From peptide libraries to optimized nonpeptide ligands in the search for S-farnesyltransferase inhibitors

A complete 331,776-member library of tetrapeptides made of 24 amino acid building blocks was synthesized robotically on solid phase and subjected to a deconvolution based on the inhibitory potency of the sublibraries in a HPLC assay of the S-farnesyltransferase activity in vitro. One of the non-natural peptide and noncysteine-containing leads Nip-Trp-Phe-His (Nip=p-nitrophenyl-L-alanine) was optimized chemically to give a proteolytically stable pseudopeptide with a 200-fold potency compared with the original lead. The final compound was converted to the C-terminal ethyl ester: p-F-C6H4-CO(CH2)2-CO-Bta-D-Phepsi[CH2NH]His-OEt (Bta = benzothienyl-L-alanine) and shown to behave as a prodrug which was hydrolyzed back to the C-terminal acid following cell penetration. The method confirmed that several structurally original leads can be discovered in large libraries when deconvolution relies upon a highly specific assay and that these leads can be optimized by chemical modification to impart the final compound the desired pharmacological and pharmacokinetic properties.