Wk 12: Alkylating + platinating agents (nitrogen mustards) Flashcards
What is the size of a clinical detectable tumour?
10^9 cells
What is the size of a lethal tumour?
10^12 cells
What is the doubling time of hodgkin’s disease + testicular teratoma?
3-6 days
What is the doubling time of breast, prostate, lung and colon cancer?
80-100 days
What is the 3 log, 1 log regrowth principle?
- In tumour w/ 10^11 cells, cycle of chemo = 10^3 cells dying
- Regrowing by 10^1 btw each cycle
Which tissues have the most rapid proliferation?
- Bone marrow
- GI mucosa
- Ovary
- Testis
- Hair follicles
Which tissues have slow proliferation?
- Liver
- Lung
- Kidney
- Endocrine glands
- Vascular endothelium
Which tissues have no proliferation?
- Muscle
- Bone
- Cartilage
- Nerve
What are the proliferation side effects of anti-cancer agents?
- Myelosuppression
- Immunosuppression
- Mucositis
- GI disturbance
- Alopecia
- Gondal damage
What are alkylating agents?
Highly reactive electrophilic compounds that form covalent bonds
What is the mode of action of alkylating agents?
- React w/ nucleophilic group on DNA (N-7 of guanine bc on major groove)
- Covalent bond forms btw DNA + agent, removing CI
- Bifunctional: causes intrastrand cross linking - stops strand unzipping
- Causes: Enzymes not to work, missing DNA base (depurination), mispairing of G-T
- Leads to apoptosis
How many H bonds do complementary base pairs form?
- AT = 2
- CG = 3
What are the groups of alkylating agents?
- Nitrogen mustards
- Nitrosoureas
- Platinum compounds
In which part of the phase are alkylating agents most effective?
- G1 + M phase
- Most base pairs not hydrogen bonded tf more susceptible to alkylation
- Impairs progression from G1 + S phase to M
What is bis-(2-chloroethyl)sulfide?
- Chemical warfare agent
- Destroys WBC + depresses bone marrow prod of WBC
- Toxic in rapidly dividing cells
What was the first anti-cancer drug?
Mustine:
- Replace sulfur by N-CH3
- Treated: lymphomas + hodgkin disease
- Administered as hydrochloride salt
What were the changes made to bis(2-chloroethyl)sulfide to make into the nitrogen mustards used for chemotherapy today?
- N = less nucleophilic + basic
- Less reactive, less toxic + better therapeutic agent
What is the mode of action of nitrogen mustards?
DNA strand cross linked tf can’t unwind
Interfered w/ transcription + replication
Inhibits cell division:
- Removal of guanine bases
- Alkylated guanine paired w/ Thymine
Ineffective repair (frameshift mutation)
Outline the signalling pathway once nitrogen alkylating groups have caused damage to the DNA
- Induces p53 + DNA damage-induced apoptotic pathway
- Converge on caspase by cytochrome C from mitochondrial inter-membrane
- Mitochondrial apoptotic pathway activation
- Upregulation of pro apoptotic genes (TRAIL) + tumour suppressor gene
What are the 2 most used nitrogen mustards?
Cyclophosphamide + ifosfamide
Give examples of nitrogen mustards
- Cyclophosphamide
- Ifosfamide
- Chlorambucil
- Melphalan
- Cyclophosphamide
What is chlorambucil?
Leukeran
- 2mg tabs (oral)
- Aromatic nitrogen mustard
- Slow acting + least toxic
- Less basic + toxic than mustine
- Less reactive: intermediate aziridinium ion less easily formed + only reacts with strong nucleophiles
Why is chlorambucil less reactive in terms of its structure?
N lp of aromatic amine = less nucleophilic than aliphatic amine due to resonance (weaker base = poorer nucleophile)
What is melphalan?
Alkeran
- Oral 5mg tabs
- Mostly: IV 50 mg anhydrous melphalan hydrochloride w/ 10ml solvent
- Derivative: L-phenylalanine
- Absorbed by AT (stereospecific)
- Plasma levels higher than other mustards
What is cyclophosphamide?
Nitrogen mustard prodrug (endoxan)
- Oral 50mg tabs
- IV/IM: powder reconstituted w/ sterile water
- Activated by liver metabolism (CYP450)
- Active agent: normustine (bifunctional)
Outline the activation of cyclophosphamide
- Bioactivation by CYP450
- Hydrolysis causes ring to open
- Further hydrolysis = releasing acrolein
- Hydrolysis = releasing normustine
What is acrolein + how does it cause hemorrhagic cystitis?
- Cyclophosphamide is a prodrug
- Activated by oxidisation by CYP450 which produces acrolein
- In glomerular filtration = water is reabsorbed, tf comparison of acrolein in blood compared to urine is concentrated
- Tf damage of acrolein in kidney tubules is much worse than when in blood
- Causes irreversible alkylation of cysteine residue -> significant renal + bladder damage -> hemorrhagic cystitis
How is the damage from acrolein resolved?
- Inc fluid intake
- Administer MESNA at 0, 4, 8 hrs of treatment (minimal cell penetration + no interference w/ cytotoxic effect)
What has a similar mode of action as cyclophosphamide?
Ifosfamide (mitoxana)
What is estramustine?
Estracyt:
- Caps w/ estramustine phosphate 140mg
- Combine estradiol + nitrogen mustard
What are the advantages of the combination of estradiol + nitrogen mustard in estramustine?
- Carbamate lowers N nucleophilicity
- Phosphate inc water sol
- Estradiol crosses membrane
- Targets cells w/ estrogenic receptor
- Alkylate as intact drug/cleavage by plasma/carbamase enzyme
What are the possible resistance pathways?
- Dec prodrug activation by CYP3A4 + CYP2B6
- Inc metabolism
- Dec entry into tumour cells
- Inc efflux from tumour cells
- Inc cellular thiol
- Inc DNA repair
- Deficient apoptotic response
