Section 7 - Platinum based Anti-Cancer Drugs Flashcards
When was cis platin approved for clinical use?
1978
What cancers does cis platin treat?
- 95% successful for testicular and ovarian cancer
- Used for head and neck, bladder, lung, cervical cancers
- lymphoma, melanoma and osterosareoma
What are the loose ‘rules’ for anti cancer activity?
- For (PdL2X2)
- cis leaving groups
- neutral complex with Pt(II) and Pt(IV) systems
- Leaving groups are moderately bound, too labile and the drug is toxic, too strong and the drug has reduced activity
- non LG is also crucial, amine groups with at least one N-H group
How are anti-cancer drugs tested?
- first in vitro on mouse leukaemia cell lines
- testing on animals
- several stages of clinical trials
- only 1 in 10000 drugs make it to approved clinical use
How is the active form of cis-platin formed?
- slow addition of one water molecule
- reversible
- 2-3 hours
What is the structure of cis platin?
PtCl2(NH3)2
How was the active species of cis platin determined?
- Pt and N NMR studies
What is mono aqua cis platin attracted to?
- positive charge means the complex is attracted to the negatively charged surface of DNA
What causes the delay in Cis Platin sensitisation?
due to the slow formation of the mono aqua complex
Where is the mono aqua complex of cis platin most likely to form?
- in plasma [Cl]= 100 mM and a slow exchange of Cl for H2O
- the molecule crosses the cell membrane by passive diffusion
- in the cytoplasm [Cl]= 4 mM so a slow ligand exchange occurs
Why is Pt-DNA binding the main focus of studies?
- diseases where DNA processes are deficient are hypersensitive to cis-platin
- Correlations have been shown between Pt-DNA adducts in blood cells and disease response in cis-platin patients
- Treatment of HeLa cells with a high dose of 195Pt(metastable) radiolabelled cis platin shows:
1 Pt per 10^5 protiiens
1 Pt per 100 RNA
1 Pt per 1 DNA
What mononucleotides does Pt bind the most strongly to?
G(N7)>A(N7)>A(N1)>C(N3)
Why is the Pt G(N7) bond so strong?
- the most electron rich centre
- bond can only be broken by a strong nucleophile e.g. cyanide
- possible H-Bonding also stabilises the adduct
How is the Pt-G(N7) adduct detected spectroscopically?
- Proton NMR
- Guanune C8-H singlet at 7.8
- Adduct singlet at 8.8 and Pt satellites
- large shift means that NMR is very useful in more complex systems
What are oligonucleotides?
- very short duplexes that are 2 to 15 nucleotides long
- used for model studies
Where does cis platin bind in oligonucleotides?
- G(N7)-p-G(N7) almost exclusively
Describe a typical experiment for studying cis-platin binding to DNA
1) in vitro incubation of DNA and cis platin
2) extraction of DNA from cells
3) digestion of DNA with enzymes
4) separation by HPLC
5) characterisation by NMR
What percentage of cis platin is cis[Pt(NH3)2(GpG)]?
- 60-65%
- G,G 1,2 intrastrand
- hard to cleave due to cycle
What percentage of cis platin is cis[Pt(NH3)2(ApG)]?
- 20 to 25%
- G,A 1,2 intrastrand
What percentage of cis platin is cis[Pt(NH3)2(GMP)2]?
- 5-10%
- G,G 1,3 intrastrand
- or interstrand (1,3 most likely)
What percentage of cis platin is cis[Pt(NH3)2(GMP)(OH)]?
- 2-5%
- monoadduct
- time dependent
What percentage of cis platin is cis[Pt(NH3)2(GMP)(protein)]?
- less than 1%
- whole cell studies only
- DNA protein binding
- can lead to side effects
Why are 1,2- intrastrand crosslinks important to anti cancer activity?
- they are the major adducts formed
- clinically inactive compounds fail to form these cross links
Describe the kinetics of the mechanism of cis platin and DNA binding
- kinetics is important
- RDS is the formation of the mono aqua complex
- binding of DNA and cation is very fast
How do 1,2- intrastrand cross-links distort DNA?
- bending towards the major groove
- unwinding of the duplex
- distortion of Watson- Crick base-pairing
- duplex destabilisation by 6.37 kcal per mol
How does distorting DNA affect biological function?
- blocks replication and inhibits transcription
- replication stops at sites one nucleotide before the Pt-G residue and at site opposite the two Pt-G residues
What types of cells are sensitive to cis-platin?
- cells deficient in DNA repair
- DNA is continually checked for errors in its sequence by DNA proteins
- Proteins can activate enzymes to prevent the errors
What is apoptosis?
- programmed cell death
- some evidence suggests it is induces by cis-platin
How do Pt 1,2 intrastrand cross links inhibit repair?
- Binding of HMG protien with the 1,2 intrastrand cis Platin DNA complex shields the DNA from repair leading to apoptosis
- binding occurs by HMG inserting into a phenyl group protruding from its backbone into the notch created when cis Platin binds with DNA
- increases the bend in DNA to 60-90 degrees
What element provides potential coordination sites for Pt complexes?
- Suphur
- strong soft-soft interactions
How do platinum and sulphur bind in cells?
- GSH (glutathione) is in cells at 3-10 mM concentrations
- GSH binds to Pt and gives a polymer with a Pt:GSH ratio of 1:2
- results in the depletion of Pt from circulation
- GS-X pump pumps GS-Pt out of tumor cells
- Is a way of intercepting Pt before it binds to DNA, which makes the drug less effective
- Cancer cells grow faster and have thinner cel membranes than normal cells so are affected more than normal cells
How do platinum and sulphur bind in kidneys?
