Solid-phase synthesis of Stat3 inhibitors incorporating O-carbamoylserine and O-carbamoylthreonine as glutamine mimics

Targeting SH2 domains in breast cancer

Breast cancer is among the most commonly diagnosed cancer types in women worldwide and is the second leading cause of cancer-related disease in the USA. SH2 domains recruit signaling proteins to phosphotyrosine residues on aberrantly activated growth factor and cytokine receptors and contribute to cancer cell cycling, metastasis, angiogenesis and so on. Herein we review phosphopeptide mimetic and small-molecule approaches targeting the SH2 domains of Grb2, Grb7 and STAT3 that inhibit their targets and reduce proliferation in in vitro breast cancer models. Only STAT3 inhibitors have been evaluated in in vivo models and have led to tumor reduction. Taken together, these studies suggest that targeting SH2 domains is an important approach to the treatment of breast cancer.

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.

STAT inhibitors for cancer therapy

Signal Transducer and Activator of Transcription (STAT) proteins are a family of cytoplasmic transcription factors consisting of 7 members, STAT1 to STAT6, including STAT5a and STAT5b. STAT proteins are thought to be ideal targets for anti-cancer therapy since cancer cells are more dependent on the STAT activity than their normal counterparts. Inhibitors targeting STAT3 and STAT5 have been developed. These included peptidomimetics, small molecule inhibitors and oligonucleotides. This review summarized advances in preclinical and clinical development of these compounds.

Structure-Activity Studies of Phosphopeptidomimetic Prodrugs Targeting the Src Homology 2 (SH2) Domain of Signal Transducer and Activator of Transcription 3 (Stat3)

Signal transducer and activator of transcription 3 (Stat3) transmits signals from growth factors and interleukin-6 family cytokines by binding to their receptors via its Src homology 2 (SH2) domain. This results in phosphorylation of Tyr705, dimerization, translocation to the nucleus, and regulation of transcription of downstream genes. Stat3 is constitutively activated in several human cancers and is a target for anti-cancer drug design. We have shown previously phosphorylation of Tyr705 in intact cancer cells can be inhibited with prodrugs of phosphopeptide mimics targeting the SH2 domain. In a series of prodrugs consisting of bis-pivaloyloxymethyl esters of 4'-phosphonodifluoromethyl cinnamoyl-Haic-Gln-NHBn, appending methyl group to the β-position of the cinnamate increased potency ca. twofold, which paralleled the increase in affinity of the corresponding phosphopeptide models. However, dramatic increases in potency were observed when the C-terminal C(O)NHBn of Gln-NHBn was replaced with a simple methyl group. In this communication we continue to explore the effects of structural modifications of prodrugs on their ability to inhibit Tyr705 phosphorylation. A set of 4-substituted prolines incorporated into β-methyl-4-phosphocinnamoyl-leucinyl-Xaa-4-aminopentamide model peptides exhibited affinities of 88-317 nM by fluorescence polarization (Pro IC₅₀ = 156 nM). In corresponding prodrugs, Pro inhibited constitutive Stat3 phosphorylation at 10 μM in MDA-MB-468 breast tumor cells. However, 4,4-difluoroproline and 4,4-dimethylproline resulted in complete inhibition at 0.5 μM. These results suggest that the prodrug with native proline undergoes metabolism that those with substituted prolines do not. In conclusion, changes in structure with minimal impact on intrinsic affinity can nevertheless have profound effects on the cellular potency of prodrug inhibitors of Stat3.

A molecular model for the differential activation of STAT3 and STAT6 by the herpesviral oncoprotein tip

Constitutive STAT signaling provides growth promoting signals in many forms of malignancy. We performed molecular modeling and molecular dynamics studies of the interaction between the regulatory Src homology 2 (SH2) domains of STAT3 and 6 with phosphorylated peptides of the herpesviral oncoprotein Tip, which facilitates Src kinase mediated STAT-activation and T cell proliferation. The studies give insight into the ligand binding specificity of the STAT SH2 domains and provide the first model for the differential activation of STAT3 or STAT6 by two distinct regions of the viral Tip protein. The biological relevance of the modeled interactions was then confirmed by activation studies using corresponding recombinant oncoproteins, and finally by respective recombinant viruses. The functional data give experimental validation of the molecular dynamics study, and provide evidence for the involvement of STAT6 in the herpesvirus induced T cell proliferation.

