Antimetabolites

Antimetabolite drugs were among the first effective chemotherapeutic agents discovered, and are folic acid, pyrimidine or purine analogues. They have similar structures to molecules the body uses in nucleic acid (DNA and RNA) synthesis. Antimetabolites are similar to chemicals needed for normal biochemical activity, but differ enough that 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 for a variety of cancer therapies, including leukemia, breast, pancreatic, ovarian, and gastro-intestinal cancers.

Antimetabolites is a broad term, and could potentially refer to any drug that interferes with metabolic pathways, by inhibiting enzymatic reactions.  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.

Antimetabolite medicines used to fight cancer (e.g. methotrexate) slow the synthesis of pyrimidine and purine that cells in the S-Phase use to build new DNA molecules.  (Methotrexate is an analog for folic acid.) They are characterized by low molecular weights.

Pyrimidine Compounds

Duschinsky synthesized and Heidelberger, in 1957, 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. It is 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 actions in liver and tumor cells. It is used as adjuvant therapy in colon and breast metastasis. Gemcitabine is an ara-C pro drug 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. Finally, decitabine is phosphorylated and directly incorporated into DNA. In cancer cells, it stops methylation by inhibiting DNA methyltransferase and induces cell death. It may also restore normal gene function controlling cell proliferation. It is used therapeutically in myelodysplastic syndrome.

Purine Compounds

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.  Scientists  conjecture that these purine antagonists stop synthesis by decreasing the production of the purine bases or may be incorporated into the DNA strands during synthesis and halt cell replication. Genetic mutation may lead to purine resistance. Fludarabine or 2-fluoro-ara-amp is an antimetabolite of the purine class. It functions as a pro-drug. It is dephosphorylated and enters the cancer cell. Fludarabine is converted to F-ara-ATP. Upon incorporation into the DNA strand, it halts strand lengthening. The drug is successfully used in treating refractory chronic lymphocytic and chronic B cell leukemias, non-Hodgkin’s lymphoma and T- cell lymphoma. 6-Mercaptopurine (6-MP) is another purine agent successfully used against acute lymphocytic leukemia. It is active in the S phase of cell proliferation. Upon incorporation 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. Without adequate amounts of the purine bases, nucleotide production stops and the cancer cell dies. There are few clinically useful purine antagonists.

Folate Antagonists

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.  Intracellulary folate is converted by the enzyme dihydrofolate reductase (DHFR) to dihydrofolate. This compound is then reduced to active folate also called tetrahydrofolate (THDF). THDF acts as a carbon carrier compound that donates methyl groups to end target molecules through the enzymatic action of thymidine synthetase (TS). DHFR is continuously used in this process and is the site where the folate antagonists function. These drugs generally function by impeding enzyme action.

Methotrexate binds to the enzyme DHFR reversibly and inactivates it. This prevents methylation and decreases available supplies of purine methotrexate structureand thymidine bases for new DNA and RNA synthesis. It is active in the S phase of cell growth. Methotrexate remains the primary folate antagonist used today even though others have been produced. 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.

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. Resistance is a major contributor to treatment failure with this drug.

Pemetrexed  is another folate antagonist is effective 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,

Adenosine deaminase inhibitors

Cladribine




Medicinal Compounds

Azacitidine

Brand/Trade Names: Vidaza

Formula: C8H12N4O5

Mechanism: demethylation

Class: antimetabolite

Administration: Oral

Notes: chemical analog of cytidine

Capecitabine

Brand/Trade Names: Xeloda

Formula: C15H22FN3O6

Mechanism:

Class: antimetabolite

Administration: Oral

Notes:

Clofarabine

Brand/Trade Names: Clolar

Formula: C10H11ClFN5O3

Mechanism: Aromatase Inhibitor

Class: antimetabolite

Administration: Intraveneous

Notes:

Cladribine

Brand/Trade Names: Leustatin

Formula: C10H12ClN5O3

Mechanism: demethylation

Class: antimetabolite

Administration: Oral

Notes: chemical analog of cytidine

Cytarabine

Brand/Trade Names: Cytosar-U

Formula: C9H13N3O5

Mechanism:

Class: antimetabolite

Administration: Intraveneous

Notes:

Decitabine

Brand/Trade Names: Dacogen

Formula: C8H12N4O4

Mechanism:

Class: antimetabolite

Administration: Intraveneous

Notes: cytidine analog.

Fluorouracil (5F0U)

Brand/Trade Names: Abdrucil, 5-FU

Formula: C4H3FN2O2

Mechanism:

Class: antimetabolite

Administration: Intraveneous

Notes:

Floxuridine

Brand/Trade Names: FUDR

Formula: C9H11FN2O5

Mechanism:

Class: antimetabolite

Administration: Intraveneous

Notes: chemical analog of cytidine

Gemcitabine

Brand/Trade Names: Gemzar

Formula: C9H11F2N3O4

Mechanism:

Class: antimetabolite

Administration: Intraveneous

Notes: chemical analog of cytidine

Hydroxyurea

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

Formula: CH4N2O2

Mechanism:

Class: antimetabolite

Administration: Intraveneous

Notes:

Leucovorin

Brand/Trade Names: folinic acid

Formula: C20H23N7O7

Mechanism:

Class: antimetabolite

Administration: Intraveneous, Oral, Intramuscular

Notes:

Mercaptopurine

Brand/Trade Names: Purinethol, Purixan

Formula: C5H4N4S

Mechanism:

Class: antimetabolite

Administration: Oral

Notes: used for leukemia

Nelarabine

Brand/Trade Names: Arranon

Formula: C11H15N5O5

Mechanism:

Class: antimetabolite

Administration: Intraveneous

Notes:

Pentostatin

Brand/Trade Names: Nipent

Formula: C11H16N4O4

Mechanism:

Class: antimetabolite

Administration: Intraveneous

Notes: purine analog

Procarbazine

Brand/Trade Names: Matulane

Formula: C12H19N3O

Mechanism:

Class: antimetabolite

Administration: Oral

Notes:

Thioguanine

Brand/Trade Names: Tabloid

Formula: C5H5N5S

Mechanism:

Class: antimetabolite

Administration: Oral

Notes:

Trifluridine/Tipiricil

Brand/Trade Names: Lonsurf

Formula: C5H5N5S

Mechanism:

Class: antimetabolite

Administration: Oral

Notes: