Drug Mech: Anticancer Drugs (Chemotherapy)

• Resting Phase:
• G₀ (GAP 0): no growth occurs

• Interphase:
• G₁ (GAP 1): cell growth
• S (DNA Synthesis): replication of DNA (duplication of each chromatid)
• G₂ (GAP 2): cell prepares to divide (each chromosome is composed of two chromatids in preparation for mitosis)

• Mitotic Phase:
• M (Mitosis): cell division (dividing chromatids between daughter cells via PPMAT = Prophase, Prometaphase, Metaphase, Telophase)

• 
• 
• 

G₁ to S to G₂ to M to cytokinesis

All cells, normal and neoplastic (abnormal growth), must traverse a number of cell cycle phases before and during cell division. A schematic of the cell cycle of a typical malignant cell is shown below.

Kinases are involved in triggering events in the cell cycle.

Kinases add phosphate to molecules (phosphorylate), and the modification can serve as a "switch" to turn events in the cell on or off.

Cdk or cyclin dependent kinases regulate the cell cycle.

• Protease enzyme: degrade proteins (for example, trypsin is a protease important in digestion)
• Transferase enzyme: transfers molecules
• Nuclease enzyme: degrade nucleic acids

p53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe this protein can cause apoptosis (cell death).

p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle.

A p53 mutation is the most frequent mutation leading to cancer. An extreme case of this is Li Fraumeni syndrome, where a genetic a defect in p53 leads to a high frequency of cancer in affected individuals.

Mutations in p53 would prevent abnormal cells from dying by apoptosis.

If the mechanism for eliminating abnormal cells is defective, then the cells will continue to divide and further damage their DNA. This process leads to cancer. Exposure to sun also causes apoptosis in damaged cells. This produces the familiar peeling and blistering associated with severe sunburn. Over many years, exposure to sun also leads to cancer. It is particularly interesting that smoke seems to cause a high frequency of p53 mutations.

Many of the effective anticancer drugs exert their action only on cells traversing the cell cycle (cycling cells); these drugs are known as cell cycle-specific (CCS) drugs. CCS drugs are not active against resting cells (i.e., cells that are in the G₀ phase).

Other anticancer drugs can sterilize tumor cells regardless of whether they are cycling or resting in the G₀ phase; these particular drugs are known as cell cycle-nonspecific (CCNS) drugs. CCNS drugs can kill both G₀ and cycling cells, although cycling cells are more sensitive.

Cycling cells are more sensitive to anticancer drugs, or in other words, cells that are not in the G₀ resting phase are more sensitive to anticancer drugs.

In general, CCS drugs are most effective in hematologic cancers (affecting blood, bone marrow and lymph nodes) and solid tumors in which a large proportion of the cells are proliferating or are in the growth fraction.

CCNS drugs, many of which bind to and damage cellular DNA, are useful in low growth fraction solid tumors as well as in high growth fraction tumors.

• Cell Cycle-Nonspecific (CCNS) are more effective than CCS because they can kill the cell regardless of what phase in the cell cycle the cell is in.
• 

All effective CCS and CCNS drugs inactivate tumor stem cells, which are often only a small fraction of the cells in a tumor.

Non-stem cells (i.e., tumor cells that have irreversibly differentiated) are considered sterile (harmless) by definition.

All effective CCS and CCNS drugs inactivate tumor stem cells, which are often only a small fraction of the cells in a tumor.

Non-stem cells (i.e., tumor cells that have irreversibly differentiated) are considered sterile (harmless) by definition.

• Antimetabolites
• Bleomycin
• Epipodophyllotoxins
• Taxanes
• Epothilones
• Vinca Alkaloids

• Alkylating Agents
• Anthracyclines
• Dactinomycin
• Mitomycin
• Camptothecins

Primary resistance is the absence of response to cancer chemotherapy on the first exposure to currently available standard anticancer drugs.

Primary resistance is a type of inherent resistance to drug therapy has been attributed to the genomic instability associated with the development of most cancers.

In other words, primary resistance is due mainly to the instability of the cancer.

Acquired resistance develops in drug-sensitive tumors.

It can be highly specific against a single drug due to alteration in the genetic apparatus (target modification) of the tumor with amplification or increased expression of one or more specific genes.

Acquired resistance can also be a multidrug resistance (ie, resistance to a variety of anticancer drugs of differing structures).

• Primary resistance is not common
• Acquired resistance is very common

It could be just one drug or it could be multidrug resistance. Acquired resistance develops in drug-sensitive tumors, but it could be resistance to a single or many drugs.

Primary resistance

• Multidrug resistance (MDR) is often attributed to increased expression of a normal gene, the MDR1 gene, for a cell surface glycoprotein (P-glycoprotein) involved in drug efflux.
• P-glycoprotein is a transport molecule that requires ATP to expel a variety of foreign molecules (not limited to anticancer drugs) from the cell.

Multidrug resistance (MDR) is often attributed to increased expression of a normal gene, the MDR1 gene, for a cell surface glycoprotein (P-glycoprotein) involved in drug efflux.

P-glycoprotein is a transport molecule that requires ATP to expel a variety of foreign molecules (not limited to anticancer drugs) from the cell.

Other mechanisms of MDR involve overexpression of the multidrug resistance protein 1 (MRP1).

MRP1 is a member of a superfamily of ATP-binding transmembrane transporters; it increases resistance to natural product anticancer drugs, such as vinca alkaloids, taxanes, and anthracyclines, by acting as a drug export pump.

