Lecture 8 Flashcards
Give the typical experiment for cis-platin binding to DNA
1) In vitro incubation of DNA and cis platin
2) Extraction of DNA from cells
3) digestion of DNA by enzymes
4) separation (HPLC)
5) characterisation by NMR
Why are 1,2- intrastrand cross links important to anti-cancer activity
1) they are the major product formed
2) clinically inactive compounds fail to form these cross links
How is DNA ‘damaged’ on cis platin binding?
- 2 guanine groups in ‘head to head’ configuration
- Dihedral angle between rings 80 degrees (base stacking disruption)
- 17 membered ring
- H bonding from NH3 to 5’-phosphate group
Effects of 1,2- intrastrand cross-links are:
- bending towards major groove
- unwinding of duplex (by around 20 degrees)
- widened minor groove
- distortion of Watson crick base pairing
- duplex destabilisation
- blocks replication and inhibits transcription
- cells undergo apoptosis (cell death)
How do 1,2- intrastrand cross-links inhibit repair?
- HMG (high mobility group) inserts a phenyl group that protrudes from its backbone into the notch created when cis platin complexes to DNA
- increased bend in DNA
Cis-Platin in cells
- glutathione (GSH) binds to Pt to give a high MW polymer with Pt:GSH ratio of 1:2
- GSH-Pt binding results in depletion of Pt from circulation
- GS-X pumps can pump out GS-Pt from tutor cells
- intercept of Pt before DNA binding making the drugs less effective
- Cancer cells grow faster and have thinner, less developed cell membranes so are affected more than normal cells
Cis-Platin in kidneys
Metallothionen (MT) is a low MW protien in which 1/3 of the residues are Cys
- bind and store metals for detoxification in kidneys
- cis platin reacts to give Pt(7-10) MT containing Pt 54 units
Why is trans-platen inactive?
- increased distance between Cl leaving groups means no 1,2 intrastrand DNA adducts can form
- trans platin lesions are more easily repaired as they cause more DNA distortion and don’t bind HMG as strongly due to lack of appropriate space
- more readily intercepted by sulphur containing species
- mono adducts are more easily displaced by trans labilizing nucleophiles such as Glu or Thiourea
Side effects
- vomiting
- hair loss
- hearing loss
- low white blood cell count
- kidney damage
- depression
long term
- kidney damage
- hearing impaired
- loss of feeling in limbs
- psycho sexual effects
Controlling the side effects of cis platin
- Complexation to sulphur inhibited by high Cl- concentration so a saline drip administers the drug and reduces kidney damage
- Rescue agents are administered 3/4 hours later and displace Pt from S containing biomolecules whilst leaving it left bound to DNA
Mechanisms of resistance
- Decreased intracellular transport
- increase interception by thiol rich species
- improved repair
How to overcome resistance
- Develop new Pt drugs
- Higher dosage of Pt drugs
- Use combination chemo with other active anti - tumor drugs
- use in combinations with drugs that surpress resistance mechanisms
Carboplatin
- less toxic so higher dosages can be used
- due to stability within the bloodstream meaning proteins are less likely to bind to it
- less potent
- only active against some tumors
- same adducts formed as cis platin
Nedaplatin
- same as carboplatin
- used in Japan more
- testicular, ovarian and cervical cancers
Oxilaplatin
- stage 3 cancer treatment after surgery
- administered with fluororacil and leucovorin for increased effectiveness
