Antimetabolites have similar chemical structures to those of molecules the body uses to create nucleic acid (DNA and RNA).  They interfere with normal cell function. Generally, antimetabolites induce cell death during the S phase of cell growth when incorporated into RNA and DNA or inhibit enzymes needed for nucleic acid production. These agents are used to treat a variety of cancers, including leukemia, breast, pancreatic, ovarian, and gastro-intestinal cancers. Antimetabolites are characterized by low molecular weights. Why can’t we just make more chemical compounds that get in the pathway? The problem for drug designers is that metabolic pathways are hard to figure out, and making compounds to interfere with them without causing other problems is fiendishly difficult. Both the pyrimidine bases (uracil, cytosine), and the purine bases (adenine, guanine) are building blocks in the synthesis of DNA and RNA nucleotides. In the replication process, nucleotides combine to form DNA strands. It is less clear how the purine antagonists function, but they may inhibit normal production of DNA.

Antimetabolite drugs were among the first effective chemotherapeutic agents discovered.

Pyrimidine Compounds

In 1957 scientists introduced 5-flurouracil (5-FU). 5-FU is a pyrimidine base containing a fluoride atom at the 5 carbon position on the ring. Uracil is a naturally occurring pyrimidine base used in nucleic acid synthesis. It is converted to thymidine by enzyme action. 5-FU is similar in structure to uracil and is converted to two active metabolites (FdUMP and FUTP) that inhibit the activity of the enzyme thymidylate synthetase. The enzyme normally converts uracil to thymidine by adding a methyl group at the fifth carbon of the pyrimidine ring. 5-FU mimics the natural base and functions to inhibit DNA synthesis. The carbon group cannot be added because of the fluoride atom at the five position. Normal DNA synthesis fails. dUTP and FdUTP are incorporated into DNA so that it cannot function normally. In addition, FUTP is incorporated into RNA leading to faulty translation of the RNA. Thus, the synthesis of multiple forms of RNA (messenger, ribosomal, transfer and small nuclear RNAs) is blocked. These combined actions on DNA and RNA are cytotoxic to the rapidly dividing cancer cells. 5-FU is used for the treatment of many malignancies: breast, head and neck, adrenal, pancreatic, gastric, colon, rectal, esophageal, liver, and G-U (bladder, penile, vulva, prostate) . 5-FU may be administered by bolus IV infusion or continuous IV infusion over two days every 2-3 weeks or by oral ingestion. In addition, it may be used to treat skin cancers (basal cell and keratosis) by topical application.

Other pyrimidine antagonists include: cytarabine, capecitabine, gemcitabine and decitabine. Cytarabine (aka arabinosylcytosine) is a deoxycytidine base compound that is converted to its active metabolite, ara-CTP. This base is incorporated into DNA and causes strand termination. The cancer cell is unable to divide. This drug has proven effective in acute non-lymphocytic, lymphocytic, myelogenous , and chronic myelocytic leukemias, as well as leptomeningeal carcinomatosis and non-Hodgkin’s lymphoma.  Capecitabine is an oral 5-FU pro-drug. It is converted to 5-FU by liver and tumor cells. It is used as adjuvant therapy in colon and breast metastasis. Gemcitabine is an ara-C prodrug which is activated by intracellular phosphorylation. This inhibits DNA and RNA synthesis. It is a first line treatment of pancreatic, metastatic breast, bladder, ovarian and non-small cell lung cancers.

Purine Compounds

There are few clinically useful purine antagonists.  Scientists conjecture that these purine antagonists stop synthesis by decreasing the production of the purine bases or that the antagonist molecules themselves may be incorporated into the DNA strands during synthesis and halt cell replication.  Without adequate amounts of the purine bases, nucleotide production stops and the cancer cell dies.

Fludarabine or 2-fluoro-ara-amp is an antimetabolite of the purine class. It functions as a pro-drug. It is dephosphorylated to F-ara-ATP and enters the cancer cell. Upon incorporation into the DNA strand, it halts strand lengthening. The drug has been employed to treat refractory chronic lymphocytic and chronic B cell leukemias, non-Hodgkin’s lymphoma, and T- cell lymphoma.

