP = adenosine monophosphate ​
CDP = cytidine diphosphate ​
dGTP = deoxy-guanosine triphosphate ​
dTTP = deoxy-thymidine triphosphate​

cAMP = 3'-5' cyclic adenosine monophosphate

DNA structure​

• base

• nucleoside

• nucleotide

Antimetabolites

are similar in structure tonative molecules (i.e. metabolites) required fornormal biochemical reactions, yet slightlydifferent to interfere with the normal functionsof cells.

(Antimetabolites) They are analogues of

• vitamins → coenzymes (e.g., folic acid )​

• nitrogenous bases, nucleosides andnucleotides

General Mechanisms of Action​

1. Antimetabolites interfere with the production ofnucleic acids by:​

• inhibiting key enzymes for nucleotide synthesis.​

• substituting normal purines or pyrimidines.​

(2) Antimetabolites inhibit DNA and RNA synthesis.​

(3) Antimetabolites inhibit cell growth and proliferation.​

Usually cell-cycle S-phase dependent.

Antimetabolites

• Folate Antagonists​

• Pyrimidine Antagonists​

• Purine Antagonists

Folate Antagonists (antifolate)

​

• Methotrexate (MTX,Trexall®) ​

• Pemetrexed (Alimta®)​

• Pralatrexate (Folotyn®)

Folic Acid​

• Folic acid (vitamin B9) is essential for cellgrowth and fetus development.​

• Reduced to tetrahydrofolate (FH4) (active)by dihydrofolate reductase (DHFR).​

FH4

coenzyme for the biosynthesis ofpurines and in the conversion of dUMP(deoxy-uridine monophosphate) to dTMP(deoxy-thymidine monophosphate).

Methotrexate

• Methotrexate (MTX), the 4-amino, 10-methyl analogueof folic acid​

• MTX remains the most widely used antifolate in cancerchemotherapy​

• Activity against a wide range of human malignancies,including many solid tumors and hematologicmalignancies

MOA of Methotrexate

• Methotrexatecompetitively inhibits thedihydrofolate reductase(DHFR), preventing theformation oftetrahydropholate (FH4).​

• DNA synthesis isinhibited because dTMPis not available andinhibited purinebiosynthesis.​

• It effects can bereversed by leucovorin(reduced folate)​

• Cells during S-phaseare most sensitive.

Leucovorin (LCV) “Rescue”​

We can give an antidote to methotrexate​

• Leucovorin (folinic acid): very similar, but not thesame as folic acid; not requiring the action of DHFR,unaffected by inhibition of this enzyme by MTX​

• “Rescue” normal cells from toxic effects of MTX;reduce adverse effects of MTX on bone marrow & GImucosa​

• Administered at the appropriate time following MTXas part of a total chemotherapeutic plan to rescuenormal cells from toxicity

LCV restores theformation of dTMP​

LCV restores purinebiosynthesis​

Transport​

• Enters cell by active transport​
  -Reduced folate carrier (RFC) ​

-Folate receptor (FR) systems​

Metabolism

• Metabolism ---> methotrexate polyglutamates which can't be transported across cell membrane & are retained within cell ​

-Occurs in tumor cells and, to a lesser extent, in normal tissues (selectivity)

Mechanisms of Resistance (MTX) ​

• Decreased antifolate transport due to adefect in the reduced folate carrier (RFC)or folate receptor protein (FR) systems​

• Decreased capacity to polyglutamate MTX​

• Increased levels of the target enzymeDHFR through gene amplification andother mechanisms

Toxicity

• Oral and GI tract ulceration.​

• Bone marrow depression.​

• Leukopenia​

• Thrombocytopenia

“Lecovorin rescue”

reduce ADRs of MTX​

Pemetrexed (Alimta®)​

• New analogue of folic acid​

• Unique multiple enzyme inhibition: notonly DHFR, but also inhibit thymidylatesynthase (TS) ​

• Differs from methotrexate in its transportproperties: Pemetrexed is more rapidlypolyglutamated than methotrexate ​

• Treating mesothelioma and lung cancer

Pyrimidine Antagonists​

• 5-Fluorouracil (5-FU) andCapecitabine (Xeloda®)​

• Cytarabine (AraC, Cytosar®)​

• Gemcitabine (Gemzar®)

Mechanism for PyrimidineAnalogs​

1. Incorporation into nucleic acids.​

2. Inhibition of the enzymes: ​

e.g, for 5-FU, ↓ thymidylatesynthase (TS) ↓ thymidinenucleotide production (i.e.,dTMP).

TS

dUMP → dTMP

5-Fluorouracil​

• 5-FU resemblespyrimidine bases(e.g., U and T).​

• Indications: Colon,rectal, gastric,hepatic, head &neck, breastcancer…​

 Active in S-phases.

MOA of 5-Fluorouracil​

5-FU is ribosylated andphosphorylated nucleotides analogs(e.g., 5-FUTP and 5-FdUMP).​

(A) 5-FUTP (instead ofUTP) is incorporatedinto RNA → inhibition ofRNA processing.​​

(B) 5-FdUMP inhibitsirreversibly thymidylatesynthase → ↓ dTMP → ↓ DNA synthesis.

