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Page 18<br />

related 4-hydroxypyrans and 4-hydroxycoumarins, the cyclic urea-<strong>based</strong> DMP323 series, and AG1284.<br />

<strong>Design</strong> and <strong>Structure</strong> of AG1343 (Nelfinavir)<br />

Analysis of the crystal structure of saquinavir with HIV PR indicated that while the nonpeptidic<br />

components of the ligand, namely the decahydroisoquinoline and the t-butylamide moieties fill the S1'<br />

and S2' subsites nearly optimally, the N-terminal portion offered the possibility for remodeling, aimed at<br />

the elimination of the peptidic character. Also, the contribution of the quinoline to the binding affinity to<br />

HIV PR was difficult to rationalize. Since the removal of quinoline resulted in a nearly 1000-fold loss in<br />

binding constant, it was concluded that the stacking interactions of the P1 phenyl ring and the P3<br />

aromatic moiety of quinoline are necessary for the conformational stability of Ro 31-8959. In an attempt<br />

to redesign the N-terminal part of the ligand, the nonpeptidic portions of the P1' and P2' were maintained<br />

but for reasons of synthetic accessibility, the decahydroisoquinoline moiety was replaced <strong>by</strong> an orthosubstituted<br />

benzylamide [35]. Crystallographic analysis of both compounds showed that saquinovir and<br />

the modified LY289612 bind essentially identically to the active site of HIV PR and their inhibition<br />

constants and antiviral activity were very similar (Table 4 and Table 3).<br />

In the first attempt to functionally substitute the P2 side chain of asparagine, the isophthalic-acidcontaining<br />

compound was modeled and the low-energy conformation of the aromatic ring, required for<br />

binding in the S2 subsite, was stabilized <strong>by</strong> a tertiary carboxamide in the P3 region of the inhibitor [36].<br />

The analysis of the binding mode and interactions of the isophthalic ring in the S2 subsite indicated a<br />

lipophilic pocket deep on the border between the S2 and S1' subsites, which could be conveniently filled<br />

with a methyl group extending from the 2 position of the ring. The resulting compound II in Table 4 lost<br />

most of the peptidomimetic character of LY289612 but retained its inhibitory potency.<br />

In an independent line of design, the relationship between the P1 phenylalanine side chain and the P3<br />

quinoline was investigated. In the crystal structure of saquinavir bound to the active site of HIV PR<br />

(Figure 4), the aromatic ring of the P1 phenylalanine makes several van der Waals contacts with<br />

residues forming the S1 subsite. Computer modeling indicated that an extension of the phenylalanine<br />

side chain to phenethyl (homophenylalanine) would lead to prohibitive close contacts of the phenyl ring<br />

with the aliphatic side chains of HIV PR. On the other hand, replacement of the γ-carbon of the<br />

homophenylalanine <strong>by</strong> sulphur, which has a more acute C-S-C bond angle, would direct the aromatic<br />

ring into the neighbouring S3 subsite without changing the desired lipophilic nature of the P1 side chain.<br />

The increased area of hydrophobic interactions in the S1 and S3 subsites <strong>by</strong> compounds with the Sphenylcysteine<br />

and<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_18.html [4/5/2004 4:44:45 PM]

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