• overexpression of MDR1 gene for cell surface P-glycoprotein
• overexpression of protein 1 (MRP1)

• P-glycoprotein
• MRP1 (protein 1)

protein 1 (MRP1)

Yes. MDR can be reversed by administering drugs that inhibit efflux transporters such as calcium channel blockers (e.g., verapamil).

calcium channel blockers (e.g., verapamil)

1) Bone Marrow Depression (Myelosuppression), due to damage to normal erythropoietic and leukopoietic cells.

2) Nausea, Vomiting, Diarrhea (due to damage to the epithelial cells of the GIT).

3) Alopecia (hair loss), due to damage to hair-follicle cells.

• bone marrow depression
• myelosuppression
• neutropenia

Bone marrow depression, also called myelosuppression, is due to damage of normal erythropoietic and leukopoietic cells.

Nausea, Vomiting, Diarrhea may be due to damage to the epithelial cells of the GI Tract.

Alopecia, or hair loss, is due to damage of hair-follicle cells.

Most anticancer drugs have myelosuppression, nausea, vomiting, diarrhea and alopecia because the cells that are affected in anticancer drugs are the cells that replicate the most. The cells affected in these areas have a high turnover rate compared to most cells in the body.

• Polyfunctional Alkylating Agents
• Antimetabolites
• Natural Products
• Hormonal Agents
• Protein Tyrosine Kinase Inhibitors

• 1) Bis(chloroethyl)amines
• - Cyclophosphamide
• - Mechlorethamine
• - Chlorambucil
• - Melphalan

• 2) Nitrosoureas
• - Carmustine (BCNU)
• - Lomustine (CCNU)
• - Semustine (Methyl-CCNU)

• 3) Aziridines
• - Thiotepa
• - Triethylenemelamine

• 4) Alkylsulfonates
• - Busulfan

• 5) Other Related Drugs
• - Procarbazine
• - Dacarbazine
• - Altretamine
• - Cisplatin, Carboplatin, & Oxaliplatin

Anticancer alkylating agents were developed based on the chemical structure and mechanism of toxicity of ‘Mustard Gas’ (or Nitrogen Mustard), a deadly chemical that was used in WW-I.

Polyfunctional alkylating agents are CCNS.

The alkylating agents exert cytotoxic effects via transfer of their alkyl groups to various cellular constituents, particularly DNA; alkylation of DNA and other cellular constituents lead to cell death.

• The general mechanism of action of these drugs
• involves intramolecular cyclization
• to form an ethyleneimonium ion
• that may directly or through formation of a carbonium ion transfer an alkyl
• group to a cellular constituent (e.g., DNA).

• The major site of alkylation within DNA is the
• N7 position of guanine; however, other bases
• are also alkylated to lesser degrees as well as phosphate atoms and proteins
• associated with DNA.

All polyfunctional alkylating agents are electrophiles (highly electrophilic substances that are usually positively charged and eager to accept electrons...they are usually acidic).

DNA, on the other hand has bases...it is neutrophilic.

Therefore, when you combine an alkylating agent (electrophile) with DNA (nucleophile) = PARTY....Covalent bond!!

Covalent binding interactions of the alkylating agents with DNA can occur on a single strand (monofunctional alkylating agents) or o both strands (bifunctional alkylating agents) of DNA through cross-linking (most major alkylating agents are bifunctional).

Most major alkylating agents are bifunctional, which is good because they are also more effective than monofunctional alkylating agents.

• The higher efficacy of bifunctional alkylating agents in cancer chemotherapy, as compared to monofunctional agents, is attributed to their ability to cross-link DNA.
• 

• Monofunctional alkylating agents: form a single covalent bond with DNA. These are not as effective as bifunctional because the cell can more easily repair the damage that just one bond causes.
• Bifunctional alkylating agents: forms two covalent bonds...one bond with each strand of DNA, which is "cross linking" DNA. Therefore, these are sometimes called "cross-linkers." Most drugs on the market are bifunctional.
• 

Methylation is distinct from alkylation in that it is specifically the transfer of one carbon, whereas alkylation can refer to the transfer of long chain carbon groups.

Alkylation of DNA results in miscoding or excision of alkylated residues; the latter leads to DNA strand breakage.

Although alkylating agents are CCNS, replicating cells (ie, cells in late G₁ and S phases) are most susceptible to alkylation.

• 1) Increased capability to repair DNA.
• 2) Decreased cell permeability to the drug.
• 3) Increased production of glutathione which inactivates the electrophilic alkylating agent via conjugation.
• 4) Increased glutathione S-transferase activity (the enzyme that catalyzes the glutathione conjugation reaction).

Glutathione is an endogenous tripeptide that protects us from toxic electrophiles (recall that electrophiles will form destructive covalent bonds with DNA).

Glutathione will inactivate an electrophile (such as an alkylating agent) via conjugation.

Glutathione S-transferase is the enzyme that catalyzes the glutathione conjugation reaction.

Cross-resistance exists among the alkylating agents; however, there are exceptions to the rule depending on the specific type of tumor.

