Signal Transducer and Activator of Transcription 5 (Stat5) protein has gained notoriety for its aberrant role in many human cancers including breast, liver, prostate, skin, head and neck. Despite the significant evidence showing Stat5's role in cancers, there has been little progress in developing small molecule inhibitors of Stat5 function. In contrast, there has been significant advancements in identifying inhibitors for the Stat3 protein. As a result, several potent and direct small molecule and oligonucleotide-based inhibitors have entered pre-clinical trials as a Stat3 therapeutic. Unfortunately, there has not been progress towards the development of potent Stat5 inhibitors.

Stat5 is activated by ligand-receptor binding, which recruits Stat5 to intracellular receptor sites via their SH2 domain. Stat5 is then phosphorylated at conserved tyrosine (Y) residues Y694 and 699, which facilitates the formation of active Stat5 dimers via reciprocal pY-SH2 domain interactions. The dimers then translocate to the nucleus and induce target gene transcription. In cancer cells, Stat5 is constitutively phosphorylated leading to the aberrant expression of anti-apoptotic Stat5 target genes.

In an effort to identify direct inhibitors of Stat5, we conducted an in vitro screen of a focused library of SH2 domain binding salicylic acid-containing inhibitors (∼150) against Stat5. Several potent (Ki < 5 μM) and Stat5 selective (>3-fold specificity for Stat5 cf. Stat1 and Stat3) inhibitors were identified which were then evaluated in K562 and MV-4–11 human leukemia cells which are known to harbour constitutively activated Stat5. BP-1–108 was identified as the most potent lead within these cell lines. The lead was further evaluated to show potent induction of apoptosis (IC50's ∼ 20 μM) which correlated with potent and selective suppression of Stat5 phosphorylation. In addition, it inhibited Stat5 target genes; Cyclin D1 & c-myc. BP-1–108 also showed no cytotoxicity against healthy bone marrow cells at concentrations up to 160 μM. These inhibitors represent the most potent and direct inhibitors of Stat5 function reported to date. An in silico docking analysis with BP-1–108 within the Stat5b crystal structure (pdb:1Y1U) revealed a tertrapodal projection of functionality facilitating access to a number of sub pockets within the Stat5 SH2 domain. Currently we are conducting a structure-activity relationship against Stat5's SH2 domain with one projection into the hydrophobic pocket using diverse heterocycles to delineate inhibitor binding to improve the potency of our inhibitors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B224.