Criteria of Toxicity

Can evaluate toxicity in three ways:

• observing exposed populations of organisms to chemicals (epidemiology)
• administration of known doses to experimental organisms (in vivo)
• exposing cell fractions, cells, cell cultures or single celled organisms to toxins (in vitro).

There are many ways of measuring toxicity, and it is a relative phenomena. As a result we need to understand some of the conventions used in quantifying toxicity and the types of measurements commonly used. One of the first criteria in characterizing toxicity is the lethality, generally determined in mice or rats. Generally want a minimum sample size of 50 - 100 to get good statistics and a high probability of representing the population.

One of the best known and most often used measures is the LD₅₀, or the dose which is lethal for 50% of a given population. The LD₅₀ appears to be and often is a rather gross measure unless carefully observed. Though the LD₅₀ is a crude measure, its value can be increased by careful observation and analysis of changes occurring during its determination. For example, additional measures of toxicity which might be obtained include, in order of increasing information content (and cost):

• muscle tone, liver palpability (both are non-invasive measures)
• fatty liver (require biopsy or autopsy), serum protein changes (require blood sample)
• binding site saturation (requires isolation from tissue samples)

Each of these can provide a measurable response which will enhance the value of the toxicity test. However, much work will generally be required to get down to the detailed biochemical mode of action. Of course the more we know, the better we can guess and the more quickly we can assess new toxins for mode of action etc.

Of course, we can now also use biomarkers based on biotechnology techniques we discussed last time, including genomics, proteomics, metabolomics etc.

Interactions of Toxins: a number of terms are used to describe the interactions of toxins. It is useful to be familiar with the following:

• Additivity: the overall effect of two toxins is the direct addition of the individual toxicities of the doses given.
• Synergism: the toxicity of the doses of two toxins is greater than the sum of the toxicities of these two doses. Synergy gives a greater toxic effect than additive toxicities.
• Potentiation: similar to synergism, but the toxins have different effects. Specifically, one substance leads to a greater effect by a second toxic substance. An example is "antiabuse" in treating alcoholism - by itself at the dose given there is no effect, but when alcohol is ingested there is a toxic response to the alcohol at small doses.
• Antagonism: one substance reduces the toxicity of another.
• Tolerance: exposure at low doses leads to a tolerance whereby subsequent exposure to toxic dose levels prove to be less toxic than normal.

Dose-Response Curves

Dose-response Curve: When lethality is plotted against log Dose the typical response is described by a sigmoidal curve (text Fig 2.3, pg 11).

[This kind of curve shape also describes a wide range of binding phenomena, the most familiar being the pH titration curve. For the pH titration the axis are interchanged. The log is of course pH, while the % response corresponds to the % acid titrated.]

We generally consider that this relationship is based on three assumptions:

1. The toxic response is a function of the concentration of the compound at the site of action.
2. The concentration at the the site of action is related to the dose.
3. The response is causally related to the compound.

Let's look at these assumptions:

1. The response is a function of the concentration at the site of action:

• The site of action may be an enzyme, a receptor site, a cell, etc.
• The interaction at this site may be reversible or irreversible.

For reversible phenomena :

T + R TR,

the response is proportional to [TR]

 

where T = toxin, R = receptor, and TR = toxin - receptor complex

 

For r = R + TR = number of receptor sites, TR/r = 1 will get the maximum response.

 

K_eq will effect the dose-response curve, but the curve will remain sigmoidal, with:

• Strong binding giving a steep curve.
• Weak binding giving a shallow curve.

For irreversible interactions the response may still be proportional to the concentration at the site of action, but the results may be quite different. Since the interaction is irreversible a single interaction knocks out the receptor. The response will now depend on how fast the receptors may be replaced, and if the replacement is slow, the dose may be cumulative.

Last modified 4 February 2010

© RA Paselk 2001