Cytotoxic therapy for cancer (DONE)

Question 1

Cancer general conisderations

Answer 1

Cancer arises from loss of normal growth control
Cancer is a disease of genetic origin, involving dynamic changes in the genome
Cells in a tumour descend from one common ancestral cell that at one point (usually decades before a tumour becomes palpable) initiated a program of inappropriate reproduction
Tumour formation in humans is a multistep process (age-dependent incidence implicating four to seven rate limiting events)

Question 2

Cancer: a genetic disease

Answer 2

The malignant transformation comes about through the accumulation of mutations in specific classes of the genes within it
Two gene classes play particularly important roles in tumour genesis: proto-oncogenes and tumour suppressor genes
Cancer cells acquire defects in regulatory circuits that govern normal cell proliferation and homeostasis
Cancer, in many senses, is a hugely complex disease; however, the principles underlying tumour genesis can be understood by the hallmarks of cancer

Question 3

The hallmarks of cancer

Answer 3

Self-sufficiency in growth signals
Insensitivity to growth-inhibitory signals
Evasion of programmed cell death (apoptosis)
Limitless replicative potential
Sustained angiogenesis
Tissue invasion and metastasis
Plus emerging hallmarks of energy metabolism reprogramming, evading immune destruction and creation of a tumour microenvironment

Question 4

Self sufficiency in growth signals

Answer 4

Normal cells only divide when instructed to do so by other cells in their vicinity, ensuring that each tissue maintains an appropriate size and architecture
Cancer cells, in contrast, become deaf to the usual controls on proliferation and follow their own agenda for growth
Growth signals are transmitted into the cell by trans-membrane receptors that bind distinctive classes of signalling molecules

Question 5

Proto-oncogenes and signalling systems

Answer 5

Many proto-oncogenes encode for proteins that relay growth-stimulatory signals from outside the cell to the nucleus
Growth factors released by cells bind to specific receptors on the surface of neighbouring cells
Receptors span the outer membrane of the cell, and convey proliferative signals to a chain of proteins in the cytoplasm
The succession of relay proteins ends in activation of transcription factors which activate genes that help to usher the cell through its growth cycle

Question 6

Insensitivity to growth-inhibitory signals

Answer 6

Cancer cells can also devise ways to evade or ignore braking signals issued by normal neighbour cells
The most important of these code for the nuclear proteins p53 and pRB
The p53 protein (the guardian of the genome) can halt cell cycle progression in response to DNA damage and induce apoptosis; mutant p53 is involved in around 50% of all cancers
The RB protein is the master brake of the cell cycle

Question 7

What do proto-oncogenes do?

Answer 7

Proto-oncogenes encourage cell growth; however, when mutated can drive excessive multiplication

Question 8

What do tumour suppressor genes do?

Answer 8

Tumour suppressor genes normally act as a brake on excessive growth; however, when mutated can contribute to inappropriate proliferation

Question 9

Cancer therapy

Answer 9

In clinical practice cancers are treated by several approaches: surgery, radiotherapy, anticancer drugs
Anticancer drugs can be divided into various classes: cytotoxic or chemotherapeutic drugs, anti-hormonal treatments, agents directed to tumour biology and gene therapy

Question 10

Aims of treatment

Answer 10

Curative- kill all the cancer cells present
Palliative- reduce tumour load and improve symptoms
Adjuvant- kill remaining cancer cells after surgery or radiotherapy

Question 11

Rational for cytotoxic therapy

Answer 11

Cells divide by an ordered sequence of molecular events termed the cell cycle
Cytotoxic drugs target these activities

Question 12

Factors affecting response to cytotoxics

Answer 12

Growth fraction
Type of tumour
Tumour burden, stage and spread
Health, age and pre-existing medical conditions

Question 13

How are cells at different stages of the cell cycle affected by cytotoxics?

