TOXICOLOGY LEC
[PRELIMS] | S.Y 2023 – 2024
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GUIDE QUESTION
Knowledge of the toxicology of poisonous agents was
published earliest in?
a.
b.
c.
d.
e.
Ebers Papyrus
De Historia Plantarum
De Materia Medica
Lex Cornelia
Treatise on Poisons and their Antidotes
TOPIC OUTLINE
I. History of Toxicology
A. Antiquity
B. 20th Century
C. After World War
D. 21st Century
II. Principles of Toxicology
A. Introduction
B. Spectrum of Undesired Effects
C. Characteristics of Exposure
D. Dose-Response Relationship
E. Variation in Toxic Response
HISTORY OF TOXICOLOGY
ANTIQUITY
● Ebers Papyrus
○ One of the oldest known writings
○ Contains information pertaining to many
recognized poisons (hemlock, opium, lead,
copper, antimony)
■ Opium - Papaver somniferum
○ Discovered in Egypt
○ Contains discussion on snake bites
● Hippocrates
○ Added a number of poisons and clinical
toxicology principles to bioavailability in
therapy and overdosage
● Theophrastus
○ A student of Aristotle
○ De Historia Plantarum (collection of plant
sources)
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Dioscorides
○ Made the 1st attempt to classify poisons in
De Materia Medica
○ A physician traveling Rome
Paracelsus
○ Philippus Aureolus Theophrastus Bombastus
von Hohenheim
○ A physician-alchemist who formulated many
revolutionary views that remain integral
structure of toxicology, pharmacology, and
therapeutics
○ "All substances are poisons; there is
none that is not a poison. The right dose
differentiate a poison from a remedy”
○ Pioneered concepts:
1. Experimentation is essential in the
examination of responses to chemicals
2. One should make a distinction between
the therapeutic and toxic properties of
chemicals
3. These properties are sometimes but
not always indistinguishable except by
dose
4. Once can ascertain a certain degree of
specificity of chemicals and their
therapeutic or toxic effect
Magiendie, Orfila, & Bernard laid the groundwork for
pharmacology
experimental
therapeutics,
and
occupational toxicology
○ Orfila - Spanish physician, Forensic
toxicology
○ Magiendide - Physician and experimental
physiologist who studied the MOA of
emetine and strychnine
■ Cardiologist
■ Source of emetine - Ipecac
● Use: emetics
● Before, ipecac was widely
used as a treatment for
poisoning (not practiced
now because it can do
more harm)
■ Strychnine
blocks
glycine
receptors
● Now used as insecticide
but can cause muscle
spasm
○ Claude Bernard - An introduction to the
Study of Experimental Medicine
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20TH CENTURY TOXICOLOGY
● 1850’s
○ Advent of anesthetics and disinfectants
■ Disinfectant - inanimate object
■ Antiseptic - animate
■ Joseph Lister - Father of antiseptics
● Applied
phenol/carbolic
acid to the wound of the
mother during delivery
○ Prevalent use of “patent” medicines lead to
several incidents of poisonings
● 1890’s
○ Discovery of radioactivity and vitamins, and
start of Preclinical trials
● 1930’s
○ The National Institutes of Health (NIH) was
established as a response to the tragic
consequence of acute kidney injury after
taking sulfonamide in glycol solutions
■ Ethylene glycol is used as an
anti-freeze
■ Metabolized
by
alcohol
dehydrogenase to glycolaldehyde.
Glycolaldehyde is metabolized by
aldehyde dehydrogenase to glycolic
acid and can cause acidosis to the
patient . Glycolic acid can also
become oxalic acid which can
precipitate inside the kidney leading
to kidney injury
AFTER WORLD WAR 2
● 1950’s
○ Commitment of US FDA to toxicology (Ban
any products found to be carcinogenic in lab
animals or humans)
● 1960’s
○ Thalidomide → Phocomelia ; attempts to
understand the effects of chemicals on the
embryo and fetus
■ Thalidomide was marketed as
treatment for morning sickness in
Europe and it caused phocomelia.
C deformity of hands
& feet
■ Thalidomide
→
Pregnancy
Category X; currently used for
resistant leprosy
21ST CENTURY TOXICOLOGY
● Sequencing of the human genome and other
organisms has markedly affected all biological
sciences
● Deeper understanding of epigenetics has provided
novel approaches in studying the fetal origin of adult
diseases (cancer, diabetes, and neurodegenerative
diseases).
PRINCIPLES OF TOXICOLOGY
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INTRODUCTION
TOXICOLOGY - Study of adverse effect of chemicals
on living organisms
Toxic substances can be classified based on its:
○ Origin (Toxins vs. Toxicant)
■ TOXINS - Naturally occurring
harmful chemicals
■ TOXICANTS - Toxic substances
that are produced or are by-product
of
human-made
activities
(synthetic)
■ XENOBIOTICS
Foreign
substances that include: a variety of
synthetic chemicals (natural or
synthetic)
○ Physical state (gas, dust, liquid)
○ Chemical stability (explosive, flammable,
corrosive)
○ General chemical structure (aromatic amine,
halogenated hydrocarbon)
○ Ability to cause significant toxicity (extremely
toxic, very toxic, slightly toxic)
○ Mode
of
action
(alkylating
agents,
cholinesterase inhibitors)
TOXICOLOGIST - an individual trained to examine
and communicate the nature of a toxicant's properties
and identify approaches to prevent or mitigate harm
done to human, animal, and environmental health
MECHANISTIC TOXICOLOGIST - identifies the
cellular, biochemical, and molecular mechanisms by
which chemicals exert toxic effects on living
organisms.
○ Study the mechanism of action
HAZARD
ASSESSMENT
TOXICOLOGIST
conducts toxicity testing that provides comprehensive
information for the evaluation of a chemical's safety
and to meet important regulatory requirements
○ Assess/test the possible hazard
REGULATORY
TOXICOLOGIST
has
the
responsibility for deciding, on the basis of data
provided by descriptive and mechanistic toxicologists,
whether a drug or other chemical poses a sufficiently
low risk (or, in the case of drugs, a favorable
risk/benefit profile) to be marketed for a stated
purpose.
○ Establish standards and policies (like FDA)
COMPUTATIONAL TOXICOLOGIST - develop and
implement computer-based models to predict adverse
health effects resulting from the interaction of
chemicals with biological organisms
OCCUPATIONAL TOXICOLOGIST - responsible for
conducting research and making recommendations
for the prevention of work-related injury and illness
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Company toxicologists
Make standards and policies appropriate for
work
FORENSIC TOXICOLOGY - covers the medicolegal
aspects of the deleterious effects of chemicals on
animals and humans
○ The expertise of forensic Toxicologists is
used to aid in establishing the cause of
death and determining its circumstances in a
post mortem investigation
CLINICAL TOXICOLOGY - physicians who receive
specialized training in emergency medicine and
poison management
○ Do first aid
ENVIRONMENTAL TOXICOLOGY - Focuses on the
impact of chemical pollutants in the environment on
biological organisms.
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CHECKPOINT 1
● What is the difference between side effects and
adverse effects?
○ Side effect - can be positive or side; not
that harmful; cannot cause death
○ Adverse effect - deleterious; can cause
death
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SPECTRUM OF UNDESIRED EFFECTS
ALLERGIC REACTIONS - an adverse reaction of the
immune system to a chemical in response to a
previous exposure to that chemical or to a structurally
similar one.
○ Genetic predisposition - if both parents are
allergic to a specific substance, the baby is
also allergic to that substance
○ Sensitization - patient must have a prior
exposure to the substance
○ Once sensitization has occurred, allergic
reactions may result from exposure to
relatively very low doses of chemicals
IDIOSYNCRATIC REACTIONS - abnormal reactivity
of an individual to a chemical based on its genetics or
other individual sensitivity factors.
