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mulate during treatment; in particular, AZT-monoP i (XIII) accumulates in cells to millimolar levels.
These metabolites were investigated as possible integrase inhibitors and it was shown that all three
phosphate derivatives inhibit with IC 50 values of 100–150 μM, although AZT itself is not inhibitory [57].
These results suggest that despite the weak inhibition by these particular compounds, nucleoside analogs
may serve as lead compounds for the development of integrase inhibitors.
It was recently observed that oligonucleotides that form guanosine quartet structures inhibit HIV
replication [58]. In light of the AZT results that suggested that there may be a nucleotide binding site on
integrase—and because integrase binds DNA—these oligonucleotides (for example, 5'-
GTGGTGGGTGGGTGGGT-3', XIV) were investigated as possible inhibitors [59]. These compounds
have the lowest inhibition constants reported to date (see Table 1), and suggest an exciting new avenue
for integrase inhibitor development.
In a completely different approach to integrase inhibitors, a synthetic peptide combinatorial library was
used to select a hexapeptide capable of inhibiting integrase proteins [43]. The first two amino acids were
selected using a library of 400 dipeptides, and the remaining amino acids selected one-by-one in an
iterative process. The optimal hexapeptide, HCKFWW (XV), inhibits HIV-1 integrase with an IC 50 of 2
μM. The peptide does not compete for DNA binding to viral DNA, nor does it represent a sequence
present in integrase itself. Although it is not expected that a peptide consisting of L-amino acids would
be a suitable drug in itself, the use of D-amino acids or peptidomimetic backbones may be fruitful
directions to pursue.
Summary
Many of the compounds identified to date that inhibit HIV integrase in vitro have common structural
features as illustrated in Figure 9. Most notably, these include hydroxy-substitued phenyl rings.
However, these compounds may inhibit in vitro activity for reasons unrelated to enzyme binding. For
example, it is difficult to know what to make of inhibition studies where the compounds added to the
assays are known to bind DNA. Do the compounds affect in vitro activity because they bind and
sequester the substrate? It is possible that they distort the DNA or intercalate between base pairs,
preventing appropriate binding to the enzyme. While this is itself is a valid basis for the design of
inhibitors against HIV infection, particularly if it targets DNA specific to the virus such as the LTR
sequences [60], it is not an approach that builds on knowledge of the three-dimensional structure of the
protein. To this end, valuable information could be obtained from studies in which direct binding of
compounds to integrase is measured. This is also true for those inhibitors that do not contain the hydroxysubstituted
phenyl pharmacophore. Binding studies could be carried
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