Discovery of a Napabucasin PROTAC as an Effective Degrader of the E3 Ligase ZFP91

Napabucasin, undergoing multiple
clinical trials, was reported
to inhibit the signal transducer and transcription factor 3 (STAT3).
To better elucidate its mechanism of action, we designed a napabucasin-based
proteolysis targeting chimera (PROTAC), XD2-149 that
resulted in inhibition of STAT3 signaling in pancreatic cancer cell
lines without inducing proteasome-dependent degradation of STAT3.
Proteomics analysis of XD2-149 revealed the downregulation
of the E3 ubiquitin-protein ligase ZFP91. XD2-149 degrades
ZFP91 with DC50 values in the nanomolar range. The cytotoxicity
of XD2-149 was significantly, but not fully, reduced
with ZFP91 knockdown providing evidence for its multi-targeted mechanism
of action. The NQO1 inhibitor, dicoumarol, rescued the cytotoxicity
of XD2-149 but not ZFP91 degradation, suggesting that
the NQO1-induced cell death is independent of ZFP91. ZFP91 plays a
role in tumorigenesis and is involved in multiple oncogenic pathways
including NF-κB and HIF-1α.