• The most common toxicities of the alkylating
• agents include:
• nausea
• vomiting
• bone marrow depression

POLYFUNCTIONAL ALKYLATING AGENTS

• Bis(chloroethyl)amines
• Cyclophosphamide
• Mechlorethamine
• Chlorambucil
• Melphalan

Cyclophosphamide

bifunctional (meaning it forms two covalent bonds with DNA instead of just one covalent bond)

orally

Yes. Cyclophosphamide is relatively a less reactive alkylating agent; it requires activation to its cytotoxic forms by the cytochrome P450 enzyme system in the liver.

In other words, cyclophosphamide has to be bioactivated by P450 enzymes in the liver.

The liver protects itself from the cytotoxic effect of cyclophosphamide by further metabolizing the active metabolites to inactive metabolites.

The active metabolites of cyclophosphamide are first delivered by the general circulation to both tumor and normal tissue, where nonenzymatic cleavage of aldophosphamide to the cytotoxic (electrophilic) forms (phosphoramide mustard & acrolein) occurs.

• In other words, cyclophosphamide is:
• 1) catalyzed by P450 enzyme in the liver
• 2) active metabolites formed in liver and 99% of the converted cyclophosphamide is dumped into immediate blood circulation
• 3) 1% stays in liver and is converted quickly to inactive metabolites

• inactive metabolite: aldophosphamide
• active metabolites: phosphoramide mustard & acrolein (electrophilic form of cyclophosphamide)

• ANTIMETABOLITES
• 1) Methotrexate
• 2) Pralatrexate
• 3) Purine Antagonists
• - 6-Thiopurines (Mercaptopurine or 6-MP)
• - Fludarabine Phosphate
• - Cladribine
• 4) Pyrimidine Antagonists
• - Fluorouracil or 5-FU
• - Capecitabine
• - Cytarabine
• - Gemcitabine

The antimetabolites are anticancer drugs that act on intermediary metabolism of proliferating cells. (If they work on proliferating cells, then they must be CCS remember).

• They take advantage of a number of quantitative differences in metabolism that exist between
• neoplastic cells and normal cells; these differences render tumor cells more susceptible to this particular class of anticancer drugs (the antimetabolites).

In other words, antimetabolites inhibit nucleic acid synthesis in the cell by competing with natural cofactors in the cell. Therefore, antimetabolites are structurally related to intermediate cofactors in the cell. The end result is antimetabolites inhibit DNA and RNA.

Cell Cycle Specific (CCS)

Antimetabolites are compounds which prevent the biosynthesis or utilization of normal cellular metabolites.

• Nearly all clinically useful antimetabolites are structurally related to metabolites and
• cofactors that are involved in the biosynthesis
• of nucleic acids.

Because of structural similarity, the antimetabolites are capable of antagonizing the normal cellular metabolites and cofactors and, as a result, inhibiting nucleic acid synthesis.

Many of the antimetabolites are enzyme inhibitors; they inhibit enzymes by either:

• 1) Binding to the catalytic site of the enzyme
• (ie, compete with the normal substrate due to structural similarity).

• 2) Binding to an allosteric regulatory site, particularly when the antimetabolite is structurally similar to the end product of a biosynthetic pathway
• (ie, feedback control).

Some antimetabolites have to be bioactivated before they can exert their cytotoxic effect.

Methotrexate is an antimetabolite.

Methotrexate is a folic acid antagonist.

• Methotrexate is a folic acid antagonist that binds to the active catalytic site of dihydrofolate reductase (DHFR) enzyme, inhibiting the enzyme and interfering with the synthesis of tetrahydrofolic
• acid (THF).

The lack of THF inhibits the synthesis of thymidylate, purine nucleotides, and the amino acids serine and methionine, thereby inhibiting the formation of DNA, RNA, and proteins.

• Intracellular formation of polyglutamate derivatives during methotrexate therapy is important for its anticancer activity. The polyglutamates are selectively retained in cancer cells and have increased inhibitory effects on enzymes involved in folate metabolism, resulting in an increase in the duration of action of methotrexate.
• 

• 1) Methotrexate inhibits an enzyme in folate metabolism: the dihydrofolate reductase enzyme (occasionally called "folate reductase").
• 2) Tetrahydrofolic acid (THF) production is now blocked.
• 3) THF is a precursor to:
• - thymidylate
• - purine nucleotides
• - amino acid serine
• - amino acid methionine
• Therefore, since THF is blocked, so is the synthesis of all of the others.
• 4) DNA synthesis is therefore blocked.
• 5) Polyglutamate derivatives are formed and retained in cancer cells.
• 6) Polyglutamate derivatives also inhibit enzymes in folate metabolism.
• 7) Polyglutamate derivatives increase the duration of action of methotrexate.
• 

• 1) Methotrexate inhibits the biosynthesis of the dihydrofolate reductase enzyme, thus inhibiting the production of DNA in cancer and normal cells.
• 2) Polyglutamates, which also inhibit folate metabolism enzymes, increase the cytotoxic effect of methotrexate.
• 

• Tumor cell resistance to methotrexate has been attributed to:
• 1) Decreased drug transport into the cell.
• 2) Decreased polyglutamate formation.
• 3) Synthesis of increased levels of DHFR via gene amplification.
• 4) Altered DHFR with reduced affinity for methotrexate.
• 5) Active efflux through activation of a MDR P170 glycoprotein transporter.

MDR P170 glycoprotein transporter will facilitate active efflux of methotrexate.

• bone marrow depression (myelosuppression) *major toxicity
• leucopenia
• thrombocytopenia

Recall methotrexate inhibits the synthesis of purines and DNA in normal cells as well as cancer cells.

