PHAR 8: Cancer Drug Therapy Flashcards
1
Q
Observe the learning outcomes of this session
A
2
Q
Summarise cancer
- definition of cancer
- tumour
A
- Cancer is a disease in which cells in the body with one or more abnormalities multiply in an uncontrolled manner.
- Over time, this abnormal growth and multiplication can give rise to a large mass of abnormal cells – a tumour – that can disrupt metabolic, signaling, and physiological functions in tissues and organs, and ultimately lead to morbidity and mortality.
- A solid tumour is an abnormal mass of tissue that usually does not contain cysts (fluid filled sac), and may be benign (not cancerous), or malignant (cancerous).
- Different types of solid tumours are named for the type of cells that form them; examples of solid tumours are sarcomas, carcinomas, and lymphomas.
- In some cases, abnormal cells translocate from the primary site of the tumour to other parts of the body, whereupon secondary tumours form (metastases).
- The abnormalities that give rise to uncontrolled multiplication and growth are many and various, with both genetic and environmental factors involved to some extent.
- Furthermore, the number of different cell types in which these abnormalities can occur is very large, with each combination presenting a different set of biochemical features.
- Taken together, it is clear that the term ‘cancer’ actually covers a very large number of diseases with a common feature – uncontrolled multiplication and growth – but with a wide range of phenotypic/clinical endpoints.
3
Q
What are some things that cancer cells do differently from normal cells that could be selectively targeted in cancer therapies?
A
- Cancer cells exhibit uncontrolled growth and multiply far more (and in most cases more rapidly) than their non-cancerous equivalents.
- They will, therefore, have a greater demand for energy and anabolic substrates to facilitate growth (e.g. production of cell membranes, proteins, etc.).
- Cancer cells will have a less well-regulated cell cycle and will need to duplicate their DNA far more frequently that non-cancerous equivalents
- To replicate, they will rely on correctly functioning replicative processes.
4
Q
What are the original six hallmarks of cancer?
What are some additional four ‘emerging hallmarks of cancer’ from 2011?
A
- inducing angiogenesis
- enabling replicative immortality
- resisting cell death
- activating invasion and metastasis
- sustaining proliferative signalling
- evading growth suppressors
In a recent update to this perspective in 2011, they included four other ‘emerging hallmarks of cancer’:
- deregulating cellular energetics
- avoiding immune destruction
- genome instability and mutation
- tumour-promoting inflammation
5
Q
Study this figure of how the 10 hallmarks of cancer provide opportunities for chemotherapy
A
6
Q
What is cancer chemotherapy?
When and why is it used?
A
- While it is good to be aware of all the possible routes we might explore to impede cancer progression (e.g. monoclonal antibodies, etc.), in this session we will focus on treatments that directly exploit anabolic, replicative, and metabolic mechanisms.
- These approaches have historically been referred to as cancer chemotherapy, and are aimed at selectively killing cancer cells over normal cells using cytotoxic agents.
- If the aim of anti-neoplastic drugs is to kill all cancerous cells, then administration of agents that are simply cytotoxic could affect normal cells as well, leading to those unwanted side-effects.
- Treating a very large tumour with such agents alone might require an intolerably high dose to be administered, and therefore in many cases, drug therapy is used after a surgical procedure to remove as much of the tumour as possible (debulking).
7
Q
Describe the stages of the cell cycle
A
- G1 phase:
- Metabolic changes prepare the cell for division.
- At a certain point - the restriction point - the cell is committed to division and moves into the S phase.
- S phase:
- DNA synthesis replicates the genetic material.
- Each chromosome now consists of two sister chromatids.
- G2 phase:
- Metabolic changes produce and assemble the cytoplasmic materials necessary for mitosis and cytokinesis.
- M phase:
- A nuclear division (mitosis) followed by a cell division (cytokinesis).
- The period between mitotic divisions - that is, G1, S and G2 - is known as interphase.
8
Q
What is the importance of targeting the cell cycle in cancer therapy?
A
- In order for a tumour to grow or for a cancer to spread / metastasise, cell proliferation is required
- proliferating cells proceed through the cell-cycle and divide.
- When considering therapeutic angles for cancer, we might, therefore, target processes in the cell-cycle in order to reduce proliferation and tumour growth.
- Not all tumours grow at a similar rate, and a varying proportion of cancerous cells may be proliferating at a given moment
- the growth fraction of a tumour refers to the percentage of cells engaged in proliferative versus resting phases at a given point in time.
