Lecture 5 - Anticancer Part 1 PDF

Anticancer Agents 1 What is Cancer? The medical term for cancer or tumor is neoplasm, which means a relatively autonomous growth of tissue. Cancer is a collection of related diseases characterized by abnormal growth and uncontrolled division of cells which le...

Anticancer Agents 1 What is Cancer? The medical term for cancer or tumor is neoplasm, which means a relatively autonomous growth of tissue. Cancer is a collection of related diseases characterized by abnormal growth and uncontrolled division of cells which leads energy consumption and loss the structure and the ability to perform normal functions. These tumors may invade adjacent tissues (metastasis) hence become difficult to control and may lead to death of patients 2 What is Cancer? Some tumors are named after the individual who first described the condition, such as Hodgkin’s disease. Some are named according to the tissue or origin, e.g: 1. A cancer that arises from mesodermal tissue is called a sarcoma, that from ecto- or endodermal tissue is called carcinoma and that from fibrous tissue is called fibroma; 2. A cancer of the blood involving the abnormal increase of leukocytes is called leukemia. 3 Etiology of cancer A lot of contributing factors have been postulated but none of them is the exact etiologic factor: Genetic factors (mutation & changed gene expression). Viral factor (human T-lymphotropic virus type I (HTLV-I) has been proved to cause a form of leukemia). Physical factors (long-term exposure to chemicals and/ or irradiation). Hormonal factors. 4 Mechanism of Cancer Formation Proto-oncogenes: are genes which normally code for proteins involved in the control of cell division and differentiation, If they are mutated, this disrupts the normal function, and the cell becomes cancerous, the protoncogene is then called “oncogene” It is generally believed that most human malignancies result from incorrect proto-oncogene expression. Tumor suppressor genes: are intended to keep oncogenes in check by halting uncontrolled cellular growth (e.g. p53). These promote cancer if attenuated or inactivated. 5 Mechanism of Cancer Formation Two fundamentally different genetic mechanisms exist consisting of: (1) Enhanced or aberrant Oncogene expression or (2) Decreased activity of Tumor Suppressor Gene (anti- oncogenes). 6  Hallmarks of Cancer The hallmarks of cancer comprise six biological capabilities acquired during the multistep development of human tumors. They include: 1. Sustaining proliferative signaling, 2. Evading growth suppressors, 3. Resisting cell death, 4. Enabling replicative immortality, 5. Inducing angiogenesis, 6. Activating invasion and metastasis. Douglas Hanahan; Hallmarks of Cancer: New Dimensions. 7 Cancer Discov 1 January 2022; 12 (1): 31–46. https://doi.org/10.1158/2159-8290.CD-21-1059 Treatment of cancer Surgery: provided it has not metastasized. Radiation therapy: it is superior to surgery since it effectively destroys a tumor and at the same time causes only minimal damage to the surrounding normal tissue. Chemotherapy (Antineoplastic, Cytotoxic, Anticancer, and Antitumor drugs): chemotherapy is mainly limited by total mass of the tumor.  The most common side effects of present day chemotherapy are nausea, hair loss, increased susceptibility to infection and many others.  Useful drugs without side effects do not yet exist. 8 Broad Classification of anticancer classes 1. Agents that inhibit or interfere with DNA synthesis, or cell division (Cytotoxic agents) 2. Hormonal therapy (For hormone related cancers) 3. Targeted anticancer therapeutics: (now include several classes of anticancer agents for example:  Signal transduction inhibitors (Kinase inhibitors)  Immunomodulators  Epigenetic therapy in cancer 9 Classification of cancer chemotherapeutics They can be grouped according to their chemical, pharmacological and mechanistic profiles into: 1. DNA cross linking agents (Alkylating agents & organometallics) 2. Topoisomerase poisons 3. Antibiotics 4. Antimetabolites 5. Mitosis inhibitors 6. Hormone –based anticancer agents 7. Signal transduction inhibitors (Tyrosine kinase inhibitors) 8. Immunomodulators 9. Epigenetic therapy in cancer 10 1) DNA Cross-linking agents (Alkylators and Organometallics) A. Nitrogen mustards B. Triazines (DNA methylators) C. Nitrosoureas