What is toxicology?
●●●

Paracelsus (1493-1541, Physician, Alchemist) is called as "The father of toxicology"●"Alle Ding sind Gift und nichts ohn Gift; alein die Dosis macht daß ein Ding kein
Gift ist"("All things are poison and nothing is without poison; only the dose makes athing not a poison")●See, https://doi.org/10.1093/toxsci/53.1.2
Definition: The scientific research to clarify the safety to human health of drugs
and chemical substances. (Since drugs and chemical substances constitutechemical environment, toxicology is important component of environmental health.)
Core problem in medicine: any drug has both therapeutic and adverse effect onhuman body, which widely varies. Suppose:●Flu drug, to cause liver failure for 10% of users, is not acceptable.
●
New cancer drug, which can cure 80%, is acceptable even if it causes mildadverse effect for everyone.
●If the efficacy of vaccine to prevent disease occurrence is 90%, fatal adverse
side effect is 0.001% and case fatality risk is 2% and prevalence is 0.1%, thevaccine is widely applicable. But if case fatality risk is 0.01%, the applicability ofthe vaccine with the same nature is questionable.
1Lethal Dose 50 (LD50) of various substances●●●Definition: the dose which kills a half of administered animals(mouse/rat) within a study period, in mg/kg body weight.
The most popular indicator of acute toxicity of substances.
(FYI) Molecular model can be made by Chimera(https://www.cgl.ucsf.edu/chimera/download.html)

0.003 mg/kg(saxitoxin, the best known
paralytic shellfish toxin)

10 mg/kg
1 mg/kg (VX nerve (Sodium cyanide)gas, chemicalweapon)
500 mg/kg(acetoaminophene)(Note) White: hydrogen, Blue: nitrogen, Red:oxygen, Grey: carbon, Yellow: sulfur, Orange:
phosphorus, Green: chlorine

118 mg/kg(Chlorpyrifos,insecticide)2,400 mg/kg(Sodiumchloride)2

Body compartment model for dynamics ofsubstances
Inhalation
Environmental
pollutants
FoodDrugs
Additives

Medicine(through skin,injections, etc.)

Injection

Ingestion
Lung

KidneyFatOrgans

Ga-InLuGa
LiverLiverIn

Plasma,Lymph

Bile
Feces

KidneyBiological half time: When substances in a body decrease with Bladderexcretion/metabolism and the rate of decrease is proportionate
to the amount in the body, the period from intake until theUrine
amount reaching a half of initial amount.

Lung

Extracellular
fluid

Alveoli

Secretaryorgan

BreathSweat, Saliva
Milk3

Biological protection system●

Via immune system●
non-specific: phagocytosis (neutrophil, monocyte, macrophage), attack
to cancer/virus infected cells (NK cell), natural immunity (IgM)●
specific: acquired immunity (B, Helper/Killer T cells)– https://doi.org/10.1016/j.mpaic.2021.06.006–●

https://doi.org/10.1016/j.imlet.2014.10.011

Via non-immune system●
enzymatic catabolization: fat-soluble -> water-soluble●
metallothionein: induced by Cu, Zn, Cd (MW 6-7K)●
superoxide elimination system: SOD, GPx, Catalase●
DNA repairing enzyme
IgA
Lyzozym

Infectious
agent
Bio-molecule(high MW)Chemicalsubstance(low MW)Transferrin
Lyzozym

Albumin
Globulin
Barrier by skin and/or
mucosanon-specificimmunity

specific
immunity

combineaccumulation

metabolicsystem

Infection
Injury, toxic

Virus-infected cells
Malignant cells
Hapten bound cellsCd-metallothionein(Data: https://www.rcsb.org/structure/4MT2)

