Daniel P. Ball

Signal transducer and activator of transcription 3 inhibitors: a patent review

Importance of the field: Aberrant activation of signal transducer and activator of transcription (Stat) 3, a member of the STAT family of proteins, is prevalent in numerous human cancers and is now widely recognized as a critical molecular abnormality and a master regulator of tumor processes. Thus, the identification of potent and selective Stat3 inhibitors will have a high commercial potential as anticancer drugs, given the many tumors in which Stat3 is implicated. Areas covered in this review: This review covers the structures and activities of direct inhibitors of Stat3 protein activity described in the patent literature since the research fields inception in 2001. The patents reviewed include peptide and peptidomimetic compounds, small molecules, oligonucleotides and platinum-based Stat3 inhibitors. What the reader will gain: Readers will gain an understanding of how Stat3 protein function has been inhibited by a wide variety of structurally diverse therapeutic compounds. Readers will learn about which classes of patented Stat3 inhibitors are most advanced toward clinical trials, and will be exposed to the proposed mechanisms of inhibition and scope of their application in treating human cancers. Take home message: Numerous groups have shown that in vivo administration of inhibitors of activated Stat3 induce human tumor regression in xenograft models. Indeed, the growing number of preclinical studies in numerous cancer types, as well as the first Phase 0 clinical trial of a Stat3 inhibitor, suggest that Stat3 is a valid and exciting therapeutic target for molecular inhibitors.

Identification of Purine-Scaffold Small-Molecule Inhibitors of Stat3 Activation by QSAR Studies

To facilitate the discovery of clinically useful Stat3 inhibitors, computational analysis of the binding to Stat3 of the existing Stat3 dimerization disruptors and quantitative structure−activity relationships (QSAR) were pursued, by which a pharmacophore model was derived for predicting optimized Stat3 dimerization inhibitors. The 2,6,9-trisubstituted-purine scaffold was functionalized in order to access the three subpockets of the Stat3 SH2 domain surface and to derive potent Stat3-binding inhibitors. Select purine scaffolds showed good affinities (KD, 0.8−12 μM) for purified, nonphosphorylated Stat3 and inhibited Stat3 DNA-binding activity in vitro and intracellular phosphorylation at 20−60 μM. Furthermore, agents selectively suppressed viability of human prostate, breast and pancreatic cancer cells, and v-Src-transformed mouse fibroblasts that harbor aberrant Stat3 activity. Studies herein identified novel small-molecule trisubstituted purines as effective inhibitors of constitutively active Stat3 and of the viability of Stat3-dependent tumor cells, and are the first to validate the use of purine bases as templates for building novel Stat3 inhibitors.

A STAT inhibitor patent review: progress since 2011.

Introduction: The clinical utility of effective direct STAT inhibitors, particularly STAT3 and STAT5, for treating cancer and other diseases is well studied and known. Areas covered: This review will highlight the STAT inhibitor patent literature from 2011 to 2015 inclusive. Emphasis will be placed on inhibitors of the STAT3, STAT5a/b, and STAT1 proteins for cancer treatment. The review will, where suitably investigated, describe the mode and the site of inhibition, list indications that were evaluated, and rank the inhibitor’s relative potency among compounds in the same class. The reader will gain an understanding of the diverse set of approaches, used both in academia and industry, to target STAT proteins. Expert opinion: There is still much work to be done to directly target the STAT3 and STAT5 proteins. As yet, there is still no direct STAT3 inhibitor in the clinic. While the SH2 domain remains a popular target for therapeutic intervention, the DNA-binding domain and N-terminal region are now attracting attention as possible sites for inhibition. Multiple putative STAT3 and STAT5 inhibitors have now been patented across a broad spectrum of chemotypes, each with their own advantages and limitations.

Applying Small Molecule Signal Transducer and Activator of Transcription-3 (STAT3) Protein Inhibitors as Pancreatic Cancer Therapeutics

Constitutively activated STAT3 protein has been found to be a key regulator of pancreatic cancer and a target for molecular therapeutic intervention. In this study, PG-S3-001, a small molecule derived from the SH-4-54 class of STAT3 inhibitors, was found to inhibit patient-derived pancreatic cancer cell proliferation in vitro and in vivo in the low micromolar range. PG-S3-001 binds the STAT3 protein potently, Kd = 324 nmol/L by surface plasmon resonance, and showed no effect in a kinome screen (>100 cancer-relevant kinases). In vitro studies demonstrated potent cell killing as well as inhibition of STAT3 activation in pancreatic cancer cells. To better model the tumor and its microenvironment, we utilized three-dimensional (3D) cultures of patient-derived pancreatic cancer cells in the absence and presence of cancer-associated fibroblasts (CAF). In this coculture model, inhibition of tumor growth is maintained following STAT3 inhibition in the presence of CAFs. Confocal microscopy was used to verify tumor cell death following treatment of 3D cocultures with PG-S3-001. The 3D model was predictive of in vivo efficacy as significant tumor growth inhibition was observed upon administration of PG-S3-001. These studies showed that the inhibition of STAT3 was able to impact the survival of tumor cells in a relevant 3D model, as well as in a xenograft model using patient-derived cells. Mol Cancer Ther; 15(5); 794–805. ©2016 AACR.