IMMEDIATE VS. DELAYED TOXICITY
IMMEDIATE - occurs rapidly after a single
administration
DELAYED - occurs after a lapse of some time
(months or years)
REVERSIBLE VS. IRREVERSIBLE TOXICITY
Depends of the ability of the injured tissue to adapt,
repair, and regenerate
● LIVER & GIT - reversible
● CNS & Heart - irreversible
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Labile - unlimited regenerative capacity (skin)
Quiescent - limited regenerative capacity (liver)
Permanent - no regenerative capacity (brain)
LOCAL VS. SYSTEMIC TOXICITY
LOCAL - site of first contact between the toxicant and
biologic substance
SYSTEMIC - absorption and distribution of toxicant
from its entry point to a distant site at which
deleterious effects are produced
CHEMICAL INTERACTIONS
ADDITIVE - Combined response of 2 chemicals are
equal to the sum of responses to each chemical given
alone (e.g., 2 + 3 = 5)
SYNERGISTIC - Combined response of 2 chemicals
are much greater than the sum of the responses to
each chemical when given alone (e.g., 2 + 2 = 20)
POTENTIATION - None substance does not produce
any toxicity on a particular tissue or system but when
added to another chemical makes that chemical much
more toxic
○ Drug B is non toxic but once you
co-administer it with Drug A, it becomes toxic
ANTAGONISM - occurs when two chemicals
administered together interfere with each other's
actions or one interferes with the action of the other
ANTAGONISM
RECEPTOR ANTAGONISM - occurs when two
chemicals that bind to the same receptor produce less
of an effect when given together relative to the
addition of their separate effects. Often termed as
blockers
○ Pharmacologic Antagonism
CHEMICAL ANTAGONISM - direct chemical reaction
between two compounds that produces a less toxic
product
○ Antagonism will directly bind with the
poison/toxin
○ Heparin
and
protamine
sulfate
→
neutralization reaction
DISPOSITIONAL ANTAGONISM - occurs when the
absorption,
distribution,
biotransformation,
or
excretion of a chemical is altered so that the
concentration and/or duration of the chemical at the
target organ is reduced
○ Inhibitors
FUNCTIONAL ANTAGONISM - occurs when two
chemicals counterbalance each other by producing
opposing effects on the same physiological function,
often through different signaling pathways
○ Physiological Antagonism
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TOLERANCE
TOLERANCE - Repeated exposure to a chemical can
reduce its pharmacologic and/or toxicologic actions
○ Chronic exposure to a toxicant or drug will
lead
to
down-regulation (reduce in
pharmacologic or toxicologic effect)
CROSS-TOLERANCE - Occurs when structurally
related chemicals cause diminished responses.
○ If you are tolerant to lorazepam, you are also
tolerant to clonazepam since they have the
same structure
DISPOSITIONAL TOLERANCE - Occurs when the
amount of chemical reaching the site of action
decreases over time, leading to the reduced
responsiveness of the tissue to stimulation
CHEMICAL/CELLULAR TOLERANCE - May result
from a lower availability of receptors and/or mediators
CHECKPOINT 2
● What is the difference between tolerance and
tachyphylaxis?
○ Both are a result of chronic exposure
○ Tolerance → down-regulation happens
slowly
○ Tachyphylaxis
→
down-regulation
happens rapidly
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Why do most injuries in the liver due to toxicants
are reversible? Recall a drug that can cause
damage to the brain/CNS.
○ Because they have limited regenerative
capacity
○ Example is Paracetamol
■ Toxic metabolite of paracetamol
is
NAPQI
(N-acetyl-parabenzoquinone imine)
■ Chemical name: N-acetyl-paraaminophenol
■ Toxic dose of paracetamol: more
than 4 grams
■ Treatment
of
choice
is
n-acetylcysteine
Why do most injuries in the brain due to toxicants
are irreversible?
○ Because they have no regenerative
capacity
CHARACTERISTICS OF EXPOSURE
Toxicity to a biological system requires that sufficient
concentration: of the "active" form of a chemical
accumulate at the site of action for a requisite period
of time
To characterize fully the potential hazard of a specific
chemical, one needs to know not only the type of
effect it produces, and the dose required to produce
that effect, but also the information about the
chemical, route of exposure, and disposition.
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ROUTE AND SITE OF EXPOSURE
The vehicle or the inert material in which the toxicant
is dissolved, and other formulation ingredients can
markedly alter chemical absorption after ingestion,
inhalation or topical exposure
In addition, the route of administration can influence
the toxicity of chemicals
An
approximate
descending
order
of
effectiveness for the route of exposure would be:
○ Intravenous > Inhalation > Intraperitoneal >
Subcutaneous > Intramuscular > Intradermal
> Oral > and Dermal
■ Intraperitoneal
→
within
or
administered through the thin,
transparent membrane that lines
the walls of the abdominal
DURATION AND FREQUENCY OF EXPOSURE
ACUTE
EXPOSURE
- <24 hours; Single
Administration
● SUBACUTE EXPOSURE - 1 month or less; repeated
exposure
● SUBCHRONIC EXPOSURE - 1 to 3 months;
repeated exposure
● CHRONIC EXPOSURE - >3 months; repeated
exposure
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Why is it important to determine the type of exposure?
● So that we can do proper treatment
● Acute exposure to benzene will cause CNS
depression but if you are exposed chronically, it can
cause leukemia
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DOSE-RESPONSE RELATIONSHIPS
GRADED-DOSE RESPONSE RELATIONSHIP Response of an individual organism to increasing
doses of a chemical (individual-dose response)
QUANTAL-DOSE RESPONSE RELATIONSHIP Characterizes the distribution of individual responses
to different doses in a population of organisms
GRADED-DOSE RESPONSE RELATIONSHIP
Response of a particular receptor-effector system
against increasing concentration of drug
● Relationship of drug concentration and effect
● Efficacy and potency parameters can be derived
● EFFICACY
○ Maximal effect a drug can produce
○ Represented by Emax
● POTENCY
○ Amount of drug required to produce a
specific effect (50%)
○ Represented by EC50 (dose that causes
50% of maximal effect) in graded
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dose-response and ED50 in quantal
dose-response curve
The lower the EC50, the more potent the
drug is
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Drug A is more effective than Drug B since it has a
higher Efficacy than drug B. Both drugs have different
potency as well.
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The 2 drugs both have different potency but they have
the same ceiling dose (same efficacy). Drug A is more
potent than drug B which means that drug A can
produce the effect with a lower dose than drug B.
QUANTAL-DOSE RESPONSE RELATIONSHIP
Minimum concentration needed to produce a specific
response in each member of a population
● Median effective dose (ED50), Median toxic dose
(TD50), and median lethal dose (LD50) can be
derived
○ The lower the LD50, the more lethal the
poison is
● Maximal effect of drug is not determined (vs. Graded
dose-response)
● THERAPEUTIC INDEX
○ Ratio of TD50 and ED50
● THERAPEUTIC WINDOW
○ Range between TD50 and ED50
● NARROW THERAPEUTIC INDEX DRUGS (WALA
Cyang PaPa VasTeD pa!)
○ Warfarin
○ Aminoglycosides
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Lithium
Amphotericin B
Carbamazepine
Phenytoin
Phenobarbital
Vancomycin
Theophylline
Digoxin
VARIATION IN TOXIC RESPONSES
Selective Toxicity
Modifying Factors
○ Genetics (Genetic Polymorphism)
○ Age
○ Sex
○ Circadian Rhythm
○ Microbiome
ANIMAL TESTING
ACUTE TOXICITY TESTING
○ Typically performed in rodents
○ These studies aim to approximate the LD of
a chemical within 14 days after a single
exposure
○ Approaches to determine the doses and
dosing includes: fixed dose (OECD 420),
stepwise method (OECD 423), up-and-down
procedure (OECD 425)
SUBACUTE TOXICITY TESTING
○ Determine toxicity of a chemical after
repeated administration
○ 10
animals/sex/dose
(rodents).
