Antitumor activity of PARP inhibitors in combination with Temozolomide in fumarate hydratase-deficient RCC

Hereditary Leiomyomatosis and Renal Cell Cancer (HLRCC) is a rare form of RCC associated with an aggressive clinical course. Despite the recent availability of therapeutic approaches active in FH-deficient RCC, most patients with advanced HLRCC die from their disease; identification of additional effective treatment strategies for these patients is, therefore, a therapeutic imperative. HLRCC is characterized by germline pathogenic variants in the fumarate hydratase (FH) gene, that encodes a key Tricarboxylic Acid (TCA) Cycle enzyme. Loss of FH activity leads to the accumulation of its substrate, fumarate. Excess fumarate serves as an oncometabolite and has been reported to suppress DNA damage repair. We evaluated the activity of PARP inhibitor-based combinations in preclinical models of HLRCC. The antitumor activity of two PARP inhibitors, Talazoparib and Niraparib , either as single agents or in combination with the alkylating chemotherapy agent temozolomide, was evaluated in several patient-derived HLRCC cell lines. In vitro viability assays demonstrated that both PARP inhibitors induced dose-dependent growth inhibition in HLRCC lines. The addition of temozolomide to either PARP inhibitor led to more pronounced reductions in cell proliferation across the HLRCC lines tested, compared to control single PARP inhibitors or temozolomide alone. At the molecular level, western blot analysis demonstrated that PARP inhibitors inhibited PARylation with concomitant upregulation of phosphorylated (ƴ) H2AX expression, and increased apoptosis. Furthermore, the effects on PARylation, ƴH2AX accumulation, and apoptosis were more pronounced in the PARP inhibitor/temozolomide combinations than with PARP inhibitors alone and were accompanied by substantial increases in cleaved PARP. These results demonstrate that PARP inhibitors have antitumor activity against HLRCC cells and that their activity is potentiated by the addition of the alkylating agent temozolomide. Additional in vitro and in vivo studies are ongoing to further define the activity of these combinations and to better understand the underlying molecular mechanisms. This study highlights the potential therapeutic utility of PARP inhibitor-based combinations in targeting DNA repair defects in HLRCC and provides the impetus for further exploration of this strategy.