Combined Actions and Interactions of Chemicals in Mixtures

(Brantom et al., 1997). A few in vitro systems, such as the HET-CAM Test (Hens
Egg Test at the Chorion Allantois Membrane), have been used to evaluate
combined effects of ocular irritants.
7.1.3.3 Examples of combined action
Contamination of the eye with surfactants and detergents represents a widespread,
but complex problem. Some surfactants, such as ordinary soap, cause immediate
stinging and burning with little or no injury. Other surfactants produce corneal
oedema and loss of corneal epithelium without any alerting discomfort. Several
cationic surfactants, e.g. benzalkonium chloride and cetylpyridinium chloride, may
produce severe delayed effects on the corneal epithelium and stroma. Additive or
synergistic effects of combined exposure to other ocular irritants can be expected,
when the barrier properties of the corneal epithelium have been compromised.
Exposure to acids and bases produce rapid, deep penetrating ocular injury as a
result of the extreme pH-changes within the tissues. Immediate effects are
dissolution of the epithelia and mottled clouding of the corneal stroma after
alkaline substances, or coagulation of epithelia after acids. Later effects include
oedema, opacification, vascularization, and degeneration of the cornea. Slight,
superficial and reversible injuries involving the corneal epithelium may cause great
discomfort due to irritation of the corneal nerve endings. More serious chemical
burns may, however, produce little pain, because destruction of the sensory nerves
of the cornea renders the cornea anaesthetic.
It is well established, that pre-treatment of the eyes of rabbits with topical
anaesthetics synergize the effects of ocular irritants. Anaesthetics reduce blinking
and tearing, thereby maintaining a higher concentration of the test-material
concentration at the surface of the eye. The anaesthetic may also increase corneal
permeability and this may bring the test agent into contact with more structures of
the eye. Some anaesthetics delay healing after ocular injury. All of these various
effects may result in increased irritation to the eye (Durham et al., 1992; Seabaugh
et al., 1993).
Ocular exposure to chemically inert solvents, especially very lipophilic solvents,
usually causes immediate stinging and smarting pain to the eyes, as it may cause
loss of some or all of the corneal epithelium. Recently, the combined effects on
ocular and nasal irritation of various solvents have been assessed. Threshold
responses of nasal irritation and eye irritation were determined in human volunteers
for single chemicals (1-oropanol, 1-hexanol, ethyl acetate, heptyl acetate, 2pentanone,
toluene, ethyl benzene, and propyl benzene), and their mixtures.
Various degrees of additive effects were observed for each of the three sensory
channels when testing mixtures. As the number of components and the lipophilicity
of such components in the mixtures increased, so did the degree of agonism.
Synergistic effects characterised the eye irritation response for the most complex
and the most lipophilic mixtures (Cometto-Muniz and Cain, 1997).
An in vitro test where the uptake of [3H]-uridine by mouse fibroblasts is measured
has been used to predict the ocular irritancy of 25 chemicals individually and in
combination. The test compounds included alcohols, ethers, esters, ketones,
amides, acids and a detergent. The concentration of the agents required to induce a
50% inhibition in uridine uptake rates after 4 hours of treatment correlated well
with published data on the chemicals' capacity to induce ocular irritation in rabbits.
Combinations of agents with differing functional groups produced additive effects
on the inhibition of uridine uptake, suggesting the utility of this approach for the
analysis of mixtures (Shopsis and Sathe, 1984).
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