3-4
animals/sex (dog)
○ 14 or 28 days
DEVELOPMENTAL & REPRODUCTIVE TOXICITY
○ DEVELOPMENTAL TOXICOLOGY - study
of adverse effects on the developing
organism occurring any time during the life
span of the organism that may result from
exposure to chemical or physical agents
before conception (either parent), during
prenatal development, or postnatally until the
time of puberty
○ REPRODUCTIVE TOXICOLOGY - study of
the occurrence of adverse effects on the
male or female reproductive system that may
result from exposure to chemical or physical
agents
○ TERATOLOGY - the study of defects
induced during development between
conception and birth
SUBCHRONIC TOXICITY TESTING
○ Determines the no observable adverse effect
level (NOAEL), lowest observable adverse
effect level (LOAEL) organ involvement
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Conducted in 2 species (rat & dog for FDA,
& rat or mouse for EPA)
○ 90 days (most common test duration)
CHRONIC TOXICITY TESTING
○ Performed similarly to subchronic studies
except the period of exposure is longer than
3 months
■ 6 months - 2 years in rodents
■ 1 year (or longer) in nonrodent
species
NEUROTOXICITY ASSESSMENT
IMMUNOTOXICITY ASSESSMENT
EYE AND SKIN IRRITATION AND CORROSION
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(Memo since it will be included in the exam↓)
FDA DRUG RISK CLASSIFICATION
CATEGORY
Warfarin,
Statins
Methotrexate,
ADR
TYPE OF
EFFECT
CHARACTERISTICS
EXAMPLE
A
Augmented
Dose-dependent
(Predictable)
Hypoglycemia
from insulin
B
Bizarre
Dose-independent
(Unpredictable)
Anaphylaxis
Aspirin-induced
asthma
C
Chronic
Use
Prolong treatment
(Long term exposure)
D
Delayed
After years of
treatment
Carcinogenesis
E
End of Use
Occurs after
withdrawal
Cushing
syndrome after
abrupt
discontinuation
of
corticosteroids
F
Failure of
Therapy
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Augmented - as you increase the dose, the effect also
increases
DESCRIPTION
A
Controlled studies in humans show no risk
to the fetus
B
No controlled studies have been conducted
in humans; animal studies show no risk to
the fetus
C
No controlled studies have been conducted
in animals or humans
D
Evidence of human risk to the fetus exists;
however, benefits may outweigh risks in
certain situations
X
X
(Absolute Contraindication)
Pregnancy
after using oral
contraceptive
pills
Controlled studies in both animals and
humans demonstrate fetal abnormalities;
the risk in pregnant women outweighs any
possible benefit
CATEGORY
A
(Safe)
EXAMPLE
Levothyroxine, Folic Acid
Levo
fo
B
(Prescribed only as
needed for maternal
health)
Metformin,
Hydrochlorothiazide
C
(Prescribed only in
pregnancy if benefit clearly
outweigh the risks)
Tramadol, Gabapentin
D
(Not recommended in
pregnancy; prescribed if
absolutely necessary)
Alprazolam, Clonazepam
Met Hy
Tra Gab
AlClo
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4.
Inappropriate repair and adaptation
*toxicity is usually noteworthy at steps 3 and 4
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GUIDE QUESTION
Toxication
(or
metabolic
activation)
is
the
biotransformation of a toxicant to a more toxic and
reactive species. Which of the following is not a reactive
chemical species commonly formed by toxication?
a.
b.
c.
d.
e.
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Electrophiles
Nucleophiles
Superoxide anions
Hydroxy radicals
Hydrophilic organic acids
DELIVERY TO THE TARGET
Before it reaches the target molecule, the toxicant is
absorbed, distributed, eliminated, and activated
Exposure at the target site is an absolute requirement
for any mechanism of toxicity
Xenobiotic disposition
○ Defined as the absorption, distribution,
biotransformation, and elimination of a
toxicant
○ Critical determinant of target organ toxicity
along with sensitivity of target sites to toxicity
INTRODUCTION
Mechanism of toxicity describe how an adverse effect
occurs
Knowledge of mechanisms of toxicity is essential for
developing risk assessments for chemical exposure,
as such data are relevant to determining the likelihood
that chemical exposure may cause harmful effects
Mechanistic toxicology data are also useful for
developing more predictive biomarkers of toxicity,
developing approaches to antagonize or prevent
toxicity, and gaining insight into fundamental
physiologic, biochemical, and molecular processes
that underlie normal and abnormal organ function
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Shows us what will happen if we are exposed to a
toxicant
4 critical events
1. Delivery of the toxicant to the target organ
2. The interaction or the reaction of the toxicant with the
target molecule
3. Cellular dysfunction and toxicity
Body has a defense mechanism to protect us from the
toxicant:
○ Presystemic elimination
○ Distribution away from target
○ Excretion
○ Detoxification
ABSORPTION & PRESYSTEMIC ELIMINATION
● Absorption is defined as the process by which
toxicants cross body membranes to enter the
systemic circulation
● Absorption occurs along the entire length of the
gastrointestinal tract and is usually determined by
the extent to which the compound exists in its
nonionized, lipid-soluble form
● In the skin, the stratum corneum layer of the
epidermis is a major barrier to absorption, but once
through this barrier, compounds are typically
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absorbed by diffusion into the venous or lymphatic
capillaries to enter the systemic circulation
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Presystemic elimination is a process by which a
toxicant is eliminated prior to reaching the systemic
circulation
○ Presystemic elimination in the git is also
known as first-pass effect
The gastrointestinal tract epithelium may eliminate or
modify a compound directly
Many toxicants absorbed from the gastrointestinal
tract pass via the portal circulation directly into the
liver where they are modified by biotransformation
enzymes and excreted into bile
DISTRIBUTION OF TOXICANTS OCCURS BY PASSIVE OR
ACTIVE PROCESSES
● Tissue distribution is the process by which a toxicant
reaches its target site
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Affected by 4 factors:
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Porosity of the Capillary Endothelium
■ Liver sinusoid VS. BBB → liver
highly fenestrated than BBB
■ Or fenestration of the capillary
endothelium
● Butas butas sa capillary
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Presence
of
Specialized Transport
Processes
Potential for Accumulation within Cellular
Organelles
Binding
to
proteins
or
Other
Macromolecules
■ Acidic drugs/Toxicants → Albumin
■ Basic
drugs/Toxicants
→
alpha-I-acid
glycoprotein
(orosomucoid)
■ Only free drugs or toxicants can
exert their action. If a drug is bound
to a macromolecule, it cannot bind
to its receptor
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ELIMINATION PROCESS AFFECT DELIVERY OF
TOXICANTS
REABSORPTION AND EXCRETION
● Enterohepatic circulation → cycle in which a
compound is excreted into bile to enter the intestine
where it is modified to facilitate intestinal reabsorption
rather than fecal elimination
○ Mostly, drugs and toxicants that underwent
glucuronidation will favor enterohepatic
circulation because they are mainly
eliminated via the bile
○ Once the glucuronide reaches the intestine,
where your gut bacteria is located,
glucuronic acid is hydrolyzed with the action
of beta glucuronidase facilitating the
reabsorption of the toxicant or drug
● In the kidney, reabsorption mechanisms are highly
pH-dependent, as the generally acidic milieu of urine
favors
reabsorption
of
weak
acids.
(Henderson-Hasselbalch Equation)
○ Acidic drugs are absorbable or reabsorbable
in an acidic medium
■ If you want to excrete → alkalinize
the urine
■ Ex. Aspirin - to facilitate its
accretion, give sodium bicarbonate,
a urinary alkalinizer.
○ Basic drugs are absorbable or reabsorbable
in an basic medium
■ To excrete - acidify the urine. You
cam give ammonium chloride, a
urinary acidifier
● The route and rate of excretion largely depend on the
physicochemical properties of the toxicant. The major
excretory organs for non-volatile compounds are the
liver and kidneys, which are most efficient at
removing water soluble, usually ionized compounds
○ Toxicant, in order to be excreted, should be
hydrophilic, charged, & ionized
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The excretion of non-volatile highly lipophilic
compounds is much less efficient than that of water
soluble compounds. Biotransformation may facilitate
elimination of these compounds, but other than
processes of accretion of highly lipophilic compounds
include accretion in the bile secretion directly into the
intestinal contents, and excretion in milk lipids in
mammary glands
Volatile lipophilic compounds diffuse through
pulmonary capillaries to be excreted in expired air
Drugs or toxicants will undergo phase 1 and phase 2
metabolism in the liver.