To reverse these effects and rescue normal cells, one particular agent is administered: Leucovorin.

Leucovorin (an important intermediate in the biosynthesis of purines and DNA) prevents the lethal effects of methotrexate on normal cells by rescuing the biosynthesis of purines and DNA.

In addition, leucovorin inhibits the active transport of methotrexate into normal cells and stimulates its efflux out of normal cells.

Leucovorin does not reach adequate concentrations in tumor cells; as a result, it will not rescue tumor cells.

• Leucovorin has three effects on normal cells when co-administered with methotrexate:
• 1) Leucovorin is a downstream intermediate in the pathway to synthesizing purines and DNA (what methotrexate blocks). Therefore, by administering Leucovorin, we can bypass the step that methotrexate inhibits and continue on to make DNA and purines in normal cells.
• 2) Leucovorin inhibits the active transport of methotrexate into normal cells.
• 3) Leucovorin stimulates active efflux of methotrexate out of normal cells.

Leucovorin will not rescue cancer cells because leucovorin does not accumulate in cancer cells like it does in normal cells.

• ANTIMETABOLITES
• Purine Antagonists
• - 6-Thiopurines (Mercaptopurine or 6-MP)
• - Fludarabine Phosphate
• - Cladribine

• 6-MP must be bioactivated first by hypoxanthine-guanine phosphoribosyl transferase (HGPRT) to the nucleotide form 6-thioinosinic acid, which in turn inhibits a number of the enzymes involved in purine nucleotide interconversions. Consequently DNA synthesis is inhibited.
• 

• 1) hypoxanthine-guanine phosphoribosyl transferase (HGPRT) converts 6-MP to the active metabolite, which is 6-thioinosinic acid (nucleotide form).
• 2) 6-thioinosinic acid (nucleotide form) inhibits several enzymes involved in purine nucleotide synthesis.
• 3) DNA synthesis is ultimately inhibited.
• 

• decrease in HGPRT enzyme activity (the enzyme that converts 6-MP to the active metabolite)
• increased levels of alkaline phosphatase (which inactivates 6-MP)

• myelosupprossion *main toxicity
• hepatotoxicity

• hypoxanthine-guanine phosphoribosyl transferase (HGPRT) activates 6-MP to the active metabolite
• xanthine oxidase enzyme deactivates 6-MP to the inactive metabolite

6-MP is converted in the body to an inactive metabolite, 6-thiouric acid, by xanthine oxidase enzyme.

The purine analog allopurinol is a potent inhibitor of xanthine oxidase.

As a result, allopurinol is capable of enhancing the activity and toxicity of 6-MP.

• Allopurinol is frequently administered with chemotherapy to cancer patients in order to prevent
• hyperuricemia (which is caused by the release of purines into the general circulation following tumor cell lysis).

Recall hyperuricemia is caused by killing a lot of cells, which causes an excess of purine bases into the circulation. This causes the metabolic proteins to convert the purine bases to uric acid, thus leading to hyperuricemia. Hyperuricemia can in turn lead to gout.

Recall hyperuricemia is a cause of killing a lot of cells, which causes an excess of purine bases into the circulation. This causes the metabolic proteins to convert the purine bases to uric acid, thus leading to hyperuricemia. Hyperuricemia can in turn lead to gout.

• ANTIMETABOLITES
• Pyrimidine Antagonists
• - Fluorouracil
• - Capecitabine
• - Cytarabine
• - Gemcitabine

• 5-FU is a uracil analog.
• 

5-FU must be bioactivated first to ribosyl and deoxyribosyl nucleotide metabolites.

• Its cytotoxicity is attributed to effects on both DNA- and RNA-mediated events.
• 

• DNA Inhibition
• 5-FU is bioconverted to fluorodeoxyuridine monophosphate (FdUMP), which inhibits thymidylate synthase enzyme and the synthesis of thymidylate. This results in inhibition of DNA synthesis.

• RNA Inhibition
• 5-FU is also bioconverted to fluorouridine triphosphate (FUTP), which is incorporated into RNA, where it interferes with RNA processing and mRNA translation.
• 

• 5-FUTP inhibits RNA synthesis
• 5-FdUMP inhibits DNA synthesis
• 

• myelosuppression
• neurotoxicity

• NATURAL PRODUCTS
• Vinca Alkaloids
• - Vinblastine
• - Vincristine
• - Vinorelbine

Vinblastine and vincristine are plant alkaloids derived from the periwinkle plant (Vinca rosea).

Vinorelbine is a semisynthetic vinca alkaloid.

• Similarities
• All vinca alkaloids exhibit the same mechanism of action and are closely related in their chemical structures.

• Differences
• Despite these similarities, there are differences among the vinca alkaloids; these differences are related to spectra of anticancer activity and toxicity.

The vinca alkaloids cause depolymerization of microtubules, which are an important part of the cytoskeleton and the mitotic spindle.

They bind specifically to the microtubule protein tubulin in dimeric form.

The drug-tubulin complex adds to the forming end of the microtubules to terminate assembly, and depolymerization of the microtubules then occurs.

Depolymerization of the microtubules results in mitotic arrest at metaphase (the M phase), dissolution of the mitotic spindle, and interference with chromosome segregation.

