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for intracellular targets. Cell-based approaches broaden the target horizon and assure the identification of compounds with desirable cell permeability. The two largest disadvantages with cell-based screening are the absence of a defined mechanism of action and the need for more numerous specificity controls. In the following section, we will discuss a cell-based screen and compare it to a mechanism-based screen.

B. Case Study: Screen for TNF-a Expression Inhibitors

An example of a cell-based expression screen run at Pharmacopeia is that for inhibitors of tumor necrosis factor-a (TNF-a) production. In a search to identify inhibitors of TNF-a expression, we adapted an assay to a high-throughput screen that measured TNF-a concentration in the culture media from stimulated THP-1 human monocytic cells (Fig. 4). Compounds were screened at a final concentra-

Figure 5 Inhibition curves showing effect of PS200981 on p38a,P,y kinases.

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