○ Phase 1 aka functionalization because you
have to expose the functional group of the
drug or toxicant (reduction, oxidation,
hydrolysis)
○ Phase 2 aka conjugation reaction because
you have to add conjugates (sulfate group,
methyl group, glucuronic acid)
The reaction that is important in enterohepatic
circulation is glucuronidation
○ Most drugs or toxicants that will undergo this
type of reaction will be excreted t the bile
○ Once the bile reaches the large intestine,
where the bacteria are mostly located,
glucuronides will be hydrolyzed using an
enzyme called beta glucuronidase
■ Present in our gut bacteria
○ Once the glucuronic acid is removed from
the toxicant or the drug, it is reabsorbed in
the hepatic portal vein papuntang systemic
circulation
TOXICATION AND DETOXIFICATION
● Although some xenobiotics are directly toxic, many
others produce toxicity through metabolites formed
following exposure. Biotransformation of xenobiotics
that increases toxicity is referred to as metabolic
activation or a toxification process
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Toxication process forms electrophiles, free radicals
(superoxide, peroxidase, hydroxyl radicals), and
Nucleophiles
Detoxification: to be eliminated from the body more
efficiently - must be hydrophilic and ionized
○ Electrophiles - conjugation with glutathione
(a thiol nucleophile)
○ Free radicals - starts with superoxide
dismutase (SOD), followed by catalase,
glutathione peroxidase, or peroxidation
○ Nucleophiles - conjugation
○ No functional group - Phase I & Phase 2
REACTION OF THE ULTIMATE TOXICANT WITH TARGET
MOLECULE
● Toxicity is initiated by a reaction between the ultimate
toxicant and its target molecule, with subsequent
events leading to dysfunction or injury
● Determinants of interaction of ultimate toxicant with
the target molecule:
○ ATTRIBUTES OF TARGET MOLECULES
■ Ability to react with the target &
adversely affect its function
■ Ability to reach an effective
concentration at target site
■ Alter the target in way that is
related to toxicity
○ TYPES OF REACTION
■ Noncovalent binding - between
nonpolar
interactions
or
the
formation of H bond and ionic
bonds; reversible
■ Covalent binding - irreversible and
permanent
(organophosphates
action on Acetylcholinesterase)
■ Electron transfer - some chemicals
can oxidize your ferrous iron to
ferric (nitrites)
■ Enzymatic reaction - botulinum
toxin can hydrolyze the fission
proteins of Acetylcholine preventing
the release of acetylcholine
○ EFFECTS OF TOXICANT ON THE TARGET
MOLECULES
■ Dysfunction of target molecules
(by binding of the toxicant to the
receptor, mimicking the effect of
indigenous ligands)
■ Destruction of target molecules
(especially
DNA
to
certain
substances that can damage DNA)
■ Neoantigen formation (urushiol
and penicillins are haptens; by
themselves, they cannot produce
allergic reaction, but once they
combine with a protein in the body,
9 of 15
they become allergenic. They can
induce allergic reaction)
CELLULAR DYSFUNCTION & RESULTANT TOXICITIES
● Toxicity will emerge here
-
Atropine is an anticholinergic. Its effect is mostly
sympathetic.
-
For atropine toxicity:
Atropine toxicity can be treated by cholinergic
substances like physostigmine
Mode of action of BZD - increase frequency of GABA
A receptor opening (Barbiturates increase the
duration of GABA A receptors)
When GABA A receptor is opened, it will
lead to influx of chloride channel leading to
hyperpolarization of the cell leading to
sedation
TOXICANTS THAT WILL ALTER CELLULAR REGULATION
● Cellular Dysregulation
○ Dysregulation of Gene Expression
-
-
-
There are several ligands, agonists, or hormones that
can affect your gene expression. One of them is
estrogen.
Once estrogen binds to nuclear receptor, nuclear
receptor will go inside the nucleus and will dimerize
leading to transcription of different proteins
There are several toxicants that can inhibit
this function such as bisphenol A,
nonylphenol,
phthalate,
DDT
(Dichlorodiphenyltrichloroethane - used as
insecticide but it was withdrawn because of
its toxicity)
Binding if endocrine disrupting chemicals following to
the estrogen receptor result to estrogenic effect
-
●
Dysregulation of Ongoing Cellular Function
○ Toxicant-Neurotransmitter Interaction
Another example of neurotransmitter toxicant
interaction
is
the
interaction
between
organophosphate and acetylcholinesterase.
Acetylcholinesterase
is
an
enzyme
responsible
for
the
breakdown
of
acetylcholine. If this is inhibited, Ach will
remain in the synapse, leading to excessive
parasympathetic activity. (SE: DUMBBELS)
Symptoms of organophosphate poisoning
can be treated by atropine. We can give
pralidoxime if aging has not yet happened.
10 of 15
-
Organophosphate, at a given time, will
irreversibly inhibit acetylcholinesterase.
Another example is cocaine, tricyclic antidepressants,
and amphetamine.
The main elimination of norepinephrine is via
reuptake to your presynaptic vesicle, that’s in
comparison to your acetylcholine (via
degradation
secondary
to
acetylcholinesterase)
secondary
to
norepinephrine transporter. This is inhibited
by cocaine, tricyclic antidepressants, and
amphetamine. If inhibited, norepinephrine
will remain at the synapse, leading to
excessive sympathetic activity.
○
-
Toxicant-Transducer Interactions
DDT, pyrethroids, ciguatoxin (toxin from fish) act on
voltage-gated Na+ channels. They facilitate the
opening of the channel leading to neuronal activation
and overexcitation and convulsion.
CELLULAR DYSFUNCTION IN THE CELL MAINTENANCE
Impaired Cellular Maintenance
● Impaired Internal Maintenance
○ Impaired ATP synthesis
■
■
REVIEW/SEARCH for the other name of the complexes
● COMPLEX
1
=
NADH
oxidoreductase/dehydrogenase
● COMPLEX
2
=
Succinate-CoQ
reductase/dehydrogenase
● COMPLEX 3 = Q-cytochrome c oxidoreductase
● COMPLEX 4 = cytochrome c oxidase
● COMPLEX 5 = ATP SYNTHASE
●
There are several toxicants that can inhibit your
electron
transport
chain.
An
example
is
2,4-Dinitrophenol and Aspirin overdose. They are
called uncouplers. It increases permeability of cells
facilitating the release of hydrogen ions needed for
ATP synthesis.
Complex I = (-) by Rotenone
Complex III = (-) by Anti(three)mycin
Complex IV = (-) by Azide, Cyanide (CN-),
Carbon Monoxide (CO) (4 letters)
Complex V = (-) by Oligomycin (5 letters)
○
Impaired Membrane Function
■ Ethanol
and organic solvent
increase membrane fluidity
Impaired External Maintenance
○ Toxicities Interfering with Cells Specialized to
Provide Support to Other Cells, Tissues or
Whole Organism
■ Inhibition of hepatic synthesis of
coagulation factors by coumarin
and warfarin
DYSREPAIR AND ADAPTATION
MECHANISMS OF REPAIR
● MOLECULAR
○ Some chemical alteration (via oxidation) and
are simply reversed (through reduction).
○ Other toxicants trigger misfolding of proteins.
Proteins (enzymes) are typically folded into a
globular form. Chaperone proteins (HSP90)
prevents unfolding secondary to toxicants
●
-
Lipid solvents destroy plasma
membrane
Hydrocarbons destroy lysosomal
membranes
●
CELLULAR
○ Autophagic removal of damaged cell
organelles (lysosomal process) - lysosomes
are aka cell-eating organelles (autophagy self eating)
TISSUE
○ Depends on the ability of the injured tissue to
repair, and regenerate
MECHANISMS OF ADAPTATION
● Adaptation is a biological process by which an
organism develops increased tolerance to the harm
itself. It involves responses acting to preserve or
regain the biological homeostasis in the face of
increased harm
● Theoretically, adaptation to toxicity may result from
biological changes causing:
○ (1) Diminished delivery of the causative
chemical(s) to the target,
○ (2) Decreased size or susceptibility of the
target,
11 of 15
○
○
(3) Increased capacity of the organism to
repair itself, and
(4) Mechanisms to compensate for the
toxicant inflicted dysfunction
TOXICITY RESULTING FROM INAPPROPRIATE REPAIR
AND ADAPTATION
● NECROSIS
○ Cell death ensues when repair mechanisms
are inefficient or the molecular damage is not
readily reversible.
○ Sometimes progression to tissue necrosis
can be interrupted by apoptosis and cell
proliferation.
● FIBROSIS
○ Fibrosis is characterized by excessive
formation and deposition of connective
tissue with abnormal composition of the
ECM (extracellular membrane)
○ In general, fibrotic conditions can develop in
any organ, but liver, lung, kidney and heart
are noteworthy
○ fibrosis - usually in permanent cell or
quiescent cells
■ due to no or limited regen cap.
●
-
Necrosis = pathologic; accompanied by an
inflammation (pus); dirty
Apoptosis = no inflammation and often physiologic;
programmed cell death; no adjacent inflammation;
step by step process
CARCINOGENESIS
Manifestations of Carcinogenesis (lead to tumor formation)
● Failure of DNA repair
● Failure of apoptosis
● Failure to terminate cell proliferation
12 of 15
●
●
GUIDE QUESTION
Which of the following statements about weak base
overdose is most correct?
a.
b.
c.
d.
