Genotoxicity and carcinogenicity of drugs Flashcards
Terms in Carcinogenesis
carcinogenesis
cancer
carcinogens
tumor
genotoxic
Carcinogenesis: new or autonomous growth of tissue
(neoplasia) resulting in formation of a lesion named
neoplasm
Cancer: malignant neoplasm, disease characterized by
mutation, modified gene expression, cell proliferation, and
aberrant cell growth
Carcinogens: physical or chemical agent that causes or
induces neoplasia
Tumor: lesion characterized by swelling or increase in size,
may or may not be neoplastic
Genotoxic: an agent or process that interacts with cellular
DNA, resulting in alteration of DNA structure
Cellular basis of cancer
- Uncontrolled and
inappropriate division of
cells - Genetic damage to critical
genes that regulate cell
growth - Exposure to chemical
carcinogens considered
important
Where are we exposed to
carcinogens ?
diet (35%)
viruses
smoking
environment
radiation
All of the time…..
What factors influence the development of cancer?What factors influence the development of cancer?
Dose, amount and length of exposure. The lower the
dose the least likely you are to develop cancer or related
diseases.
* Environmental or “lifestyle” factors.
* Cigarette smoking (co-carcinogen)
* Alcohol consumption (co-carcinogen)
* Diet, high fat consumption, natural antioxidants
* Geographic location, industrial areas, UV light
* Therapeutic drugs, some are known carcinogens
* Inherited conditions
Human carcinogens - Environmental
Aflatoxins Fungus, contaminated peanut and grain
* Asbestos
* Benzene
* Cadmium
* Coal tar
* Creosote
* DDT
* Polycyclic aromatic hydrocarbons
* Radon
* Solar radiation
Human carcinogens - Drugs/Therapeutic agents
- Adriamycin (doxorubicin)
- Androgenic steroids
- Chlorambucil
- Cisplatin
- Cyclophosphamide
- Cyclosporin A
- Diethylstilbestrol
- Ethylene oxide
- Melphalan
- Tamoxifen
How is chemical carcinogenicity determined?
Epidemiological studies determine the relationship
between a cancer suspect chemical and a human
population over a long period of time.
* Animal studies directly induce cancer in test animals
using a large sample of animals, usually of two or more
species with varying dose and time parameters.
* Experiments with animals are based on the premise that
chemicals that produce cancer in animals will have
similar effects on human cells. Most known human
carcinogens produce cancer in experimental animals.
The process of carcinogenesis
Initiation
(irreversible)
malignant
metastases
More
mutations
Progression
(irreversible)
Promotion
(reversible)
Initiation Stage
First stage of cancer process
* Rapid
* Irreversible
* First step: DNA modification by initiator (binding of carcinogen or its
functional group)
* Middle step: single DNA replication without previous or simultaneous DNA
repair
* Final step: formation of stable heritable changes in the cell DNA=
mutations
Promotion
Epigenetic event, change in gene expression without
change in DNA. Promotion is reversible.
* Mitogenic (Not mutagenic). Stimulates proliferation.
Causes both mutated and normal cells to proliferate.
* Enhances the effect of the genotoxic initiating agent by
establishing clones of initiated cells.
* Long delay possible between administration of initiating
agent and promoting agent.
* Examples: ROS and redox active xenobiotics and
metals, phorbol esters (e.g. TPA), polycyclic aromatic
compounds (e.g. Dioxin), peroxisome proliferators
(oxidized fats), endocrine disruptors (e.g. estradiol,
diethylstilbestrol)
Progresssion Stage
- Third (final) stage of the carcinogenesis process
- Conversion of the benign preneoplastic lesions into a neoplastic cancer
- Result of occurrence of genotoxic events inducing chromosomal
damage such as aberrations and translocations - Mediated by increase in DNA synthesis and cell proliferation in
the preneoplastic lesions (promotion stage)
Chromosome aberrations: breaks and structural changes.
Examples of progressor agents: inflammation, asbestos fibers,
benzene, benzoyl peroxide, other peroxides, oxidative stress,
inflammation
Chemical carcinogens
Genotoxic
* Possess the ability to damage DNA
* Can lead to mutations in critical genes resulting in
development of cancer
* Majority of known human carcinogens are genotoxic
Non Genotoxic (epigenetic)
* Don’t damage DNA
*Alter the balance between cellular growth and death
* Lots of non genotoxic chemicals positive in animal
studies
Carcinogens classification with regards to their
mode of action
- Primary (direct acting) carcinogens: no need for
metabolic activation to act as cancer causative agent - Secondary carcinogens (indirect acting): have
to be metabolically activated - Co-carcinogens: increase carcinogenic
potential by stimulating biotransformation of a
certain substance - Promotors: suppress apoptosis; stimulate proliferation
Direct Acting Carcinogens
Direct-acting agents require no metabolic conversion to
become carcinogenic. Have highly reactive eletrophile
groups that directly damage DNA, leading to mutations
and eventually cancer.
