Mechanism of Action of Commonly Used Anti-cancer Agents Flashcards
mustards were first used to treat what in the 1940s
leukemia and lymphoma
examples of nitrogen mustards
chlormethamine, mechlorethamine, mustine
very reactive and excessively toxic
nucleophilicity is controlled by what
steric, electronic and hydrogen bonding effects
alkylating agents are potent
electrophiles
intrastrand definition
linking between two bases on the same strand
interstrand definition
cross-linking of two separate strands
alkylating agents react with many nucleophiles other than DNA bases…
Thiols especially reactive, amines also reactive
Cysteine and lysine residues in proteins. Protein adducts occasionally trigger an allergic response
Glutathione in cells can react with alkylating agents and ”quench” their activity.=
toxicity to cancer cells results from _____________ and _____________. DNA-protein cross-links also inhibit DNA function
DNA alkylation; DNA cross-linking
cells are more susceptible in late G1 and S phases of cell cycle but alkylating agents are considered ____________
non cell cycle specific
mechlorethamine (mustargen, mustine, chlormethine) facts
First used clinically in the 1940’s – An important prototype that resulted in the development of field of chemotherapy. Not used commonly anymore.
Extremely reactive compound – half-life in plasma ~ 1 min
- Rapidly alkylates all nucleophiles – modifies DNA, RNA and protein
- Antitumor activity correlates with DNA cross-links
Side effects for all alkylators
- Myelosuppression
- Nausea and vomiting
- Carcinogenic and teratogenic
Two strategies to reduce reactivity and increase selectivity of nitrogen mustards
- decrease nucleophilicity of nitrogen by adding aryl groups
- pro-drug strategy
examples of compounds that decrease nucleophilicity of nitrogen by adding aryl groups
chlorambucil
bendamustine
melphalan
examples of compounds that utilize the pro-drug strategy
cyclophosphamide
for chemically stable prodrugs…
…requires hydroxylation by hepatic Cytochrome P450
cyclophosphamide facts
most useful and commonly used alkylating agent
SE modest compared to most alkylating agents:
- mild bone marrow toxicity: sparing to marrow stem cells (and platelets) because of high ADH levels in these cell types
- hemorrhagic cystitis: acrolein is toxic to bladder mucosa
also in this class: ifosfamide has longer half life than cyclophosphamide, but also has increased CNS toxicity
what is mesna
Cyclophosphamide is toxic to the bladder
-Acrolein accumulates in urine and damages bladder mucosa
Mesna containing a charged (anionic) sulfonate group so it does not penetrate cells
- Anion transporters in proximal tubule excrete via kidney
- Mesna accumulates in the urine
The free thiol on Mesna reacts with and inactivates acrolein metabolites in urine
Administered with cyclophosphamide to block hemorrhagic cystitis
what are nitrosoureas
Chemically unstable - non-enzymatic conversion to diazonium ion
Diazonium ion-highly electrophilic and has extremely short half-life
-Conversion to diazonium ion must take place inside cell
examples: bis-chlorethylnitrosourea (BCNU) = carmustine
also in this class = lomustine
nitrosoureas: highly lipophilic alkylating agents which means they…
…readily cross the blood-brain barrier
used to treat glioblastoma multiforme and other brain tumors. structurally different DNA adducts so different DNA repair mechanisms
toxicities typical of other alkylating agents: significant bone marrow toxicity: myelosuppression is delayed (3-4 weeks) and prolonged. requires longer interval between doses than other agents. suggests that toxicity is greater to earlier progenitor cells
alkyl sulfonates facts
Given in high doses with cyclophosphamide before a bone marrow transplant – designed to eradicate all hematopoetic cells
Pulmonary fibrosis - “busulfan lung”
example compound: busulfan
Mitomycin C (Mutamycin)
Aziridine-containing natural product
Similarities to nitrogen mustard compounds. Functions as an alkylating agent, although toxicity pattern differs slightly from other alkylating agents
Myelosuppression is dose-limiting
mitomycin C can form ___________ adducts
bifunctional
What alkylator reacts via the more reactive (and positively charged) aziridinium?
