Biomedical Engineering Reference
In-Depth Information
that pathway is difficult to inhibit (i.e., not very druggable), then the possibility of
finding a compound that specifically inhibits the target pathway without inhibiting
other pathways is low. Similarly, a certain cell subset might be found to act aberrantly
in a disease state. However, targeting that cell type might also invariably lead to
adverse side effects in other cell types that are more druggable. Therefore, although
target identification is perhaps the most important part of drug discovery, prudent
selection of targets requires prior knowledge of the landscape of druggable signaling
pathways and cell types, so that the chances of successfully identifying an efficacious
drug are maximized.
15.5.5 Needle in a Haystack: Drug Discovery in Rare Cell Types
One of the inherent challenges of drug discovery is attempting to target a cell type that
is rare in the body, such as a hematopoetic stem cell, a cancer stem cell, or an antigen-
specific T cell. These types of cells comprise 0.1% or less of total circulating blood
cells. However, these might be important drug targets, particularly with the potential
of stem cells to ameliorate many disparate diseases. These cell types are difficult to
analyze due to their rarity and a lack of in vitro model systems of their behavior. Here
again, the phospho flow platform provides a method for measuring the effects of
compounds directly on these cell types in the mixture of primary cell types in the
blood.
As an example of this capability, one of the natural product molecules we
identified in initial screens, streptonigrin, proved to have potent inhibitory effects
on B cells relative to T cells and neutrophils when we analyzed cell signaling in
murine splenocytes [16]. In the murine spleen, B cells comprise approximately
60% of the total white blood cells. Therefore, even if we had measured the entire
leukocyte population as a whole, we would have identified streptonigrin as an
inhibitor of Jak-Stat signaling. However, what was more illustrative was when we
treated mice with streptonigrin and measured its inhibitory effects on cytokine
signaling directly in the blood. In the blood, B cells comprise only 5% of the total
population. Therefore, when we analyzed the entire white blood cell population
present in the blood, streptonigrin did not have an appreciable effect on signaling.
However, by using surface markers to identify the various cell subsets, we
observed a clear and potent inhibitionofsignalinginBcells,butnotinanyof
the other cell subsets.
This example shows how the proportion of cell types in the blood can dramatically
affect the apparent activity of a drug. If the drug has selectivity for one cell type
relative to another, it will appear to be more potent/effective if it inhibits a more
prevalent cell type. If it inhibits a rare cell type, it will appear to have no effect on the
general population. This can lead to misleading conclusions about drug action and
toxicity.
Because many diseases, especially immunological diseases, are restricted to
particular cell types, we believe that drug discovery must take into account the
effects of compounds on one cell type relative to others. It cannot be assumed that
because a lead compound inhibits a kinase in enzymatic assays and appears to
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