Understanding the cell cycle - chemotherapy

and beyond

Julie Mycroft

Principal Pharmacist

Paediatric Oncology

Royal Marsden NHS Trust

Cancer treatment modalities

Can be used alone or in combination

Outcome measured in terms of survival rates and response rates

Aims of treatment

Chemotherapy To reduce tumour burden

Improvement in symptoms

Eradication of metastases → cure

Hormone Therapy Manipulate hormone

environment leading to regression of tumours sensitive to hormones

Adjuvant setting

Palliation in advanced disease

Neoadjuvant

History of chemotherapy development

1946 Nitrogen mustard given to treat lymphomas

1947 Antifolates introduced 1949 Methotrexate introduced 1950s 5-Fluoro-uracil synthesised 1952 6-mercaptopurine

described 1954 Actinomycin D introduced 1960s Combination chemo cured

childhood ALL and HD Recent Years Many new agents

Focus changes to optimising timing and usage and modulating toxicity

The cell cycle

Phases of the cell cycle

G0 resting phase

G1 early growth phase

S DNA synthesis

G2 later growth phase

M Mitosis

Cell division – mitosis (1) Prophase

Chromatin condenses into chromosomes. Each chromosome duplicates and consists of 2 sister chromatids. Nucleus breaks down

Metaphase Chromosomes align and are

held by microtubules attached the mitotic spindle and to the centromere

Cell division – mitosis (2) Anaphase

The centromeres divide. Sister chromatids separate and move toward the corresponding poles

Telophase Daughter chromosomes arrive

at the poles and the microtubules disappear. The condensed chromatin expands and the nuclear envelope reappears

The cytoplasm divides, the cell membrane pinches inwards and two daughter cells are produced

The Cell Cycle and Tissue Growth

The rate of cell division in human tumours varies considerably from one disease to another

Majority of common cancers increase in size slowly compared to sensitive normal tissues such as BM and GI epithelium

The relationship between cell cycle and cell death affects tumour growth

Chemotherapy Effects

Cytotoxic drugs produce their effects by damaging the reproductive potential of cells

The more rapidly growing tumours are more likely to respond to drug treatment this accounts for leukaemias, lymphomas and

testicular cancers being more responsive than colonic / pancreatic cancers

Growth Fraction

At a given time, the number of cells in a population that are actively passing through the cell cycle divided by the total number of cells in the population = growth fraction

The greater the growth faction, the more likely the treatment will produce cell death

Kinetics of cell killing Fractional Cell kill

hypothesis A given dose of cytotoxic

drug kills a given proportion of cells, not a given number

Smaller tumours require fewer cycles of chemotherapy than larger ones

Pulsed intermittent therapy Maximises tumour cell killing

whilst allowing normal tissues damaged by the drug to recover

Cytotoxic Drug Classification

Cell Cycle Phase-Specific Agents active in a particular phase of cell cycle Depend on the production of some type of unique

biochemical blockade of a particular reaction occurring in a single phase of the cell cycle

Cell Cycle Phase-Non-specific Agents

Cytotoxic effect exerted irrespective of cell cycle state equally effective in large tumours in which cell growth is low dose dependent single dose has same effect as repeated fractions totalling

the same amount

Phase Specificity of Cytotoxic Drugs

Phase of cell cyle Effective agents

G1 Steroids, asparaginase

S phase Antimetabolites

G2 Bleomycin, etoposide

Mitosis Vinca alkaloids, taxanes

Phase non-specific Alkylating agents, nitrosoureas, antibiotics, procarbazine, dacarbazine, platinums

Mechanisms of Action (1)

Alkylating agents and nitrosureas

Highly reactive molecules Interfere with replication by covalently linking an

alkyl group (R-CH2+) to nucleic acids and

proteins of the base pairs of the cellular DNA causes the strands of DNA to cross-link either

within a strand or between strands. Mechanism of toxicity - impairment of DNA

replication Examples

e.g cyclophosphamide, chlorambucil, melphalan, nitrosoureas eg carmustine

alkylator-like agents - cisplatin, carboplatin, procarbazine, dacarbazine

Mechanisms of Action (2)

Antitumour Antibiotics

Disrupt normal replication by binding to DNA intercalating between the base pairs blocking the transcription of DNA

Breaks in DNA may also occur Examples

Anthracyclines ie Doxorubicin Epirubin, Mitoxantrone, Actinomycin D Bleomycin,

Mechanisms of Action (3)Antimetabolites Interfere with normal

synthesis of nucleic acids Cell cycle phase specific (S

phase) folate antagonist - MTX pyrimidine antagonists -

cytarabine, 5-fluorouracil, capecitabine, gemcitabine

purine antagonists - cladribine, mercaptopurine, thioguanine, fludarabine

adenosine deaminase inhibitor - pentostatin

Mechanisms of Action (4)Disrupt the M phase of the cell

cycleVinca alkaloids Inhibit the assembly of

microtubules by binding to tubulin resulting in the dissolution of the mitotic spindle required for chromosome division

e.g vincristine, vinblastine, vinorelbine

Taxanes Bind to stabilised microtubules

once they have formed, resulting in arrest of normal mitotic cell division and subsequently cell death

e.g paclitaxel, docetaxel

Mechanisms of Action (5)

Camptothecans Inhibit type I DNA

topoisomerase. Act predominantly in

the S phase e.g topotecan,

irinotecan

Epipodophyllotoxins Inhibit type II DNA

topoisomerase and prevent cells from entering mitosis

Produce protein-associated DNA double strand breaks

e.g etoposide

Mechanisms of Action (6) – Misc. Asparaginase

L-Asparagine Deficiency

Aspartic Acid + Ammonia

L-Asparagine(cell produced)