Answer 13

Cells in cell cycle- killed by most cytotoxic drugs
Cells in G0- can be killed once they enter cell cycle
Cells terminally differentiated- unaffected by cytotoxic drugs

Question 14

Growth fraction

Answer 14

Cytotoxic drugs are more effective if the cancer is dividing faster as more cells are in growth phase
Burkitts’ lymphoma- doubles in 24 hours- 96-98% of cells in cell cycle
Breast cancer- double in 3 months- 5-60% of cells in cell cycle
Prostate cancer- double in 8 months- 0.5-15% of cells in cell cycle

Question 15

Tumour size

Answer 15

Geometric resistance increases with size limiting drug and oxygen delivery to large tumours
Blood flow across a capillary bed controls drug delivery

Question 16

Tumour stage, grade and spread

Answer 16

Stage local or invasive
Metastatic spread
Histological grade: increasing anaplasia, increasing mitotic activity, increasing genetic instability

Question 17

Health, age and pre-existing medical condition

Answer 17

Cytotoxic therapy is debilitating and not suitable for old/frail individuals
Liver disease- drug metabolism, serum binging proteins affected
Anaemia and immune disorders- cytotoxics exacerbate
Kidney disease- drug excretion, toxicity problems
Lung/heart disease- precludes use of certain cytotoxics

Question 18

Factors affecting drug delivery to drug tumours

Answer 18

Blood flow across a capillary bed
Geometric resistance within the tumour
Oral administration altered by drug absorption and first pass metabolism in the liver
Drug delivery can also be influenced by plasma protein binding

Question 19

Therapeutic index- schedule and route of administration

Answer 19

Intermittent intensive course are better than continuous (daily therapy)
Intravenous infusion of cytotoxics often best
Regional administration achieve higher drug concentrations in the vicinity of the tumour e.g. intraperitoneal for ovarian cancer, intrapleural for mesothelioma

Question 20

Therapeutic index altered by drug metabolism

Answer 20

Many cytotoxic drugs require chemical or enzymatic activation in either normal or tumour tissue

Question 21

Therapeutic index altered by drug resistance

Answer 21

Decreased drug uptake/increased drug removal
Decreased drug activating enzymes enzymes/increased drub inactivating enzymes
Increased levels of target enzyme
Altered affinity of target
Increased DNA repair
An alternative metabolic pathway

Question 22

What are the targets for cytotoxic drugs?

Answer 22

Specific enzymes involved in the synthesis of nucleotides
Nucleic acids- DNA and RNA
Microtubule dynamic involved in mitosis

Question 23

Classes of cytotoxic drugs

Answer 23

Antimetabolites- target enzymes involved in nucleotide synthesis
Drugs targeting DNA structure and template activity
Mitotic arrest agents target microtubules

Question 24

Sites of action of cytotoxic drugs

Answer 24

Antimetabolites- nucleotide synthesis
Alkylating agents- DNA
Intercalating agents- DNA transcription and duplication
Topoisomerase inhibitors- DNA transcription and duplication
Mitotic inhibitors- mitosis

Question 25

Combination chemotherapy

Answer 25

Single drug treatments are very rare Combination therapy is used for virtually all neoplasms Convincing rational for combination therapy is its clinical success Bone marrow toxicity is the main dose limiting factor

Question 26

Aim of combination therapy

Answer 26

Different targets and mechanisms of action and resistance- activity Compatible side effects on different organ targets- safety

Question 27

Combination chemotherapy guidelines

Answer 27

Active against the tumour type Act on different biochemical targets Act on different phases of the cell cycle Affect different organs and tissues Have minimum or no overlapping toxicity Are optimum for each drug with consistent timing of the doses Therapy cycles dictated by bone marrow recovery Clinical response measure of successful combination

Question 28

Antimetabolites

Answer 28

Analogues of substrates and cofactors of the enzymes involved in nucleotide synthesis Mercaptopurine and Thioguanine- purine analogues inhibit several enzymes in purine synthesis 5-Fluorouracil- a pyrimidine analogue inhibits thymidylate synthase Methotrexate- a cofactor analogue inhibits an enzyme generating folate essential for purine synthesis Drugs work in S phase of the cell cycle

Question 29

Mechanism of action of 5FU

Answer 29

Covalently inactivates thymidylate synthase | A stable ternary covalent complex is formed between 5-FdUMP, reduced folate derivative and thymidylate synthase

Question 30

Alkylating agents

Answer 30

Nitrogen mustards, nitrosureas and the platinum antitumour compounds Form strong chemical bonds with electron rich atoms such as nitrogen in DNA mainly the N7 position of deoxyguanylates Using drugs with two alkylating groups results in covalent cross linking of adjacent strands of DNA Most alkylating agents require chemical or enzymatic activation Primary cytotoxic action is inhibition of DNA synthesis because of the crosslinking in the strands All phases of the cell cycle are susceptible

