AC-4-130

This compound is potent and has nice selectivity for STAT5B over STAT3, but there is no analysis in the literature of other targets of this molecule, so it should be used with some caution. As for in vivo testing, the compound appears to be well tolerated by mice, but I have not seen a pharmacokinetic analysis to determine, for example, exposure and how well correlated that is to the cellular IC50 values.

In the “Leukemia” key publication (doi: 10.1038/s41375-017-0005-9) the authors present the STAT5 inhibitor AC-4-130 as a new therapeutic agent for haematological malignancies. They showed an effect in mono therapy in different cellular models as well as in vivo and also demonstrated a potential of synergistic effects in combination with other therapeutic agents. Despite these impressive pre-clinical results, in my opinion, the small molecule AC-4-130 misses some critical requirements for the suitability as chemical probe. An in vitro activity was only shown with a non-quantitative thermal shift assay, instead of a more common Kd-value. Moreover, the fluorine NMR experiment suggesting the non-covalent binding to STAT5 also shows an irreversible interaction with STAT3 demonstrating a certain promiscuity of the molecule. Also, the binding mode was only proposed by docking models. A final validation in form of a co-crystal structure would be more applicable (but is also not mandatory).

In summary, with the current available data I do not recommend AC-4-130 as a chemical probe for STAT5, unless more sophisticated data on on-target activity (e.g. Kd) and selectivity are available.

The probe represents one of the first STAT5 inhibitors targeting the SH2 domain. However, while cellular data are convincing, current literature does not characterise the probe well in vitro using binding and selectivity assays that would be expected for a high quality probe molecule. Direct interaction with the STAT5 Sh2 domain is only demonstrated by an NMR STD experiment at high concentration, no direct binding data (eg ITC) are available and “on target” activity has not been demonstrated in a cellular context. Structural information of the binding mode of this probe is only provided by docking but not by experimental data.