• nausea
• vomiting
• bone marrow suppression
• alopecia

• neurotoxicity (including neuropathy, ataxia, seizures, coma)
• myelosuppression *milder & less significant than in vinblastine

Vinblastine has much more significant myelosuppression than what is found with vincristine.

myelosuppression with neutropenia

• NATURAL PRODUCTS
• Epipodophyllotoxins
• - Etoposide
• - Teniposide

Etoposide and teniposide are semisynthetic derivatives of podophyllotoxin, which is extracted from the mayapple root (Podophyllum peltatum).

• Both agents exhibit the same mechanism of action
• and are closely related in their chemical structures.

Etoposide and teniposide block cell division in the late S-G2 phase of the cell cycle. Therefore, these drugs are Cell Cycle Specific (CCS).

• They inhibit topoisomerase II enzyme, which
• results in DNA damage through strand breakage induced by the formation of a ternary complex of drug, DNA, and enzyme.

Etoposide and teniposide, the epipodophyllotoxins, inhibit topoisomerase II enzyme.

• nausea
• vomiting
• alopecia
• bone marrow depression

• NATURAL PRODUCTS
• Camptothecins
• - Topotecan
• - Irinotecan

The camptothecins are natural products derived from a tree, Camptotheca acuminata.

They inhibit the activity of topoisomerase I, the key enzyme responsible for cutting and religating single DNA strands.

Inhibition of Topo I results in DNA damage.

• nausea
• vomiting
• diarrhea
• myelosuppression
• arthralgia (joint pain)

• Camptothecins (Topotecan & Irinotecan): inhibit Topoisomerase I
• Epipodophyllotoxins (Etoposide & Teniposide) : inhibit Topoisomerase II

• NATURAL PRODUCTS
• Taxanes
• - Paclitaxel
• - Docetaxel

Paclitaxel (Taxol®) is a natural product derived from the pacific yew tree (Taxus brevifolia).

Docetaxel is a semisynthetic taxane derived from the European yew tree (Taxus baccata).

Both agents exhibit the same mechanism of action and are structurally related.

The taxanes function as mitotic spindle poisons through high-affinity binding to microtubules with enhancement of tubulin polymerization.

This promotion of microtubule assembly by the taxanes leads to inhibition of mitosis and cell division.

• arrythmias
• bone marrow depression
• peripheral sensory neuropathy (nerve sensations)

• neurotoxicity
• bone marrow depression (neutropenia)

• NATURAL PRODUCTS
• Epothilones
• - Ixabepilone

Epothilones are cytotoxic macrolides (antibiotics) derived from bacterial fermentation.

Ixabepilone (Ixempra®), a semisynthetic epothilone analog, has been approved for treatment of advanced breast cancer.

Like the taxanes, ixabepilone binds to tubulin of microtubules to induce microtubule polymerization and stabilization, which lead to arrest of cells in the G2-M phase of the cell cycle and the induction of apoptosis.

Epothilones and the taxanes bind differently to tubulin because they are not structurally related.

As a result, the epothilones are not affected by the common mechanisms of resistance that affect the taxanes.

Studies have shown that ixabepilone is active against tumors that are resistant to paclitaxel.

The epothilones are not affected by the common mechanisms of resistance that affect the taxanes.

Studies have shown that ixabepilone is active against tumors that are resistant to paclitaxel.

• peripheral sensory neuropathy
• neutropenia (bone marrow depression)
• *notice: same toxicities as the taxanes

• NATURAL PRODUCTS
• Anthracyclines
• - Doxorubicin
• - Daunorubicin
• - Idarubicin
• - Epirubicin

• The anthracyclines are among the most widely used cytotoxic anticancer drugs. Daunorubicin and doxorubicin were the first agents in this class to be introduced.
• 

• The anthracyclines exert their cytotoxic effect through four major mechanisms:
• 1) Inhibition of topoisomerase II.
• 2) High-affinity binding to DNA through intercalation, leading to inhibition of the synthesis of DNA and RNA as well as DNA strand breakage. In other words, the anthracyclines insert themselves (intercalate) between DNA bases in the double helix, then they act non-covalently via a hydrophobic bond to inhibit DNA.
• 3) Binding to phospholipid cellular membranes to alter fluidity and ion transport.
• 4) Generation of semiquinone free radicals and oxygen free radicals through an enzyme-mediated reductive process. Generation of free radicals has been shown to be the cause of the cardiac toxicity of the anthracyclines. In other words, the anthracyclines act as oxidizing agents in the presence of iron, and therefore produce reactive oxygen species (ROS), which are toxic and damage DNA.

Intercalation is the reversible inclusion of a molecule (or group) between two other molecules (or groups). In other words, a molecule is reversibly inserting itself between two other molecules.

Covalent binding, in contrast, is irreversible.

Anthracyclines have aromatic rings that are flat!

Therefore, because the molecular structure is basically 2-D (rather than 3-D), the anthracyclines can insert themselves (like a piece of paper) between the DNA bases on the double helix structures of DNA.

There, the anthracyclines will interact non-covalently (rather than covalently) via hydrophobic bonds to inhibit DNA.

• Inhibit Topoisomerase I
• Camptothecins (Topotecan & Irinotecan)

• Inhibit Topoisomerase II
• Epipodophyllotoxins (Etoposide & Teniposide)
• Anthracyclines (Doxorubicin, Daunorubicin, Idarubicin, Epirubicin)

• myelosuppression with neutropenia
• cardiotoxicity

• Two forms of cardiotoxicity are observed during anthracycline therapy:
• 1) Acute Cardiotoxicity which occurs
• within the first 2-3 days of therapy and is characterized by arrhythmias and myocarditis. This form of cardiotoxicity is usually transient and is asymptomatic in most cases.
• 2) Chronic Cardiotoxicity which results in a dose-dependent, dilated cardiomyopathy associated with heart failure. This form of cardiotoxicity is caused by increased production of free radicals within the myocardium.