Aspartic Acid+ L-Glutamine

L-AsparagineSynthetase

Normal Cell

L-A´ase

Cell Proliferation Cell Death

Tumor Cell

Combination Chemotherapy (1)

Early studies used single agents, but remissions were short and relapse was associated with drug resistance

Combination chemotherapy is used to try and improve rate and duration of response by combining drugs with different mechanisms of action. This also helps prevent resistance mechanisms

Despite knowledge of cell kinetics, most regimens have been decided on empirically

Combination Chemotherapy (2)

The combination of drugs is chosen based on some common principles Use drugs that are known to be effective as single agents Use drugs with non-overlapping toxicity pulsed intermittent therapy should be used to allow the

GIT and the bone marrow to recover Use the optimal dose and schedule for each individual

agent If possible use drugs with synergistic killing effects Use drugs which work at different phases in the cell cycle Follow schedules that are supported by experience or

observation, not just theory

Short-term toxicity

Haematopoietic System Bone marrow suppression occurs

when the pool of stem cells has been damaged by chemotherapy.

The store of mature blood cells in the bone marrow lasts for around 8 to 10 days following treatment, after which leucopenia and thrombocytopenia can develop

Gastro-intestinal tract Nausea and vomiting are common in

patients treated with intravenous alkylating agents, doxorubicin and cisplatin.

Hair loss Scalp cooling may be used

Long term toxicity

Impaired gonadal function Procarbazine and alkylating agents men – decreased spermatogenesis women – ovarian failure

Pulmonary fibrosis Busulfan, Bleomycin

Organ damage Liver damage – antimetabolites Cardiac damage – anthracyclines

Second Cancers Alkylating agents, etoposide,

anthracyclines

Drug Resistance

Cells in a solid tumour are not uniformly sensitive to a cytotoxic drug

As the tumour grows, greater heterogeneity develops and cell mutation occurs

Host defence mechanisms and the use of cytotoxic drugs exert a selection pressure encouraging the survival of the resistant cells , which grow and multiply

Cellular Mechanisms of Resistance

Mechanism DrugEfficient repair to damaged DNA alkylating agents

Decreased uptake by cell MTX, doxorubicin

Increased drug efflux epipodophyllotoxin

(p-glycoprotein) vincs, anthracyclines

Decreased intracellular activation 6MP,5-FU

Increased intracellular breakdown cytarabine

Bypass biochemical pathways MTX, 6MP, asparaginase

Gene amplification or over- MTX, nitrosoureas

production of blocked enzyme

Other Mechanisms of Drug Resistance

Diminished vascularity

Only a small proportion of cells may be in cycle, allowing time for repair from cytotoxic damage before cell division

Hormone Therapy Beatson demonstrated that

some inoperable breast cancers regressed after removal of the ovaries (oophorectomy)

Many years later Huggins showed that metastatic prostatic cancer regressed after removal of the testes (orchidectomy).

In breast cancer, hormone receptor status is clinically important in management

Principals of hormone therapy (1) Receptor proteins for steroid

hormones are found in both the cytoplasm and the nucleus

The interaction between these hormones and their receptor proteins promotes cell growth and division.

The steroid hormone crosses the cell membrane and forms a complex with a receptor in the cytoplasm.

This activated complex passes into the nucleus where it binds to a protein, leading to the production of messenger RNA (mRNA) and then protein.

Finally DNA is synthesised and the cell divides

Principals of hormone therapy (2) This interaction provides the rationale

for a number of ways in which hormone manipulation can modify tumour growth. It may be possible to:

lower the plasma concentration of hormone by removing the source of production, e.g. the testes or ovaries

prevent the hormone from binding to receptor via competitive inhibition or by reducing synthesis of the receptors

block binding of the hormone/receptor complex to DNA in the nucleus

The precise mode of action of agents used in hormone therapy is often unclear.

Approaches to hormone therapy (1)

Lowering plasma hormone concentration

Radiotherapy Radiotherapy to the ovaries induces the menopause – the ovaries stop producing eggs and the female sex hormones.

SurgeryIn breast cancer can involve:

the ovaries (oophorectomy) the adrenals (adrenalectomy)

the breast tissue (mastectomy)

In prostate cancer:surgical removal of the testes (orchidectomy)

Medical treatmentAromatase inhibitors

Analogues of luteinizing hormone–releasing hormone (e.g. goserelin and leuprorelin)

Approaches to hormone therapy (2)

Blocking the action of circulating hormones

Anti-oestrogens and anti-androgenswork by blocking the binding of hormones to their receptors.

Anti-oestrogens (e.g. tamoxifen) Anti-androgens (e.g. flutamide, megestrol acetate)

Additive hormone therapies

The action of circulating hormones can also be blocked by additive hormone therapies, which in breast cancer include oestrogens, androgens, glucocorticoids and progestogens.

Summary

A knowledge of the cell cycle is important to understanding the mechanism of action of cancer chemotherapy

Combination chemotherapy is used to try and improve rate and duration of response by combining drugs with different mechanisms of action.

Manipulating the interaction between hormones and cell growth provides a means for treating hormone sensitive cancers

Acknowledgements

The Institute of Cancer Research - interactive education unit module 4 “An approach to therapies”

‘Cancer and it’s Management’ - 3rd Edition Souhami R and Tobias J (Blackwell Science)