This preview shows page 1. Sign up to view the full content.
Unformatted text preview: than 96
h. In practice they may have durations of 7 days to 60
days or for the life-span of the organism. Ecotoxicology
LC50 as a function of time with
has not yet attained the level of sophistication of
mammalian toxicology where full life -span tests are routinely conducted in rodents. The most
likely reason for this is the expense of maintaining environmentally appropriate organisms such
as fish and the cost of treating the effluent from their tanks.
Time From a pharmacokinetic point of view, the chronic test should have a duration of at least three
half-lives in the organism. This would allow the compound to almost attain equilibrium in the
organism and thus allow the expression of responses at the highest body dose likely to be
attained. Lifetime tests are designed to test for delayed toxic effects such as reproductive
problems, developmental abnormalities, and cancer.
3.1.5 Selectivity Selectivity of toxic action is very important in a ssessing the effects of toxic substances in the
environment. Selectivity is well illustrated in the case of pesticides, which are designed to
protect some organisms while controlling others. ―Selective pesticides‖ has long been the cry of
the Integrated Pest Management (IPM) special ist, the environmentalist, and of the general public.
It is perhaps relevant to first define selectivity in the context of this discussion. Selectivity can
be defined as an increase in toxicity towards one organism (the pe st in the case of pesticides),
combined with a reduction in hazard to the user and/or the organisms being protected. This type
of selectivity is the first aim of all drug and pesticide develop ment and manufacture and often is
achieved as the result of t he specific receptor systems found only in the pest organism. An
example of this is seen in the selectivity of the herbicide glyphosate for plants, which possess the
target enzyme ( See Chapter 1, Section 1.7.8 ) Receptor -mediated modes of toxicity usually result
in greater toxicity to organisms that possess the receptor system and smaller, background,
toxicity as a result of their physical or narcotic actions ( Lipnick 1993) in non-receptor organism
(Figure 3-14). Narcotic
Narcotic Response Receptor-mediated Concentration Selective toxicity towards a class or group
of pests is an obvious ad vantage in selective
control and is a characteristic of many
fungicides and herbicides where the non target organisms are also in the same class
as the pests. With insects, it is obvious that
the insecticide must not harm the plants to
be protected; but it is also an advantage for
integrated pest management if the
insecticide is not toxic to beneficial insects
(bees) and other useful, or important,
organisms. Figure 3-146
Illustration of differences in sensitivity between organisms
with and those without the receptor system The disadvantages of too great a selectivity
towards certain pests are mostly economic
— too small a market may be available to
support development costs.
especially relevant today with the in creasing complexity and expense of toxicology in supp...
View Full Document
This document was uploaded on 04/07/2014.
- Fall '14
- The Land