Tankyrases 1 and 2 are members of the poly(ADP-ribose) polymerase (PARP) family of enzymes that modulate Wnt pathway signaling. While amide- and lactam-based nicotinamide mimetics that inhibit tankyrase activity, such as XAV939, are well-known, herein we report the discovery and evaluation of a novel nicotinamide isostere that demonstrates selectivity over other PARP family members. We demonstrate the utilization of lipophilic efficiency-based structure-efficiency relationships (SER) to rapidly drive the evaluation of this series. These efforts led to a series of selective, cell-active compounds with solubility, physicochemical, and in vitro properties suitable for further optimization.