Valproate augments Niraparib killing of tumor cells

PARP1 inhibitors are approved therapeutic agents in ovarian carcinomas, and have clinical activity in some breast cancers. As a single agent, niraparib killed ovarian and mammary tumor cells via an ATM-AMPK-ULK1 pathway which resulted in mTOR inactivation and the formation of autophagosomes, temporally followed by autolysosome formation. In parallel, niraparib activated a CD95-FADD-caspase 8 pathway, and collectively these signals caused tumor cell death that was suppressed by knock down of Beclin1, ATG5, CD95, FADD or AIF; or by expression of c-FLIP-s, BCL-XL or dominant negative caspase 9. The HDAC inhibitors AR42 and sodium valproate enhanced niraparib lethality in a greater than additive fashion. HDAC inhibitors enhanced niraparib lethality by increasing activation of the ATM-AMPK-ULK1-autophagy and CD95-FADD-caspase 8 pathways. Knock down of eIF2α, ATM, AMPKα, ULK1, Beclin1 or ATG5 reduced tumor cell killing by the niraparib plus HDAC inhibitor combination. Blockade of either caspase 9 function or that of cathepsin B partially prevented cell death. As a single agent niraparib delayed tumor growth, but did not significantly alter the tumor control rate. Tumors previously exposed to niraparib had activated the ERK1/2 and AKT-mTOR pathways that correlated with increased plasma levels of IL-8, MIF, EGF, uPA and IL-12. Collectively our findings argue that the addition of HDAC inhibitors to niraparib enhances the anti-cancer activity of the PARP1 inhibitor niraparib.