Relative inhibitory activity for EPAC1-mediated Rap1b-bGDP exchange. Dose-dependent inhibition of EPAC1 GEF activity by compound 1 (black), 8 (blue), 14 (red), 22 (green), 26 (brown) and 33 (purple), in the presence of 20 μM cAMP. Relative GEF activity were presented as normalized reaction rate constant (means ± SD, n = 3) described in the method.