Development of chemical tool compounds targeting the non-catalytic functions of tankyrase

Abstract

Tankyrase (TNKS1 and TNKS2) is a promising target in anti-cancer drug discovery due to its role in regulating cellular processes which are dysregulated in cancer. In addition to catalysing the poly(ADP-ribosyl)ation of substrate proteins, tankyrase is a scaffolding protein which recruits substrate proteins through its ARC domain, and undergoes SAM domain mediated polymerisation. Whilst tankyrase inhibitor development has focused on antagonists of the catalytic PARP domain, the limitations of this approach have been suggested in recent literature highlighting the contribution of catalysis-independent mechanisms to cellular tankyrase functions, The aim of this work is the development of chemical tool compounds targeting the non-catalytic functions of tankyrase. The development of substrate binding antagonists of the tankyrase ARC domain from a literature-reported fragment hit has been pursued using fragment-based drug discovery. Hit optimisation was undertaken through iterations of in silico guided fragment design, synthesis, and testing in biophysical NMR assays against TNKS2 ARC4. Whilst modification of the furan motif did not lead to an increase in binding affinity, quninoxaline substitution led to fragment analogues with improved binding affinity (Kd = 240 uM and Kd = 120 uM) which maintain competitive binding in a specific sub-site of the substrate recognition pocket of TNKS2 ARC4. A higher throughput fluorescence polarisation assay was then established and was used to determine the potency of higher affinity compounds based on lead fragments. Targeted degradation of tankyrase has been pursued by the synthesis of PROTACs from a potent catalytic tankyrase inhibitor in conjunction with a cereblon E3 binding ligand. The synthesis and profiling of heterobifunctional compounds based on the catalytic tankyrase inhibitor XAV939 was undertaken. No evidence of tankyrase degradation in cells was observed, however compounds with a reduced hydrogen-bond donor count showed binding to tankyrase and cereblon in both biochemical and cellular target engagement assays, therefore demonstrating cellular permeability.