General principles of toxicology and dose-time relationships

Question 1

What is toxicology and what is the focus of toxicology today?

Answer 1

The study of the nature, effects and detection of chemicals with adverse (toxic) effects on biological systems/living organisms. Today the focus is the SAFE use of chemicals.

Question 2

Poison is an overall term for toxic agents, what is the difference beween: toxin, toxicant, xenobiotic and venom?

Answer 2

• A toxin is a compound produces by an animal, plant, fungi or bacteria (any other organism than humans).
• A toxicant or xenobiotic is a anthropogenically produced (man-made) compound.
• Venom is a toxin injected by a bite.

Question 3

Why is the dose so important in the definition of poison?

Answer 3

Because anything can be toxic in a high enough dose, even common things such as sugar (LD50=30g/kg), salt (LD50=3 g/kg) or water (LD50=90g/kg)!

Question 4

What is tolerance?

Answer 4

Tolerance is a decreased responsiveness to a toxic effect of a compound due to prior exposure to that compound.

Question 5

What are the two mechanisms behind tolerance?

Answer 5

The two mechanisms are:
1. A decreased amount of toxicant reaching the site where the toxic effect is produced (dispositional tolerance).
2. Reduced tissue-specific responsiveness
Both can be caused by for example enzyme inhibition or induction or increased synthesis of a protein that can handle the compound.

Question 6

What is the difference between dose and dosage in toxicology?

Answer 6

“Dose” refers to the specific quantity of a compound that enters the organism (often expressed as an amount per volume or mass), while “dosage” refers to the rate of application of a dose and therefore also include the time aspect.

Question 7

What is meant by “exposure”?

Answer 7

Exposure in toxicology refers to the point where the substance enters the systemic circulation and appears in the blood.

Question 8

What different routes of exposure are there?

Answer 8

• GI-tract/ingestion
• Lungs/inhalation
• Skin: dermal, topical, percutaneous
• Direct to the blood stream – intravenous

Question 9

Both duration and frequency plays a role in exposure, explain the difference between acute, subacute, subchronic and chronic exposure.

Answer 9

• Acute – less than 24 hours – usually single
• Subacute – 24 hours to 1 month - repeated
• Subchronic – 1 to 3 months - repeated
• Chronic – more than 3 months

NOTE! It’s important to differentiate between response/effect and exposure. E.g. an acute exposure can have chronic effects.

Question 10

Why is it important to include frequency in exposure?

Answer 10

Because one dose X can have an effect while five repeated doses x/5+x/5+x/5+x/5+x/5 = x may not give
the same effect, might be more or less of an effect even though the total dose is the same.

Question 11

What is meant by the toxic concentration interval for a compound? Exemplify.

Answer 11

The toxic concentration interval is the concentration interval where we see toxic effects of the compound. A compound with slow elimination (A) will remain within the toxic concentration interval for a longer time than a compound with very fast elimination (B). Repeated doses of compound A will lead to a longer time in the toxic concentration interval (accumulation of A) than repeated doses of B, since it can all be eliminated in between doses and therefor never remain in the toxic concentration interval. So the time in between doses for these two compounds would differ a lot in practice.

Question 12

Define “dose-response relationship”.

Answer 12

The correlation between the characteristics of exposure and the spectrum of effects. For example, that a higher dose gives a higher response.

Question 13

There are two types of dose-response relationships, what are they called and what are they?

Answer 13

The two different types of dose time relationships are:

• Graded: A dose-response relationship for an individual, which is a continuous response. Often the higher the dose, the higher the response.
• Quantal: A dose-response relationship for a population, which refers to the distribution of responses to varying doses of a chemical in a population of individuals, an “all or nothing measure” since one individual is either a responder or a non-responder at a given dose. Visualized as a dose-frequency relationship (in the shape of a sigmoid curve since few respond to low doses and all respond to a high enough dose).

The quantal dose-response relationship is often normally distributed (which means that the mean ± 3 SD represents 99.7% of the population).

Question 14

You can get some information directly by looking at a cumulative dose-response curve (quantal), what can you tell from the angle of the slope?

Answer 14

The variation in sensitivity of the population is what determines the angle of the slope of the sigmoid curve. A steep slope means that there is little variation and a shallow slope means that there is big variation.

The variation is determined by natural gene diversity causing tolerance/sensitization?

Question 15

What is the aim of probit transformation?

Answer 15

Probit transformation aims to transform the sigmoid dose-response curve to a straight line (log scale for X axis), which makes it easy to determine parameters such as EC50, LD50 etc. (certain characteristics of the compound for a given percentage of pop). 50% = 5 in probit units.