Acute Cardiotoxicity

Acute Cardiotoxicity

Chronic Cardiotoxicity

Chronic Cardiotoxicity

• 1) A decrease in the dose
• 2) The use of slow, continuous infusion
• 3) The use of Dexrazoxane (Zinecard®, Totect®) has been shown to protect against the cardiotoxicity of anthracyclines because dexrazoxane is a potent iron-chelating agent (like tetracylines). If iron is suppressed through chelation, then there are no free radicals produced from the iron!

The use of Dexrazoxane (Zinecard®, Totect®) has been shown to protect against the cardiotoxicity of anthracyclines because dexrazoxane is a potent iron-chelating agent (like tetracylines). If iron is suppressed through chelation, then there are no free radicals produced from the iron!

• NATURAL PRODUCTS
• Mitomycin

Mitomycin is a CCNS alkylating agent.

Mitomycin is Cell Cycle Nonspecific (CCNS).

Mitomycin is metabolically activated via an enzymatic reduction reaction to generate an alkylating electrophilic species that is capable of cross-linking DNA.

Hypoxic tumor stem cells of solid tumors exist in an environment conducive to reductive reactions and, as a result, are more sensitive to the cytotoxic effects of mitomycin than normal cells and oxygenated tumor cells.

• thrombocytopenia
• leucopenia

• NATURAL PRODUCTS
• Bleomycin

Bleomycin is a peptide that contains a DNA-binding region and an iron-binding domain at opposite ends of the molecule.

Bleomycin is a CCS anticancer drug that causes accumulation of cells in the G2 phase of the cell cycle.

It acts by binding to DNA, which results in single strand and double-strand breaks following free radical formation; this leads to inhibition of DNA synthesis.

The fragmentation of DNA is due to oxidation of a DNA-bleomycin-Fe (II) complex and leads to chromosomal aberrations.

• The dose-limiting toxicity for bleomycin is pulmonary toxicity which is characterized
• by:
• pneumonitis with cough
• dyspnea
• infiltrates

• The incidence of pulmonary toxicity is increased:
• 1) in patients older than 70 years of age
• 2) with cumulative doses greater than 400 units

In rare cases, pulmonary toxicity can be fatal.

• HORMONAL AGENTS
• Antiestrogens
• - Tamoxifen (SERM)
• - Raloxifene (SERM)
• - Fulvestrant (SERD)

• Tamoxifen is a selective estrogen-receptor
• modulator (SERM).

• Tamoxifen has estrogen-agonist effects in bone and uterus; it has anti-estrogen effects in
• the breast tissue.

Tamoxifen has estrogen-agonist effects in bone and uterus; it has anti-estrogen effects in the breast tissue.

The anti-estrogen tamoxifen has proved to be extremely useful for the treatment of both early-stage and metastatic breast cancer. It is also used as a chemopreventive agent in women at high risk for breast cancer.

Yes. Tamoxifen is first line of therapy.

It is given orally.

Raloxifene, a related SERM, differs from tamoxifen mainly in having anti-estrogen effects in the uterus.

Recall that tamoxifen has estrogen agonist effects in the uterus.

• Tamoxifen and raloxifene are both indicated for
• and equally effective in the prevention of breast cancer in women at high risk for development of the disease (in the ‘STAR’ trial, both drugs reduced the risk of developing invasive breast cancer by about 50% in postmenopausal women who are at increased risk of developing the disease).

• Raloxifene may have a more favorable adverse-effect profile, with fewer thromboembolic events (known toxicities of SERMs, including deep vein
• thrombosis and pulmonary embolism) and less uterine hyperplasia (endometrial cancer) when compared with tamoxifen.

Tamoxifen acts as a competitive partial agonist (ie, an antagonist) of estrogen and binds to the estrogen receptors of estrogen-sensitive tumors.

Tamoxifen has a much longer biologic half-life (7-14 days) than estradiol.

Tamoxifen has a tenfold lower affinity for the estrogen receptor than does estradiol, indicating the importance of ablation (removal) of endogenous estrogen for optimal antiestrogen effect.

Remove endogenous estrogen so tamoxifen does not have to compete with estrogen for the binding site. Tamoxifen has tenfold lower affinity for the estrogen receptor.

Yes. Tamoxifen is bioactivated in the body by CYP2D6.

Inhibitors of CYP2D6, such as the selective serotonin reuptake inhibitors (SSRIs), may lower the success rate of tamoxifen in preventing recurrence of breast cancer.

The use of SSRIs is common in women taking tamoxifen for breast cancer, both to treat depression and to decrease hot flashes.

• SSRIs that are potent inhibitors of CYP2D6 are: Fluoxetine(Prozac®)
• Paroxetine (Paxil®)
• Sertraline (Zoloft®) inhibits CYP2D6 to a lesser extent

• SSRIs that are weak inhibitors of CYP2D6 are: Citalopram (Celexa®)
• Escitalopram (Lexapro®)

For women who need to take a SSRI with tamoxifen, citalopram or escitalopram might be the safest choice.

