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Background CD123 (IL-3 receptor alpha-chain) is a therapeutic target for hematological malignancies based on high expression levels in acute myeloid leukemia (AML), blastic plasmacytoid dendritic cell neoplasm (BPDCN), and other cancers. The anti-CD123 antibody-drug conjugate (ADC), IMGN632, comprises a humanized monoclonal antibody covalently linked to a DNA - alkylating cytotoxic payload which is currently in phase 1 evaluation for relapsed/refractory CD123-positive hematological malignancies (NCT03386513). Novel approaches to enhance the efficacy of ADCs are of significant therapeutic interest. Our laboratory has previously demonstrated that the Poly ADP Ribose (PARP) inhibitor, olaparib, synergistically enhances the activity of the CD33-targeted ADC, IMGN779, in preclinical AML models (Portwood S et al, ASH 2016). Based on the hypothesis that PARP inhibition will synergize with DNA damaging mechanism of IMGN632, we investigated the ability of olaparib and other PARP inhibitors (PARPi) in clinical development (talazoparib, niraparib, rucaparib, and veliparib) to enhance the therapeutic efficacy of IMGN632 across diverse human AML cell lines and primary relapsed/refractory AML samples. Materials and Methods CD123 expression on human AML cell lines (HEL, HL60, MV411, Molm13, EOL-1, THP-1, and Kasumi-1) was quantified by flow cytometry using QuantriBrite beads. AML cells were continuously cultured for 72-96 hours with varying doses of IMGN632 (range 100pM - 100nM) and specific PARP inhibitors (range 100pM -15μM) alone and in combination. Cell viability was measured using a WST-8 colorimetric assay. Primary clinically annotated CD123+ AML cells from patients with relapsed/refractory disease were obtained under IRB-approved protocols from the Roswell Park Hematologic Procurement Shared Resource and cultured short-term in the presence of multiple cytokines plus IMGN632 +/- PARP inhibitors. Apoptosis (Annexin V/PI), cell cycle, and DNA damage (H2AX) were evaluated by flow cytometry. Additive vs. synergistic effects were determined by combination indices using Compusyn software. PARP trapping was evaluated by Western blot analysis in nuclear lysates obtained from IMGN632 +/- PARP inhibitors treated AML cells. Results High expression levels of CD123 (range 937 - 2231 CD123 molecules/cell) were detected on multiple human AML cell lines (HEL-luc, MV411, Molm13, EOL-1, and THP-1) relative to unstained negative controls. Western blot analysis of nuclear lysates from AML cells demonstrated that all PARP inhibitors had varying degrees of PARP trapping on DNA. Continuous single agent 5-day treatment with all tested PARP inhibitors resulted in dose dependent in vitro inhibition of AML cell line growth with IC50 values ranging from 360 nM (talazoparib, most potent) to 78uM (veliparib, least potent). Combination therapy using PARP inhibitors (doses ranging from 300nM - 15uM) and IMGN632 (10nM) consistently resulted in enhanced anti-leukemic effects over monotherapy (Figure 1 for example). Synergistic anti-proliferative effects were obtained across all tested AML cell lines (n=5) with combination indexes ranging from 0.3-0.7 by Compusyn analysis. Combination therapy correlated with enhanced DNA damage, tumor cell apoptosis, and cell cycle arrest of AML cells. Moreover, IMGN632 and PARPi (olaparib or talazoparib) resulted in single agent activity against clinically annotated primary relapsed/refractory AML patient samples with evidence of synergistic effects when combined in vitro. Conclusions Addition of PARP inhibitors to IMGN632, a novel anti-CD123 antibody-drug conjugate, further enhances DNA damage effects and consistently results in synergistic in vitro anti-leukemic effects across multiple CD123+ AML cell lines and primary AML patient samples. Further studies investigating this novel combinatorial approach in specific molecular subtypes of AML with variable baseline sensitivities to PARPi are currently ongoing. Our results strongly support future investigation of PARPi as a novel class of agents with the potential to significantly enhance the efficacy of DNA-alkylating ADCs and/or cytotoxic chemotherapy for hematological malignancies. Figure. Figure. Disclosures Sloss: ImmunoGen: Employment. Watkins:ImmunoGen Inc.: Employment. Kovtun:ImmunoGen Inc.: Employment. Adams:ImmunoGen Inc.: Employment. Wang:Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Speakers Bureau; Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy; Novartis: Speakers Bureau; Jazz: Speakers Bureau; Jazz: Speakers Bureau; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees.