* Examples are: cancer chemotherapeutic drugs (e.g.
alkylating agents)
* Used as Rx of e.g. leukemia, lymphoma, Hodgkin
lymphoma, and ovarian carcinoma, non-neoplastic
disorders, such as rheumatoid arthritis or Wegener
granulomatosis.
* May evoke later a second form of cancer, usually
leukemia
Indirect-Acting Carcinogens
The designation indirect-acting agent refers to chemicals
that require metabolic activation and conversion to an
ultimate carcinogen before they become active.
* Indirect-acting agents are not active until converted to an
ultimate carcinogen by endogenous metabolic pathways
e.g. endogenous enzymes like cytochrome P450
oxygenase.
* Examples: Benzopyrene, polycyclic hydrocarbons,
aromatic amines and Azo dyes, Aflatoxin B1,
insecticides, fungicides, nitrites used as food
preservatives.
Induction of metabolizing enzymes
May be a reason for tissue-, and/or species-selectivity of
carcinogens
* Metabolites may be ligands for receptors
* Production of reactive oxygen species
Chemical carcinogens
The direct-acting carcinogens interact with
macromolecules through the covalent bond formation
between an electrophilic form of the carcinogen and the
nucleophilic sites in proteins (e.g. S, O, and N atoms in
cysteine, tyrosine, and histidine, respectively) and nucleic
acids (e.g. N and O atoms in purine or pyrimidine), such
as N-methyl-N-nitrosourea, a chemically-reactive alkylating
agent.
* Some agents can intercalate into the DNA double helix by
forming tight noncovalent bonds (e.g. daunorubicin).
* Most of carcinogens are indirectly-acting. Thus, metabolic
activation of certain carcinogenic agents is necessary to
produce the “ultimate carcinogen” that actually reacts with
crucial molecules in target cells
Metabolic activation of aflatoxin
Aflatoxin B1, a toxin from a mold (Aspergillus flavus oryzae) that grows on
grain and peanuts when they are stored under humid tropical conditions. It is
thought to be a contributory cause of liver cancer in the tropics
Metabolic activation of Tamoxifen
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Most Frequently Attacked Nucleophilic Centers in DNA
Adduct formation - N7 of adenine and guanine, C8 of guanine; polycyclic
aralkylated molecules on exocyclic N6 of adenine or N2 guanine → substitution
Alkylations – exocyclic O6 of guanine and O4 thymine
N1 adenine or N3 cytosine
Beside molecules that covalently bind to DNA
bases, mutations may be induced, and
carcinogenesis initiated, by agents that interact
with DNAby Van der Waals forces intercalating
between two base-pairs.
Example: doxorubicin, ethydium bromide
Beside molecules that covalently bind to DNA
bases, mutations may be induced, and
carcinogenesis initiated, by agents that interact
with DNAby Van der Waals forces intercalating
between two base-pairs.
Example: doxorubicin, ethydium bromide
- An insertion of the flat planar rings of a polycyclic
hydrocarbon between the stacked bases of doublehelical DNA may distort the helix, leading to a frameshift
mutation during DNA replication past the point of the
intercalation. - Alkylated bases in DNA can mispair with the wrong base
during DNA replication. For example, O6 methyguanine
pairs with thymine instead of cytosine during DNA
replication, leading to a base transition (i.e. GC→AT)
type of mutation during the next round of DNA
replication
Potential biological consequences of DNAadduct formation
Many of the base adducts formed by carcinogens involve
modifications of N-3 or N-7 positions on purines that
induce an instability in the glycosidic bond between the
purine base and deoxyribose. This destabilized structure
can then undergo cleavage by DNA glycosylase,
resulting in loss of the base.
* Interaction with some carcinogens has been shown to
favor a conformational transition of DNA from its usual
double-helical B form to a Z-DNA form. This could alter
the transcribability of certain genes, since B→Z
conformational transitions are thought to be involved in
regulating chromatin structure.
Testing for genotoxicity/carcinogenicity
Tiered approach
* In vitro: e.g. AMES test, mouse lymphoma assay
* Short term in vivo tests: e.g mouse micronucleus assay,
sister chromatid exchange (SCE) assay
* Long term (2 – 3 year) in vivo carcinogenicity assays