Recall these SSRIs are weak inhibitors of CYP2D6, the same enzyme that metabolizes tamoxifen to its active form.

Tamoxifen is well tolerated; its toxicities are generally mild and include menopausal symptoms and thromboembolic events (deep vein thrombosis and pulmonary embolism).

Tamoxifen therapy increases the risk/incidence of estrogen-sensitive endometrial hyperplasia and cancer (uterine cancer).

• HORMONAL AGENTS
• Antiestrogens
• - Tamoxifen (SERM)
• - Raloxifene (SERM)
• - Fulvestrant (SERD)

Fulvestrant is a selective estrogen-receptor downregulator (SERD).

Fulvestrant was approved for the treatment of postmenopausal women with hormone receptor-positive metastatic breast cancer that has progressed despite first-line anti-estrogen therapy such as tamoxifen.

No. Fulvestrant is second line of therapy.

• SERDs, unlike SERMs, are devoid of any
• estrogen-agonist activity. SERDs are ‘pure anti-estrogens’ or ‘pure ER-antagonists’.

SERDs are expected to have a better safety profile, faster onset, and longer duration of action than the SERMs.

SERDs are antagonists of estrogen receptors.

SERDs, unlike SERMs, are devoid of any estrogen-agonist activity. SERDs are ‘pure anti-estrogens’ or ‘pure ER-antagonists’.

• SERDs are expected to have:
• a better safety profile
• faster onset
• longer duration of action
• ...than the SERMs

Fulvestrant is a steroidal anti-estrogen that binds to the ER (estrogen receptor) with an affinity more than 100 times that of tamoxifen.

Binding of fulvestrant to the ER (estrogen receptor) sterically hinders receptor dimerization, leading to inhibition of receptor dimerization and an increase in ER (estrogen receptor) degradation (turnover).

Unlike tamoxifen, fulvestrant reduces the number of ER molecules in cells, both in vitro (test tube) and in vivo (within the living) (ie, downregulation of ER).

Recall that fulvestrant has a different mechanism of action than tamoxifen and down-regulates estrogen receptors.

• ER down-regulation abolishes ER-mediated transcription, completely suppressing
• the expression of estrogen-dependent genes.

This likely explains why fulvestrant is effective against tamoxifen-resistant breast cancer.

Fulvestrant is generally well tolerated.

• Most common adverse effects of fulvestrant include:
• nausea
• vasodilation (hot flashes)
• headache

• HORMONAL AGENTS
• Antiandrogens
• - Flutamide
• - Bicalutamide
• - Nilutamide

They are nonsteroidal antiandrogen agents that bind to the androgen receptors of androgen-sensitive tumors and block androgen effects.

They are used, in combination with radiation therapy, for the treatment of early-stage prostate cancer and metastatic prostate cancer.

The antiandrogens are used in combination with radiation therapy.

• Toxicities of the antiandrogen agents include:
• mild nausea
• hot flashes
• gynecomastia
• variable degrees of decreased libido and potency

• HORMONAL AGENTS
• Gonadotropin-Releasing Hormone Agonists
• - Leuprolide
• - Goserelin
• - Triptorelin

Gonadotropin Releasing Hormone

The GnRH agonists, which include Leuprolide, Goserelin, Triptorelin, are synthetic peptide analogs of gonadotropin-releasing hormone (GnRH).

They are used for the treatment of advanced prostate cancer and as part of neoadjuvant therapy of early-stage prostate cancer.

Neoadjuvant therapy is the administration of therapeutic agents before the main treatment.

They function as GnRH agonists and are more potent than the natural hormone.

They stimulate a transient release of the follicle-stimulating hormone (FSH) and luteinizing hormone (LH), followed by inhibition of the release of FSH and LH. This leads to suppression of testosterone production in the testes.

The GnRH agonists are more potent than the natural hormones.

In men, 2-4 weeks of GnRH agonist therapy results in castration levels of testosterone.

One important complication with GnRH agonist therapy is a transient flare of disease which results from the initial surge of LH and FSH (and the transient initial increase in testosterone levels).

This initial surge of testosterone can be avoided by temporary (2-4 weeks) administration of androgen receptor blockers or by the use of GnRH antagonists.

• hot flashes
• impotence
• gynecomastia

In addition, GnRH agonists significantly increase the risk of diabetes and heart disease (MI, stroke, and sudden death) among men who use these drugs (i.e., men who use androgen deprivation therapy) to treat prostate cancer.

The androgens (sex hormones) in men, which is mainly testosterone.

• HORMONAL AGENTS
• Gonadotropin-Releasing Hormone Antagonists
• - Degarelix

Degarelix (Firmagon®) is a synthetic decapeptide GnRH receptor antagonist.

Degerelix is used for rapid medical castration of men with advanced prostate cancer.

It binds reversibly to the GnRH receptor in the anterior pituitary gland. As a result, it inhibits the release of gonadotropins (LH and FSH), which in turn suppresses testosterone production in the testes.

Degerelix binds reversibly.

• Follicle Stimulating Hormone (FSH)
• Luteinizing hormone (LH)

GnRH antagonist therapy rapidly reduces serum testosterone levels without the transient initial increase observed with GnRH agonist therapy.

Other than avoidance of the initial flare of disease, GnRH antagonist therapy offers no advantage compared with GnRH agonists.

