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 (IC₅₀ 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 IC₅₀ of 69 nM, the highest affinity Stat3 inhibitor reported to date.