Question 16

Potency and efficacy are terms used to compare toxic effects of two (or more) chemicals. What do they mean and how are they different?

Answer 16

Potency refers to the amount of the compound needed to produce a certain effect, meaning a more potent compound requires a lower amount to produce an effect than a less potent compound. (X-axis in dose-response curve)

Efficacy refers to the maximal response that can be elicited by the compound, meaning a compound with high efficacy creates a higher response than a compound with low efficacy. (Y-axis in dose-response curve).

Question 17

There are many measures of toxicity that are useful to quantify toxicity of different compounds. These are important for risk assessment of for example medical products, nutritional additives and environmental pollutants. Explain what the toxicity parameters LD50 and LC50 mean.

Answer 17

LD50: lethal dose in 50% of the population, e.g. the dose from which 50% of the pop would die from if exposed. (Mostly expressed in mg/kg BW or mmol/kg BW)

LC50: lethal concentration in 50% of the pop, the concentration in the body that 50% of the population would die from if exposed. (Mostly expressed in mmol/l, μg/m3).

Both require a quite large number of tested individuals but the way of exposure differs, for LD50, the exposure is through applied dose such as inhalation/skin/GI-tract while for LC50 the exposure is from the living medium (water/air etc.)

Question 18

Explain what the toxicity parameters ED50 and EC50 mean.

Answer 18

ED50/EC50: effective dose/concentration in 50% of population, e.g. the dose/concentration needed to get a desired effect in 50% of the population. Both graded and quantal responses are possible, example graded: enzyme activity, example quantal: anaesthesia (sleep/no sleep).

Question 19

Explain what the toxicity parameter IC50 mean.

Answer 19

IC50: The concentration causing a 50 % reduction/inhibition of the studied variable. Only for graded responses, such as enzyme activity of gene expression.

Question 20

Explain what the toxicity parameters NOAEL and LOAEL mean.

Answer 20

NOAEL: No observed adverse effect level, meaning the HIGHEST tested level/conc/dose for which NO ADVERSE EFFECTS were observed.

LOAEL: Lowest observed adverse effect level - meaning the LOWEST tested level/conc/dose for which ADVERSE EFFECTS WERE observed.

Question 21

Explain what the toxicity parameters NOEC and LOEC mean and

Answer 21

NOEC: No observed effect concentration, meaning the highest tested concentration for which no effect was observed.

LOEC: Lowest observed effect concentration, meaning the lowest tested concentration for which effects were observed.

Question 22

What differences are there between NOAEL/LOAEL vs NOEC/LOEC?

Answer 22

NOAEL/LOAEL are for adverse effects and are more commonly used in toxicity testing applicable to humans, NOEC/LOEC are for effects in general and are more common in ecotoxicology. All these measure TESTED CONCENTRATIONS ONLY, which means they are seldom true, as the true values lie in between tested concentrations.

Question 23

The two most common dose-response curve shapes are monotonic- and U-shaped dose-response curves, what are the fundamental differences between these?

Answer 23

Dose-response curves are called:

• “monotonic” when the effect increases with increasing dose.
• “U-shaped” when low doses cause effects that decrease when dose
  increases and increasing doses increase the effect (can also be vice versa), e.g. vitamins which is toxic in too low doses (deficiency) and toxic in high doses (surplus), compounds for which homeostasis (-> AROI (acceptable range of oral intake)) is needed.

Question 24

How does the time relate to the dose-response relationship?

Answer 24

The response is always dependent on the dose AND the time:

• Longer exposure time gives higher response
• Shorter exposure time gives lower response

Question 25

Since there is always some variation in how populations respond to a compound, we need a way to measure how a compounds therapeutic effect compares to its toxic effect in a population. This measurement is called the therapeutic index, but how is it calculated?

Answer 25

Therapeutic index, TI = TD50/ED50, the ratio between the toxic dose (TD) for 50% of pop (or LD50 for animals) and effective dose (ED) for 50% of pop. TI express the relative safety of the compound, a high TI is preferred = high TD and low ED.

Although, it doesn’t take the slopes/variation of the curves into account, so if the TD variation is big and ED variation is small, there is a large chunk that would experience toxic effects.

Question 26

Since the slopes of the curves ED and TD (or LD) aren’t being taken into account in the therapeutic index, we generally use Margin of Safety (MoS) as the measure of safety. How is the MoS calculated?

Answer 26

The Margin of Safety is calculated as MoS = TD1 /ED99 (or MoS = LD 1 /ED 99) for drugs, meaning the ratio between the dose which is toxic to 1% of the pop over the dose which is effective in 99% of the pop. A high MoS is a good MoS!

Note: for nondrug chemicals, the MoS is calculated as MoS = NOAEL/Exposure.