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Absorption pathway●

via gastrointestinal tract●
most materials absorbed from gastrointestinal organs go throughportal vein to liver, then are metabolized
in mucosa of oral cavity, tongue surface, and mucosa of lower
rectum, materials are directly absorbed●
stomach easily absorb fat-soluble/acidulous substances
via lung●
alveoli absorbs air pollutants●
some materials (eg. mercury) are more effectively absorbed asvapor from lung than as liquid from gastrointestinal tractvia skin●usually low absorption efficiency due to simple diffusion●
exceptions: sarin, tetrachlorocarbon, paraquat (herbicide)●●●

5Intra-body kinetics
●

●
●

●

Whether a chemical substance has toxic effect or not depends on (1)sensitivity of host organ, and (2) concentration of the substance there●
The concentration depends on intra-body kinetics composed of 4 factors(Absorption, Distribution, Metabolism, Excretion)
Critical concentration: lowest concentration to harm tissue
Target organ: the first organ where the substance accumulates up to criticalconcentration●
Cadmium (Cd)●Chronic exposure -> Itai-itai disease (affecting bone)●
Most cadmium accumulates liver, subsequently kidney: thus target organs
are them●
Lead (Pb): hematopoietic system (bone marrow) -> decrease ofhemoglobin and increase of reticulocytes●
Paraquat: lung●
Inorganic arsenic (As): No mutagenecity but carcinogenecity, probably
inhibiting macrophage/NK cells
The highest concentration is not necessarily seen in target organ, because
the sensitivity varies by organ
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Distribution●Distribution in the body differs by substances
●
●

●
●

DDT, thiopental accumulates to fat tissue
Inorganic mercury is more distributed to kidney, secondly liverand spleen, but methylmercury are equally distributed to anyorgans (incl. brain, fetus)
Cadmium accumulates liver and kidney (not in bone)

Why differs by substances?
●●

Host factors: various blood flow to each organ, tissue barrier(BBB, BPB)
Material factors: MW, fat-solubility, binding capacity with bloodelements and tissue cells–fat-soluble substances have longer biological half-life7

Metabolism●●●

Catabolization basically increases the excretion by increasing thepolarity
Basically reducing toxicity, but rarely the metabolic products havehigher toxicity (metabolic activation)
Stages of metabolic reaction●●●

1st phase: Increasing polarity by oxidation, reduction, orhydrolysis. In liver, most active. a kind of heme-proteins,
cytochrome P450 in hepatic microsome is most important in
oxdation
2nd phase: Cohesion with endogeneous substances likeglucuronic acid, increasing ability of excretion
3rd phase in excretion
8Genetic polymorphisms of metabolicenzymes
Enzymes

Molecules

Substrate (externaltoxin)

Frequencies ofdeletion type

Effects of deletion

Cytochrome P450

CYP2C19

Mephenytoin, etc.

3% of Caucasian
20% of JapaneseMore adverse
effectAlcoholdehydrogenase

ADH1

Ethanol

4-20% of Cauc.
90% of Japan.

More aldehyde
production

Aldehydedehydrogenase

ALDH2

Acetoaldehyde

Rare in Cauc.
40% of Japan.

"Flusher"N-acetyl transferase

NAT2

Isoniazid (antituberculosis)

60% of Cauc.
12% of Japan.

More adverse
effectGlutathione-Stransferase

GSTM1, GSTT1,GSTP1

EpoxideGSTM1=50%,
GSTT1=38%

Cancer induction
by smoking

UDP-glucuronide
transferase

UGT1A1

Bilirubin?

Crigier-Najjar
syndrome

Thiopurine-methyltransferase

TPMT

Anti-leukemia,immunosuppr.

Deletion-homo
0.2-0.3% Cauc.

