Platinum drugs for chemo

Question 1

Cisplatin becomes activated intracellularly by the _____ of one of the two chloride leaving groups and subsequently covalently binds to DNA forming DNA _____.

Answer 1

Aquation (water molecule incorporated into compound)

Adducts

Question 2

Cisplatins ability to form DNA adducts activates various signal transduction pathways including… (3)

Answer 2

DNA damage recognition and repair
Cell cycle arrest (as impede replication and transcription)
Programmed cell death/apoptosis

Question 3

What do DNA adducts cause?

Answer 3

DNA unwinding and bending which is recognised by several cellular proteins (some of which are involved in DNA repair pathways)

Question 4

Cisplatin is notoriously toxic to the _____ (______) and GI tract.

What technique was used to try and overcome this?

What drug was developed that is devoid of this issue?

Answer 4

Kidneys (nephrotoxicity)

Prehydration techniques: giving large amounts of water before chemotherapy

Carboplatin

Question 5

What is dose limiting for carboplatin?

Answer 5

Myelosuppression - decreased bone marrow function that results in lower numbers of RBC and WBC and platelets

Mainly:
Thrombocytopenia - decrease in number of platelets

Question 6

Platinum drugs bind preferentially to ____ on position ____ of _____.

Answer 6

Nitrogen
N7 position
Guanine

Question 7

They form major _____ strand ______ (adjacent guanines on same strand of DNA).
and minor _____ strand ______ (binds guanine on opposite DNA strands).

Answer 7

Intrastrand crosslinks

Interstrand crosslines

Question 8

What is the major cellular outcome of DNA adduct formation?

Answer 8

Apoptotic cell death (and cell cycle arrest)

Question 9

What step activated platinum drugs?

Answer 9

Aquation (incorporation of an H2O molecule which displaces a chloride)

Question 10

List 3 types of resistance seen with platinum agents before they can exert their action.

Answer 10

1) Reduced cellular uptake due to reduced levels of CTR1 (copper transporter)
- Platinum drugs are taken up either by this transporter or by passive diffusion
- Reduced levels of this transporter means less uptake of platinum agents so less action (less adducts formed etc)

2) ‘Mopping up’ of platinum drugs by glutathione or metallothioneins
- Some cancer cells produce high levels of glutathione and metallothioneins
- Platinum drugs bind these preferentially due to the presence of sulphur
- This reduces the availability of platinum drugs to bind guanine residues on DNA so less DNA adducts are formed = less action of drug

3) Increased efflux/removal of drug from cells
- Platinum drugs are removed from cells by copper exporters ATP7A and ATP7B
- They are also effluxes by glutathione S conjugate export by the GS-X pump (MRP2)
- This can reduce levels of platinum agents in cells hence another mechanism of resistance

Question 11

What happens if you knockdown ERCC1 (NER endonuclease) and then treat with Cisplatin?

Answer 11

• Enhanced cellular sensitivity to cisplatin due to reduced NER of cisplatin induced DNA lesions
• ERCCI is needed to combine with XPF to make the 5’ incision into the DNA strand relative to the site of platininated DNA
• If this does not occur then NER is reduced so the damage to the DNA remains (adducts remain)

Question 12

What is the implication of ovarian cells that have increased expression of ERCC1 and XPF?

Answer 12

• Increased levels of ERCC1 and XPF means that more NER occurs
• ERCC1 and XPF are required to make the incision 5’ into DNA strand relative to the site of platinated DNA to remove the damaging adduct
• If more of this occurs then the drugs action is reversed

= Resistance mechanism

Question 13

List the 4 main strategies to circumvent platinum drug resistance in cancer patients:

Answer 13

1) Improved platinum drugs
2) Improved deliver of platinum to tumours
3) Co-administration of platinum drugs with pharmacological modulators of resistance mechanisms
4) Combine platinum drugs with molecularly targeted drugs

Question 14

Give an example of a combination of platinum drug with molecularly targeted agent

Answer 14

Either:
Bevacizumab (targets VEGF)
In combination with carboplatin-paclitaxel

Trastuzumab (HER2 receptor antibody)
In combination with cisplatin
Synergise to reduce DNA repair of damage caused by platinum agent (less repair of DNA adducts)

Question 15

What does Bevacizumab do?

Answer 15

Blocks growth of new blood vessels to tumours

It binds VEGF to prevent it interacting with receptors on endothelial cells that line blood vessels

Activation of receptors usually leads to formation of new blood vessels to tumours so this doesn’t happen

This result in increased sensitivity to cytotoxic drug and prevents metastasis!

Question 16

Cisplatin binds to adjacent guanines in major groove of DNA and as a result bends DNA in direction of major groove. This distortion of DNA allows access to DNA binding proteins in minor groove. What does this then do?

Answer 16

This stabilises the interactions between DNA and binding proteins.

These proteins enforce the bending of DNA and leads to enforced architectural changes

And can also lead to masking DNA lesions such as cisplatin DNA adduct from NER OR deplete proteins involved in transcription!

• Could be reason for toxicity of agents!