A phosphopeptide mimetic prodrug targeting the SH2 domain of Stat3 inhibits tumor growth and angiogenesis

Signal transducer and activator of transcription 3 (Stat3) is constitutively activated in a number of human cancers and cancer cell lines. Via its Src homology 2 (SH2) domain, Stat3 is recruited to phosphotyrosine residues on intracellular domains of cytokine and growth factor receptors, whereupon it is phosphorylated on Tyr705, dimerizes, translocates to the nucleus and is reported to participate in the expression of genes related to angiogenesis, metastasis, growth and survival. To block this process, we are developing cell-permeable, phosphatase-stable phosphopeptide mimics, targeted to the SH2 domain of Stat3, that inhibit the phosphorylation of Tyr705 of Stat3 in cultured tumor cells (Mandal et al., J. Med. Chem. 54, 3549-5463, 2011). At concentrations that inhibit tyrosine phosphorylation, these materials were not cytotoxic, similar to recent reports on JAK inhibitors. At higher concentrations, cytotoxicity was accompanied by off-target effects. We report that treatment of MDA-MB-468 human breast cancer xenografts in mice with peptidomimetic PM-73G significantly inhibited tumor growth, which was accompanied by reduction in VEGF production and microvessel density. No evidence of apoptosis or changes in the expression of the canonical genes cyclin D1 or survivin were observed. Thus selective inhibition of Stat3 Tyr705 phosphorylation may be a novel anti-angiogenesis strategy for the treatment of cancer.

Signal transducer and activator of transcription 3 (STAT3): a promising target for anticancer therapy

Signal transducer and activator of transcription 3 (STAT3) is an oncogenic protein whose inhibition is sought for the prevention and treatment of cancer. In this review, the validated therapeutic strategy to block aberrant activity of STAT3 in many tumor cell lines is evaluated by presenting the most promising inhibitors to date. The compounds are discussed in classes based on their different mechanisms of action, which are critically explained. In addition, their future clinical development as anticancer agents is considered. Furthermore, the efforts devoted to the comprehension of the structure-activity relationships and to the identification of the biological effects are brought to attention. The synthetic and technological approaches recently developed to overcome the difficulties in the obtainment of clinically suitable drugs are also presented.

Antagonism of the Stat3-Stat3 protein dimer with salicylic acid based small molecules

More than 50 new inhibitors of the oncogenic Stat3 protein were identified through a structure-activity relationship (SAR) study based on the previously identified inhibitor S3I-201 (IC₅₀ =86 μM, K(i) >300 μM). A key structural feature of these inhibitors is a salicylic acid moiety, which, by acting as a phosphotyrosine mimetic, is believed to facilitate binding to the Stat3 SH2 domain. Several of the analogues exhibit higher potency than the lead compound in inhibiting Stat3 DNA binding activity, with an in vitro IC₅₀ range of 18.7-51.9 μM, and disruption of Stat3-pTyr peptide interactions with K(i) values in the 15.5-41 μM range. One agent in particular exhibited potent inhibition of Stat3 phosphorylation in both breast and multiple myeloma tumor cells, suppressed the expression of Stat3 target genes, and induced antitumor effects in tumor cells harboring activated Stat3 protein.

Potent and selective phosphopeptide mimetic prodrugs targeted to the Src homology 2 (SH2) domain of signal transducer and activator of transcription 3

Signal transducer and activator of transcription 3 (Stat3), a target for anticancer drug design, is activated by recruitment to phosphotyrosine residues on growth factor and cytokine receptors via its SH2 domain. We report here structure-activity relationship studies on phosphopeptide mimics targeted to the SH2 domain of Stat3. Inclusion of a methyl group on the β-position of the pTyr mimic 4-phosphocinnamide enhanced affinity 2- to 3-fold. Bis-pivaloyloxymethyl prodrugs containing β-methylcinnamide, dipeptide scaffolds Haic and Nle-cis-3,4-methanoproline, and glutamine surrogates were highly potent, completely inhibiting phosphorylation of Stat3 Tyr705 at 0.5-1 μM in a variety of cancer cell lines. The inhibitors were selective for Stat3 over Stat1, Stat5, Src, and p85 of PI3K, indicating ability to discriminate individual SH2 domains in intact cells. At concentrations that completely inhibited Stat3 phosphorylation, the prodrugs were not cytotoxic to a panel of tumor cells, thereby showing clear distinction between cytotoxicity and effects downstream of activated Stat3.

Structure-Based Design of Conformationally Constrained, Cell-Permeable STAT3 Inhibitors

We report herein the structure-based design of a class of conformationally constrained, potent, cell-permeable small-molecule inhibitors to target the SH2 domain in STAT3. Compound 11 (CJ-1383) binds to STAT3 with a K(i) value of 0.95 µM, dose-dependently inhibits cellular STAT3 signaling and cancer cell growth, and induces apoptosis in the MDA-MB-468 cancer cell line with constitutively activated STAT3.