6-Mercaptopurine (6-MP) is another purine agent used against acute lymphocytic leukemia. It is active in the S phase of cell proliferation. When it is incorporated into DNA and RNA, the nucleic acids are rendered useless. 6-MP may also act through inhibition of de novo synthesis of the purine bases. Genetic mutation may lead to purine resistance.

Adenosine deaminase inhibitors are a class of compounds that can be classified under the purine antagonist umbrella.  These include Cladribine and Pentostatin.

Folate Antagonists

An early advancement in cancer research came in the mid-20th Century when pioneering researcher Sidney Farber was able to induce remission in children with leukemia using the antimetabolite drug aminopterin.  In 1953 another milestone was achieved with the first cure of a solid tumor entirely with chemotherapy, the antimetabolite methotrexate.  Folic acid is a necessary compound for the production of nucleotides. It was empirically observed in patients with leukemia that diets low in folate produced lower white cell counts than observed in leukemic patients on normal folate diets. In 1948, a folate antagonist was found effective in childhood leukemia. The antifolate medication methotrexate became an early chemotherapy drug. Between cells folate is converted by the enzyme dihydrofolate reductase (DHFR) to dihydrofolate. This compound is then reduced to tetrahydrofolate (THDF). THDF acts as a carbon carrier compound that donates methyl groups to end target molecules through the enzymatic action of thymidine synthetase. DHFR is continuously used in this process and is the site where the folate antagonists function. The drugs impede enzyme action and hence interfere with nucleotide formation.

Methotrexate binds to DHFR reversibly and inactivates it. This prevents methylation and decreases available supplies of purine and thymidine bases for new DNA and RNA synthesis. Methotrexate is active in the S phase of cell growth. It is effective in many malignancies. Breast, head and neck, colorectal, non-Hodgkin’s lymphomas, osteosarcoma, bladder and choriocarcinoma are treated with methotrexate. It is also used in acute lymphocytic leukemia, and some types of meningeal carcinomas. Methotrexate is the most common folate antagonist used today even though others have been developed. Drug resistance is a primary complication of treatment with methotrexate. Decreased drug transport into the cell can occur. A lower and less effective dose of methotrexate is observed intracellulary. Genetic mutations and alterations in gene activity may occur as well which alter binding constants to the enzymes or causes increases in the DHFR enzyme within the cell.

Pemetrexed is a folate antagonist used in the treatment of mesothelioma and non-small cell lung cancer. Pemetrexed is combined with cisplatin (an agent which promotes DNA cross-linking) to treat those cancers. Pemetrexed acts like methotrexate. It hinders multiple enzymes needed for de novo production of the thymidine and purine nucleotides. Normal DNA and RNA production is prevented. Pralatrexate is also approved for use against cancer, but it is less often used.

Demethylation Agents

Azacitidine and Decitabine work through demethylation. In normal cellular metabolism, cytosine methylation is an epigenetic mark for maintenance of gene silencing across cellular divisions. However, this chemically stable modification may be removed from DNA through demethylation. Azacitidine and Decitabine are chemical analogs of cytidine. Like other antimetabolites they can become incorporated into the nucleic acid.  Once part of the cell’s DNA, it stops methylation by inhibiting DNA methyltransferase and thereby induces cell death. It may also restore normal gene function controlling cell proliferation.

Medicinal Compounds

There are 19 currently approved cancer medicines that can be classified as antimetabolites, of which 4 are antifolates.

Azacitidine (aka Azacytidine)

Brand/Trade Names: Vidazaaz

Manufacturers: Celgene, Apicore LLC, Archimica, Ash Stevens, Inc., Aspire Lifesciences Pvt Ltd

Formula: C₈H₁₂N₄O₅

Mechanism: demethylation. Cytidine analogue.

Class: antimetabolite, DNA methyltransferase inhibitor

Administration: Oral

Notes: Chemical analog of cytidine.  An epigenetic drug.  Approved by the FDA in 2004.  Used for treatment of myelodysplastic syndromes and leukemia.

Capecitabine

Brand/Trade Names: Xeloda

Manufacturers: Genentech, Aarti Industries Ltd, Acebright India Pharma Private Limited, Apotex Pharmachem,  Archimica

Formula: C₁₅H₂₂FN₃O₆

Mechanism: antimetabolite

Class: Pyrimidine analogue

Administration: Oral

Notes:  Approved by the FDA in 2005.  Used for treatment of breast cancer and colorectal cancer.

