Tankyrases 1 and 2 are central biotargets in the WNT/β-catenin signaling and Hippo signaling pathways. We have previously developed tankyrase inhibitors bearing a 1,2,4-triazole moiety and binding predominantly to the adenosine binding site of the tankyrase catalytic domain. Here we describe a systematic structure-guided lead optimization approach of these tankyrase inhibitors. The central 1,2,4-triazole template and trans-cyclobutyl linker of the lead compound 1 were left unchanged, while side-group East, West, and South moieties were altered by introducing different building blocks defined as point mutations. The systematic study provided a novel series of compounds reaching picomolar IC50 inhibition in WNT/β-catenin signaling cellular reporter assay. The novel optimized lead 13 resolves previous atropisomerism, solubility, and Caco-2 efflux liabilities. 13 shows a favorable ADME profile, including improved Caco-2 permeability and oral bioavailability in mice, and exhibits antiproliferative efficacy in the colon cancer cell line COLO 320DM in vitro.