PARP inhibition by talazoparib results in leukemia cell death via induction of myeloid differentiation Free

Acute myeloid leukemia (AML) is an aggressive hematopoietic malignancy characterized by the uncontrolled proliferation of immature myeloid progenitors arrested in differentiation. Despite the recent advent of multiple agents for AML, not all patients are eligible for or tolerate therapy due to underlying disease biology and toxicity. The overall survival of AML patients remains poor, with a dire need for novel approaches. Inhibitors of poly (ADP-ribose) polymerase (PARP) are approved for the treatment of multiple solid tumors based on their ability to exploit synthetic lethality by preventing DNA repair in cancers with underlying genomic defects. Talazoparib is a potent PARP inhibitor which has demonstrated clinical efficacy in ovarian and breast cancers with germline BRCA1/2 mutations.

Here we examined the potential of PARP inhibitors to impact the growth of a panel of genetically and mutational diverse human acute myeloid leukemia (AML) cells. Across multiple cell lines, talazoparib consistently exhibited more potent dose dependent anti-proliferative effects on AML growth than other PARP inhibitors (olaparib,niraparib, rucaparib). Induction of AML cell death correlated with the degree of PARP blockage with associated apoptosis and decreased fraction of cells cycling in mitosis.

Importantly, talazoparib treatment of human AML cells also induced significant myeloid differentiation of myeloblasts. Treatment of the AML cell line MOLM13 with talazoparib for seven days resulted in reduced numbers of immature monoblasts and promonocytes and a concomitant increase in late promonocytes and maturing monocytes. This was reflected in dose-dependent upregulation of CD11b and CD14 surface protein expression by flow cytometry and reduced CDK2 protein levels. Similar effects, specifically enhanced apoptosis and upregulation of CD11b expression, were observed following talazoparib therapy of primary AML patient samples cultured in vitro with marked reduction of colony formation units. These findings were further confirmed in vivo in a systemic xenograft model established with MOLM13-bioluminescent human AML cells in NOD-SCID immunodeficient mice. Treatment with talazoparib in leukemia-bearing mice for 4 weeks significantly reduced tumor burden and markedly prolonged overall survival as compared with vehicle treated animals. Moreover, examination of leukemia cells within the marrow of talazoparib-treated mice demonstrated morphological evidence of myeloblast differentiation.

To elucidate the underlying processes responsible for this drug-induced differentiation, we performed transcriptional profiling of MOLM13 cells treated with talazoparib versus vehicle. RNA sequencing demonstrated that talazoparib significantly upregulated several genes including SPI-1, HLA-DMA, and IL1-beta. These elevated levels were then confirmed by qRT-PCR.

Next, we selected SPI-1, a transcription factor essential for normal myeloid differentiation, for further evaluation. Dysregulation of SPI-1 dysregulation has previously been associated with disrupted myeloid differentiation and accumulation of abnormal blasts in AML.

Using RNA interference techniques, we generated single cell clones of MOLM13 with significantly reduced expression of SPI-1 protein by western blot analysis. Talazoparib treatment of these SPI-1 knockdown AML clones demonstrated markedly less induction of CD11b surface protein expression as compared with parenteral controls, suggesting that talazoparib relies upon SPI-1 dependent processes to drive terminal myeloid differentiation. Investigation of the contribution of other genes in the SPI-1 differentiation pathway to talazoparib activity are ongoing.

We demonstrate here that talazoparib exerts potent anti-tumor activity across an array of human AML cell lines, patient samples, and an in vivo xenograft model in part due to induction of myeloid differentiation. This was characterized by attenuated growth, prominent morphological and molecular hallmarks, and increased cell death. Our results suggest that talazoparib may represent a novel mutation-agnostic differentiation therapy for acute myeloid leukemia.Further exploration of the molecular mechanisms underlying PARP inhibitor induced myeloid differentiation in leukemia are ongoing.