Decoy-
oligodeoxynucleotides (D-ODNs) can target undruggable
transcription factors, such as STAT3. However, challenges in D-ODN delivery and potency hampered their translation. To overcome these limitations, we conjugated STAT3-specific D-ODN to
thalidomide (Tha), a known
ligand to cereblon (CRBN, a component of
E3 ubiquitin ligase) to generate a
proteolysis-targeting chimera (STAT3D
PROTAC ). STAT3D
PROTAC downregulated STAT3, but not STAT1 or STAT5, in target cells. Computational modeling of the STAT3D
PROTAC ternary complex predicted two surface lysines on STAT3, K601 and K626 as potential ubiquitination sites for the
PROTAC bound
E3 ligase. Accordingly, K601/K626 point mutations in STAT3, as well as
proteasome inhibitors, and CRBN deletion alleviated STAT3D
PROTAC effect. Next, we conjugated STAT3D
PROTAC to a CpG
ligand targeting
Toll-like receptor 9 (TLR9) to generate myeloid/B-cell-selective C-STAT3D
PROTAC conjugate. Naked C-STAT3D
PROTAC was spontaneously internalized by TLR9 + myeloid cells, B cells as well as human Ly18 and mouse A20
lymphoma cells, but not by T cells. C-STAT3D
PROTAC decreased STAT3 levels to 50% at 250 nM and over 85% at 2 µM dosing in myeloid cells. We also observed significantly improved downregulation of STAT3 target genes involved in
lymphoma cell proliferation and/or survival ( BCL2L1, CCND2, MYC ). Finally, we assessed the antitumor efficacy of C-STAT3D
PROTAC compared to C-STAT3D or scrambled control (C-SCR) against human
lymphoma xenotransplants. Local C-STAT3D
PROTAC administration triggered
lymphoma regression while control treatments had limited effects. Our results underscore feasibility of using
PROTAC strategy for cell-selective, decoy
oligonucleotide-based targeting of STAT3 and potentially other tumorigenic
transcription factors for
cancer therapy.