• hot flashes
• weight gain
• increase in hepatic transaminases (liver enzymes)

• hepatic means "of the liver"
• transaminase is an enzyme that catalyzes an amine reaction
• therefore, hepatic transaminases are liver enzymes

Long-term androgen deprivation therapy can prolong the QT interval, which could be problematic in patients taking other drugs that also prolong the QT interval.

• HORMONAL AGENTS
• Aromatase Inhibitors
• - Aminoglutethimide
• - Anastrozole
• - Letrozole
• - Exemestane

Aromatase enzyme converts the adrenal androgen androstenedione to estrone.

This aromatization of an androgenic precursor (androstenedione) into an estrogen (estrone) occurs in body fat.

Estrone (E1, and also oestrone) is an estrogenic hormone secreted by the ovary as well as adipose tissue.

Estrone is one of several natural estrogens, which also include estriol and estradiol. Estrone is the least abundant of the three hormones; estradiol is present almost always in the reproductive female body, and estriol is abundant primarily during pregnancy.

Since estrogens promote the growth of breast cancer, estrogen synthesis in adipose tissue can be important in breast cancer growth in postmenopausal women.

Anticancer aromatase inhibitors act as inhibitors of estrogen synthesis in adipose tissue and are used for the treatment of breast cancer in postmenopausal women.

postmenopausal women

• HORMONAL AGENTS
• Aromatase Inhibitors
• - Aminoglutethimide
• - Anastrozole
• - Letrozole
• - Exemestane

Aminoglutethimide is a nonsteroidal inhibitor of the aromatase enzyme.

• 1) estrogen synthesis in adipose tissue
• 2) adrenal steroidogenesis (corticosteroid synthesis), which include glucocorticoids and mineralcorticoids

• Coricosteroids are steroid hormones produced in the adrenal cortex.
• Glucocorticoids (cortisol)
• Mineralcorticoids (aldosterone)

It is normally administered with hydrocortisone to prevent symptoms of adrenal insufficiency.

• adrenal insufficiency
• myelosuppression

• HORMONAL AGENTS
• Aromatase Inhibitors
• - Aminoglutethimide
• - Anastrozole
• - Letrozole
• - Exemestane

Anastrozole is a selective nonsteroidal inhibitor of aromatase.

selective

aromatase

It is used for the treatment of postmenopausal women with metastatic breast cancer that is ER-positive (ER = estrogen receptor), as well as women whose tumors have progressed while on tamoxifen therapy. It is also used as adjuvant therapy of postmenopausal women with hormone-positive, early-stage breast cancer.

• ER positive metastatic breast cancer
• adjuvant therapy for ER positive early stage breast cancer
• second line therapy for those that have been on tamoxifen

Unlike aminoglutethimide, anastrozole has no inhibitory effect on adrenal glucocorticoid or mineralocorticoid synthesis.

• hot flashes
• mild nausea
• arthralgias (joint pain)

Letrozole is another selective aromatase inhibitor that has the same mechanism of action, toxicities, and clinical indications as anastrozole.

• HORMONAL AGENTS
• Aromatase Inhibitors
• - Aminoglutethimide
• - Anastrozole
• - Letrozole
• - Exemestane

Exemestane is a steroidal hormonal agent that binds to and irreversibly inactivates the aromatase enzyme.

irreversibly

It is indicated for the treatment of advanced breast cancer in postmenopausal women whose tumors have progressed on tamoxifen therapy.

No. Exemestane is second line after tamoxifen.

There is no cross-resistance between exemestane and the nonsteroidal aromatase inhibitors.

• hot flashes
• mild nausea
• headache

• PROTEIN TYROSINE KINASE INHIBITORS
• - Imatinib
• - Dasatinib
• - Sunitinib
• - Sorafenib
• - Gefitinib
• - Erlotinib
• - Lapatinib
• - Nilotinib
• - Temsirolimus
• - Everolimus
• - Pazopanib

Protein kinases are critical components of signal transduction pathways that transmit information concerning extracellular or cytoplasmic conditions to the nucleus, thereby influencing gene transcription and/or DNA synthesis.

• Tyrosine kinases are classified into:
• 1) proteins that have an extracellular ligand binding domain (receptor tyrosine kinases)
• 2) enzymes that are confined to the cytoplasm and/or nuclear cellular compartment (nonreceptor tyrosine kinases)

Abnormal activation of specific protein tyrosine kinases has been demonstrated in many human cancers, making them attractive molecular targets for cancer therapy.

• Imatinib has inhibitory activity against:
• 1) the platelet-derived growth factor receptor (PDGFR) tyrosine kinase
• 2) the cytoplasmic ABL tyrosine kinase
• 3) the receptor tyrosine kinase KIT

• Imatinib is effective in treating cancers in which the
• ABL, KIT, or PDGFR have dominant roles in driving the proliferation of the tumor.

This dominant role is defined by the presence of a mutation that results in constitutive (constant) activation of the kinase.

Constitutive means it is constantly active, so a constitutive kinase or protein would be constantly turned on.

• Imatinib shows remarkable therapeutic efficacy in
• patients with:
• chronic myelogenous leukemia (CML) gastrointestinal stromal tumors (GISTs)
• chronic myelomonocytic leukemia (CMML)
• hypereosinophilic syndrome (HES)

orally

Acquired resistance to imatinib results predominantly from mutations in the kinase domain.

• nausea
• vomiting
• edema
• muscle cramps