II- Antimetabolites

II- ANTIMETABOLITES (Structural Analogues) Antimetabolites are structurally related to normal cellular components MOA: block one or more of the metabolic pathways involved in DNA synthesis Most antimetabolites interfere with nucleic acid synthesis (nucleotide synthesis)

II- ANTIMETABOLITES They inhibit the synthesis purine or pyrimidine nucleotide or by competing with them in DNA or RNA synthesis Their maximal cytotoxic effects are S-phase and are therefore cell-cycle specific drugs (CCS)

Examples of Antimetabolites Folic acid Antagonists: Methotrexate (MTX) Purines antagonists: Mercaptopurine (6-MP) Pyrimidines antagonists: Fluorouracil (5-FU)

Folic Acid Folic acid is needed for the synthesis of the  thymidine, required for DNA synthesis Also, folate is essential for the synthesis of purine nucleotides which in turn are essential for DNA synthesis and cell division Methotrexate competitively inhibits dihydrofolate reductase (DHFR), an enzyme that participates in the tetrahydrofolate synthesis.[7] The affinity of methotrexate for DHFR is about one thousand-fold that of folate. DHFR catalyses the conversion of dihydrofolate to the active tetrahydrofolate. Folic acid is needed for the de novo synthesis of the nucleoside thymidine, required for DNA synthesis. Also, folate is needed for purine base synthesis, so all purine synthesis will be inhibited. Methotrexate, therefore, inhibits the synthesis of DNA, RNA, thymidylates, and proteins.

Inhibition of dihydrofolate reductase (DFR)

A. Folic Acid Antagonists Methotrexate (MTX) MTX is structurally related to folic acid and acts as an antagonist of that vitamin by inhibiting dihydrofolate reductase This enzyme converts folic acid to its active, coenzyme form (FH4); tetrahydrofolic acid Methotrexate has a higher affinity than Folate for dihydrofolate reductase and thus inhibits the enzyme = depleting intracellular FH4

MTX is cytotoxic during the S-phase of the cell cycle. Lack of this cofactor (FH4) interrupts the synthesis of thymidine, purine nucleotides Thereby interfering with the formation of DNA, RNA, and protein leading to cell death  MTX is cytotoxic during the S-phase of the cell cycle. MTX has a greater toxic effect on rapidly dividing cells (GIT, oral mucosa, BM)

Therapeutic Uses Methotrexate, is effective against acute leukemia, choriocarcinoma, non Hodgkin’s lymphoma in children, breast cancer and head & neck carcinoma To induce abortion with misoprostol It is effective against certain inflammatory diseases, such as: Rheumatoid arthritis Severe psoriasis

Adverse Effects Depression of the bone marrow Damage to the epithelium of the GIT; stomatitis, nausea, vomiting and diarrhea Erythema, rash, urticaria, alopecia High doses of MTX may cause renal damage (crystalluria) so it is important to keep the urine alkaline + hydration Intrathecal administration lead to meningeal irritation, stiff neck, headache Hepatic and pulmonary toxicity

Leucovorin rescue Large doses of MTX must be followed by 'rescue' with folinic acid (a form of FH4) Rescue normal cells by Leucovorin =folinic acid Folinic acid bypasses the blocked enzyme and replenish the folate pool

Tumor resistance to MTX Decreased drug transport into the cell Altered dihydrofolate reductase enzyme -- lower affinity for MTX Increase in dihydrofolate reductase enzyme concentration in the cell

II- ANTIMETABOLITES They inhibit the synthesis purine or pyrimidine nucleotide or by competing with them in DNA or RNA synthesis Their maximal cytotoxic effects are S-phase and are therefore cell-cycle specific drugs (CCS)

B. Purine Antagonists 6-Mercaptopurine impairs the synthesis of purine nucleotide After oral administration, 6-mercaptopurine undergoes first-pass metabolism, inactivated by xanthine oxidase in the liver Clinical uses:   For remission induction and maintenance therapy of acute lymphatic leukemia.