- It can be expected that response to chemotherapic agents that target processes in the cell-cycle will depend to some degree on the growth fraction, as it will be primarily proliferating cells that are affected.
- As might be expected, if the growth fraction is low, the fraction of cells killed by chemotherapy will typically be quite small,
- and conversely, cancers with a higher growth fraction, will be more responsive to chemotherapy because they have a greater number of proliferating cells.
- Drugs that specifically target processes involved in the cell cycle are particularly effective for high growth fraction malignancies
- As we will see later, several types of drugs function by disrupting cell-cycle process.
- Drugs that act via non-cell-cycle specific mechanisms can be effective for both low- and high-growth fraction malignancies, as the rate of proliferation is less of an influence on the proportion of cells that may be affected at a given point in time.
9
Q
What name is given to the cell-cycle stage in which metabolic changes produce and assemble the cytoplasmic materials necessary for mitosis and cytokinesis?
A
- G2
10
Q
Which of the following is the cell-cycle stage in which metabolic changes prepare the cell for division?
A
- G1
11
Q
Which of the following is the cell-cycle stage after which each chromosome consists of two sister chromatids?
A
- S
12
Q
Which of the following is the cell-cycle stage in which nuclear division is followed by cell division?
A
- M
13
Q
Why might a chemotherapeutic agent that acts by inhibiting mitosis be ineffective in treating a slow-growing, solid tumour?
A
- The growth fraction of the tumour is low and therefore the proportion of cells that are likely to be affected at a given point in time is relatively low.
14
Q
List the five main types of anti-cancer drugs
A
- Alkylating and intercalating agents
- Antibiotics
- Antimetabolites
- Microtubule inhibitors
- Hormones
15
Q
What are alkylating agents?
A
- alkylating agents can incur damage to DNA or chemically modify it to prevent cell division
- then cellular machinery detects this damage and initiates programmed cell death
- these agents exert their cytotoxic effects by covalently binding to particular macromolecules in the cell (most importantly is covalent binding to DNA)
- Alkylation of DNA is a crucial cytotoxic reaction that is lethal to the tumour cells.
16
Q
What are bifunctional agents?
A
- bifunctional agents are alkylating agents that can bind and react at two separate sites.
- The drug binds with (say, two guanines) leading to cross-linkages between guanine residues in the DNA chain which facilitates DNA strand breakage.
- This interferes with transcription and replication of DNA.
17
Q
What is the common substructure here of these bifunctional alkylating agents?
A
- All have a similar tertiary nitrogen atom with two chloroethane groups attached.
- These are fundamental to the ability of these compounds to react at two separate sites.
18
Q
Study how bifunctional agents react
A
19
Q
Side effects for alkylating agents are often particularly severe, variable, and seemingly related to many separate aspects of the body.
Why might this be?
A
- DNA damage caused by these agents may not be targeted well (i.e. not acting only at a specific site/tissue), and therefore interfere with DNA in normal cells.
20
Q
What are some anti-tumour antibiotics?
How do they work?
A
- dactinomycin
- doxorubicin
- these anti-tumour antibiotics owe their cytotoxic action primarily to their interactions with DNA, leading to disruption of DNA function
21
Q
Describe how the anti-tumour antibiotic: dactinomycin works
- side effects
A
- This drug intercalates into the minor groove of the double helix between guanine-cytosine base pairs of DNA, forming a stable dactinomycin-DNA complex.
- The complex interferes primarily with DNA-dependent RNA polymerase, although at high doses, dactinomycin also hinders DNA synthesis.
- The drug also causes single-strand breaks, possibly due to action on topoisomerase II or by generation of free radicals.
- The major dose-limiting toxicity is bone marrow depression.
- Other adverse reactions include nausea, vomiting, diarrhoea, and alopecia.
22
Q
Describe how the anti-tumour antibiotic doxorubicin works
- side effects
A
- the mechanism of action of doxorubicin as it functions in several different ways:
- Intercalation of DNA: The drugs insert non-specifically between adjacent base pairs and bind to the sugar-phosphate backbone of DNA.
- This causes local uncoiling and, thus, blocks DNA and RNA synthesis.
- Binding to cell membranes: This action alters the function of transport processes.
- Generation of oxygen radicals: Cytochrome P450 reductase (POR; present in cell nuclear membranes) catalyses reduction of the anthracyclines to semiquinone free radicals.
- These in turn reduce molecular O2, producing superoxide ions and hydrogen peroxide, which mediate single-strand scission (cutting) of DNA.
- side effects:
- There is evidence to suggest that giving this antibiotic in large doses or for a prolonged period can cause irreversible, dose-dependent cardiotoxicity, thought to result from the generation of free radicals and association lipid peroxidation.
