MICROTEACHING

Sanjeev Sharma 2-3-2010

Cancer cell and Chemotherapy

• In which phase of cell cycle DNA replication occurs ?

• What is the mechanism of action of cytarabine ?

• How bortezomib acts?

PHASES OF CELL CYCLE

• G0 phase (resting stage)- The cell has not yet started to divide. Cells spend much of their lives in this phase . Depending on the type of cell, G0 can last for a few hours to a few years. When the cell is signalled to reproduce, it moves into the G1 phase

• G1 phase- During this phase, the

cell starts making more proteins and growing larger, so the new cells will be of normal size. This phase lasts about 18 to 30 hours.

• S phase – In this phase, the chromosomes containing the genetic code (DNA) are copied so that both of the new cells formed will have matching strands of DNA. This phase lasts about 18 to 20 hrs.

• G2 phase- In this phase, the cell checks the DNA and prepares to start splitting into cells. It lasts from 2 to 10 hrs.

• M-phase-In this phase, which lasts only 30 to 60 mins, the cell actually splits into 2 new cells.

• Topoisomerases- catalyze transient breaks in DNA molecule and unwind the double helix

• DNA polymerases- carry out the discontinuous replication of DNA strands

The Ubiquitin-Proteasome Pathway

Tumor Growth Patterns

• Tumors grow exponentially at first• As tumors get larger, the growth rate slows

due to lack of oxygen and nutrients• As tumors get very large, many cells are not

proliferating, and some have died due to lack of oxygen and nutrients

Characteristics of cancer cell division• Growth fraction - at any

particular time some cells are going through the cell cycle whereas other cells are resting.

• The ratio of proliferating cells to cells in G0, is called the growth fraction.

• A tissue with a large percentage of proliferating cells & few cells in G0 has a high growth fraction.

• Conversely, a tissue composed of mostly of cells in G0 has a low growth fraction.

Chemotherapy Effect on Cellular Reproduction

CLASSIFICATION OF CANCER CHEMOTHERAPY

Alkylating agents- Cyclophosphamide, Ifosfamide, Chlorambucil

Antimetabolites- Folate antagonists- Methotrexate Purine antagonists- 6 MP Pyrimidine antagonists- Cytarabine Vinca alkaloids- Vincristine, Vinblastine Epipodophyllotoxins- Etoposide Antibiotics- Daunorubicin,Doxorubicin, Mitoxantrone Hormonal- Steroids Miscellaneous/Others- Hydroxyurea, L-

Asparaginase,Rituximab, Imatinib, ATRA

Classification on the basis of effect on cell cycle

• Cell cycle active, phase specific

• Cell cycle active, phase non-specific

• Non cell cycle active

S phase- Methtrexate, 6MP, Cytarabine

M phase- VCR

Alkylating agents, anthracyclines

Steroids, Asparaginase

Mechanism of action of chemotherapeutic agents

Alkylating agents

• Alkylating agents (cyclophosphamide) works by 3 different mechanisms- 1) It attaches alkyl groups to DNA bases. This alteration causes DNA fragmentation by repair enzymes

Cyclophosphamide

Bases alkylated

DNA fragmented

Alkylating agents contd…

2) Formation of cross-bridges,which prevent DNA from being separated for synthesis or transcription

3) Induction of mispairing of nucleotides leading to mutations

Cross bridge formed

Mispairing

Methotrexate MTX prevents the formation

of THF, causing an intracellular deficiency of folate coenzymes and accumulation of the toxic inhibitory substrate, DHF polyglutamate

The one carbon transfer reactions for purine and thymidylate synthesis cease, interrupting DNA and RNA synthesis

Cytarabine• Cytarabine is converted

to its triphosphate form which acts by two ways-

1) Inhibition of DNA polymerase

2) Is incoporporated into the DNA, where it terminates strand elongation-DNA synthesis stops in S phase

Ara-C Ara-C

Vinca AlkaloidsBinds to the microtubular protein tubulin and terminates assembly of the mitotic spindle- interference with chromosome segregation resulting in mitotic arrest at metaphase .M phase specific

Anthracyclines1) Intercalation: Doxorubicin intercalates between adjacent nucleotides along the DNA forming a tight DNA-drug interaction. This interaction disrupts DNA synthesis and transcription. 2) Enzyme inhibition: Doxorubicin binds and inhibits topoisomerase II,a key enzyme involved in DNA synthesis.3) Oxygen free radicals are also produced which damage DNA and prevent DNA synthesis.

DNA-Drug interaction

Doxorubicin

Asparaginase• Causes catabolic

depletion of serum asparagine to aspartic acid and ammonia

• Resulting in reduced blood asparagine levels and inhibition of protein synthesis

Imatinib

Imatinib (STI571) occupies the ATP-binding site (also known as the phosphate- or P-loop) of BCR-ABL complex. ATP is displaced and substrate is not phosphorylated.

ATRA• The fusion protein

complexes(PML-RARA) are not activated by physiological concentrations of RA so the corepressors are not dissociated and transcription does not occur.

• The high dosage of RA is enough to bind to the PML-RARα fusion protein and dissociate the SMRT/HDAC complex so that repression no longer occurs

Bortezomib• NF-kB is an important regulatory

protein that promotes the transcription of a variety of growth promoting and angiogenic factors

• In normal cells NF-kB exists in an inactivated state bound to inhibitor IkB.

• In MM increased proteasomal degradation of IkB increases NF-kB

• Bortezomib blocks proteasome, elevates IkB, which inactivates NF-kB, promoting tumor cell apoptosis

Rituximab

Anti CD-20 monoclonal antibody (Rituximab) binds to CD-20 on malignant cells and leads to Apoptosis by – Antibody dependent cell mediated cytotoxicity(ADCC)AndComplement dependent cytotoxicity(CDC)

COMBINATION CHEMOTHERAPY

Advantages of combination therapy: 1. Suppression of drug resistance - less chance of a

cell developing resistance to multi drugs than to single drug.

2. Increased cancer cell kill - administration of drugs with different mechanisms of action.

3. Reduced injury to normal cells - by using a combination of drugs that do not have overlapping toxicities, we can achieve a greater anticancer effect than we could by using any one agent alone.

Summary

THANKS