BQU57 is a derivative of RBC8. It show selectivity for Ral relative to the GTPases Ras and RhoA and inhibit tumour xenograft growth. The binding of BQU57 to RalB–GDP was with a dissociation constant (Kd) of 7.760.6 mM by using isothermal titration calorimetry (ITC). It was similar to the results from surface plasmon resonance (SPR), which had a Kd value of 4.761.5 mM 1.
RBC8 or BQU57 can effectively inhibited the colony formation in soft agar of the Ral-dependent lines H2122 and H358, but not H460 or Calu-6. The IC50 value of RBC8 was 3.5 mM in H2122 cells and 3.4 mM in H358 cells; and the IC50 value of BQU57 was 2.0 mM in H2122 cells and 1.3 mM in H358 cells. RBC8 or BQU57 treatment showed no further inhibition of colony formation after RAL knockdown 1.
RBC8 and BQU57 acted specifically through the GDP-bound form of Ral proteins. RBC8 and BQU57 inhibited both Ral A and Ral B activation in both the H2122 and H358 cell lines by a Ral pull-down assay using RALBP1-bound agarose beads 1.
The inhibition of Ral activity and tumour growth by these compounds were evaluated in human lung cancer xenografts in mice. RBC8 and BQU57 showed good properties in vivo. RBC8 and BQU57 entry into tumour tissue 3 h after dosing, and were detectable in tumour tissue 1. BQU57 (10, 20 and 50mg per kg bodyweight) was intraperitoneal injected into H2122 tumour xenografts, and the activation of Ral in tumour extracts was analysed in RALBP1 pull-down assays. Both RalA and RalB were inhibited by RBC8 and BQU57. But no inhibition of Ras or RhoA activity was happened 1.
References:
1. Yan C, Liu D, Li L et al. Discovery and characterization of small molecules that target the GTPase Ral. Nature. 2014 Nov 20;515(7527):443-7.