Antimetabolite drugs were among the first effective chemotherapeutic agents discovered and are folic acid, pyrimidine or purine analogues. They are characterized by low molecular weights. They have similar structures as naturally occurring molecules used in nucleic acid (DNA and RNA) synthesis. Antimetabolites are similar to chemicals needed for normal biochemical activity, but differ enough so 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, ovarian and gastro-intestinal cancers. (See web sites and PDR for chemical structures ).
A. 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 pyramidine 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: arabinosylcytosine, capecitabine, gemcitabine and decitabine. Arabinosylcytosine or cytarabine is a deooxycytidine 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 leptominingeal 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 a 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 methytransferase and induces cell death. It may also restore normal gene function controlling cell proliferation. It is used therapeutically in myeloplastic syndrome.
B. 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 are joined to one another to form DNA strands. It is less clear how the purine antagonists function, but they may inhibit normal production of DNA. It is conjectured 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 rephosphorylated 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.
C.FOLATE ANTAGONIST CLASS
Folic acid is an necessary element 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. Following this development, methotrexate was made available. Methotrexate had less toxic side effects and was observed to be clinically more effective. Intracellulary, folate is converted by the enzyme dihydrofolate reductase (DHFR) to dihyrodrofolate. 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.
The most commonly used folate antagonist is methotrexate. It binds to the enzyme DHFR reversibly and inactivates it. This prevents methylation and decreases available supplies of purine and 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 occurs and 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 increases in the DHFR enzyme within the cell. Resistance is a major contributor to treatment failure with this drug.
One other 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 the above named cancers. Premetrexed 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.