Dissecting the molecular determinants of clinical PARP1 inhibitor selectivity for tankyrase1.

Poly ADP ribosyltransferases play a critical role in DNA repair and cell death, and PARP1 is a particularly important therapeutic target for the treatment of breast cancer due to its synthetic lethal relationship with BRCA1/2. Numerous PARP1 inhibitors have been developed, and their efficacy in cancer treatment is attributed to both the inhibition of enzymatic activity and their ability to trap PARP1 on to the damaged DNA, which is cytotoxic. Of the clinical PARP inhibitors, talazoparib is the most effective at trapping PARP1 on damaged DNA. Biochemically, talazoparib is also suspected to be a potent inhibitor of PARP5a/b (tankyrase1/2), which is an important regulator of Wnt/β-catenin pathway. Here we show using competition experiments in cell lysate that, at a clinically relevant concentration, talazoparib can potentially bind and engage tankyrase1. Using surface plasmon resonance, we measured the dissociation constants of talazoparib, olaparib, niraparib and veliparib for their interaction with PARP1 and tankyrase1. The results show that talazoparib has strong affinity for PARP1 as well as uniquely strong affinity for tankyrase1. Finally, we used crystallography and hydrogen deuterium exchange mass spectroscopy to dissect the molecular mechanism of differential selectivity of these PARP1 inhibitors. From these data, we conclude that subtle differences between the ligand binding sites of PARP1 and tankyrase1, differences in the electrostatic nature of the ligands, protein dynamics, and ligand conformational energetics contribute to the different pharmacology of these PARP1 inhibitors. These results will help in the design of drugs to treat Wnt-β-catenin pathway-related cancers, such as colorectal cancers.