D. Organoplatinum complexes 11 A) Nitrogen mustards Historically, alkylating agents were important in the early development of cancer chemotherapies. Victims of sulfur mustard gas, bis(2-chloroethyl)sulfide, in World War I were found to have a severe lymphoid aplasia as well as pulmonary irritation. This led to clinical trials of the related but less toxic and more soluble nitrogen mustard derivative (substitution of sulfur with methylamino group, mechlorethamine), which produced tumor regressions in lymphoma patients. Many derivatives of the nitrogen mustards have been synthesized with various improvements. Clinical use of nitrogen mustard today is mostly limited to the treatment of lymphomas, especially Hodgkin’s disease. 12  A) Nitrogen Mustards; Structure & mechanism of action Bis (β-Chloroethylamines) 13 Nitrogen Mustards; Mechanism of action Both aliphatic and aromatic nitrogen mustards react with 7- position of guanine in each of double strands of DNA, causing cross-linking (bifunctional alkylating agents), This interferes with separation of the strands and prevents mitosis. Thus, these bifunctional alkylating agents are more effective antitumor agents than their monofunctional analogs; However increasing the number of alkylating sites on the agent beyond two does not appear to increase antitumor activity 14 A) Nitrogen mustards; Specific drugs Cl N COOH Cl i- Mechlorethamine HCl ii- Chlorambucil N-methyl bis(2-chloroethyl)amine Aliphatic nitrogen mustards will have lone pair more available, hence, increase rate of aziridinium ion formation, increase reactivity and hence toxicity Conversely, aromatic nitrogen mustards have their nitrogen lone pair consumed in resonance, hence slows rate of intramolecular nucleophilic attack, and aziridinium ion formation, hence permits oral administration and attenuate severity of side effects 15  1 Cl A) Nitrogen mustards O O 2P N iii) Cyclophosphamide (Endoxan) NH 3 Cl Steps of Biotransformation 16  1 Cl O O iii) Cyclophosphamide 2P N NH 3 Cl Steps of Biotransformation  It is inactive in vitro and must be converted to an active form by metabolic processes (Bioprecursor).  The drug undergoes metabolic activation in the liver catalyzed by the cytochrome P-450 microsomal enzymes. It is converted to 4- hydroxycyclophosphamide, which is spontaneously tautomerized to aldophosphamide.  In tumor tissues, aldophosphamide is hydrolyzed to yield phosphoramide, the active antitumor mustard, and acrolein. 17 iii) Cyclophosphamide Side effects Acrolein is believed to be responsible for the cystitis produced by cyclophosphamide. (Kidney and bladder damage) (very electrophilic, alkylates Cys residues in kidney and bladder) Therefore, cyclophosphamide is co-administered with N- acetylcysteine or 2-mercaptoethanesulfonate (mesna). Both are thiols that neutralize acrolein by giving non-toxic conjugate addition to its double bond. 18 Structure activity relationship of nitrogen mustards 1. Bis(2-chloroethyl) is essential for activity. 2. Halogen other than chlorine decrease the activity. 3. Ethylene moiety between the nitrogen and chlorine is essential for activity due to the facility of the formation of aziridinium ion. 4. Methylene or trimethylene moiety abolishes the activity. 19 I) Alkylating agents B) Nitrosoureas (Carmustine & Lomustine) They are highly lipid soluble and easily transported across the BBB, making them useful in brain tumors.  Carmustine: (BiCNu): 1,3-bis(2-chloroethyl)-1-nitrosourea.  Lomustine: (CeeNu): 1-(2-chloroethyl)-3-cyclohexyl-1- nitrosourea. O O Cl Cl Cl N N N N H H NO NO 20 (B) Nitrosoureas; Mechanism of action Chemical decomposition of these agents in aqueous solution yields two reactive intermediates, a chloroethyldiazo- hydroxide and an isocyanate group. The former decomposes further to a relatively reactive chloroethyl carbonium ion that alkylate DNA, 21 (C) Nitrosoureas Mechanism of action Chemical decomposition of these agents in aqueous solution yields two reactive intermediates, a chloroethyldiazohydroxide and an isocyanate group. The former decomposes further to a relatively reactive chloroethyl carbonium ion that alkylate DNA, whereas the isocyanate moiety carbamoylate amino acid (lysine residues), The nitrosoureas have limited clinical utility because of tendency to cause more prolonged myelosuppression than other alkylators. 