Molecular disruption of oncogenic signal transducer and activator of transcription 3 (STAT3) protein

Signal transducer and activator of transcription protein 3 (STAT3) is a latent cytosolic transcription factor that is widely recognized as being a master regulator of the cellular functions that lead to the cancer phenotype. Constitutively activated STAT3 protein activity is routinely observed in human cancers, promoting uncontrolled cell proliferation and suppressing apoptosis. Until relatively recently, inhibition of STAT3 transcriptional activity was achieved indirectly via suppression of upstream kinase activators and extracellular cytokine and (or) growth factor stimuli. However, activated STAT3 forms transcriptionally functional STAT3-STAT3 dimers, providing a valid juncture for targeted downstream molecular inhibition. STAT3's prominent role in cancer has seen a decade of innovative and novel approaches to targeting constitutively active STAT3 protein-protein complexes. This mini-review outlines the progress made towards identifying molecular agents capable of silencing aberrant STAT3 signalling through the disruption of STAT3 complexation events.

Structure-affinity relationships of glutamine mimics incorporated into phosphopeptides targeted to the SH2 domain of signal transducer and activator of transcription 3

In cancer cells, signal transducer and activator of transcription 3 (Stat3) participates in aberrant growth, survival, angiogenesis, and invasion signals and is a validated target for anticancer drug design. We are targeting its SH2 domain to prevent docking to cytokine and growth factor receptors and subsequent signaling. One of the important elements of the recognition sequence, pTyr-Xxx-Xxx-Gln, is glutamine. We incorporated novel Gln mimics into a lead peptide, pCinn-Leu-Pro-Gln-NHBn, and found that a linear, unconstrained side chain and carboxamide are necessary for high affinity, and the benzamide can be eliminated. Replacement of Gln-NHBn with (R)-4-aminopentanamide or 2-aminoethylurea produced inhibitors with equal or greater potency than that of the lead, as judged by fluorescence polarization (IC(50) values were 110 and 130 nM, respectively). When Pro was replaced with cis-3,4-methanoproline, the glutamine mimic, (4R,5S)-4-amino-5-benzyloxyhexanamide resulted in an IC(50) of 69 nM, the highest affinity Stat3 inhibitor reported to date.

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.

Conformationally constrained peptidomimetic inhibitors of signal transducer and activator of transcription. 3: Evaluation and molecular modeling

Signal transducer and activator of transcription 3 (Stat3) is involved in aberrant growth and survival signals in malignant tumor cells and is a validated target for anticancer drug design. We are targeting its SH2 domain to prevent docking to cytokine and growth factor receptors and subsequent signaling. The amino acids of our lead phosphopeptide, Ac-pTyr-Leu-Pro-Gln-Thr-Val-NH(2), were replaced with conformationally constrained mimics. Structure-affinity studies led to the peptidomimetic, pCinn-Haic-Gln-NHBn (21), which had an IC(50) of 162 nM (fluorescence polarization), compared to 290 nM for the lead phosphopeptide (pCinn = 4-phosphoryloxycinnamate, Haic = (2S,5S)-5-amino-1,2,4,5,6,7-hexahydro-4-oxo-azepino[3,2,1-hi]indole-2-carboxylic acid). pCinn-Haic-Gln-OH was docked to the SH2 domain (AUTODOCK), and the two highest populated clusters were subjected to molecular dynamics simulations. Both converged to a common peptide conformation. The complex exhibits unique hydrogen bonding between Haic and Gln and Stat3 as well as hydrophobic interactions between the protein and pCinn and Haic.

Structural basis for the binding of high affinity phosphopeptides to Stat3

Signal transducer and activator of transcription 3 (Stat3) is constitutively active in a number of cancers where it participates in aberrant transcription of prosurvival, cell cycling, and angiogenesis genes. Since Stat3 initiates its signaling activity through binding of its SH2 domain to phosphotyrosine residues on cell surface receptors, inhibitors targeting this region of the protein are potential chemotherapeutic agents. To date, no NMR or X-ray crystallographic structures of high-affinity phosphopeptides complexed with the Stat3 SH2 domain are available to aid in the development of peptidomimetic antagonists. Examination of the crystal structures of several STAT proteins and the complex of Stat1 with Ac-pTyr-Asp-Lys-Pro-His-NH(2) led to a hypothesis that the specificity determinant for Stat3, glutamine at position pY+3 in pTyr-Xxx-Xxx-Gln sequences, resides in a unique pocket on the protein surface at the juncture of the third strand of the central beta-sheet and a unique, STAT specific alpha-helix. Docking of Ac-pTyr-Leu-Pro-Gln-NHBn to the SH2 domain of Stat3 using molecular modeling showed that the Gln binds tightly in this pocket and participates in a network of hydrogen bonds. Novel interactions between the peptide main chain and the protein were also discovered. Phosphopeptide structure-affinity studies using unnatural amino acids and glutamine derivatives provide evidence for the peptide-protein interactions revealed by the model and lend support to the binding hypothesis.