Ortunities for increasing inhibitor selectivity.Aoyagi-Scharber et al.Acta Cryst. (2014). F70, 1143?BMNstructural communications4. DiscussionRecent efforts in PARP inhibitor design have indeed centered on targeting sequence-variable and/or structure-variable regions outside the nicotinamide-binding pocket for enhanced specificity (Steffen et al., 2013; Ekblad et al., 2013). The aforementioned variable D-loop (Fig. 4a) has been pursued as a druggable site for designing nextgeneration selective inhibitors (Andersson et al., 2012). The aromatic D-loop residue, like Tyr889 in PARP1 and Tyr455 in PARP2 (Fig. 3b), which types -stacking interactions with the unique fluorophenyl group of BMN 673, is missing in PARP3 and tankyrases 1/2. The D-loop in PARP3 and tankyrases is also shorter and assumes ?distinct conformations (Fig. 4a; Lehtio et al., 2009; Wahlberg et al., 2012; Karlberg, Markova, et al., 2010; Narwal et al., 2012). Structural superposition indicates that the D-loop of PARP3 or tankyrases should undergo conformational modifications in order to accommodate the fluorophenyl moiety of BMN 673 inside the NAD+-binding pocket (Fig. 4a). BMN 673, which fits in the exceptional binding space with structure and sequence diversity, thus opens up new possibilities for selective inhibition of ADP-ribosyltransferase enzymes. Targeting the noncatalytic function of PARP1/2 provides an alternative method for designing selective and potent PARP inhibitors. A crystal structure of important PARP1 domains in complex having a DNA double-strand break revealed that inter-domain communication is mediated by the N-terminal -helical bundle domain (Langelier et al., 2012), towards which the triazole substituent of BMN 673 points (Fig. 3b). Interestingly, BMN 673 is 100-fold far more efficient than other clinical PARP1/2 inhibitors at trapping PARP1/2 on DNA damage sites, a potentially key mechanism by which these inhibitors exert their cytotoxicity (Murai et al., 2014). The truth is, BMN 673 exhibits exceptional cytotoxicity in homologous recombination-deficient cells compared with other PARP1/2 inhibitors using a comparable capacity to inhibit PARP catalysis (Shen et al., 2013). The co-crystal structures of catPARP1 and catPARP2 in complicated with BMN 673 reported here reveal that this hugely potent inhibitor occupies a distinctive space inside the extended NAD+-binding pocket (Fig. 4b). Elucidating potential long-range structural effects that BMN 673, with its novel chiral disubstituted scaffold, may have on DNA binding and/or DNA damage-dependent allosteric regulation might aid within the Traditional Cytotoxic Agents Inhibitor Molecular Weight improvement of new-generation PARP inhibitors with enhanced selectivity. We thank Drs Ying Feng, Daniel Chu and Leonard Post for their scientific knowledge and input. We gratefully acknowledge Dr Gordon Vehar for important comments around the manuscript. We especially thank Tracy Arakaki, Thomas Edwards, Brandy Taylor, Ilyssa Exley, Jacob Statnekov, Shellie Dieterich and Jess Leonard (Emerald BioStructures) for the crystallographic function. MA-S, BKY, BW, YS and PAF are workers of, and have equity interest in, MEK Activator web BioMarin Pharmaceutical Inc., which can be establishing BMN 673 as a prospective commercial therapeutic.Emsley, P. Cowtan, K. (2004). Acta Cryst. D60, 2126?132. Emsley, P., Lohkamp, B., Scott, W. G. Cowtan, K. (2010). Acta Cryst. D66, 486?01. Ferraris, D. V. (2010). J. Med. Chem. 53, 4561?584. Gandhi, V. B., Luo, Y., Liu, X., Shi, Y., Klinghofer, V., Johnson, E. F., Park, C., Giranda, V. L., Penning, T.