Structure–Activity Studies of Phosphopeptidomimetic Prodrugs Targeting the Src Homology 2 (SH2) Domain of Signal Transducer and Activator of Transcription 3 (Stat3)
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Signal transducer and activator of transcription 3 (Stat3) transmits signals from growth factors and interleukin-6 family cytokines by binding to their receptors via its Src homology 2 (SH2) domain. This results in phosphorylation of Tyr705, dimerization, translocation to the nucleus, and regulation of transcription of downstream genes. Stat3 is constitutively activated in several human cancers and is a target for anti-cancer drug design. We have shown previously phosphorylation of Tyr705 in intact cancer cells can be inhibited with prodrugs of phosphopeptide mimics targeting the SH2 domain. In a series of prodrugs consisting of bis-pivaloyloxymethyl esters of 4′-phosphonodifluoromethyl cinnamoyl-Haic-Gln-NHBn, appending methyl group to the β-position of the cinnamate increased potency ca. twofold, which paralleled the increase in affinity of the corresponding phosphopeptide models. However, dramatic increases in potency were observed when the C-terminal C(O)NHBn of Gln-NHBn was replaced with a simple methyl group. In this communication we continue to explore the effects of structural modifications of prodrugs on their ability to inhibit Tyr705 phosphorylation. A set of 4-substituted prolines incorporated into β-methyl-4-phosphocinnamoyl-leucinyl-Xaa-4-aminopentamide model peptides exhibited affinities of 88–317 nM by fluorescence polarization (Pro IC50 = 156 nM). In corresponding prodrugs, Pro inhibited constitutive Stat3 phosphorylation at 10 μM in MDA-MB-468 breast tumor cells. However, 4,4-difluoroproline and 4,4-dimethylproline resulted in complete inhibition at 0.5 μM. These results suggest that the prodrug with native proline undergoes metabolism that those with substituted prolines do not. In conclusion, changes in structure with minimal impact on intrinsic affinity can nevertheless have profound effects on the cellular potency of prodrug inhibitors of Stat3.
KeywordsSignal transducer and activator of transcription 3 Stat3 Src homology domain 2 SH2 domain Peptidomimetic Phosphopeptide Prodrug
β-Methyl pCinn or [2E] 3-(4-phosphoryloxyphenyl)-2-butenamide
Tyr705 phosphorylated Stat3
- SH2 domain
Src homology 2 domain
Signal transducer and activator of transcription 3
We are grateful to the National Cancer Institute (CA096652) for support of this work. Partial support by a Grant from the Center for Targeted Therapy of The University of Texas M. D. Anderson Cancer Center and the Texas Institute of Drug and Diagnostic Development at The University of Texas at Austin is acknowledged. We also acknowledge the NCI Cancer Center Support Grant CA016672 for the support of both our NMR facility and the Translation Chemistry Core Facility which performed the mass spectrometry. Funding as an Odyssey Fellow (Z.R.) was supported by the Odyssey Program and the Cockrell Foundation Award for Scientific Achievement at UTMDACC.
Conflict of interest
The authors have no association with commercial entities associated with this work.
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