Phosphatase-stable peptidomimetic prodrug inhibitors of signal transducer and activator of transcription 3

Abstract

1306

Signal transducer and activator of transcription 3 (Stat3) transmits signals from IL-6 family cytokines, EGF, Src, etc., is constitutively activated in head and neck, breast, prostate, and other cancers, and is a target for cancer drug design. To develop small molecule inhibitors of Stat3, we have targeted the Stat3 SH2 domain with phosphopeptide-based inhibitors. Molecular modeling of a lead peptide, Ac-pTyr-Leu-Pro-Gln-NHBn, docked to the Stat3 SH2 domain [1] suggested additional modifications that would enhance binding affinity between the inhibitor and the protein. A series of peptidomimetics incorporating these modifications showed 2- to 3-fold increases in binding affinity with IC₅₀ values ranging from 35 - 120 nM in fluorescence polarization assays. Several of these inhibitors were converted to phosphatase-stable, cell-permeable prodrugs by replacing the phosphate with the phosphonodifluoromethyl group and capping the phosphonate with pivaloyloxymethyl (POM) groups. One such compound, PM-252F, when added to MDA MB-468 and BT-20 breast cancer cells completely inhibited constitutive Stat3 phosphorylation at 100 nM and inhibited cell growth with an IC₅₀ of 1-2 μM. PM-252F also inhibited proliferation of primary CLL cells by inducing apoptosis. Thus, we have successfully converted our lead phosphopeptide to a phosphatase-stable and cell-permeable small molecule inhibitor of Stat3.
[1] Biopolymers, 2007, in press