Urinary excretion would be accelerated by
administration of NH4Cl, an acidifying agent
Urinary excretion would be accelerated by giving
NaHCO3, an alkalinizing agent
Less of the drug would be ionized at blood pH
than at stomach pH
Absorption of the drug would be slower from the
stomach than from the small intestine
INTRODUCTION
DISPOSITION
● Absorption,
distribution,
biotransformation,
&
elimination of chemical or xenobiotics
● Determines the toxicants concentration at the site of
action
TOXICOKINETICS
● Quantitative characterization of xenobiotic disposition
● Mathematical representation of disposition
○ Example: volume of distribution and half life
● Toxicants usually pass through cells
● Basic unit of cell membrane is a lipid bilayer
composed primarily by phospholipids, glycolipids, &
cholesterol
○ Polar heads – oriented towards outer and
inner surface
○ Hydrophobic tails – oriented inward and face
each other
○ Numerous
proteins are inserted or
embedded in the plasma membrane:
■ Integral protein- Transverse the
whole bilayer. If it is removed, it will
disrupt the plasma membrane.
Hence, it is an integral part of the
plasma membrane.
■ Peripheral action – It can be in the
inner or outer surface only of the
membrane. If it is removed, it will
not disrupt the plasma membrane.
● Most of the toxicants, for them to exert effect, they
must pass several barriers like the plasma membrane
(composed of two →bilayer)
PERMEATION
Movement of drug/toxicant molecules into and within
the biologic environment
Before a drug can be absorbed, it must permeate
DIFFUSION
● Movement of solutes from higher to lower
concentration (downhill)
● Do not use transport proteins and energy (passive)
● Governed by Fick's law
○ Aqueous Diffusion
■ Across epithelial membrane tight
junction and endothelial lining of
blood vessel
○ Lipid Diffusion
■ Passive movement across the lipid
bilayer
TRANSPORT BY SPECIAL CARRIERS
○ NOT governed by Fick's Law
○ Facilitated Diffusion – Downhill, Passive →
(SLC)
○ Active Transport – Uphill, Active → (ABC)
ENDOCYTOSIS
● Type of movement of molecule if the molecule is very
large
● Once a large molecule is in contact with plasma
membrane, it will bind to a receptor which will trigger
the engulfing of the plasma membrane = it will eat the
molecule
● Other name is “cell eating”
Diagram representing the different movement of molecules
Simple diffusion and facilitated diffusion is the
movement of molecules from high to low
concentration = it will follow concentration gradient
and will not use any energy
Active transport is against the concentration gradient
(from high to low concentration) and will use energy
-
Saturability
13 of 25
-
-
When all of the transport carriers or proteins
are occupied, they cannot transfer any solute
anymore.
Active Transport
It’s against the concentration gradient
It uses transport protein and energy
It is saturable
Table 1. Human ABC Transporters: Gene Family Overview and
Major Transporters Involved in Xenobiotic Disposition
ABC
Subfamily
Genes in
Family
Gene Symbols
A
12
ABCA1-10, 12, 13
B
11
ABCB1-11
C
13
ABCC1-13*
D
4
ABCD1-4
E
1
ABCE1
F
3
ABCF1-3
G
5
ABCG1, 2, 4, 5, 8
FICK’S LAW OF DIFFUSION
● Predicts the rate of movement of molecules across a
barrier
● Concentration and Surface area are directly
proportional to rate of movement
○ ↑ Concentration and Surface area = ↑rate of
movement across a membrane
● Thickness is inversely proportional to rate of
movement
○ ↑Thickness = ↓ rate of movement across a
membrane
Question: Compare which has the higher solute movement:
50 mg Drug A or 500 mg Drug B.
Answer: 500 mg Drug B
Explanation: Since according to Fick’s Law, the higher the
concentration, the higher the solute movement.
HENDERSON-HASSELBALCH EQUATION
14 of 25
●
●
●
●
Electrostatic charge of an ionized molecule attracts
water dipoles and results in the polar, relatively
water-soluble and lipid-soluble complex.
○ Ionized (charged) → Polar molecules → lipid
insoluble (Lipophilic, Hydrophilic)
○ Non-ionized (uncharged) → lipid soluble
(Lipophilic, Hydrophobic) → absorbable
(LUNA: Lipophilic, Uncharged, Non-ionized,
non-polar = Absorbable)
Most toxicants and drugs exist as weak bases or
weak acids → pH of the medium determines the
fraction
of
molecules
charged
(ionized/nonabsorbable)
of
uncharged
(non-ionized/absorbable)
Henderson-Hasselbach equation predicts the fraction
of molecules in an ionized state.
WEAK ACIDS (WA)
○ Neutral molecules that can dissociate into
anion and proton (H+)
○ PROTONATED
waka
acids
→
uncharged/non-ionized → lipophilic →
absorbable (LUNA)
■ pH = pKa + log [A-/HA]
○ ABSORPTION
■ Weak acids are readily absorbable
in acidic medium
○ ION TRAPPING
■ Weak acids are readily excreted in
basic medium
○
○
○
○
Pyrimethamine
● When it combines with protons it forms a cation.
● The more absorbable form is Deprotonated form
(no protons) = uncharged.
Question
● A patient will experience Aspirin poisoning. What
will you give to facilitate excretion of the aspirin?
● Answer: Sodium bicarbonate
○ Weak bases are readily absorbable in
basic medium.
○ It is absorbable in the small intestine
compared to stomach
○ Readily excreted in acidic medium
●
●
●
●
●
●
Aspirin is a weak acid. It can exist as an anion plus
a proton.
In a medium where protons predominate, weak
acids exist as neutral or non-ionized
Example of medium in the body with many protons:
○ Stomach – because HCl with dissociate
into Cl- and H+
Mnemonics:
○ AAA
■ Acidic drugs are
■ Absorbable in
■ Acidic medium
○ AEB
■ Acidic drugs are more
■ Excretable in
■ Basic medium
○ BAB
■ Basic drugs are
■ Absorbable in
■ Basic medium
○ BEA
■ Basic drugs are more
■ Excretable in
■ Acidic medium
Neutral molecules that can form a cation by
combining with a proton (H+)
DEPROTONATED
weak
bases
uncharged/non
ionized
→
Lipophilic
absorbable (LUNA)
■ pH = pKa + log [/HB+]
ABSORPTION
■ Weak bases are readily absorbable
in basic medium
ION TRAPPING
■ Weak bases are readily excreted in
acidic medium
■ Mostly exist as ionized form
●
●
●
ABSORPTION
Movement of toxicants from the site of administration
to the systemic circulation.
Main sites: GIT, Lungs, & Skin
○ Toxicants that enter through the GIT must
pass through the liver.
○ Some of these toxicants will be metabolized
before they reach the systemic circulation
which is called as “first pass effect” or also
known as “presystemic elimination”
GASTROINTESTINAL (GI) TRACT
GIT may be viewed as a tube transversing the body.
Although it is within the body, its contents remain
outside the body. Absorption of toxicants can take
place along the entire GIT
○ Organic acid or base- absorb by simple
diffusion where it exists in its most lipid
soluble form
○ In unionized and ionized form
Numerous transporters are expressed in the GIT
○ Particles and particulate matter – particle
size is the main determinant (rather than
LUNA)
○ Compared to organic acid and base their
absorption is more dependent on their sites.
WEAK BASES (WB)
15 of 25
●
●
Chemicals
can
alter
absorption
Antidiarrheal)
First-pass effect/Presystemic elimination
(Laxatives,
●
SKIN
Skin comprises of 2 major layers
Epidermis
● Thin outer portion, composed of several layers.
Outermost layer of epidermis is composed of dead
cells with keratin
● Stratum corneum
○ Toxicants move across via Passive diffusion
●
LUNGS
Toxicants that are absorbed by the lungs are gases,
vapors of volatile or volatilizable liquids, & aerosols
GASSES & VAPORS
● Vapors – gas form of substance that can also exist as
a liquid or solid at atmospheric pressure & normal
temperature
● Most organic solvents evaporate and produce vapor &
some solids can sublimate into a gaseous form
● Once inhaled, gasses first pass through the nose that
partially protects the lungs from injury
● Once inhaled into the lungs, gas molecules diffuses
from alveolar space into the blood
● Through nostrils it protects the lungs from the
toxicants
AEROSOLS
● Absorption is determined by aerosol size & water
solubility of any chemicals
● Site of deposition of aerosols and particulates depend
largely on the size
○ 5 mcm or larger (coarse particles) deposited in nasopharyngeal region
○ 2.5 mcm (fine particles) - tracheobronchial
region
○ 1 mcm and smaller -alveolar sacs
● In the nasopharyngeal region, the defense
mechanism is sneezing and secretion of mucus
● In the tracheobronchial region, the defense
mechanism includes coughing.