MOA of 6-mercaptopurine 6-MP is activated inside cell 6-MP by Hypoxanthine Guanine Phospho-Ribosyl Transferase (HGPRTase) Thiol inosine monophosphate Toxic nucleotide (TIMP) E needed for purine synthesis  TIMP Incorporated into DNA “Non functional” Tumor cell death

Adverse effects: Drug interaction: Principal toxicity is BM depression N & V, diarrhea Hepatotoxicity Drug interaction: Allupurinol; is used to inhibit xanthine oxidase, to prevent hyperuricemia associated with tumor cell lysis, inhibits the breakdown of 6-MP and increases 6-MP effects, toxicity so dose of 6-MP must be reduced by at least 25%

C. Pyrimidine antagonists Fluorouracil (5-FU) It is converted to the corresponding deoxynocleotide (5-FdUMP) which inhibits thymidylate synthetase Inhibition of thymidylate synthetase leads to suppression of the formation of thymidine nucleotides. This results in inhibition of DNA synthesis through “Thymidine-less death”

Cytotoxic effects on both RNA and DNA MOA of Fluorouracil 5FU  5-FdUMP inhibits Thymidylate synthetase  synthesis of  thymidine nucleotide   DNA synthesis Cytotoxic effects on both RNA and DNA Thymidine-less death

Therapeutic Uses Fluorouracil is used primarily in the treatment of slowly growing solid tumors such as colorectal cancer, cancer of breast, ovary, pancreas A cream incorporating fluorouracil is used topically for treating skin cancers

Toxic Effects: It may lead to nausea, vomiting, anorexia, diarrhea and alopecia Severe ulceration of the oral and gastrointestinal mucosa Bone marrow depression

Microtubule inhibitors

Microtubules Inhibitors Mitototic spindle is formed during cell division

Microtubules Inhibitors CCS Vinca alkaloids Vinblastine Vincristine Taxanes Paclitaxel Podophyllotoxins Etoposide

Vinblastine and Vincristine Belongs to Vinca alkaloids Both drugs are cell cycle-specific (CCS) because they block mitosis in metaphase

Therapeutic uses Adverse Effects: Vinblastine is used in testicular carcinoma, Hodgkin’s disease Vincristine is used in the treatment of acute leukemia in children, Wilm’s tumor, and Hodgkin’s and non-Hodgkin’s lymphomas Adverse Effects: Common adverse effects for both drugs include cellulitis or phlebitis if the drugs extravasate during injection Nausea, vomiting, diarrhea, and alopecia

Toxicity of Vinblastine & Vincristine Myelo-supression Peripheral neuropathy: Depressed deep tendon reflex, paresthesia, foot drop Vincristine NOT myelosuppressant

Side effects; neutropenia ‼, peripheral neuropathy Taxanes Paclitaxel (Taxol) shows good activity against metastatic breast cancer and advanced ovarian cancer Side effects; neutropenia ‼, peripheral neuropathy Filgrastim ( granulocyte colony stimulating factor)

IV. Cytotoxic Antibiotics

Substances of microbial origin that prevent mammalian cell division Some antibiotics that affect DNA in both microbial and mammalian cells can be used in cancer chemotherapy

Cytotoxic Antibiotics (Antitumor Antibiotics) Dactinomycin (Actinomycin D) Doxorubicin (Adriamycin) Bleomycin As a rule, they should not be given together with radiotherapy, as the cumulative toxicity is very high

Dactinomycin (Actinomycin D) Dactinomycin was the first antibiotic used in cancer chemotherapy It affects cells in all phases of the cell-cycle i.e. CCNS Dactinomycin is given intravenously, it remains unchanged and is concentrated in the liver and excreted in bile It does not cross the BBB

Mechanism of action of Dactinomycin It intercalates, in the minor groove of DNA, between adjacent guanine-cytosine pairs thus preventing transcription (through an effect on topoisomerase II that essentially unwinds the DNA helix for replication) D D Minor Groove  

Therapeutic Uses Adverse Effects: It is mainly used for treating paediatric cancers with other drugs in the treatment of Wilm’s tumor, gestational choriocarcinoma Adverse Effects: Extravasation during injection produces severe irritation and cellulitis (leucopenia and thrombocytopenia)

Adverse Effects (cont.) Bone marrow depression is the major dose-limiting toxicity (leucopenia and thrombocytopenia) Nausea, vomiting, diarrhea, oral ulcers alopecia and skin eruptions may also be noted Dactinomycin sensitizes to radiation; inflammation at sites of prior radiation therapy may occur

