Internal tandem duplications within the juxtamembrane domain of fms-like tyrosine kinase 3 (FLT3-ITD) are present in approximately 25% of patients with acute myeloid leukemia (AML) and is an independent predictor of poor prognosis. FLT3-ITD is a constitutively active form of the receptor and aberrantly signals through signal transducer and activator of transcription (STAT) 5 which feeds-back to stabilize FLT3 through the downstream effector, Pim-1 kinase. FLT3-specific tyrosine kinase inhibitors (TKI) have clinical effects in FLT3-ITD AML but responses are limited due to the development of resistance through various means, including point mutations in the FLT3 tyrosine kinase domain commonly at D835 in the activation loop and F691 in the gate-keeper residue. Novel therapeutic strategies are therefore required for overcoming TKI-resistance. Poly (ADP-ribose) polymerase (PARP) 1 functions by poly-ADP-ribosylating (PAR) itself and other proteins primarily to catalyze DNA repair, however, PARP can also regulate the activity and function of proteins involved in other various processes. Notably, PARP inhibitors (PARPi) are novel cancer therapeutics that induce synthetic lethality in cancers with BRCA mutations. Recent studies also show that concurrent treatment of PARPi and TKI can effectively induce synthetic lethality in BRCA-proficient TKI-sensitive FLT3-ITD AML. However, the effects of PARPi in TKI-resistant disease has yet to be explored. We previously reported elevated reactive oxygen species (ROS) in FLT3-ITD AML leading to upregulated PARP1 and we now show that, compared to parental TKI-sensitive Molm14 cells, D835Y and F691L point-mutated TKI-resistant forms have significantly increased ROS production, and subsequently increased DNA damage and PARP1 protein expression. Interestingly, TKI-resistant cells also have elevated phospho-STAT5 (pSTAT5) and subsequent CRISPR-Cas9 knockout of PARP1 leads to reduction in pSTAT5 as well as the downstream targets, Pim-1 and FLT3. Significantly, STAT5 protein was found to immunoprecipitate with PAR and PARP1, suggesting for the first time that PARP1 may play a direct role in regulating STAT5. Finally, we have found that low doses (5-50nM) of the potent PARPi, Talazoparib (Tal), in combination with low dose (10nM) of the TKIs, Quizartinib (Quiz) or Gilteritinib (Gilt), is synergistically lethal in the otherwise TKI-resistant cells. Additionally, sequential treatment of TKI-resistant cells with Tal (72hr) prior to treatment with Quiz (72hr) or Gilt (72hr) was still synergistic whereas the reverse is not. This suggests a novel model whereby Tal may resensitize cells to TKI by decreasing PARP1 effects on pSTAT5 activity which feeds back to reduce FLT3 receptor activation. Therefore, this combination has potential for treatment of TKI-resistant AML and studies are underway to translate these findings with the intent of developing an effective therapy for patients with this disease.

Citation Format: Anna J. Dellomo, Rena G. Lapidus, Mariusz Karbowski, Maria R. Baer, Tami J. Kingsbury, Feyruz V. Rassool. PARP inhibitor resensitizes TKI-resistant AML to TKI [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1428.