- Metllothionen (MT) is a low molecular weight protien (6-7 k)
- 1/3 of residues are Cys
- Cys residues bind and store metal ions and are used in the kidneys for detoxification
- MT reacts with cis-platin to give Pt7-10MT containing PtS4 units
What adducts does trans platin form with DNA?
- The longer distance between the two Cl leaving groups (4 vs 2.8) results in trans-platin being unable to form 1,2-intrastrand DNA adducts
- can form 1,3-intrastrand (GpNpG), 1,4-intrastrand and interstrand links as well as monoadducts.
Why does trans platin not show any anticancer activity?
- Trans-platin lesions are more easily repaired since they lead to more radical distortion of DNA. They don’t bind HMG proteins as strongly as cis-platin lesions, possibly due to the lack of an appropriate space for intercalation of the HMG protein phenyl group.
- Trans-platin is more readily intercepted by sulphur-containing species (e.g., glutathione) leading to removal of Pt from the cancer cells, due to trans effect
- 3.
Trans-platin monoadducts are more easily displaced by the action of trans- labilizing nucleophiles such as glutathione or thiourea.
What are the short term effect of cis platin?
Emetogensis – severe vomiting and nausea
Hair loss
Ototoxicty – hearing loss / tinnitus
Myelosupression – low white blood cell count
Nephrotoxicity – kidney damage Neurotoxicity - depression
What are the chronic effects of cis platin?
Kidney damage
Hearing impaired
Peripheral neuropathy – loss of feeling in limbs
Psycho-sexual difficulties
What causes the side effects and what does this limit?
- These side effects are associated with DNA and protein bound platinum (similar to other heavy metal poisoning).
- This limits the maximum dose of cis-platin to ~100mg per day for up to 5 consecutive days.
How can the toxic effects of cis platin be controlled?
- by inhibiting the formation of Pt- protein complexes
- by ‘rescuing’ Pt from these Pt-protein complexes
Give examples of how to control cis platin side effects
- RS-, RSH complexation is inhibited by high Cl- concentration. Hence cis-platin is administered in a saline drip. This reduces kidney damage dramatically.
- Rescue agents can be administered 3-4 hours after cis-platin treatment. These agents displace Pt from sulphur containing biomolecules but do not affect Pt-DNA complexes.
What are the three major mechanisms of cis platin resistance
A - Decreased intracellular transport
B - Increase interception by thiol rich species
C - Improved repair
How can cis platin resistance be overcome?
1) Develop new Pt drugs
2) increase the dosage of Pt drugs
3) use combination chemo with other active anti-tumor drugs
4) use in combination with other agents know to modulate the mechanisms of cis platin resistance
What is carboplatin? Draw its structure
- the first of the second generation anti-cancer drugs
What are the effects of carboplatin compared to cis platin?
- less toxic than cis platin
- higher dosage can be used
- ## can still cause myelosupression
Why is carboplatin less toxic than cis platin?
- added stability in the bloodstream
- prevents binding to proteins
What are the disadvantages of carboplatin?
- less potent than cis platin (4x the dose required)
- 10x more expensive
How is carboplatin administered?
- intravenous infusion
What is carboplatin effective against?
- the same cancers as cisplatin as it forms the same adducts to DNA
Describe Nedaplatin and draw its structure
- similar clinical properties to carboplatin
- used in Japan for testicular, ovarian and cervical cancers
Describe oxaliplatin and draw its structure
- recently approved
- used to treat stage 3 colorectal cancers after surgery
- administered in combination with 5-fluorouracil and leucovorin for increased effectiveness
What are third generation anti cancer drugs?
- decreased toxicity, increased activity against resistance and the ability to be taken orally
- new Pt compounds with decreased reactivity towards nucleophiles, new biodistribution and different boding modes to DNA
Why are new sterically hindered complexes being formed?
- leads to decreased substitution reaction rates
- slower reaction with sulphur containing biomolecules
- fewer side effects and can overcome type B resistance
Describe an example of a sterically hindered complex and draw its structure
- ZD0473
- treats lung and ovarian cancers
- the 2-methyl group sits above the plane of the molecule leading to axial steric hinderance
- forms 1,2- intrastrand links with the DNA similar to cis platin
Why are Pt(IV) complexes being developed
- more inert to ligand substitution than Pt(II)
- less likely to react with biomolecules so they have fewer side effects
- ## active against some resistant cells
How do Pt(IV) complexes bind to DNA?
- Some studies show Pt(IV)-DNA complexes resulting in large kinks
- more likely that intra and extracellular agents reduce them to Pt(II) to form cis platin like adducts
Give examples of Pt (IV) complexes, draw their structures
- Satraplatin, tetraplatin and iproplatin
-
Describe Satraplatin
- orally administered
- water and lipid soluble
- treats prostate, lung and ovarian cancers
Why are complexes with a second DNA binding function being developed?
- by attaching DNA-intercalators such as doxorubicin and anilioarinide the compounds could localise on DNA before aquation
- can also stabilise adducts
What is an alternative approach to DNA binding functions?
- adding hydrogen bonding groups to the complex
- bind to the DNA phosphate backbones
- or attachment of proteins that bind in the minor grove of DNA
Why are trans Pt complexes being developed?
- statically hindered ligands reduce the kinetic activity of trans platinum
- aquation rates approach those of cis platin
- overcomes the problem of trans molecules reacting with biomolecules
- result in activity against improved repair
Why are multinuclear Pt complexes being developed?
- initially dinuclear Pt complexes were found to be active
- make a hair pin structure with B-DNA
- too toxic for clinical trials
- more Pt nuclei added
Give an example of a Pt multinuclear complex
- BBR3464
- phase II trials for melanoma, lung cancer and pancreatic cancer
- 10 x more potent than cis platin
- active against resistant cells
- forms long range interstrand and intrastrand crosslinks (up to 6 base pairs apart)
- significant duplex unwinding