○ Pseudo
Columnar
ciliated
epithelium
contains ciliated cells which is a type of cell
that produces mucus
○ Toxicants can be trapped in the mucus, and
it will be swept up by the cilia. It will be
swallowed, sneezed, or spit. This is called
the mucus ciliary; it will bring toxicants up.
○ In~ 2.5 mcm or smaller the defense
mechanism are the alveolar macrophages
Dermis
● Thick middle portion composed of connective tissues
● Hydrophilic toxicants are more likely to penetrate
through the skin appendages
● In some books, there are 3 layers. The 3rd layer is
called the subcutaneous layer.
● The epidermis is composed of 5 or sometimes 6
layers
● Toxicants that are very lipophilic can pass through the
epidermis through passive diffusion
● The Stratum Lucidum is found in very thick skin (E.g.
palms and soles)
ABSORPTION OF TOXICANTS AFTER SPECIAL ROUTES
OF ADMINISTRATION
● Chemicals may be administered by other routes,
including
○ Intravenous
■ toxicant directly introduced to the
systemic circulation
○ Intraperitoneal
■ Rapid absorption due to peritoneal
and mesenteric blood supply
■ Most chemical enters the liver via
portal circulation → First pass effect
○ Subcutaneous & Intramuscular
■ slower absorption rates
●
●
DISTRIBUTION
Movement of toxicant from systemic circulation to the
tissue (target action)
The rate of distribution to organs or tissues is
determined primarily by blood flow and rate of
diffusion out of the capillary bed into the cells of an
16 of 25
●
●
●
●
●
●
organ/tissue & affinity of a xenobiotic for various
tissues
Where is the fastest distribution of toxicants? In the
brain or fats?
○ Brain, Liver Kidneys - ↑blood flow,
↑distribution
○ This is in comparison to bone, skin and fats
with low distribution
Protein binding:
○ Acidic drug – binds to Albumin
○ Basic drugs – binds Alpha 1 acid
glycoprotein also know as orosomucoid
The initial distribution is dominated by blood flow,
whereas the eventual is determined largely by affinity
In order for a drug to bind to its receptor, it should be
a free drug. But there are certain drugs/toxicants that
are bound to protein, if bound to protein it can exert its
pharmacologic/toxic effect.
Acidic Drug - bound to albumin
Basic Drugs - bound to alpha 1-acid glycoprotein
APPARENT VOLUME OF DISTRIBUTION
● Volume in which the amount of drug would need to
uniformly dissolved in order to produce the observed
blood concentration
● If chemical distributes only to the plasma
compartment (no tissue distribution), it has a high
plasma concentration → Low Vid
● If chemical is distributed throughout the body the
effective plasma concentration is low→ High Vid
● Binding to and/or dissolution to various storage sites
of the body (Fat. Liver, Bone) am important factors in
determining distribution
STORAGE OF TOXICANTS IN TISSUES
● Only free fraction of chemicals will bind to the target
molecule, binding to or dissolving in certain body
constituents greatly alters the distribution of
xenobiotics
● The compartment where a toxicant is concentrated
but is not the major site of toxicity for that chemical is
describe as a storage depot
● As chemical is biotransformed or excreted from the
body more is released from the storage site → Longer
1/12
● Storage depot
○ PLASMA PROTEINS
○ LIVER & KIDNEY
○ FAT
○ BONE(lead can be stored in the bone)
EXCRETION
URINARY EXCRETION
● Toxic compounds are excreted in urine by the same
mechanisms the kidney uses to remove the end
product of intermediary metabolism from the body
(Glomerular filtration, tubular excretion, and Active
tubular secretion)
● Depending of the physicochemical properties of the
compounds, it may be reabsorbed across the tubular
cells back into the bloodstream
● Xenobiotics can also be excreted into urine by active
secretion
● If the drug and urine is acidic, reabsorption happens.
Therefore, it all depends on the pH of the medium and
the physicochemical of the agent.
FECAL EXCRETION
● Excretion of toxicants via the feces can result from
direct elimination of non-absorbed compounds in the
GIT, from delivery to the GIT via bile, and from
secretion into luminal contents from the enterocytes
● There are certain drugs that are eliminated via bile,
especially those that underwent Phase II,
Glucuronidation. So, inside the stomach, and most
especially inside the colon, wherein there is a large
population of bacteria that hydrolyzes the glucuronic
acid from the toxicant, leading to reabsorption.
EXHALATION
● Substances that exist predominantly in the gas phase
at body temperature are eliminated mainly by the
lungs
● The usual route of excretion for highly lipophilic and
volatile toxicants.
OTHER ROUTES
● CSF
● Milk
● Sweat and Saliva
○ Rifampin is excreted through this route,
representing red or orange discoloration
17 of 25
●
●
●
BIOTRANSFORMATION
It is the conversion of chemicals to more water
soluble compounds.
It’s the end product of metabolism- to eliminate or to
excrete the compound
A more water-soluble compound is excretable
Xenobiotic
● a chemical compound (drug, pesticide, carcinogen)
that is foreign to a living organism
Endogenous
● chemical growing or originating from within.
● Originates or produce inside the body
○ Example: Epinephrine (produced in the
adrenal medulla)
Substrate
● a substance to be catalyzed
● A substrate is a substance that is acted upon by the
enzyme - “substance to be catalyzed”
○ Enzyme catalyzes that reaction
-
●
●
Ethanol or ethyl alcohol is acted upon by alcohol
dehydrogenase into acetaldehyde
HOW ENZYMES WORK?
Help to speed chemical reactions by lowering
activation energy (without consuming itself)
Allowing the reaction to occur quickly
Reaction between Substrate A and Substrate B
Y-axis = energy use
x-axis = time.
The reaction between the two substances, without the
enzyme, uses a lot more energy in comparison to the
reaction with enzyme.
This allows the reaction to occur more
quickly
Enzymes lower activation energy without
consuming itself
●
●
Enzymes play a vital role in biotransformation
Transformation/Metabolism of Xenobiotics can either
be beneficial or harmful
○ Depending on the dose and circumstances
2 PHASES OF METABOLISM
PHASE I
● Addition of a functional group
● Aka Functionalization reaction
○ Because of the addition/exposure of the
functional group of a substance which results
to a slightly polar molecule that can be
inactive or active
PHASE II
● Conjugation of the modified xenobiotic with another
substance
● Aka Conjugation reaction
○ Because of the addition of conjugates to the
parent molecule
○ These conjugates are either glucuronic acid
from glucuronidation, sulfate from sulfation,
methyl group from methylation, etc.
● After phase 2 reaction, the metabolite is usually very
polar and inactive. Hence, it is readily excretable
● Conjugated products
○ Larger molecule than substrate
○ Generally polar in nature = water soluble
○ Have polar ability to cross cell membranes
■ This means they are less likely to
be re-absorbed
PHASE I REACTIONS
HYDROLYSIS
● Reaction with the addition of water to a reactant (OH
+ H)
● Substances that will undergo hydrolysis are the
following
○ Esters
○ Amines
○ Hydrazine
○ Carbamates
● Ex:
procaine → p-aminobenzoic acid +
diethylaminoethanol
18 of 25
○
○
Procaine - An ester local anesthetic
■ Local anesthetics can be divided
into esters and amines
When it is hydrolyzed it will yield to para
aminobenzoic
acid
(PABA)
and
diethylaminoethanol
■
■
■
1.
2.
3.
4.
ENZYMES INVOLVED IN HYDROLYSIS
Carboxylesterases (found in serum & tissues)
a. Hydrolyze endogenous lipid compounds
b. Generate
pharmacologically
active
metabolites (usually)
Cholinesterases
a. Limit the toxicity of organophosphate
i.
Acetylcholinesterase → Degrades
acetylcholine
ii.
Butyrylcholinesterase → Degrades
acetylcholine
but
is
usually
metabolizes xenobiotics
Epoxide hydrolase
a. Detoxify electrophilic epoxides (cause
cellular toxicity and genetic mutations)
Beta-glucuronidase
a. Hydrolyzes xenobiotic glucuronides from
phase II reaction
i.