Combination of leucovorin with 5-FU ​

• Leucovorin: tetrahydrofolate (FH4) derivative (reducedfolate)​

• Acts as a coenzyme for Thymidylate Synthetase(TS)​

• When given with 5-FU or capecitabine​

• Allows 5-FU to form a more stable bond to TS​

• Inhibition of TS is enhanced​

• Cytotoxic effects of 5-FU are enhanced

Combination of leucovorin with 5-FU ​Indications

• ↑ cytotoxic effects of fluorouracil (synergistic effect)​

• Usually given just before 5-FU

Capecitabine (Xeloda)​

• Oral 5-FU prodrug​

• The activation of capecitabine follows a pathway withthree enzymatic steps​

• Enzymatically converted to 5-FU in the tumor bytumor-specific enzyme​

• Produces therapeutic results similar to intravenousbolus 5-FU, with less mucositis andmyelosuppression (it is more tumor specific)​

• The most widely prescribed oral drug for colorectalcancer

MOA of xeloda

more tumor specific

Cytarabine

Cytosine arabinoside ​(AraC).​

• Arabinose replacesribose (cytidine) ordeoxyribose(deoxycytidine).​

• Only for leukemiasand lymphomas​

• Cell-cycle S-phasespecific.

MOA Cytarabine

AraC is phosphorylated → AraCTP:​

1. AraCTP is incorporated into DNA andRNA → inhibits chain elongation andligation.​

2. AraCTP is a competitive inhibitor ofDNA polymerase → ↓ DNA synthesis.​

​

Gemcitabine

• Gemcitabine (2’,2’-difluorodeoxycytidine,dFdC) is structurallysimilar to cytarabine. ​

• Different mechanism ofaction → wider antitumoractivity.​

• S and G1/S cell-cyclephase specific.

MOA of Gemcitabine

Gemcitabine is phosphorylated by kinase togemcitabine di- (dFdCDP) and tri-phosphate(dFdCTP):​

1. dFdCDP inhibits the ribonucleotide reductase→ ↓ dCTP pool → ↓ DNA synthesis.​

(2) dFdCTP inhibits DNA polymerases → ↓ DNAsynthesis and induces apoptosis.​

(3) dFdCTP is incorporated into DNA → inhibitschain elongation and ligation.​

All ribonucleotides (e.g. CDP) are reduced to deoxyribonucleotides (e.g.dCDP) by a single enzyme, ribonucleotide reductase (its substratesare diphosphate forms, i.e. ADP, GDP, UDP and CDP).

5-Fluorouracil Toxicity

Nausea, oral and GI​ ulceration, bone marrow depression​

Capecitabine

Myelosupression, nausea,​ and vomiting(less than 5-FU)

Cytarabine Toxicity

Nausea, vomiting, bone marrow​, depression, megaloblastosis, leukopenia,​

and thrombocytopenia​

Gemcitabine Toxicity

Nausea, vomiting, diarrhea,​ and myelosupression​

Purine Antagonists​

• 6-Thioguanine​

• 6-Mercaptopurine​

• Fludarabine Phosphate

Mechanism of Action forPurine Analogs​

• After ribosylation andphosphorylation → DNAincorporation.​

• DNA incorporation → inhibitionof DNA synthesis.​

• Inhibition of key enzymes ofpurine nucleotide synthesis

6-Thioguanine (6-TG)​ and​ 6-Mercaptopurine (6-MP) MOA​

6-Thioguanine and 6-Mercaptopurine are ​

analogs of Guanine (=O → -SH).​

• For childhood acute leukemias.​

• Triphosphate nucleotides formed from 6-MP and 6-TG→ DNA incorporation  inhibition of DNA synthesis. ​

Fludarabine​ phosphate MOA

• Fludarabine phosphate is firstdephosphorylated(extracellular). ​

• The intermediate is thenphosphorylated (intracelluarly).​

• The triphosphate metaboliteinhibits DNA polymerase;incorporated into DNA and RNA;diphosphate metabolite inhibitsribonucleotide reductase  DNA synthesis.​

• For chronic lymphocyticleukemia (CLL)

Thioguanine and Mercaptopurine Toxicity

Well tolerated, ​bone marrow depression (↑doses) (↑ TPMT-deficient patients) ​

Fludarabine Toxicity

Myelosupression,​ neurotoxicity (high doses)​

Pyrimidine Analogs​

• Cytarabine​

• Gemcitabine​

• Fluorouracil​

• Capecitabine​

• Floxuridine​

• Azacitidine*​

• Decitabine*

Folic Acid Analogs​

• Methotrexate

• Pemetrexed

• Pralatrexate

Purine Analogs​

• Fludarabine​

• Cladribine​

• Clofarabine​

• Pentostatin​

• Mercaptopurine​

• Azathioprine​

• Thioguanine​

• Nelarabine