Question 31

Cyclophosphamide mechanism of action

Answer 31

More selective mustard: P=O group should decrease availability of nitrogen lone pair Acrolein removed by co-administration of 2-mercaptoethanesulphonate as a sacrificial nucleophile Wide spectrum of activity ranging from malignant lymphomas and lymphoblastic leukaemia to carcinomas of the bronchus, breast, ovary and various sarcomas

Question 32

Mechanism of action: platinum compounds

Answer 32

Series of reactions to form active compound in which chloride ions are replaced by a hydroxyl group

Question 33

Effects of alkylating agents on DNA

Answer 33

Crosslinked guanine bases- DNA replication impaired Guanine N7 alkylation can give an enol tautomer which can bind to thymidine- mismatching of bases leading to defective coding of proteins Guanine N7 alkylation can cause cleavage of imidazole ring- excision of guanine residue leading to DNA breakage

Question 34

DNA intercalators: anthracyclines and antibiotics

Answer 34

Mainly antibiotics of microbial origin and include- doxorubicin, daunorubicin, belomycin, actinomycin D, mitomycin C The planar aromatic ring structure can insert into the space between successive nucleotide base pairs distorting the a helix of DNA impairing binding of essential enzymes Binding is reversible and stabilised by the hydrophobic interaction between the opposing aromatic rings of the drug and adjacent DNA bases, also by ionic binding between drug and charged centre of DNA Intercalators act in late G1, S and early G2 phases of cell cycle

Question 35

Mechanism of action of the topoismoerases

Answer 35

Topoisomerase enzyme binds, cuts strand, strand rotates and re-anneals strand of supercoiled DNA The enzymes action results in removal of torsional stress such as unknotting and relaxation

Question 36

Topoisomerase I inhibitors: Camptothecines

Answer 36

A Camptothecin derivative CPT 11, Irinotecan is converted in vivo to active 7-ethyl derivative Stabilization of cleavable complex- inhibition of ligation step Replication fork arrest and irreversible DNA breakage

Question 37

Topoismoerase II inhibitors: epipodophyllotoxins

Answer 37

Glycosides derivatives etoposide and teniposide are cytotoxic and are used in chemotherapy regimes Topoisomerase II principle function is to catalyse the separation of daughter DNA strands just prior to mitosis The drug (etoposide) stabilises enzyme DNA complex, inhibiting the annealing process leaving DNA double strand breaks not amenable to repair They cause arrest of cell cycle in G2 phase

Question 38

Mitotic inhibitors

Answer 38

Mitotic inhibitors arrest the cells in mitosis and target the microtubules Microtubule dynamics indicate that microtubules are inherently polar structures that rapidly assemble and disassemble. In cells, one end of a microtubule is usually anchored to an organising centre (aster), the other end is either growing or shrinking

Question 39

Mitotic inhibitors B Taxanes

Answer 39

Taxanes (Taxol) bind with high affinity to polymerised microtubules of the mitotic spindle The interaction is reversible They bind to B-tubulin at specific amino acid residues They block in M phase of cell cycle, inhibiting spindle dissolution

Question 40

Mechanism of action of mitotic inhibitors

Answer 40

Vinca alkaloids prevent assembly of microtubule to form spindle in prophase Taxanes inhibit disassembly of the spindle in telophase

Question 41

Example of antimetabolites- purine analogues

Answer 41

6-thoioguanine | 6-mercaptopurine

Question 42

Example of antimetabolites- pyrimidine analogues

Answer 42

5-FU | Cytosine araboniside

Question 43

Example of antimetabolites- folate antagonists

Answer 43

Methotrexate

Question 44

Example of alkylating drugs- nitrogen mustard compounds

Answer 44

Melphalan Clorambucil Cyclophosphamide

Question 45

Example of DNA intercalators- anthracyclines, antibiotics

Answer 45

``` Doxorubicin Daunorubicin Bleomycin Actinomycin D Mitomycin C ```

Question 46

Example of topoisomerase I inhibitors- camptothecines

Answer 46

Irinotecan

Question 47

Example of topoisomerase II inhibitors- epipodophyllotoxins

Answer 47

Etoposide | Teniposide

Question 48

Example of mitotic inhibitors

Answer 48

Vinca alkaloids | Taxanes