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As predicated, the (S) acid was a 30-fold more potent inhibitor of PNP than the (R) form. X-ray
crystallographic analysis of the complexes revealed that the (S) acid was oriented properly for optimal
interactions with all three subsites (Figure 4F), whereas the (R) acid was not. This series of compounds
contains the most potent membrane-permeable inhibitors of PNP yet reported [27].
V. Summary
Recently, scientists at BioCryst have successfully completed a project to design and synthesize potent
inhibitors of the enzyme Purine Nucleoside Phosphorylase (PNP) using the three-dimensional structure
of the active site. Crystallographic and modeling methods have been combined with organic synthesis to
produce inhibitors. Our experience in creating a set of potential drugs—one of which (BCX-34) is now
in human trials for treating psoriasis and a form of T-cell lymphoma—illustrates the process and the
power of structure-based design.
This structure-based inhibitor design approach led to a number of inhibitors more than 100 times more
potent than any membrane-permeable inhibitor available at the beginning of this project. During the two
and half years of this project, about 60 active compounds were synthesized. This is a remarkably small
number compared with the extensive synthesis programs generally involved in drug discovery by trial
and error techniques. The large number of active compounds and the enhancement of inhibitor potency
stand as proof that crystallographic and modeling techniques are now capable of playing a critical role in
the rapid discovery of novel therapeutic agents. The entire protocol, from choosing the target to creating
a drug suitable for clinical trials, can probably be accomplished today in two or three years.
A. Obstacles Encountered and Lessons Learned
Crystallographic analysis was based primarily on the results of difference Fourier maps in which the
interactions between residues in the active site and the inhibitor could be characterized. During these
studies, about 35 inhibitor complexes were evaluated by x-ray crystallographic techniques. It is
noteworthy that the resolution of the PNP model extends to only 2.8 Å and that all of the difference
Fourier maps were calculated at 3.2 Å resolution, much lower than often considered essential for drug
design. Crystallographic analysis was facilitated by the large solvent content that allowed for free
diffusion of inhibitors into enzymatically active crystals.
Initial inhibitor modeling attempts using the native PNP structure were far less successful than
subsequent analyses in which coordinates for the guanine-
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