Model Organisms in Drug Discovery

ELUCIDATING THE MECHANISM OF COMPOUND ACTION 161
components in the pathway, rendering the animals more sensitive or resistant
to the compound. If a mutation in one gene makes animals resistant to the
compound-induced phenotype, the compound is likely to target the same
biological network as the wild-type version of the mutant gene product.
However, compound-induced phenotypes often do not fall into a characterized
pathway, and a mutagenesis screen is undertaken to identify genes that,
when mutated, effect the compound-induced phenotype. Resistant and
sensitive strains are identified and the underlying gene mutations and genes
involved are isolated. The protein products produced from these genes then
become candidates for the drug target of action. An important control is to
test if mutants obtained are cross-resistant to other compounds, suggesting
mutations in genes involved in non-specific drug transport.
Once potential protein candidates for a compound’s target are identified in
model systems, the identification of mammalian orthologs of those proteins
may be complicated by the duplication of gene families in the evolution of the
mammalian genomes. In these cases, identifying the true ortholog among a
number of highly related proteins may be challenging. Once a candidate gene
list is identified, experiments are conducted to see if the orthologous
mammalian gene(s) is involved in the compound’s activity. By analogy, one
may expect that disruption of the mammalian ortholog may confer similar
compound resistance or sensitivity. However, artifacts do arise because a
compound may have unrelated effects in C. elegans and in mammalian cells
due to differences in the species protein complement. In general, we have
found that compounds have comparable effects. Once the list of candidate
genes has been narrowed down and the signaling pathway has been implicated
in both the mammalian and model systems, biochemistry is utilized to show a
direct binding of the compound to the target protein. This type of approach
may yield the mechanism of the compound’s therapeutic effect but may also
reveal off-target activities of the compound that may lead to potential
toxicological effects.
A prototype example of MOA analysis in C. elegans is provided by studies
on the drug Prozac. Prozac is well known to inhibit the mechanism by which
neuronal cells recycle serotonin by interfering with a reuptake protein, but it
was controversial as to whether all the therapeutic effects of Prozac could be
accounted for by the serotonin pathway or whether the reported side-effects
were due to unknown drug targets. Choy and Thomas (1999) tested the
hypothesis that Prozac might have multiple protein targets and several
mechanisms of action. They applied the compound to both wild-type animals
and C. elegans that were mutant for the production of serotonin. In wild-type
animals Prozac had effects consistent with the known mechanism of Prozac’s
action through effects on serotonin reuptake. Interestingly, however, they
found that Prozac had effects on animals lacking in serotonin altogether,
indicating a unique mechanism separate from the serotonin system. A genetic