22 I) Alkylating agents (C) Triazenes (DNA methylators) i) Dacarbazine: (DTIC), ii)Temozolomide (TMZ) O NH2 N N N N H N(CH3)2 23 i) Dacarbazine (DTIC) Mechanism of action: (DNA methylators) Activation of DTIC by a microsomal enzyme in the liver can produce a methyldiazohydroxide, a precursor to methyl carbonium ion. CONH2 N CONH2 CH3 N N liver N N N CH3 -CH2O H CH3 CYP450 N N N N H O H CONH2 N CONH2 N CH3 + N HO N N CH3 N N N N H NH2 H + - H H OH HO + N2 + CH3+ 24 alkylates nucleic acid Activation of Temozolomide (TMZ) 25 DNA cross linking agents Organoplatinum complexes They contain an electron deficient metal atom (Pt) that can accept electrons from the DNA nucleophiles. i) Cis-platinum (cis-platin):  It was the first heavy-metal compound to be introduced into clinical cancer chemotherapy.  Today cis-platinum, in combination with other agents, has led to highly active and often curative in patients with testicular, ovarian, and head and neck cancer.  Its activity and toxicity resemble those of the alkylating agents. 26  Organoplatinum complexes i) Cis-platinum (cis-platin): Mechanism of action The two chlorine atoms are displaced by water to give the active cytotoxic hydrated form, which are easily attacked by DNA nucleophiles (N7 of guanines) (formation of cross-linking). 27 Organoplatinum complexes i) Carboplatin, Picoplatin & Oxaliplatin These are second generation cisplatin analogues with less nephrotoxicity or neurotoxicity and with less emetic potential than the parent compound. they are indicated for treatment of ovarian and colorectal cancers. 28 2) Topoisomerase poisons Topoisomerases are enzymes that control the degree of DNA supercoiling. Topoisomerase IIα cleaves double stranded DNA during replication phase, also it repairs its own damage after replication is complete. Topoisomerase I; cuts and religates a single stranded DNA. Topoisomerase poisons stimulate DNA cleavage but inhibit DNA resealing activity of the enzymes, leaving the DNA irreversibly damaged and unable to replicate 29 2) Topoisomerase poisons Three chemically distinct classes of anticancer agents act as topoisomerase poisons: 1) Camptothecins 2) Epipodophyllotoxins 3) Anthracyclines (Anticancer antibiotics) 30 2) Topoisomerase poisons 31 3) Antibiotics a) Anthracyclines: doxorubicin & daunorubicin They are antibiotics and obtained from cultures of Streptomyces Peucitius. The most important drugs in this class are doxorubicin and daunorubicin. In general, these compounds have a planar anthraquinone nucleus attached to an amino sugar 32 a) Anthracyclines: doxorubicin & daunorubicin Mechanism of action They act by different postulated mechanisms: 1) DNA-RNA Binding. Initially they were found to bind DNA by intercalation between base pairs perpendicular to the long axis of the double helix. This intercalation causes partial unwinding of the helix and thus disrupts DNA polymerases and transcription. 2) They act as Topoisomerase II poisons 33 a) Anthracyclines: doxorubicin & daunorubicin Mechanism of action Other postulated mechanisms for anthracyclines: 3) Free-radical generation. Free-radical formation (highly reactive compounds with an unpaired electron) occurs during the metabolism of anthracyclines. 4) Membrane interaction: When anthracyclines bind to cell membrane, changes in membrane glycoproteins, transmembrane flux of ions, and membrane morphology have been demonstrated in a variety of cells. 5) Metal ion chelation: The anthracyclines are capable to chelate divalent cations such as calcium and ferrous ions by virtue of the quinone and phenolic functions. 34 Important Note to remember The first three classes of anti cancer agents (crosslinking agents – topoisomerase poisons and antibiotics) all act by damaging existing DNA and inhibiting its ability to replicate 35 4) Anti-metabolites  Anti-metabolites stop the de novo synthesis of DNA by inhibiting the formation of essential nucleotides.  They act by inhibiting key enzymes in nucleotide biosynthesis, or biosynthesis of DNA or arrest chain elongation  Antimetabolites usually are closely related in structure to the metabolite (substrate) that is antagonized. 