Chronic Toxicity

Chronic Toxicity can be quantified in similar ways and can reveal evidence of accumulation and therefore estimates of body t_1/2. For chronic toxicity measure TD₅₀ or LD₅₀ for a specific time period, such as 90 days, and compare to the same total dose delivered in a single dose. Determine the chronicity factor.

Chronicity factor: (LD₅₀)_{1 dose}/(LD₅₀)_{90 doses} with units of mg/kg vs. mg/kg/day.

• Note in each case we have the same total dose top and bottom.
  • If CF = 90 then the toxin is absolutely cumulative. (e.g. for a compound with LD₅₀ = 90 mg/kg, if 90 1 mg/kg doses also gave LD₅₀, then CF = 90/1 = 90)
  • If CF > 2 then relatively cumulative.
  • If CF < 2 then relatively non-cumulative.

Accumulation can be due to accumulation of the compound in vivo or an accumulation of effect!

Example: tri-o-cresyl phosphate (TOCP), a cholinesterase inhibitor and neurotoxin. This toxin has the same effect in chickens in one 30 mg/kg dose or in 30 1 mg/kg dose delivered daily for 30 days. Here either situation could hold, because the t_1/2 of replacement for cholinesterase is > 30 days.

Transport across membranes: [overhead Fig 9.10, M&vH] (Timbrell fig. 3.2)

• Membrane structure:
  • Lipid bilayer provides barrier for transport of polar molecules, but will allow passage of non-polar molecules. (However, non-polar molecules have low solubility in aqueous solution at membrane surface, so exposure to non-polar is restricted. Overall then, between the aqueous environment and the lipid bilayer barrier, we have a very effective isolation from the outside world.)
  • Protein channels and specific binding/transport provide transport of polar molecules across the membrane, both non-specific and specific.
  • Note, have similar membranes enclosing various compartments within the cell as well.
• Mechanisms of transport:
  • Filtration.
  • Passive diffusion.
  • Facilitated diffusion: This is a catalyzed diffusion dependent on specific binding to a carrier molecule, commonly a protein. It is generally very specific, only catalyzing the transport of molecules which bind to the specific carrier active site. An example is the glucose carrier, which catalyzes the rapid equilibrium of glucose concentrations across membranes. Note that transport is driven by concentration gradients, there is no direct involvement of energy.
  • Active transport: This is very similar to facilitated diffusion in the sense that a specific carrier is involved. however in the case of active diffusion, particles are "pumped" against a concentration gradient with the expenditure of energy (e.g. ATP hydrolysis).
  • Pinocytosis: This involves the invagination of membrane to form a vesicle inside the cell. Both particles bound to the cell surface, and particles trapped from the bulk fluid during the invagination process are included in the vesicle. The bulk particles are obviously transported without specificity.

Factors affecting transport:

Molecular Properties

1. size/shape
2. lipid solubility/hydrophobicity
3. structural similarity to endogenous molecules
4. charge/polarity

Filtration

Filtration involves the flow of substances along a concentration gradient through pores. The only specificity will then be size - can the substance fit through the pore. The flow is filtered because large molecules or particles cannot pass through the pore.

Passive diffusion

1. Concentration gradients.
2. Lipid solubility
3. Ionization.

1. Concentration gradient:

Fick's Law: Rate of diffusion = k(A(C₁ - C₂)/d

A = area, d = thickness, C₁ = [ ] outside, C₂ = [ ] inside, k = constant.

Generally will be first order with k = 0.693/t_1/2, where t is the time of penetration. However, can get deviations with multiphasic processes.

Effects of Ionization on Distribution

Generally only non-ionized form passively absorbed across membrane: pH partition theory.

The proportion ionized depends on [H⁺] and K_a

HA H⁺ + A^-

and A + H₂O HA⁺ + OH^-

It is often more convenient to use log transformation.

Last modified 18 February 2010

© RA Paselk 2001