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Page 20<br />

S-naphthylcysteine derived side chains in P1 resulted in a substantial increase in the inhibition constants<br />

[37]. The increase in the binding affinity to the low picomolar range in enzyme inhibition assay, allowed<br />

for subsequent truncation of the P3 quinoline moiety. The final compound from this miniseries<br />

(compound III in Table 4) consisted of the ortho-substituted benzamide in the P1' and P2', Snaphthylcysteine<br />

in P1 and asparagine in P2. Despite reduced molecular weight, the inhibition constant<br />

of this compound for HIV PR was comparable to LY289612.<br />

The observation that a larger, nonpeptidic moiety in the P1 could eliminate the need for the P3 side<br />

chain led to hybrid molecules that incorporated ring structures as the P2 component and maintained the<br />

P1 S-naphthylcysteine side chain of compound III. In this miniseries several bicyclic functionalities<br />

were modeled as the P2 substituents and one example, compound IV utilizing a tetrahydroquinoline<br />

group, is shown in Table 4 [38]. In subsequent modeling, it was noticed that the P2 bicyclic functionality<br />

might be replaced <strong>by</strong> 2,3-disubstituted phenyl rings. In particular, a methyl substitution in position 2<br />

would increase the area of hydrophobic interaction in a manner previously observed in the isophthal<br />

series. Addition of a hydrophilic functionality attached at position 3 could increase the solubility of the<br />

compound and contribute to the binding constant <strong>by</strong> forming a hydrogen bond with the carboxylate<br />

oxygen of Asp30. A compound with a 2-methyl-3-hydroxy substitution pattern was synthesized and<br />

showed an improved inhibition constant of 3 nM in the HIV PR enzyme assay (Table 4). The crystal<br />

structure of compound V with HIV PR was solved and indicated the predicted binding mode with the<br />

possibility of a stacking interaction between the P2 phenyl and the P1 thio-naphthyl groups and the<br />

expected hydrophobic and hydrogen-bonding interactions of the P2 moiety with the protein side chains<br />

in the corresponding specificity pocket [38].<br />

As with the optimized compounds from other series, compound V suffered from low aqueous solubility.<br />

The replacement of the P1' aryl group <strong>by</strong> the basic amine-containing decahydroisoquinoline<br />

dramatically increased the solubility and allowed for truncation of the P1 S-naphthylcysteine side chain<br />

to S-phenylcysteine without any loss of inhibitory activity. The resulting compound VI, AG1343 or<br />

nelfinavir, has an inhibition constant of 1.9 nM in the HIV PR enzyme assay and respectable antiviral<br />

activity with an IC 90 of 60 nM [39]. The nonpeptidic character, pH-dependent solubility profile, and the<br />

small molecular weight of nelfinavir may contribute to its good pharmacokinetic profile in humans<br />

[40,41]. Currently, this compound is being tested and is in the advanced phase of clinical trials.<br />

The crystal structure of nelfinavir bound to the active site of HIV PR is shown in Figure 5. The general<br />

binding mode of this compound, in particular the path of the backbone, is similar to the binding mode of<br />

peptidyl inhibitors. Nevertheless, the lack of any peptide bonds utilizing naturally occurring amino<br />

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