Pijus K. Mandal

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.

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.

Crystal structure of unphosphorylated STAT3 core fragment

Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcriptional factors that play an important role in cytokine and growth factor signaling. Here we report a 3.05 A-resolution crystal structure of an unphosphorylated STAT3 core fragment. The overall monomeric structure is very similar to that of the phosphorylated STAT3 core fragment. However, the dimer interface observed in the unphosphorylated STAT1 core fragment structure is absent in the STAT3 structure. Solution studies further demonstrate that the core fragment of STAT3 is primarily monomeric. Mutations corresponding to those in STAT1, which lead to disruption of the core fragment interface and prolonged tyrosine phosphorylation, show little or no effect on the tyrosine phosphorylation kinetics of STAT3. These results highlight the structural and biochemical differences between STAT3 and STAT1, and suggest different regulation mechanisms of these two proteins.

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.

STAT3 restrains RANK- and TLR4-mediated signalling by suppressing expression of the E2 ubiquitin-conjugating enzyme Ubc13

The transcriptional regulator STAT3 curbs pro-inflammatory cytokine production mediated by NF-κB signaling in innate immune cells, yet the mechanism by which this occurs has been unclear. Here we identify STAT3 as a pivotal negative regulator of Ubc13, an E2 ubiquitin-conjugating enzyme that facilitates TRAF6 K63-linked ubiquitination and NF-κB activation. Ubc13 accumulates intracellularly in the absence of STAT3. Depletion of Ubc13 in Stat3-deficient macrophages subdues excessive RANKL- or LPS-dependent gene expression, indicating Ubc13 overexpression mediates enhanced transcriptional responses in the absence of STAT3. In RANKL-activated macrophages, STAT3 is stimulated by autocrine IL-6 and inhibits accrual of Ets-1, Set1 methyltransferase and trimethylation of histone H3 lysine 4 (H3K4me3) at the Ube2n (Ubc13) promoter. These results delineate a mechanism by which STAT3 operates as a transcriptional repressor on Ube2n, thus modulating NF-κB activity by regulation of Ubc13 abundance. Our data suggest this pathway plays important roles in bone homeostasis and restraint of inflammation.

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 (KI = 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.

Targeting the Src Homology 2 (SH2) Domain of Signal Transducer and Activator of Transcription 6 (STAT6) with Cell-Permeable, Phosphatase-Stable Phosphopeptide Mimics Potently Inhibits Tyr641 Phosphorylation and Transcriptional Activity.

Signal transducer and activator of transcription 6 (STAT6) transmits signals from cytokines IL-4 and IL-13 and is activated in allergic airway disease. We are developing phosphopeptide mimetics targeting the SH2 domain of STAT6 to block recruitment to phosphotyrosine residues on IL-4 or IL-13 receptors and subsequent Tyr641 phosphorylation to inhibit the expression of genes contributing to asthma. Structure-affinity relationship studies showed that phosphopeptides based on Tyr631 from IL-4Rα bind with weak affinity to STAT6, whereas replacing the pY+3 residue with simple aryl and alkyl amides resulted in affinities in the mid to low nM range. A set of phosphatase-stable, cell-permeable prodrug analogues inhibited cytokine-stimulated STAT6 phosphorylation in both Beas-2B human airway cells and primary mouse T-lymphocytes at concentrations as low as 100 nM. IL-13-stimulated expression of CCL26 (eotaxin-3) was inhibited in a dose-dependent manner, demonstrating that targeting the SH2 domain blocks both phosphorylation and transcriptional activity of STAT6.

Synthesis and in Vitro Evaluation of a Peptidomimetic Inhibitor Targeting the Src Homology 2 (SH2) Domain of STAT6

An improved synthesis of a phosphopeptidomimetic prodrug targeting the Src Homology 2 (SH2) domain of signal transducer and activator of transcription 6 (STAT6) is reported. In our convergent methodology, we employed a phosphotyrosine surrogate active ester harboring pivaloyloxymethyl groups, which efficiently coupled to tert-butylglycinyl proline diarylamide. Biological evaluation of 1 has not been reported. We show that it inhibits STAT6 phosphorylation in intact human bronchial epithelial cells, suggesting potential application in the treatment of asthma.

Hydrodehalogenation of alkyl iodides with base-mediated hydrogenation and catalytic transfer hydrogenation: Application to the asymmetric synthesis of N-protected α-methylamines

We report a very mild synthesis of N-protected α-methylamines from the corresponding amino acids. Carboxyl groups of amino acids are reduced to iodomethyl groups via hydroxymethyl intermediates. Reductive deiodination to methyl groups is achieved by hydrogenation or catalytic transfer hydrogenation under alkaline conditions. Basic hydrodehalogenation is selective for the iodomethyl group over hydrogenolysis-labile protecting groups, such as benzyloxycarbonyl, benzyl ester, benzyl ether, and 9-fluorenyloxymethyl, thus allowing the conversion of virtually any protected amino acid into the corresponding N-protected α-methylamine.

Efficient synthesis of apricoxib, CS-706, a selective cyclooxygenase-2 inhibitor, and evaluation of inhibition of prostaglandin E2 production in inflammatory breast cancer cells.

An efficient synthesis of apricoxib (CS-706), a selective cyclooxygenase inhibitor, was developed using copper catalyzed homoallylic ketone formation from methyl 4-ethoxybenzoate followed by ozonolysis to an aldehyde, and condensation with sulfanilamide. This method provided multi-gram access of aprocoxib in good yield. Apricoxib exhibited potency equal to celecoxib at inhibition of prostaglandin E2 synthesis in two inflammatory breast cancer cell lines.