Clofarabine

Brand/Trade Names: Clolar

Manufacturers: Sanofi-Aventis US LLC Apicore LLC  Biophore India Pharmaceuticals Pvt Ltd  Emcure Pharmaceuticals  Hangzhou Longshine Bio-Tech Co., Ltd

Formula: C₁₀H₁₁ClFN₅O₃

Mechanism: antimetabolite

Class: Purine analogue

Administration: Intravenous

Notes: Approved by the FDA in 2004.  Used for treatment of acute lymphoblastic leukemia.

Cladribine

Brand/Trade Names: Leustatin, Litak

Manufacturers: Reliable Biopharmaceutical Corporation, Ahk Tech, Aspen Biopharma Labs Pvt Ltd, Biophore India Pharmaceuticals Pvt Ltd, Cilag AG

Formula: C₁₀H₁₂ClN₅O₃

Mechanism: antimetabolite, adenosine deaminase inhibitor

Class: Purine analogue

Administration: Oral

Notes:  Approved by the FDA in 1993.  Used for treatment of hairy cell leukemia, acute myeloid leukemia, chronic lymphocytic leukemia, and lymphoma.

Cytarabine

Brand/Trade Names: Cytosar-U, Arabinosylcytosine

Manufacturers: Pharmacia & Upjohn Company LLC, Ahk Tech, Apotex Pharmachem Inc.,  Archimica,  Asahi Kasei Finechem

Formula: C₉H₁₃N₃O₅

Mechanism: antimetabolite

Class: Cytosine analogue

Administration: Intravenous

Notes:  Approved by the FDA in 1992.   Used for treatment of leukemia.

Decitabine

Brand/Trade Names: Dacogen

Manufacturers: Otsuka America Pharmaceuticals, Fujifilm Diosynth Biotechnologies

Formula: C₈H₁₂N₄O₄

Mechanism: antimetabolite, demethylation

Class: Cytosine analogue, DNA methyltransferase inhibitor

Administration: Intravenous

Notes: Cytidine analog. An epigenetic drug.  Approved by the FDA in 2006.  Used for myelodysplastic syndromes.

Fludarabine

Brand/Trade Names: Fludara

Manufacturers: Actavis Pharma Inc, Archimica,   Ash Stevens, Inc.,   Aspen Biopharma Labs Pvt Ltd,  Beijing Lunarsun Pharmaceutical Co.

Formula: C₁₀H₁₃FN₅O₇P

Mechanism: antimetabolite

Class: Purine analogue

Administration: Intravenous

Notes: purine analog.  Approved by the FDA in 1991.   Used for chronic lymphocytic leukemia, lymphoma.

Fluorouracil (5F0U)

Brand/Trade Names: Abdrucil, 5-FU

Manufacturers: Teva Parenteral Medicines, AMCOL Health and Beauty Solutions, Biotechnica Pharma Global, DCS Pharma AG, DSM Nutritional Products

Formula: C₄H₃FN₂O₂

Mechanism:  antimetabolite

Class: Pyrimidine analogue

Administration: Intravenous

Notes: Approved by the FDA in 1962.  Used for treatment of bladder, anal, breast, gastrointestinal, thymic, cervical, head and neck, and colorectal cancer

Floxuridine

Brand/Trade Names: FUDR

Manufacturers: Zhejiang Hisun Pharmaceutical, ORGANICA Feinchemie GmbH Wolfen, Reliable Biopharmaceutical Corporation

Formula: C₉H₁₁FN₂O₅

Mechanism: antimetabolite

Class: Pyrimidine analogue

Administration: Intravenous

Notes: Chemical analog of pyrimidine.  First approved by the FDA in 1970.  Used for stomach, colon, and kidney cancer.

Gemcitabine

Brand/Trade Names: Gemzar

Manufacturers: Evonik Corporation, Arch Pharmalabs, Archimica, Arene Lifesciences Limited, Aspen Biopharma Labs Pvt Ltd

Formula: C₉H₁₁F₂N₃O₄

Mechanism: antimetabolite

Class: Pyrimidine analogue

Administration: Intravenous

Notes: Approved by the FDA in 1996.  Approved for treatment of sarcoma, pancreatic cancer, ovarian cancer, breast cancer, and non-small cell lung cancer.