23
Q
What are antimetabolites?
A
- Antimetabolites are drugs that are structurally related to normal cellular components and interfere with normal metabolic processes.
- In the context of cancer chemotherapy, antimetabolites that affect the availability of the normal purine or pyrimidine nucleotide precursors by inhibiting their synthesis, and therefore more specifically affect proliferating cells.
- Typically, antimetabolites have a very similar chemical structure to the substrates for the metabolic processes they inhibit.
24
Q
What are the three main types of antimetabolites used in cancer chemotherapy
- give examples of each type
A
- Folate antagonists (e.g. methotrexate),
- Pyrimidine analogues (e.g. 5-fluorouracil),
- Purine analogues (e.g. mercaptopurine).
25
Q
Using methotrexate as an example, describe how folate antagonist antimetabolites work
A
- Folate plays an important role in a variety of metabolic reactions
- Methotrexate is structurally related to folic acid
- Mechanism of action of methotrexate:
- The drug blocks the dihydrofolate reductase enzyme leading to deficiency of numerous coenzymes which play important roles in the synthesis of purine.
- (dUMP:deoxyuridine monophosphate, dTMP: deoxythymidine monophosphate, MTF: methyltetrahydrofolate, DHF: Dihydrofolate, THF: Tetrahydrofolate).
- once the dihydrofolate reductase enzyme is inhibited, it deprives the cells of the various folate coenzymes resulting in decreased biosynthesis of DNA, RNA and protein, eventually leading to cell death
26
Q
Using mercaptopurine as an example, describe how purine analogue antimetabolites work
A
- First of all, mercaptopurine is converted into the nucleotide 6-mercaptopurine (Figure 11, A) ribose phosphate known as 6-thioinosinic acid (thio-IMP; Figure 11, B)
- Thio-IMP is the active metabolite that blocks the synthesis of AMP, XMP and phosphoribosylamine, which are necessary for purine ring biosynthesis.
- The mechanism for this is via the dehydrogenation of thio-IMP to thio-GMP, and subsequent phosphorylation to di- and tri-phosphates.
- These metabolites can be incorporated into RNA and DNA but trigger cell death as they do not function like a normal nucleotide.
27
Q
Using 5-fluorouracil as an example, describe how pyrimidine analogue antimetabolites work
A
- This drug enters the cell through a carrier-mediated transport system and is converted to the corresponding deoxynucleotide
- 5-flurodeoxyuridine monophosphate (5-FdUMP) which competes with deoxyuridine monophosphate for thymidylate synthase.
- 5-FdUMP acts as a pseudosubstrate and is trapped with the enzyme and its coenzyme N5, N10-methylene tetrahydrofolic acid in a ternary complex that cannot proceed to release products.
- DNA synthesis decreases due to the lack of thymidine, leading to imbalanced cell growth and cell death.
28
Q
Why are microtubule inhibitors used in cancer therapeutics?
A
- Microtubules, the polymeric structures made from tubulin that form part of the cellular cytoskeleton, are critical for cell function and division.
- Given their importance in cell division as the primary components of the mitotic spindle, they make a prime target for chemotherapeutic intervention.
29
Q
What is vincristine?
- Describe how it works
- derived from?
A
- vincristine is a microtubule inhibitor
- This compound is one of the so-called Vinca alkaloids as they are derived from the Vinca rosea plant.
- Vinca alkaloid drugs block mitosis in metaphase.
- Their binding to the microtubular protein, tubulin, is GTP-dependent and blocks the ability of tubulin to polymerize to form microtubules.
- Instead, paracrystalline aggregates consisting of tubulin dimers and the alkaloid drug are formed.
- The resulting dysfunctional spindle apparatus, frozen in metaphase, prevents chromosomal segregation and cell proliferation.
33
Q
Describe how the hormone/antagonist: prednisone works
A
- Prednisone itself is inactive and reduced to prednisolone by 11-β-hydroxysteroid dehydrogenase
- note: this is an example of a prodrug
- This steroid then binds to intracellular receptor that then dimerise, migrate to the nucleus and interact with DNA to modify gene transcription
- inducing synthesis of some proteins and inhibiting synthesis of others.
35
Q
Describe how the hormone/antagonist: Tamoxifen works
A
- Tamoxifen binds to the estrogen receptor, but the complex is transcriptionally not productive.
- That is, the complex fails to induce estrogen-responsive genes, and RNA synthesis does not ensue.