Doxorubicin (Adriamycin) Doxorubicin (Adriamycin) is the most commonly used member of the anthracycline antibiotics, given IV Mechanism of Cytotoxic Action: Doxorubicin exerts its cytotoxic actions through 3 mechanisms: 1. intercalates in the DNA, to stabilize the DNA-topoisomerase II complex thus halting replication

binding to cell membranes to alter fluidity and ion transport generation of oxygen free radicals (hydrogen peroxide) through interaction with molecular oxygen This action may be responsible for cardiac toxicity (dysrhythmias and heart failure) The heart is devoid of superoxide dismutase (SOD) which protects tissues against oxygen free radicals

Clinical indications of Doxorubicin One of the most important and widely used anti- cancer drug, used in combination with other agents in treatment of sarcomas and cancer of breast, lung , ovary and thyroid gland It is useful also in acute lymphoblastic leukemia, and lymphomas

Adverse Effects: Extravasation causes tissue necrosis The drug may lead to severe alopecia at standard dosage *** Stomatitis and GIT disturbances Increased skin pigmentation Doxorubicin causes bone marrow depression which is of short duration, with rapid recovery The drug has dark red color and leads to red color of the urine

Adverse Effects (cont.) Doxorubicin cause cumulative, dose- dependent cardiac toxicity, leading to dysrhythmias cardiomyopathy and heart failure due to generation of free radicals and lipid peroxidation Dexrazoxane , an iron chelator , decrease the formation of superoxide radicals is used to protect against the cardiotoxicity of the drug

Bleomycin: a metal-chelating antibiotic DNA- bleomycin-Fe2+ complex intercalate between base pair Fe DNA- bleomycin-Fe3+ B The liberated electrons react with oxygen Generate free radicals (superoxide, hydroxyl radicals) Fe B DNA strands fragmentation

Clinical uses of Bleomycin Bleomycin is most effective in the G2 phase of the cell cycle and mitosis, but it is also active against non-dividing cells (i.e. cells in the G0 phase) CCNS It is used to treat germline cancer Testicular carcinoma Hodgkin’s lymphoma

Adverse effects of Bleomycin Pulmonary fibrosis Skin toxicity: Alopecia , blisters and hyperkeratosis Hypersensitivity reactions; fever, chills,… It is not a myelosuppressant

Pulmonary & skin toxicity ‼ Hydrolase ( bleomycin inactivating enzyme) is deficient in the lung and skin .

V. Miscellaneous Anticancer Agents

V- Miscellaneous Agents L-Asparaginase Hydroxyurea Mitotane Retenoic acid derivatives Interferon

L- Asparaginase Asparagine is amino acid required for protein synthesis Most normal cells can synthesize asparagine Neoplastic cells require an external source of asparagine (from the plasma) because of their limited capacity to make sufficient asparagine to support growth

L-Asparginase is derived from bacteria L-Asparaginase L-asparaginase Asparagine aspartic acid +Ammonia L-Asparginase is derived from bacteria L-asparaginase hydrolyzes serum asparagine and thus deprives the tumor cells of this nutrient required for protein synthesis Most normal cells can synthesize asparagine and thus are less susceptible to the action of asparaginase

Theraputic uses: L-asparginase is used to treat childhood acute lymphocytic leukemia in combination with vincristine and prednisone Adverse effects: Hypersensitivity reactions, liver affection and pancreatitis

Hydroxyurea Hydroxyurea is an analog of urea. MOA: blocks the incorporation of the thymidine nucleotide into the DNA strand. used in chronic myeloid leukemia administered orally.

Miscellaneous Agents Mitotane Reduces excessive steroid secretion used for Adrenal carcinoma Retenoic acid derivatives Remissions -- acute leukemia

Drug combinations

The administration of combinations of drugs in the treatment of cancer produces better results than a single drug A combination of drugs with different toxic effects and affecting different biochemical pathways has higher anti-tumor activity without additive toxicity Therapy is given intermittently to allow recovery of normal tissue i.e. bone marrow and immune system that has been affected by the drugs

Examples of drug combinations Acute lymphocytic leukemia: vincristine, methotrexate and 6-mercaptopurine Advanced breast cancer : Doxorubicin, cyclophosphamide with or without flurouracil Non-Hodgkin's lymphomas: Cyclophosphamide, doxorubicin, vincristine