Suppression ofbone marrow

9Excretion
●
●

●

●

Mainly from Kidney and Liver
Excretion to urine: 25% of blood -> glomerulus -> 20%
filtration (<MW 60000)
https://www.lecturio.com/concepts/glomerular-filtration/
Excretion to bile: from liver. Higher polarity materials are
directly excreted into feces, lower polarity materials arecohesively coupled with glutathione or glucronic acid (afterreabsorption from intestine; enterohepatic circulation), thenconveyed to bile with transporters like MRP2 (Phase III)
Other pathways of excretion: Intestine (PCB, DDT, etc.),
Breastmilk (fat-soluble substances), breath, skin, saliva, tears

10Classification of toxicity●

General toxicity (in terms of period to expression)●●●Acute: single exposure cause a toxic response within a shortlatent priod. Evaluated with LD50, LC50●

Subacute: 1-3 months repeated exposures cause it.

●Chronic: several months to a year exposure cause it.

●Intergenerational: expression in the next generation

Special toxicity (in terms of toxic responses)●

Carcinogenecity: initiation / promotion●

Mutagenecity: causing the mutation of genes

Misc.
●

Reproductive toxicity, Neurotoxicity, Immune-toxicity, etc.
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Evaluation of toxicity●●Target●human●experimental animal

Types of testing (cf. OECD guideline)●

Acute oral test: observe 2w after admin, sectio, LD50●

Subacute: everyday admin 2-4w, sectio, NOEL●●

Chronic: Rodent+Non-rodent, at least 1yr repeatedly,NOEL, ADI, TDI
Misc: Carcinogenic test, Mutagenic test, Biomonitoring, etc.

12Indicators of toxicity●●

Acute toxicity●

LD50: lethal dose 50 = amount to kill a half●

LC50: lethal concentration 50 = concentration to kill a half●

ED50: effective dose 50 = amount to affect a halfWith threshold●●

Less than threshold, no toxic effectWithout threshold●●●

Within the tested doses, maxium dose with no observable effect is NOEL
(or NOAEL for adverse effect)
The level should be adjusted by safety factor or uncertainty factor for thepossible effect in the larger population or genetic variation
Virtually Safe Dose (VSD): setting the acceptable risk level. The amountto cause less risk than that is to be acceptable.
13Reference doses (RfD)●●●

ADI (acceptable daily intake): For the substances to beintentionally used by human-beings, the daily intake level may
have no risk even the human continue to have that level
TDI (tolerable daily intake): For the substances which are notintentionally used but taken as environmental pollutants, thedaily intake level cause no risk even the one continue to havethat level
Units are mg/kg body weight/day●NOAEL/NOEL/LOAEL for the most susceptible animal
experiment are devided by safety factor (for ADI) or
uncertainty factor (for TDI). Usually the factors are 10.

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Dose-Response Relationships
The responses of host animals may change with the dose.
●
In population level, the proportion of responded animals
changes with dose (toxic load). The relationship is usually Sshape. Approximated with cumulative logarithmic normal
distribution.
●The dose to make 50% respond is ED50●
The dose to kill 50% is LD50●
ED50 or LD50 is estimated by probit/logit analysis.
●ndead/ntotal

1.0
0.8
MF

0.6
0.4
0.2
0.0
0100
dose

Calculation example: See,
http://minato.sip21c.org/envhlth/dr2.R
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Future Perspectives of Toxicity Testing●●●

Source: Collins FS, Gray GM, Bucher JR: Transforming environmental health protection.
Science, 319: 906-7, 2008.
https://dx.doi.org/10.1126%2Fscience.1154619
Hartung T (2009) "Toxicology for the twenty-first century" Nature
https://www.nature.com/articles/460208a
Krewski D et al. (2020) “Toxicity testing in the 21st century: progress in the past decade andfuture perspectives” Arch. Toxicol. https://doi.org/10.1007/s00204-019-02613-4

Human experience1 ～ 3/yr

Rodent toxicity
10 ～ 100/yr

Alt. animal exp.
100 ～ 10000/yr

Biochem/cellin vitro>10000/yr

Mouse,
Rat

Zebrafish, fruitfly,
C.elegans, etc.

Biomolecular
Cell culture

Transgenic

Data
Human

prediction

Directly related for humanPrioritize

Computer toxicol.
(molecular simulation)

Specify the critial
toxic pathway
higher efficiency

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