Structure-activity relationships of small phosphopeptides, inhibitors of Grb2 SH2 domain, and their prodrugs

Catalytic Asymmetric Benzylation of Azomethine Ylides Enabled by Synergistic Lewis Acid/Palladium Catalysis

The synergistic chiral Lewis acid/achiral Pd catalyst system was successfully applied in the enantioselective benzylation of various imine esters, giving a range of α-benzyl-substituted α-amino acid derivatives in satisfactory yield with excellent enantioselectivity. It is worth noting that this strategy exhibits good tolerance for bicyclic and monocyclic benzylic electrophiles. Furthermore, the utility of this synthetic protocol was demonstrated by the expedient preparation of enantioenriched antihypertensive drug α-methyl-l-dopa.

The consequences of selective inhibition of signal transducer and activator of transcription 3 (STAT3) tyrosine705 phosphorylation by phosphopeptide mimetic prodrugs targeting the Src homology 2 (SH2) domain

Herein we review our progress on the development of phosphopeptide-based prodrugs targeting the SH2 domain of STAT3 to prevent recruitment to cytokine and growth factor receptors, activation, nuclear translocation and transcription of genes involved in cancer. We developed high affinity phosphopeptides (K_I = 46–200 nM). Corresponding prodrugs inhibited constitutive and IL-6 induced Tyr705 phosphorylation at 0.5–1 μM in a variety of human cancer cell lines. They were not cytotoxic at 5 μM in vitro but they inhibited tumor growth in a human xenograft breast cancer model in mice, accompanied by reduced VEGF expression and angiogenesis.

Selective targeting of disease-relevant protein binding domains by O-phosphorylated natural product derivatives

Phosphorylation-dependent protein binding domains are crucially important for intracellular signaling pathways and thus highly relevant targets in chemical biology. By screening of chemical libraries against 12 structurally diverse phosphorylation-dependent protein binding domains, we have identified fosfosal and dexamethasone-21-phosphate as selective inhibitors of two antitumor targets: the SH2 domain of the transcription factor STAT5b and the substrate-binding domain of the peptidyl-prolyl isomerase Pin1, respectively. Both compounds are phosphate prodrugs with documented clinical use as anti-inflammatory agents in humans and were discovered with a high hit rate from a small subgroup within the screening library. Our study indicates O-phosphorylation of appropriately preselected natural products or natural product derivatives as a generally applicable strategy for the identification of non-reactive and non-peptidic ligands of phosphorylation-dependent protein binding domains. Moreover, our data indicate that it would be advisable to monitor the bioactivities of clinically used prodrugs in their uncleaved state against phosphorylation-dependent protein binding domains.

Benzopyrazine derivatives: A novel class of growth factor receptor bound protein 7 antagonists

Growth factor receptor bound protein 7 (Grb7) is an adapter protein that functions as a downstream effector of growth factor mediated signal transduction. Over-expression of Grb7 has been implicated in a variety of cancers such as breast, blood, pancreatic, esophageal, and gastric carcinomas. Inhibition of Grb7 has been shown to reduce the migratory and proliferative potential of these cancers, making it an attractive therapeutic target. Starting with a known peptide antagonist, the present work reports the application of a succession of computational ligand design tools comprising a ligand shape based similarity search, molecular docking and a 2D-similarity search to identify small molecular antagonists of the Grb7-SH2 domain from the NCI chemical database. Binding to the Grb7-SH2 domain was then experimentally tested using melting point shift assays and isothermal titration calorimetry. Overall, a total of 11 benzopyrazine based small molecular antagonists were identified with affinity for the Grb7-SH2 domain. Representative compounds tested using ITC were revealed to possess moderate binding affinity in the low micromolar range. Finally, the lead compound (NSC642056) was found to reduce the growth of a Grb7-expressing breast cancer cell line with an IC(50) of 86μM. It is expected that the identified antagonists will be useful additions to further explore the function of Grb7 and for the development of inhibitors with therapeutic potential.

Synthesis of phosphatase-stable, cell-permeable peptidomimetic prodrugs that target the SH2 domain of Stat3

The synthesis of prodrugs targeted to the SH2 domain of Stat3 is reported. Using a convergent strategy, the pivaloyloxymethyl phosphonodiester of pentachlorophenyl 4-phosphonodifluoromethylcinnamate, a phosphotyrosine surrogate, was synthesized and used to acylate peptidomimetic fragments that were prepared on solid supports. Two prodrugs described here inhibited the phosphorylation of Stat3 in breast tumor cells.

Grb2 signaling in cell motility and cancer

BACKGROUND

Metastasis is the primary cause of death in most human cancers, and understanding the molecular mechanisms underpinning this multistep process is fundamental to identifying novel molecular targets and developing more effective therapies.

OBJECTIVE/METHODS

Here we review the role of growth factor receptor-bound protein 2 (Grb2) in cancer and specifically in metastasis-related processes, and summarize the development of anticancer therapeutics selectively targeting this adapter protein.