Xenobiotic glucuronides are polar
and inactive molecules. They are
readily excretable.
ii.
Once they are acted upon by
beta-glucosidase in the intestine, it
will hydrolyze the glucuronic acid
from the parent compound, then the
parent
compound
can
be
reabsorbed via the enterohepatic
circulation.
b. Enterohepatic circulation
REDUCTION
● Substrate gains electrons
○ Their valance decreases, that’s why they are
called oxidizing agents
● Occur with xenobiotics in which oxygen content is low
● Reduction reactions frequently results in activation of
a xenobiotic than detoxification
○ Most toxicants/chemicals that will undergo
reduction usually results in an increase in
pharmacologic action or toxicity
● Example: Azo reduction nitrogen – nitrogen double
bonds
○ Nitro reduction- NO2
■ Both are catalyzed by CYP450 &
NADPH- quinone oxidoreductase
○ Nitrobenzene + H2 → aniline + 02
Nitrobenzene will reduce to form
aniline
Aniline
can
cause
methemoglobinemia where the
ferrous components of hemoglobin
are converted to ferric.
Methemoglobin has a reduced
capacity to carry oxygen compared
to hemoglobin
OXIDATION
● Reactions in which substrate loses electrons
● Gain valence (VILEORA)
○ Valence Increase, Loses Electron, Oxidation,
Reducing Agent
1.
2.
-
-
ENZYMES INVOLVED IN OXIDATION
Alcohol dehydrogenase
a. Primary alcohol → aldehydes
b. Secondary alcohols → ketones
c. Primary alcohols when they are oxidized by
alcohol dehydrogenase, they will yield an
aldehyde. If it’s a secondary alcohol, it will
yield ketones
Aldehyde dehydrogenase
a. Aldehydes
→
carboxylic
acids
(NAD-cofactor)
b. Aldehydes can be further oxidized into a
carboxylic
acid
using
aldehyde
dehydrogenase
c. Both alcohol dehydrogenase and aldehyde
dehydrogenase will use NAD as cofactor
d. While CYP2E1 uses NADPH as cofactor
It shows the different metabolism pathways of ethanol
or alcohol.
The major metabolism pathway of ethanol is through
alcohol dehydrogenase (ADH) which will result in
acetaldehyde.
Pathogenesis of liver damage among
alcoholics. Some of the alcohol will be
metabolized using CYP2E1. However, this
enzyme (CYP2E1) degenerates reactive
oxygen species that can damage the liver.
The minor metabolism is through catalase
19 of 25
-
3.
4.
5.
Once acetaldehyde is form, it is metabolized
to acetic acid, a carboxylic acid using the
aldehyde dehydrogenase (ALDH)
Monoamine oxidase (MAO)
a. Oxidative deamination of primary, secondary,
and tertiary amines, including serotonin,
epinephrine, norepinephrine and some
xenobiotics
Prostaglandin H synthase
a. Arachidonic acid → (cyclooxygenase)→
prostaglandins
b. Prostaglandins- mediates the formation of
inflammation and platelet clog formation or
platelet aggregation
Cytochrome P450 (CYP)
a. Found in hepatic smooth ER microsomes
b. Heme containing
c. Classified into subfamilies based on amino
acid sequence identity.
d. Named in a specific manner
e. It is responsible mostly for phase I reaction
f. The most common CYP is CYP3A4.
i.
Most of the drug is metabolized by
this enzyme
g. CYP2D6, an important enzyme in degrading
CNS drugs
FACTORS THAT CONTRIBUTE TO DECREASE CYP
ENZYME ACTIVITY
1. A genetic mutation
a. gives rise to the poor and intermediate
metabolizer genotypes.
2. Exposure to environmental factors (infectious
disease or an inflammation process)
a. suppresses CYP enzyme expression
b. E.g. Hepatitis
3. Exposure to a xenobiotic
a. inhibits or inactivates a pre-existing CYP
enzyme
b. By inhibiting cytochrome P450, one drug can
impair the biotransformation of another
leading to an increase in plasma level of
other drug, leading to an exaggerated
pharmacologic or toxicologic response to the
second drug, and decrease its excretion
FACTORS THAT CONTRIBUTE TO INCREASE CYP
ENZYME ACTIVITY
1. Gene duplication leading to over-expression of a
CYP enzyme.
2. Exposure to drugs and other xenobiotics that induce
the synthesis of cytochrome P450
3. Stimulation of pre-existing enzyme by a xenobiotic.
a. Induction of cytochrome P450 by xenobiotics
increases CYP enzyme activity.
b.
By inducing cytochrome P450, one drug can
stimulate the metabolism of a second drug
and thereby decrease plasma levels,
decrease or ameliorate its therapeutic effect,
increase in excretion
INDUCERS
INHIBITORS
Increases the number of
CYP
→
increase
in
metabolism → decrease in
pharmacologic action
Decrease the number of
CYP
→
decrease
in
metabolism → increase in
pharmacologic action
(PPRCO)
(MEDICKAV-G)
● Phenytoin
● Phenobarbital
● Rifampicin
● Carbamazepine
● Omeprazole
● Metronidazole
● Erythromycin
● Disulfiram
● Isoniazid
● Cimetidine / Chloram
● Ketoconazole
● Amiodarone / Allopurinol
● Valproic acid
● Grape Juice
1.
Phenytoin + Paracetamol
Q1. What will happen if you co-administer Phenytoin and
Paracetamol?
● Answer: Since Phenytoin is an enzyme inducer, it
will induce formation of CYP 450 or stimulate the
action of pre-existing CYP.
● This will lead to an increase the metabolism of
Paracetamol leading to a decrease in plasma level
and decrease in pharmacologic and toxic effects
and increase in excretion
● ↑ metabolism
● ↓ Plasma
● ↓ pharmacologic and toxic effect
● ↑ excretion
2.
Omeprazole + Amlodipine
Q2: What will happen if you co-administer Omeprazole and
Amlodipine at the same time?
● Answer: Since Omeprazole is an enzyme inducer,
it will induce the formation of CYP and stimulate
the existing CYP leading to greater metabolism of
Amlodipine
3.
Ketoconazole + Losartan
What will happen if you co-administer Ketoconazole and
Losartan at the same time?
● Answer: Since Ketoconazole is an enzyme
inhibitor, it will inhibit the action of pre-existing
CYP, leading to these effects in Losartan:
○ ↓ metabolism
○ ↑ in plasma levels
○ ↑ in pharmacologic action and toxicity of
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○
4.
Losartan
↓ in excretion
●
●
Metronidazole + Gliclazide
What will happen if you co-administer Metronidazole and
Gliclazide at the same time?
● Answer: Since metronidazole is an enzyme
inhibitor, it will inhibit the metabolism of Gliclazide
●
ENVIRONMENTAL FACTORS KNOWN TO AFFECT CYP
LEVELS
● Medications
● Foods
● Social Habits (alcohol consumption, cigarette,
smoking)
○ Acute Alcohol consumption is an inducerinduces the formation of enzymes
○ Chronic alcohol consumption is an inhibitorinhibits the formation of enzymes
● Disease status (diabetes, inflammation, viral %
bacterial infection, hyperthyroidism, hypothyroidism)
● It is possible that two or more CYP enzymes can
contribute the metabolism of a single compound
●
●
●
●
●
●
Information on which human CYP enzyme
metabolizes a drug can help predict or explain drug
interactions
Inducers of cytochrome P450 increase the rate of
xenobiotic biotransformation
P450 induction lowers blood/toxicant levels, which
compromises the therapeutic goal of drug therapy but
does not cause an exaggerated response to the drug
P450 induction can cause pharmacokinetic tolerance
whereby larger drugs doses must be administered to
achieve therapeutic blood levels due to increased
drug biotransformation
PHASE II REACTIONS
CONJUGATION
● Conjugations result in a large increase in xenobiotic
hydrophilicity – greatly facilities excretion of foreign
chemicals (except methylation & acetylation)
○ Hydrophilicity means it loves water; hence it
is readily excretable.
