N-terminal carboxyl and tetrazole-containing amides as adjuvants to Grb2 SH2 domain ligand binding

High affinity binding of peptides to Src homology 2 (SH2) domains, often requires the presence of phosphotyrosyl (pTyr) or pTyr-mimicking moieties in the N-terminal position of the binding ligand. Several reports have shown that N(alpha)-acylation of the critical pTyr residue can result in increased SH2 domain binding potency. For Grb2 SH2 domains which recognize pTyr-Xxx-Asn-NH(2) motifs, significant potency enhancement can be incurred by N(alpha)-(3-amino)Z derivatization of tripeptides such as pTyr-Ile-Asn-NH(2). Using ligands based on the high affinity pY-Ac(6)c-Asn-(naphthylpropylamide) motif, (where Ac(6)c=1-aminocyclohexanecarboxylic acid), additional reports have shown moderate potentiating effects of N(alpha)-oxalyl derivatization. The current study examined variations of the N(alpha)-oxalyl theme in the context of a Xxx-Ac(6)c-Asn-(naphthylpropylamide) platform, where Xxx=the hydrolytically stable pTyr mimetics phosphonomethyl phenylalanine (Pmp) or carboxymethyl phenylalanine (Cmf). The effects of N(alpha)-(3-amino)Z derivatization were also investigated for this platform, to ascertain whether the large binding enhancement reported for tripeptides such as pTyr-Ile-Asn-NH(2) could be observed. In ELISA-based extracellular Grb2 SH2 domain binding assays, it was found for the Pmp-based series, that extending the oxalyl carboxyl out by one methylene unit or replacing carboxyl functionality with a tetrazole isostere, resulted in binding potency greater than the parent N(alpha)-acetyl-containing compound, with enhancement approximating that observed for the N(alpha)-oxalyl derivative. When Cmf was used as the pTyr mimetic, only modest differences in IC(50) values were observed for the series. Examination of the N(alpha)-(3-amino)Z derivatized Pmp-Ac(6)c-Asn-(naphthylpropylamide), showed that binding affinity was reduced relative to the parent N(alpha)-acetyl analogue, in contrast to the reported significant enhancement of affinity observed with other peptide ligands. Treatment of MDA-453 tumor cells, which are mitogenically driven through erbB-2 tyrosine kinase-dependent pathways, with Pmp-containing inhibitors resulted in growth inhibition, with the N(alpha)-oxalyl and N(alpha)-malonyl-containing compounds exhibiting IC(50) values (4.3 and 4.6 microM, respectively) approximately five-fold lower than the parent N(alpha)-acetyl-containing compound. Tetrazole and N(alpha)-(3-amino)Z-containing inhibitors were from two- to four-fold less potent than these latter analogues in the growth inhibition assays.