- The result is a depletion (down-regulation) of estrogen receptors, and the growth-promoting effects of the natural hormone.
44
Q
Describe acquired resistance
A
- Acquired resistance means that drug resistance is induced by drug treatment, arising from either adaptation of the tumour cells or somatic mutation.
- Following chemotherapy, the drug sensitive cells die while the insensitive cells continue to grow.
- Resistance then develops gradually as the insensitive cells become the dominant population in the cancer and the disease no longer reacts to a therapy that was previously very potent.
- Once this occurs, the cancer is much more difficult to treat.
45
Q
Describe intrinsic resistance
A
- if the majority of cells within the cancer contain a pre-existing adaptation that renders them insensitive to the cytotoxic drug of choice, the resistance is said to be intrinsic (i.e. exists prior to the first treatment).
- Germline mutations giving rise to intrinsic resistance are commonly used to inform treatment decisions
- for example the famous BRCA2 mutation in breast cancer
- It is often possible to switch to an alternate therapy with a better resistance profile
46
Q
What are some ways in which cancer cells might confer resistance to anticancer drugs?
A
- pharmacokinetic factors such as its absorption, distribution, metabolism and elimination can physically limit the amount of a systemically administered drug that can reach a tumour.
- However, in the tumour itself, the anticancer activity of a drug can be limited by poor drug influx or excessive efflux, drug inactivation or lack of activation, changes in expression levels of the drug target, activation of adaptive pro-survival responses, or a lack of cell death induction due to dysfunctional apoptosis.
56
Q
Make sure you understand the mechanism of action, describe/explain side effects of these drugs, strucuture
A
57
Q
A 45-year-old woman, who had undergone a radical mastectomy for infiltrating ductal carcinoma, started adjuvant combination therapy with cyclophosphamide, methotrexate and fluorouracil.
Methotrexate acts by inhibition of …
A
- purine and pyrimidine biosynthesis
58
Q
A 58 year old woman has been diagnosed with locally advanced breast cancer and has been recommended for chemotherapy. She has 5 years history of myocardial infarction and congestive heart failure.
Which anti-cancer agent might be best avoided?
A
- doxorubicin
60
Q
Which of the following drugs DOES NOT cause cytotoxic effects by interference in DNA transcription?
A
- Tamoxifen
61
Q
The drug which metabolised to a cytotoxic product to cause its action is…
A
- 5-fluorouracil
62
Q
Anticancer drug resistance is most commonly attributed to which of the following?
A
- Decrease in the amount of drug taken up by the cell
63
Q
Which anti-cancer drug mechanism of action is through alkylation of DNA which leads to strand breakage resulted in cell death?
A
- Cyclophosphamide
64
Q
The drug which acts during M phase of cell cycle is:
A
- Vincristine
65
Q
What are the two main superfamilies of efflux transporters?
A
- solute-carrier proteins (SLC)
- ABC (ABC binding cassette) transporters
- some types in the image
66
Q
Give some examples of ABC transporters
- its typical substrates
- sites in the body
A
68
Q
Describe the structure of PgP
A
- size: 1280 amino acid residues, MW = 170kDa
- total of 12 transmembrane domains
- contains N-terminal glycosylated residues
- two ATP binding sites
- well conserved sequences for ATP binding
- walker A and walker B motifs
69
Q
Describe the mechanism of drug transport by PgP
A
- substrates of PgP are transported across cell membranes and thereby removed from cells in an ATP-dependent mechanism
- process:
- substrates diffusing into the cell partition into the lipid bilyaer of the membrane
- upon reaching the inner (cytoplasmic) leaflet of the membrane, teh substrate will interact with an available PgP molecule
- the substrate enters the internal drug-binding site of the PgP through a protal region
- the substrate binds to the drug-binding site
- ATP binds to the ATP binding sites of the PgP
- hydrolysis of ATP to ADP + Pi provides the energy for causing a conformational change
- the substrate in the drug-binding site is exposed and becomes unbound from PgP
- hydrolysis of a further ATP to ADP + Pi returns the protein to the initial concentration
70
Q
What are some examples of PgP substrates
A
- vinblastine: antimicrotubule agent
- doxorubicin: cancer chemotherapeutics
- dexamethasone: steroidal anti-inflammatory
- imatinib: cancer chemotherapeutic
71
Q
Why can PgP bind to substrates that are exogenous structures?
A
- many plants produce toxins
- for some of our medications, we use these toxins these plants produce
- over time, we have produced efflux transporters to get rid of these poisons
- we are using these ‘poisons’ to kill cancer cells
