Known GRK2 inhibitors. The A, B, C and D rings pack in the adenine, ribose, polyphosphate, and hydrophobic subsites of the kinase domain, respectively. That is, the A ring interacts with the hinge, whereas the D ring interacts in a pocket defined by the P-loop, the αB and αC helices in the small lobe, and by the DFG loop in the large loop.