Model Organisms in Drug Discovery

Compound learning set for assay validation
In addition to confirming the quality of a C. elegans assay, the assay must also
be pharmacologically validated. Devgen has used a learning set of about 250
CNS drugs to include drugs with mode of actions that modulate drinking and
drugs that should not affect drinking. These classes of drugs have been used to
validate the C. elegans ‘drinking assay’. Serotonin reuptake inhibition should
increase drinking and consequently 485% of the tested SSRIs and 475% of
all 5-HT reuptake inhibitors have been shown to enhance drinking rates in
the assay. Because dopamine negatively regulates drinking, inhibitors of
dopaminergic signaling should also enhance drinking, but indirectly. Fortyfive
percent of the dopamine antagonists tested have been shown to influence
drinking rates. The third important neurotransmitter that increases drinking
rates is acetylcholine. Consequently, none of the tested antagonists were
detected in the screen. A range of unrelated CNS compounds have been
tested, including adrenergic antagonists, opioids and histaminergics. Fifteen
percent of the adrenergic modulators were shown to be enhancers of drinking
rates and must be considered as false positives. The conclusion is that the
drinking screen for enhancers is highly sensitive because it identifies most of
the SSRIs for which the assay has been configured. It should be noted that this
C. elegans assay is able to identify reliable human drugs for a specific target
and that the assay is another example of the high level of conservation
between C. elegans and human pharmacology.
Another question is the number of false positives that can be expected.
Thirty-three percent of all hits obtained in the ‘drinking assay’ are the desired
SSRIs (Figure 3.5). In addition, 51% of the hits act specifically on the biology
under investigation because a serotonin receptor agonist also increases
drinking. Although these hits may be still of interest, we will show later how to
filter out dopaminergics and serotonin receptor modulators. The remaining
16% of the hits are false positives, which need to be filtered out as well. The
data validate the drinking assay as highly robust, sensitive and selective. We
will describe the C. elegans screening platform and a screening campaign with
the drinking assay.
The C. elegans screening platform
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The HTS equipment used in a C. elegans screening unit includes the same
robotics and technology found in standard HTS laboratories but with one
important difference (Seethala, 2001): instead of targets or cells being present
in the wells for screening, the wells are filled with living animals swimming
through the medium. The growth and handling requirements associated with
the use of living C. elegans animals limits the time available for screening to 3