Hydroxyurea

Brand/Trade Names: Hydroxycarbamide, Hydrea, Droxia, Mylocel

Manufacturers: Qilu Pharmaceutical Co.,  Archimica, Bristol-Myers Squibb, Nishchem International Pvt.,  Olon Spa

Formula: CH₄N₂O₂

Mechanism: antimetabolite

Class:

Administration: Intravenous

Notes: Approved by the FDA in 1967.  Used for treatment of chronic myelogenous leukemia, squamous cell carcinoma of the head and neck, melanoma, and ovarian cancer.

Mercaptopurine

Brand/Trade Names: Purinethol, Purixan

Manufacturers: Aarti Industries Ltd, Ajinomoto Company,   Changzhou Siyao Pharmaceuticals Co., Fermion Oy, Stason Pharmaceuticals, Inc.

Formula: C₅H₄N₄S

Mechanism: antimetabolite

Class: Purine analogue

Administration: Oral

Notes: Approved by the FDA in 1953.  Used for treatment of acute lymphoblastic leukemia.

Nelarabine

Brand/Trade Names: Arranon

Manufacturers: Novartis Pharmaceuticals Corporation, ALP Pharm Beijing Co.,  Biophore India Pharmaceuticals Pvt Ltd, ChemWerth Inc, F.I.S. Fabbrica Italiana Sintetici SpA

Formula: C₁₁H₁₅N₅O₅

Mechanism: antimetabolite

Class: Purine analogue

Administration: Intravenous

Notes:  Approved by the FDA in 2005.  Used for treatment of T-cell acute lymphoblastic leukemia and T-cell lymphoblastic lymphoma.

Pentostatin

Brand/Trade Names: Nipent

Manufacturers: Hospira, Inc Apicore LLC  Biotechnica Pharma Global  ChemPacific Corp

Formula: C₁₁H₁₆N₄O₄

Mechanism: antimetabolite, adenosine deaminase inhibitor

Class: Purine analogue

Administration: Intravenous

Notes: Approved by the FDA in 1991.  Used for hairy cell leukemia.

Thioguanine

Brand/Trade Names: Tabloid

Manufacturers: Glaxo Operations UK Limited t/a Glaxo Welcome Operations

Formula: C₅H₅N₅S

Mechanism: antimetabolite

Class: Purine analogue

Administration: Oral

Notes: Approved by the FDA in 1966.  Used for treatment of acute myeloid leukemia.

Trifluridine/Tipiricil

Brand/Trade Names: Lonsurf

Manufacturers: Taiho  Pharmaceuticals  Co Ltd

Formula: C₅H₅N₅S

Mechanism: antimetabolite

Class: Pyrimidine analogue

Administration: Oral

Notes:  Approved by the FDA in 2015.  Used to treat colorectal cancer and stomach adenocarcinoma.

Antifolates

Pralatrexate

Brand/Trade Names: Folotyn

Manufacturers: Allos Therapeutics, Epoch Labs, Haoyuan Chemexpress Co., Hetero Drugs Limited. Mac-Chem Products (India) Pvt.

Formula: C₂₃H₂₃N₇O₅

Mechanism: antimetabolite

Class: Antifolate Administration: Intravenous Notes:  Approved by the FDA in 2009.  Used for treatment of lymphoma.

Pemetrexed

Brand/Trade Names: Alimta

Manufacturers: Eli Lilly and Company, Acebright India Pharma Private Limited. Arene Lifesciences Limited, Aspire Lifesciences Pvt Ltd, BrightGene Bio-Medical Technology Co.

Formula: C₂₀H₂₁N₅O₆

Mechanism: antimetabolite

Class: Antifolate

Administration: Intravenous Notes:  Approved by the FDA in 2004.  Used for treatment of mesothelioma and non-small cell lung cancer.

Methotrexate

Brand/Trade Names: MTX, amethopterin

Manufacturers: Bedford Labs, Biesterfeld Spezialchemie GmbH, Biovectra Inc, CF Pharma Ltd., Excella GmbH

Formula: C₂₀H₂₂N₈O₅

Mechanism: antimetabolite

Class: Antifolate

Administration: Intravenous

Notes: Approved by the FDA in 1953.  Used by breast cancer, osteosarcoma, leukemia, lung cancer, lymphoma, head and neck cancer, and stomach cancer. PDF list of antimetabolites.