nitrogen mustards
Monoalkylating agents definition
alkylates DNA but does NOT cross-link DNA
still inhibits DNA replication by blocking activity of replication enzymes on DNA
example compound: temozolomide, decarbazine (DTIC)
temozolomide (TMZ)
Inhibits DNA, RNA and protein synthesis
Can be combined with other alkylating (cross-linking) agents
Toxicities typical of other alkylating agents but generally less severe
- Bone marrow toxicity is milder
- Nausea and vomiting is severe
- Temozolomide is 100% bioavailable (Readily crosses blood-brain barrier – used for brain cancers)
temozolomide fact to remember
readily crosses blood-brain barrier. Used for brain cancers
decarbazine
Also converted to MTIC intermediate, but requires activation via N-demethylation catalyzed by hepatic P-450 enzyme
-N-demethylated intermediate is unstable and has a short half-life
Dacarbazine is poorly absorbed (IV administration)
TMZ resistance
promoter methylation predicts whether glioblastoma patients respond to TMZ
Temozolomides alkylate DNA via a diazonium intermediate. What crosslinker also alkylates via a diazonium?
carmustine
platinum drugs
Originally described as a therapeutic when a soluble platinum complex generated at platinum electrodes inhibited cell division in E. coli
Covalent crosslinkers, but not alkylating agents (no alkyl groups)
Cisplatin is the original prototype Square planar complex with leaving groups having cis geometry (not trans!)
platinum drugs continued
Cisplatin undergoes reversible hydrolysis in aqueous solution
- Equilibrium favors Cisplatin in plasma (where there is a high chloride concentration)
- Equilibrium favors aquo form inside cell (low chloride concentration)
- Aquo form is highly reactive and a potent electrophile
Aquo form reacts rapidly with other nucleophiles, especially thiols
platinum crosslink geometry
Aquo form reacts primarily at Guanine N-7 and Adenine N-7 in DNA
Because of bond lengths and angles, cross-links are often INTRASTRAND (but can be interstrand)
Intrastrand cis-Pt cross-link imposes severe geometrical constraints on DNA
- Introduces sharp bend in cross-linked strand
- Lesion not readily repaired by standard DNA repair enzymes
Cisplatin
Highly effective agent for many solid tumors
Drug requires non-enzymatic conversion to the active aquo form
Aquo form produces primarily intrastrand cross-links
Cross-links generally form more slowly than for other alkylating agents
Some tumor cells more sensitive in G1 than in S phase
Cisplatin SE profile
Dose-limiting nephrotoxicity (proximal tubule)
Severe nausea and vomiting (centrally mediated)
Minimal bone marrow toxicity (so good in combination with other drugs)
Peripheral neuropathy – related to cumulative dose
Ototoxicity (hearing loss)
Drug Resistance - General Mechanisms
Multiple cellular mechanisms are involved but the major mechanisms are specific for alkylating agents and platinum drugs
Increased expression of DNA repair enzymes
- Excision repair – excise alkylated base(s) and replace
- Alkyl group removal – Guanine O-alkyl transferase
Increased intracellular concentration of non-protein thiols, especially glutathione
- Glutathione is a cysteine-containing tripeptide (-Glu-Cys-Gly)
- Free thiols have extremely high reactivity toward electrophilic intermediates
- Thiols intercept the reactive intermediates of alkylating agents
- Free thiol levels > 10-fold higher in alkylating agent-resistant cells
Increased expression of cellular glutathione S-transferase (GST)
- GST’s are a family of Phase II metabolic enzymes – P1-1 is key isozyme
- GST catalyzes the reaction of glutathione with alkylating agents (parent drugs as well as reactive intermediates)
