Tankyrases 1 and 2 (TNKS1/2) are promising pharmacological biotargets with possible applications for the development of novel anticancer therapeutics. A focused structure–activity relationship study was conducted based on the tankyrase inhibitor JW74 (1). Chemical analoging of 1 improved the 1,2,4-triazole based core and led to 4-{5-(E)-2-{4-(2-chlorophenyl)-5-5-(methylsulfonyl)pyridin-2-yl-4H-1,2,4-triazol-3-yl}ethenyl-1,3,4-oxadiazol-2-yl}benzonitrile (G007-LK), a potent, “rule of 5” compliant and a metabolically stable TNKS1/2 inhibitor. G007-LK (66) displayed high selectivity toward tankyrases 1 and 2 with biochemical IC50 values of 46 nM and 25 nM, respectively, and a cellular IC50 value of 50 nM combined with an excellent pharmacokinetic profile in mice. The PARP domain of TNKS2 was cocrystallized with 66, and the X-ray structure was determined at 2.8 Å resolution in the space group P3221. The structure revealed that 66 binds to unique structural features in the extended adenosine binding pocket which forms the structural basis for the compound’s high target selectivity and specificity. Our study provides a significantly optimized compound for targeting TNKS1/2 in vitro and in vivo.