<div><p>Natural killer (NK) cells are innate lymphocytes with cytotoxic activity. Understanding the factors regulating cytotoxicity is crucial for improving NK-cell adoptive therapies. Here, we studied a previously unknown role of p35 (CDK5R1), a coactivator of cyclin-dependent kinase 5 (CDK5) in NK-cell function. p35 expression was thought to be neuronal-specific and the majority of studies are still focused on neuronal cells. Here, we show that CDK5 and p35 are expressed in NK cells and are kinase-active. NK cells from p35 knockout mice were analyzed and showed significantly increased cytotoxicity against murine cancer cells, while they did not show any differences in cell numbers or maturation stages. We confirmed this using human NK cells transduced with p35 short hairpin RNA (shRNA), showing similar increase in cytotoxicity against human cancer cells. Overexpression of p35 in NK cells resulted in moderate decrease in cytotoxicity, while expressing a kinase-dead mutant of CDK5 displayed increased cytotoxicity. Together, these data suggest that p35 negatively regulates NK-cell cytotoxicity. Surprisingly, we found that TGFβ, a known negative regulator of NK-cell cytotoxicity, induces p35 expression in NK cells. NK cells cultured with TGFβ exhibit reduced cytotoxicity, while NK cells transduced with p35 shRNA or mutant CDK5 expression exhibited partial reversal of this inhibitory effect pointing to an interesting hypothesis that p35 plays an important role in TGFβ-mediated NK-cell exhaustion.</p>Significance:<p>This study reports a role for p35 in NK-cell cytotoxicity and this might help to improve NK-cell adoptive therapy.</p></div>

<p>Murine NK cells from p35 k/o mice display heightened cytotoxicity. <b>A,</b> Murine splenocytes or bone marrow cells from WT or p35 k/o mice were stained for CD3 and NK1.1 and mixed with counting beads, then analyzed via flow cytometry. Gating on NK1.1⁺CD3⁻ cells, as well as the counting beads, allowed determination of mNK concentration and the total mNK cell count in each compartment. ns, not significant. Graphs display mean ± SD, <i>n</i> = 3 mice, unpaired two-tailed <i>t</i> test. <b>B,</b> mNK cells were isolated from WT, p35 k/o, or p35 Het mice and cocultured with calcein AM-labeled B16F10 cells at different E:T ratios for 4 hours. Fluorescence intensity of supernatant corresponding to calcein release from dead cells was measured to calculate NK-mediated cytotoxicity. **, <i>P</i> < 0.01. Graphs display mean ± SD, <i>n</i> = 3 biological cocultures (<i>n</i> = 2 for WT 1:1 ratio, p35 k/o 1:1 ratio, and p35 k/o in WT versus p35 Het versus p35 k/o experiment), and multiple unpaired two-tailed <i>t</i> tests with correction for multiple comparisons using Holm-Šídák method (WT vs. p35 k/o) or one-way ANOVA with Tukey multiple comparisons test (WT vs. p35 Het vs. p35 k/o). <b>C,</b> Murine splenocytes or bone marrow cells were stained for CD11b, CD27, CD3, and NK1.1 and analyzed via flow cytometry after gating on NK1.1⁺CD3⁻ cells. Representative flow plots are shown. Percentages of mNK cells in each subset (CD11b⁺CD27⁻, CD11b⁻CD27⁺, and CD11b⁺CD27⁺) for each mouse were recorded and graphed. ns, not significant. Graphs display mean ± SD, <i>n</i> = 8 mice, multiple unpaired two-tailed <i>t</i> tests with correction for multiple comparisons using Holm-Šídák method. <b>D,</b> Murine splenocytes were stained for CD3, NK1.1, and NKG2D, NKp46, NKG2A, 2B4, LY6C, or LY49C/F/I/H. Representative histograms of activating/inhibitory receptor expression are shown, gated on live NK1.1⁺CD3⁻ cells. <b>E,</b> Isolated mNK cells pooled from multiple mice were seeded at a concentration of 2e6 cells/mL and cultured <i>ex vivo</i> in RPMI media supplemented with 1,000 U/mL IL2. An additional 1,000 U/mL IL2 was added 2 days later without changing the media, followed by collection of supernatant after culture for an additional 2 days. Cytokine concentration was determined using a flow-based, multiplex murine cytokine release assay. ns, not significant; *, <i>P</i> < 0.05; **, <i>P</i> < 0.01. Graphs display mean ± SD, <i>n</i> = 3 biological cultures, unpaired two-tailed <i>t</i> tests.</p>