Aspects of Peptidomimetics - Peptide and Protein Design for Biopharmaceutical Applications - page 98

The U-II pharmacophore was used by several companies to screen libraries,
resulting in various nonpeptide U-II agonists and antagonists, which were
recently reviewed by Blakeney and Carotenuto [327,364]. One of the first
reported examples used the three-point pharmacophore model, composed
of the Trp 8 ,Lys 9 and Tyr 10 side chains, to screen the Aventis compound
collection [365]. Compound S6716 was identified as a potent U-II receptor
antagonist. Some more recent examples are included in Figure 3.51. The
first reported nonpeptide agonist was the isochromanone AC-7954. Its
structure served as lead for the development of the more potent 4-phenyl-
benzamide [366]. The Johnson & Johnson group idendified a phenylpiper-
idine-benzoxazinone lead from a library screening. It was optimized
to yield a low-nanomolar antagonist against both rat and human U-II
receptors [367].
Many of the concepts that were discussed in the previous sections have
been applied to the design of inhibitors of the interaction between the
tumour-suppression gene p53 and the human or murine double minute-2
(HDM2 or MDM2) oncogene [368-370]. The p53 protein regulates cell
proliferation by induction of growth arrest or apoptosis in response to
DNA damage or stress stimuli. The HDM2 protein binds to the transac-
tivation domain of p53 and downregulates its activity. The disruption of
this protein-protein interaction has been intensively studied as an
approach for cancer therapy, because it would allow the upregulation
of the p53 response. Binding to the MDM2 protein occurs through the
N-terminal transactivation domain of p53. A crystal structure of the
complex between MDM2 and a 15-residue sequence of p53 revealed a
narrow hydrophobic cleft in the MDM2 protein to which three hydro-
phobic side chains of the p53 sequence, Phe 19 , Trp 23 and Leu 26 , make
direct contact. These three residues are aligned along one face of the
amphipatic a-helix that is adopted by the p53 N-terminal domain. This
discovery stimulated research toward peptide mimetics that display three
hydrophobic groups in an orientation that mimics their presentation in
the a-helix.
A team at Novartis screened peptide libraries, obtained via phage display,
and identified a 12-mer peptide with a greater inhibition than that of the
wild-type p53 sequence [371]. Truncation of this peptide sequence revealed
an octapeptide sequence Ac-Phe-Met-Asp-Tyr-Trp-Glu-Gly-Leu-NH 2 as
the minimal sequence retaining micromolar affinity for HDM2. Exploiting
the propensity of a,a-disubstituted amino acids to induce helical conforma-
tions, the Asp and Gly residues were replaced by a-aminoisobutyric acid
(Aib) and 1-aminocyclopropanecarboxylic acid (Ac 3 c) residues, respec-
tively, resulting in an increased affinity. Subsequent replacement of the Tyr