Hit It before It Starts: Targeting a (3 + 1) Hybrid Non-canonical G-Quadruplex to Silence PARP1 in Cancer.

Error-prone DNA repair pathways promote genomic instability and cancer progression. Notably, Poly(ADP-ribose) polymerase-1 (PARP1)-directed alternative non-homologous end joining plays a pivotal role in cancer, including Multiple Myeloma (MM), conferring vulnerability to PARP1 inhibitors (PARPis). However, despite their clinical efficacy, PARPi-mediated trapping of PARP1 contributes to hematologic toxicities and can be circumvented by resistance mechanisms, highlighting the need for alternative strategies to modulate PARP1 activity. Here, we investigated the non-canonical G-quadruplex (G4) formed within the PARP1 gene promoter and characterized by an adenine bulge as a potential therapeutic target. Computational modeling and biophysical characterization identified a small molecule capable of targeting this distinctive G4 architecture. In MM cells, the compound effectively downregulated PARP1 expression, perturbed downstream DNA repair pathways, and induced DNA damage-associated apoptosis, while sparing healthy blood cells. Overall, our findings reveal a previously unexplored G4-binding mode with potential therapeutic relevance in cancer.