36 4) Antimetabolites Antimetabolites include: A. Pyrimidine antagonists (inhibit synthesis of dTMP) (either directly or indirectly (DHFRI) B. Purine antagonists (inhibit synthesis of AMP & GMP) C. DNA polymerase inhibitors (OR chain elongation inhibitors) D. DNA methyltransferase inhibitors (DNMT) 37 4.A) Pyrimidine antagonists (Inhibitors of dTMP synthesis) 38 4.A) Pyrimidine antagonists (Inhibitors of dTMP synthesis) i) Fluorouracil (5-FU) Mechanism of action:  5-FU must be phosphorylated to its nucleotide to be active (5-FUMP or 5-FdUMP)  It inhibits thymidylate synthetase, a key enzyme in the biosynthesis of DNA. (biosynthesis of dTMP)  when converted to its nucleotide (5-FU monophosphate) (5- FUMP) and FdUMP, these bind firmly to thymidylate synthetase and inhibits the synthesis of DNA. 39 4.A) Pyrimidine antagonists (Inhibitors of dTMP synthesis) i) Fluorouracil (5-FU) Mechanism of action 40 4.A.ii) Indirect inhibitors of dTMP biosynthesis Dihydrofolate reductase inhibitors (DHFRIs) (Antifolates) i) Methotrexate: 41 Dihydrofolate reductase inhibitors (DHFRIs) (Antifolates) i) Methotrexate: Mechanism of action:  Methotrexate acts as an antifolate by binding almost irreversibly to the enzyme DHFR and preventing the formation of the coenzyme tetrahydrofolic acid (THF), which is in turn converted to the essential cofactor 5,10-MTHFA, cofactor for thymidylate synthetase enzyme.  Methotrexate binds 103 fold to DHFR stronger than do dihydrofolic acid.  Resistance to methotrexate develops by an increase in dihydrofolate reductase (DHFR), which results from gene amplification, or by defect transport into tumor cells. 42 Dihydrofolate reductase inhibitors (DHFRIs) (Antifolates)) i) Methotrexate THF is converted to the essential cofactor 5,10-MTHFA: 43 Dihydrofolate reductase inhibitors (DHFRIs) (Antifolates) ii) Pemetrexed It works by inhibiting two enzymes used in purine and pyrimidine synthesis — thymidylate synthetase (TS) and Dihydrofolate reductase (DHFR), 44  4.B) Purine antagonists: i) 6-Mercaptopurine, ii) 6-Thioguanine These agents interfere with purine biosynthesis. Both 6-MP and 6-TG must be activated to their respective monophosphate ribonucleotides i) 6-Mercaptopurine (6MP)  6-MP must be converted into its ribonucleotide, 6- thioinosinate (6-TI),  6-TI inhibits the conversion of inosine monophosphate (IMP) to adenine monophosphate (AMP) and to xanthine monophosphate (XMP), hence limit the availability of XMP to form guanine monophosphate (GMP), thereby interfering with the supply of purine precursors for nucleic acid synthesis. 45 4.B) Purine antagonists: i) 6-Mercaptopurine (6MP) Mechanism of action: 46 4.B) Purine antagonists: i) 6-Mercaptopurine (6MP) Catabolism: Metabolic degradation of 6-MP by xanthine oxidase gives 6- thioxanthine, which is further oxidized by xanthine oxidase to afford 6-thiouric acid. Allopurinol, an inhibitor of xanthine oxidase, It is administered as an adjuvant with 6-MP to prevent nephrotoxicity and acute gout resulting from uric acid kidney toxicity. 47 4.B) Purine antagonists: i) 6-Mercaptopurine (6MP) Catabolism: 48  4.C) DNA polymerase inhibitors / DNA chain elongation inhibitors e.g. Cytarabine - Gemicitabine These DNA nucleoside analogs are initially converted to their triphosphate active forms They are mistakenly incorporated into the growing DNA chain thus arresting further elongation and /or inhibiting DNA polymerase key enzyme in DNA elongation. 49 5) Mitosis inhibitors a) Vinca alkaloids The Vinca alkaloids are a family of important antitumor agents from plants. They have complex structures composed of an indole containing moiety. Vincristine and vinblastine have well established in the treatment of cancer. Mechanism of action: They appear to exert their major antitumor effect by binding to critical microtubular proteins within cells. Because these proteins are essential contractile proteins of the mitotic spindle of dividing cells, this binding leads to mitotic arrest. 50 5) Antimitotic agents a) Vinca alkaloids Vincristine and vinblastine: 51 5) Antimitotic agents b) Taxanes i) Paclitaxel (Taxol):  Taxanes one of the most important classes of anticancer agents.  The only source of Taxol was the bark of yew tree, and taxol was rare commodity until methods of synthesis was reported in 1994. It was produced semisynthetically.  Taxol is an antimitotic agent, acting against the spindle assembly of dividing cells, but in different fashion from that of vinca alkaloids. 52 53