Identification of Bidentate Salicylic Acid Inhibitors of PTP1B

PTP1B is a master regulator in the insulin and leptin metabolic pathways. Hyper-activated PTP1B results in insulin resistance and is viewed as a key factor in the onset of type II diabetes and obesity. Moreover, inhibition of PTP1B expression in cancer cells dramatically inhibits cell growth in vitro and in vivo. Herein, we report the computationally guided optimization of a salicylic acid-based PTP1B inhibitor 6, identifying new and more potent bidentate PTP1B inhibitors, such as 20h, which exhibited a > 4-fold improvement in activity. In CHO-IR cells, 20f, 20h, and 20j suppressed PTP1B activity and restored insulin receptor phosphorylation levels. Notably, 20f, which displayed a 5-fold selectivity for PTP1B over the closely related PTPσ protein, showed no inhibition of PTP-LAR, PRL2 A/S, MKPX, or papain. Finally, 20i and 20j displayed nanomolar inhibition of PTPσ, representing interesting lead compounds for further investigation.

Signal transducer and activator of transcription 3 (STAT3) inhibitor, S3I-201, acts as a potent and non-selective alkylating agent

The Signal Transducer and Activator of Transcription 3 (STAT3) oncogene is a master regulator of many human cancers, and a well-recognized target for therapeutic intervention. A well known STAT3 inhibitor, S3I-201 (NSC 74859), is hypothesized to block STAT3 function in cancer cells by binding the STAT3 SH2 domain and disrupt STAT3 protein complexation events. In this study, liquid chromatography tandem mass spectrometry analysis revealed that STAT3, in the presence of S3I-201, showed a minimum of five specific sites of modification, cysteines 108, 259, 367, 542, and 687. Moreover, a prepared fluorescently labeled chemical probe of S3I-201 (DB-6-055) revealed that S3I-201 non-specifically and globally alkylated intracellular proteins at concentrations consistent with S3I-201s reported IC50. These data are consistent with the hypothesis that S3I-201 is a sub-optimal probe for interrogating STAT3-related cell biology.

Disarming an Electrophilic Warhead: Retaining Potency in Tyrosine Kinase Inhibitor (TKI)‐Resistant CML Lines While Circumventing Pharmacokinetic Liabilities

Pharmacologic blockade of the activation of signal transducer and activator of transcription 3 (STAT3) in tyrosine kinase inhibitor (TKI)‐resistant chronic myeloid leukemia (CML) cell lines characterized by kinase‐independent resistance was shown to re‐sensitize CML cells to TKI therapy, suggesting that STAT3 inhibitors in combination with TKIs are an effective combinatorial therapeutic for the treatment of CML. Benzoic acid‐ and hydroxamic acid‐based STAT3 inhibitors SH‐4‐054 and SH‐5‐007, developed previously in our laboratory, demonstrated promising activity against these resistant CML cell lines. However, pharmacokinetic studies in murine models (CD‐1 mice) revealed that both SH‐4‐054 and SH‐5‐007 are susceptible to glutathione conjugation at the para position of the pentafluorophenyl group via nucleophilic aromatic substitution (SNAr). To determine whether the electrophilicity of the pentafluorophenyl sulfonamide could be tempered, an in‐depth structure–activity relationship (SAR) study of the SH‐4‐054 scaffold was conducted. These studies revealed that AM‐1‐124, possessing a 2,3,5,6‐tetrafluorophenylsulfonamide group, retained STAT3 protein affinity (Ki=15 μm), as well as selectivity over STAT1 (Ki>250 μm). Moreover, in both hepatocytes and in in vivo pharmacokinetic studies (CD‐1 mice), AM‐1‐124 was found to be dramatically more stable than SH‐4‐054 (t1/2=1.42 h cf. 10 min, respectively). AM‐1‐124 is a promising STAT3‐targeting inhibitor with demonstrated bioavailability, suitable for evaluation in preclinical cancer models.

A 2,6,9-hetero-trisubstituted purine inhibitor exhibits potent biological effects against multiple myeloma cells

A focused library of hetero-trisubstituted purines was developed for improving the cell penetrating and biological efficacy of a series of anti-Stat3 protein inhibitors. From this SAR study, lead agent 22e was identified as being a promising inhibitor of MM tumour cells (IC50s <5μM). Surprisingly, biophysical and biochemical characterization proved that 22e was not a Stat3 inhibitor. Initial screening against the kinome, prompted by the purine scaffolds history for targeting ATP binding pockets, suggests possible targeting of the JAK family kinases, as well for ABL1 (nonphosphorylated F317L) and AAK1.