Name the alkylators
- Classical (nitrogen mustards/nitrosoureas)
- Non-classical/other
Nitrogen mustards: chlorambucil, cyclophosphamide, mechlorthamine, melphalan, ifosfamide
Nitrosoureas: CCNU, BCNU, streptozotocin
Other: Procarbazine, dacarbazine (DTIC), temozolomide, busulfan, Aziridines (thio-TEPA)
MOA:
MOA:
1. Act through covalent binding of ALKYL groups (-CH2Cl) to intracellular molecules
- Generate highly reactive positively charged intermediates – > these combine with ELECTRON RICH NUCLEOPHILIC GROUPS (ie amino, phosphate, sulfhydryl, hydroxyl moieties)
**Nucleophilic groups are potential sites of alkylation on almost all biologic molecules
**Alkylation of BASES in DNA is major cause of toxicity
Activity of most alkylators enhanced by these 5 things:
Enhance alkylator activity:
- Radiation
- Hyperthermia
- Nitroimidazoles
- Glutathione depletion
- DNA repair inhibition
Alkylating reactions can occur by 2 mechanisms SN1 and SN2. Describe each.
SN1 (1) rate limiting step = formation of carbonium ion that can rapidly react with a nucleophile (2) follows 1st order kinetics t/f rate depends solely on CONCENTRATION of alkylating agent
SN2 - follow second order kinetics, t/f rate depends on concentration of both alkylating agent AND nucleophile
NOTE: nucleophiles donate electrons to form chemical bonds
What are the main differences between monofunctional and difunctional alkylators?
- Alkylators are mono- or bifunctional (ie contain 1 or 2 reactive groups)
- Only bifunctional agents can form crosslinks between biological molecules (t/f most useful)
- Bifunctional agents – > DNA interstrand crosslinks that prevent cell replication unless repaired
- Monofunctional agents – > cytotoxic via mismatch-repair mediated processes (cells lacking MMR are methylating tolerant)
Why do cells with mutated p53 have greater resistance to alkylating agents?
DNA checkpoint proteins (p53) are responsible for recognition of DNA alkylation and strand breaks.
Most alkylators enter cells by X; exceptions include X and X
Many alkylators are LIPID soluble and enter cells by PASSIVE DIFFUSION.
Exceptions include:
1. Mechlorethamine - uptake depends on CHOLINE TRANSFER SYSTEM
- Melphalan - transported by at least two transport systems that carry LEUCINE and other AAs
Are alkylators cell cycle specific or non-specific?
Alkylators are cell-cycle NON-SPECIFIC since they bind directly to DNA.
Sensitivity to alkylators depends on X (pharmacokinetics)
Dependent on AUC (is relatively independent of schedule of administration)
What does monofunctional mean?
one reactive group
- -causes single strand breaks in DNA
- -damages bases by inducing abnormal base pairing
alkylation at N7 results in what?
alkylation at N7 results in conformation changes (base pairing G-T; gene miscoding) - mispairing!
Resistance to alkylators (general, from T&H)
6
- Decreased transport across membrane
- Increased intracellular thiol concentrations (ie glutathione) – > work by reacting w alkylating agents and reduce likelihood of interacting w DNA
- Increased detoxification of reactive intermediates
- Alterations in DNA repair enzymes (ie guanine-06-alkyltransferase)
- defects in cell cycle arrest/apoptosis
- increased AKT expression – > leads to inhibition of apoptosis and p53
Name the bifunctional alkylators (2 chloroethyl groups, able to crosslink):
The nitrogen mustards (mechlorethamine, melphalan, chlorambucil, cyclophosphamide, ifosfamide)
BCNU (a nitrosourea)
Busulfan?
Name the monofunctional alkylators (1 chloroethyl group)
CCNU
Procarbazine, dacarbazine, temozolamide, aziridines (thio-TEPA)
Nitrogen mustard MOA (with regard to bifunctionality)
- Nitrogen mustards are bifunctional with TWO CHLOROETHYL GROUPS – > 1 group undergoes first order reaction and loses a Cl – > becomes a highly reactive positive intermediate
- The intermediate can bind covalently to electronegative group on DNA base = ALKYLATION
- The other group can become reactive intermediate and bind another base = INTERSTRAND CROSSLINKING
What is the site of alkylation for nitrogen mustards?
N7 position of guanine – > can lead to mispairing with thymine or to strand breakage
Is chlorambucil more toxic to cycling or non-cycling cells?
Equally toxic to cycling and non-cycling cells – > may lead to delayed or cumulative effect on bone marrow dt toxicity of hematopoietic cells.
Is melphalan absorption predictable or unpredicatable?
Melphalan absorption is unpredictable - some respond IV when don’t PO
How is melphalan detoxified?
Any concerns for organ dysfunction?
Melphalan detoxified via SPONTANEOUS HYDROLYSIS but increased toxicity seen with RENAL dysfunction
How is melphalan uptake into cells mediated?
AA active transport system - resistance to melphalan may develop dt changes w this system
Is melphalan more toxic to cycling or non-cycling cells?
Melphalan is equally toxic to cycling and non-cycling cells – > may lead to delayed or cumulative effect on bm dt toxicity to hematopoietic cells
Do cells with mutated p53 have greater or less resistance to alkylating agents?
Cells with mutated p53 have GREATER resistance to alkylators.
Name 4 mechanisms of resistance to nitrogen mustards (chabner)
- increased capacity to repair alkylated lesions (ex: O6-alkyl transferase - nitrosoureas, busulfan)
- increased expression of glutathione-associated enzymes
- increased ALDH (cyclophosphamide)
- Decreased expression or mutation of p53
What is the oral bioavailability of cyclophosphamide, ifosfamide, melphalan, busulfan?
cyclophosphamide 100%, ifosfamide none, melphalan 30% variable, busulfan 50% +
