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HAMNO is a novel protein interaction inhibitor of replication protein A (RPA). RPA is involved in the ATR/Chk1 pathway. HAMNO alone inhibits colony formation in both HNSCC cell lines in the low micromolar range. HAMNO combined with etoposide significantly inhibits colony formation to a greater degree than HAMNO alone. After UMSCC38 cells are exposed to HAMNO, increased pan-nuclear γ-H2AX staining occurs in a dose dependent manner. Cancer derived UMSCC38 cells, as well as another cancer cell line, UMSCC11B, have prominent γ-H2AX staining, particularly after incubation with 20 μM HAMNO. Both UMSCC38 and OKF4 cells present increased γ-H2AX staining after addition of HAMNO, with the greatest increase in signal occurring in S-phase.

In mice, HAMNO slows the progression of UMSCC11B tumors. Ser33 of RPA32, an ATR substrate, is highly phosphorylated after two hours of treatment with 20 μM of etoposide, which is reduced with the addition of 2 μM HAMNO, and is nearly absent at higher concentrations, demonstrating an in vivo effect of HAMNO as an inhibitor of RPA32 phosphorylation by ATR.