● Most conjugation enzymes are mainly located in the
cytosol
● Phase II reaction is also known as conjugation
reaction because you have to add conjugates to
parent molecules which usually results to an inactive
and very polar metabolite that is readily excretable
(except for methylation & acetylation)
●
Requires
the
co-substrate
uridine
diphosphate-glucuronic acid (UDP-glucuronic acid)
Reaction
is
catalyzed
by
UDPglucuronosyltransferases (UGTs)
○ UGTs transfer glucuronic acid from the UDPglucuronic acid to the toxicant
Endogenous substrate includes bilirubin, steroid
hormones, and thyroid hormones
Conjugates are polar, water-soluble metabolites
○ Once this metabolite reaches the large
intestine, some of them are acted upon by
beta glucuronidase (bacterias) in the large
intestine
○ This will lead to the hydrolysis of glucuronic
acid and the reabsorption of parent molecule
→ enterohepatic circulation
Excreted from the body in bile or urine
Cofactor availability can limit the rate of
glucuronidation of drugs that are administered in high
doses and are conjugated extensively, such as aspirin
and acetaminophen
○ Aspirin and acetaminophen are usually
metabolized via glucuronidation
SULFONATION (SULFATE CONJUGATION)
● Catalyzed by sulfotransferases which produces a
highly water-soluble sulfuric acid ester
● The
co-substrate for the reaction is 3’phosphoadenosine- 5’-phosphosulfate (PAPS) which
is synthesized from inorganic sulfate
● Involves the transfer of sulfonate from PAPS to the
xenobiotic
● Conjugates are excreted mainly in urine
● Sulfonation is an effective means of decreasing the
pharmacologic and toxicologic activity of xenobiotics
METHYLATION
● Minor pathway of metabolism/ biotransformation. It
results in lipophilic metabolites
● Decreases the water solubility of xenobiotics, it will
result in lipophilic metabolites that are less likely to be
excreted
○ It increases the plasma concentration of the
drug and can lead to an increase in toxicity
● Masks functional groups that might otherwise be
conjugated by other enzymes
● The
co-substrate
for
methylation
is
S-adenosylmethionine (SAM)
○ We got methyl group from SAM and transfer
to parent compound via O-Methylation,
N-Methylation, S- Methylation
● Methylation can also lead to increased toxicity
ACETYLATION
GLUCURONIDATION
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●
●
●
●
●
●
N-acetylation is a major route of biotransformation for
xenobiotics
○ Aromatic amine → aromatic amide
○ Hydralazine → hydrazide
N-acetylation of certain xenobiotics, such as isoniazid,
facilitates their urinary excretion
○ Isoniazid - anti-tb drug
N-acetylation
is
catalyzed
by
cytosolic
Nacetyltransferase (NAT requiring the co-substrate
acetyl - coenzyme A (acetyl CoA)
○ We get the acetyl group from this (acetylCoA) molecule and transfer it to the parent
compound using the NAT
NAT 1 and NAT2 (acetyltransferase in humans)
Slow NAT2 acetylators are predisposed to drug
toxicities
○ Means that these people metabolize drugs
slower than others
There are 2 types of acetylators:
○ 1. Fast acetylators – Philippine, Orientals,
Asian
○ 2. Slow acetylators – Europeans
DRUG TOXICITIES
● Excessive hypotension from hydralazine
● Peripheral neuropathy from isoniazid and dapsone
○ Vit B6 can be the treatment to
isoniazid-induced peripheral neuropathy
● Systemic
lupus
erythematosus
(SLE)
from
hydralazine and procainamide
○ “Drug induced lupus effect” – there are drugs
that can induce lupus as HIPS
■ Hydralazine
■ Isoniazid
■ Procainamide
■ Sulfa Drugs
● Toxic effects if co-administration of anticonvulsant
phenytoin with isoniazid
AMINO ACID CONJUGATION
● Conjugation of xenobiotics containing
○ Carboxylic acid group with the amino group
of amino acids glycine, glutamine and
taurine
○ Aromatic hydroxylamine with the carboxylic
acid group of amino acids serine and proline
● Amino acid conjugates of xenobiotics are eliminated
primarily in urine
● Conjugation of hydroxylamines with amino acids is
catalyzed by cytosolic aminoacyl-tRNA synthetases
and requires ATP
●
Catalyzed by a family of glutathione S-transferases
that are present in most tissues
TYPES OF CONJUGATION REACTIONS
1. Displacement reactions - glutathione displaces an
electron-withdrawing group
2. Addition reactions - glutathione is added to an
activated double bonds or strained ring system
●
●
●
●
-
-
Glutathione conjugates formed in the liver can be
effluxed into bile and blood, and they can be
converted to mercapturic acids in the kidneys and
excreted in urine.
Conjugation with glutathione represents an important
detoxification reaction:
○ Because electrophiles are potentially toxic
species that can bind to critical nucleophiles
(protein and nucleic acids) causing cellular
damage and genetic mutations
Glutathione is a cofactor for glutathione peroxidase
important in protecting cells against lipid and
hemoglobin peroxidation. (Primary use of Glutathione)
○ Side Effect is defined as any effect outside or
other than therapeutic effect
Conjugation with glutathione enhances the toxicity of
a xenobiotic by:
○ Releasing a toxic metabolite
○ Being inherently toxic
○ Being degraded to a toxic metabolite
Glutathione can inhibit the enzyme, Tyrosinase.
Tyrosinase is needed to form Melanin,
responsible for the dark pigment of our skin.
Once we administer a huge amount of
Glutathione, it will inhibit Tyrosinase.
Glutathione is primarily used as a substrate for
conjugation reactions.
Side effect of Glutathione: Whitening
Glutathione is primarily used now for dermatologic
side effect
GLUTATHIONE CONJUGATION
● Tripeptide glutathione comprises of glycine, cysteine,
and glutamic acid
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●
●
●
TYPES OF ANTIDOTES
PHYSIOLOGICAL – opposite effects as that of poison
CHEMICAL – changes the chemical nature of poison
MECHANICAL – prevent absorption of poison
SKIN DISCOLORATION
ASSOCIATED SUBSTANCES
Yellow
Picric acid, Nitric acid
Bleaching white
Phenol
Ash gray
Mercuric chloride, physostigmine
Deep brown
Bromine
Brown black
Sulfuric acid, Iodine, Silver nitrate
Bluish gray
Silver salts
Blue
Cyanotics (opium, aniline, sulfides)
Pale bonds on fingernails (Mee’s line)
Arsenic
Boiled lobster appearance
Boric acid
URINARY CHANGES
ASSOCIATED SUBSTANCES
Dark yellow
Picric acid
Yellow brown
Aloe, Senna
Green blue
Methylene blue, phenol, triamterene, amiloride
Wine (Red brown)
Caffeine, Benzene, Rifampicin, Lead, Mercury, Carbon
tetrachloride
BOWEL CHANGES
ASSOCIATED SUBSTANCES
Black
Charcoal, Bismuth, Iron, Manganese dioxide, Silver nitrate
White
Aluminum hydroxide
Blue
Boric acid, Methylene blue, Iodine
Green
Indomethacin, Iron, Cupric sulfate
BLOOD CHANGES
ASSOCIATED SUBSTANCES
Cherry red blood
Carbon monoxide, Cyanide
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Dark red blood
Nicotine
Chocolate blood
Aniline
VISUAL CHANGES
ASSOCIATED SUBSTANCES
Optic neuritis (Red-green blindness)
Ethambutol
Purple vision
Digitalis, Marijuana
Partial or total blindness
Methanol, Formic acid
Bloodshot eyes
Marijuana
ODOR CHANGES
ASSOCIATED SUBSTANCES
Garlic odor
Phosphorous, Selenium, Tellurium, Arsenic
Mousy urine
Coniine
Bitter almond odor
Cyanide
Shoe polish
Nitrobenzene
Rotten eggs
Hydrogen sulfide
Mothballs
Naphthalene
Wintergreen
Methylsalycylate
Pear-like
Chloral hydrate
SYNTHETIC POISONS
SOURCE
Ethylene glycol
Anti-freeze
Picric acid
Colorant in textile
Aniline
Crayons
Nitrobenzene
Shoe polish
Hydrogen sulfide
Sewer gas
Asbestos
Fire retardant
Naphthalene
Moth balls
Eosin
Lipstick
POISON
ANTIDOTE
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Aniline or Nitrites
1% Methylene blue (0.1mg/kg)
Cyanide
Lilly Cyanide Kit (Amyl nitrite → Sodium Nitrite →Sodium
thiosulfate
Ethanol
5-10% glucose (for hypoglycemia) + thiamine
Formaldehyde
Diluted ammonia; Ammonium carbonate
Irone
Deferoxamine
Methanol
Ethanol
25 of 25