RESULTS/CONCLUSION

Grb2 is a key molecule in intracellular signal transduction, linking activated cell surface receptors to downstream targets by binding to specific phosphotyrosine-containing and proline-rich sequence motifs. Grb2 signaling is critical for cell cycle progression and actin-based cell motility, and, consequently, more complex processes such as epithelial morphogenesis, angiogenesis and vasculogenesis. These functions make Grb2 a therapeutic target for strategies designed to prevent the spread of solid tumors through local invasion and metastasis.

Development of Binding Assays for the SH2 Domain of Grb7 and Grb2 Using Fluorescence Polarization

Adaptor proteins Grb7 and Grb2 have been implicated as being 2 potential therapeutic targets in several human cancers, especially those that overexpress ErbB2. These 2 proteins contain both a SH2 domain (Src homology 2) that binds to phosphorylated tyrosine residues contained within ErbB2 and other specific protein targets. Two assays based on enzyme-linked immunosorbent assay and fluorescence polarization methods have been developed and validated to find and rank inhibitors for both proteins binding to the pY1139. Fluorescence polarization assays allowed the authors to determine quickly and reproducibly affinities of peptides from low nanomolar to high micromolar range and to compare them directly for Grb7 and Grb2. As a result, the assays have identified a known peptidomimetic Grb2 SH2 inhibitor (mAZ-pTyr-(αMe)pTyr-Asn-NH2) that exhibits the most potent affinity for the Grb7 SH2 domain described to date. ( Journal of Biomolecular Screening 2008:112-119)

Structure-based design of potent Grb2–SH2 domain antagonists not relying on phosphotyrosine mimics

Development of Grb2-SH2 domain antagonists is considered to be an effective and non-cytotoxic strategy to develop new antiproliferative agents because of their potential to shut down the Ras signaling pathway. We developed a concise route for the efficient synthesis of G1TE analogs on solid phase. Using this route, a series of cyclic peptides that do not rely on phosphotyrosine or its mimics were designed and synthesized based upon the phage library-derived cyclopeptide, G1TE. Considering that Gly7 plays prominent roles for G1TE binding to the Grb2-SH2 domain, we introduced different amino acids in the 7th position. The D-Ala7-containing peptide 3 demonstrates improved binding affinity by adopting favorable conformation for protein binding. This can be rationalized by molecular modeling. The optimization at the Leu2 position was also studied, and the resulting cyclopeptides exhibited remarkably improved binding affinity. Based upon these global modifications, a highly potent peptide ligand 9 was discovered with a Kd = 17 nM, evaluated by Biacore binding assay. This new analog is one of the most potent non-phosphorus-containing Grb2-SH2 antagonists reported to date. This potent peptidomimetic provides a new template for the development of non-pTyr containing Grb2-SH2 domain antagonists and acts as a chemotherapeutic lead for the treatment of erbB2-related cancer.

Affinity chromatography for purification of the modular protein growth factor receptor-bound protein 2 and development of a screening test for growth factor receptor-bound protein 2 Src homology 3 domain inhibitor using peroxidase-linked ligand

Growth factor receptor-bound protein 2 (Grb2) is an adapter protein involved in the Ras-dependent signaling pathway that plays an important role in human cancers initiated by oncogenic receptors. Grb2 is constituted by one Src homology 2 domain surrounded by two SH3 domains, and the inhibition of the interactions produced by these domains could provide an antitumor approach. In evaluating chemical libraries, to search for potential Grb2 inhibitors, it was necessary to elaborate a rapid test for their screening. We have developed, first, a batch method based on the use of an affinity column bearing a Grb2-SH3 peptide ligand to isolate highly purified Grb2. We subsequently describe a very rapid 96-well screening of inhibitors based on a simple competition between purified Grb2 and a peroxidase-coupled proline-rich peptide.

Development of Grb2 SH2 Domain Signaling Antagonists: A Potential New Class of Antiproliferative Agents

Aberrant signaling through protein-tyrosine kinase (PTK)-dependent pathways is associated with several proliferative diseases. Accordingly, PTK inhibitors are being developed as new approaches for the treatment of certain cancers. Growth factor receptor bound protein 2 (Grb2) is an important downstream mediator of PTK signaling that serves obligatory roles in many pathogenic processes. One of the primary functions of Grb2 is to bind to specific phosphotyrosyl (pTyr)-containing sequences through its Src homology 2 (SH2) domain. Agents that bind to the Grb2 SH2 domain and prevent its normal function could disrupt associated PTK signaling and serve as alternatives to kinase-directed inhibitors. Starting from the X-ray crystal structure of a lead peptide bound to the Grb2 SH2 domain, this review will summarize important contributions to these efforts. The presentation will be thematically arranged according to the region of peptide modified, proceeding from the N-terminus to the C-terminus, with a special section devoted to aspects of conformational constraint.