PPIs with Noncontinuous Epitopes

The cytokine IL-2 plays a key role in growth, differentiation, and activation of T-cells. The antibodies daclizumab and basiliximab, which recognize the a5 subunit of the IL-2 receptor (IL-2Ra) and block binding of IL-2, have been clinically effective as immunosuppressive agents [122], However for reasons of cost, administration, and possible side effects, they are used only pre- and post-transplantation, and in the acute phase of transplant rejection. This state of affairs has precipitated interest in small-molecule inhibition of this interaction. 

Several other X-ray structures of EL-2 complexes attest to its surface malleability [127]. This serves as an excellent example of the capacity of a hydrophobic hot spot to undergo rearrangement to focus its surface into a shape more complementary to a small molecule. 

The Boehringer Ingleheim HPV project was discontinued during lead optimization [140] nevertheless, this effort shows how in the absence of structural validation, a variety of assays can be used to identify and verify inhibitors. It also illustrates the importance of information gained through crystal structures and structural analysis, and the ability of small molecules to exploit the conformational mobility of hydrophobic hot spots. 

This arrangement of hydrophobic side chains has proven to be compact enough for several groups to have found hit compounds from HTS [153-156], focused library screening [157, 158], and computational screening [159-161]. Many of 

Compounds from some of the above series have been optimized to low nanomolar affinity levels. The Johnson and Johnson group disclosed Nutlin-3 with a K, of 36 nM [153], and Nutlins have shown anticancer effects in vitro and in vivo [168]. Further development of spirooxindoles produced MI-219 [169], which has a 5 nM K,. and also shows effects in vitro and in vivo. Many compounds exhibit potent cellular activity and good pharmacokinetics. Several other series have appeared in recent years [170], and a few compounds have reached clinical trials [171]. 

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