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Cancer homeostasis depends on a balance between activated oncogenic pathways driving tumorigenesis and engagement of stress-response programs that counteract the inherent toxicity of such aberrant signaling. While inhibition of oncogenic signaling pathways has been explored extensively, there is increasing evidence that overactivation of the same pathways can also disrupt cancer homeostasis and cause lethality. We show here that inhibition of Protein Phosphatase 2A (PP2A) hyperactivates multiple oncogenic pathways and engages stress responses in colon cancer cells. Genetic and compound screens identify combined inhibition of PP2A and WEE1 as synergistic in multiple cancer models by collapsing DNA replication and triggering premature mitosis followed by cell death. This combination also suppressed the growth of patient-derived tumorsin vivo. Remarkably, acquired resistance to this drug combination suppressed the ability of colon cancer cells to form tumorsin vivo. Our data suggest that paradoxical activation of oncogenic signaling can result in tumor suppressive resistance.

Protein phosphatase 2A (PP2A), a serine/threonine phosphatase involved in the regulation of apoptosis, proliferation, and DNA-damage response, is overexpressed in many cancers, including small cell lung cancer (SCLC). Here we report that LB100, a small molecule inhibitor of PP2A, when combined with platinum-based chemotherapy, synergistically elicited an antitumor response both in vitro and in vivo with no apparent toxicity. Using inductively coupled plasma mass spectrometry, we determined quantitatively that sensitization via LB100 was mediated by increased uptake of carboplatin in SCLC cells. Treatment with LB100 alone or in combination resulted in inhibition of cell viability in two-dimensional culture and three-dimensional spheroid models of SCLC, reduced glucose uptake, and attenuated mitochondrial and glycolytic ATP production. Combining LB100 with atezolizumab increased the capacity of T cells to infiltrate and kill tumor spheroids, and combining LB100 with carboplatin caused hyperphosphorylation of the DNA repair marker γH2AX and enhanced apoptosis while attenuating MET signaling and invasion through an endothelial cell monolayer. Taken together, these data highlight the translational potential of inhibiting PP2A with LB100 in combination with platinum-based chemotherapy and immunotherapy in SCLC.

PURPOSE: Glioblastoma (GBM) carries a dismal prognosis despite standard multimodal treatment with surgery, chemotherapy and radiation. Immune checkpoint inhibitors, such as PD1 blockade, for treatment of GBM failed to show clinical benefit. Rational combination strategies to overcome resistance of GBM to checkpoint monotherapy are needed to extend the promise of immunotherapy to GBM management. Emerging evidence suggests that protein phosphatase 2A (PP2A) plays a critical role in the signal transduction pathways of both adaptive and innate immune cells and that inhibition of PP2A could enhance cancer immunity. We investigated the use of a PP2A inhibitor, LB-100, to enhance antitumor efficacy of PD1 blockade in a syngeneic glioma model. METHODS: C57BL/6 mice were implanted with murine glioma cell line GL261-luc or GL261-WT and randomized into 4 treatment arms: (i) control, (ii) LB-100, (iii) PD1 blockade and (iv) combination. Survival was assessed and detailed profiling of tumor infiltrating leukocytes was performed. RESULTS: Dual PP2A and PD1 blockade significantly improved survival compared with monotherapy alone. Combination therapy resulted in complete regression of tumors in about 25% of mice. This effect was dependent on CD4 and CD8 T cells and cured mice established antigen-specific secondary protective immunity. Analysis of tumor lymphocytes demonstrated enhanced CD8 infiltration and effector function. CONCLUSION: This is the first preclinical investigation of the effect of combining PP2A inhibition with PD1 blockade for GBM. This novel combination provided effective tumor immunotherapy and long-term survival in our animal GBM model.

Chimeric antigen receptor (CAR)-engineered T cells represent a promising modality for treating glioblastoma. Recently, we demonstrated that CAR-T cells targeting carbonic anhydrase IX (CAIX), a protein involved in HIF-1a hypoxic signaling, is a promising CAR-T cell target in an intracranial murine glioblastoma model. Anti-CAIX CAR-T cell therapy is limited by its suboptimal activation within the tumor microenvironment. LB-100, a small molecular inhibitor of protein phosphatase 2A (PP2A), has been shown to enhance T cell anti-tumor activity through activation of the mTOR signaling pathway. Herein, we investigated if a treatment strategy consisting of a combination of LB-100 and anti-CAIX CAR-T cell therapy produced a synergistic anti-tumor effect. Our studies demonstrate that LB-100 enhanced anti-CAIX CAR-T cell treatment efficacy in vitro and in vivo. Our findings demonstrate the role of LB-100 in augmenting the cytotoxic activity of anti-CAIX CAR-T cells and underscore the synergistic therapeutic potential of applying combination LB-100 and CAR-T Cell therapy to other solid tumors.

Mounting evidence suggests that inhibition of protein phosphatase-2A (PP2A), a serine/threonine phosphatase, could enhance anticancer immunity. However, drugs targeting PP2A are not currently available. Here, we report that a PP2A inhibitor, LB-100, when combined with anti-PD-1 (aPD-1) blockade can synergistically elicit a durable immune-mediated antitumor response in a murine CT26 colon cancer model. This effect is T-cell dependent, leading to regression of a significant proportion of tumors. Analysis of tumor lymphocytes demonstrates enhanced effector T-cell and reduced suppressive regulatory T-cell infiltration. Clearance of tumor establishes antigen-specific secondary protective immunity. A synergistic effect of LB-100 and aPD-1 blockade is also observed in B16 melanoma model. In addition, LB-100 activates the mTORC1 signaling pathway resulting in decreased differentiation of naive CD4 cells into regulatory T cells. There is also increased expression of Th1 and decreased expression of Th2 cytokines. These data highlight the translational potential of PP2A inhibition in combination with checkpoint inhibition., Protein phosphatase 2A (PP2A) has been proposed as a target for cancer immunotherapy. Here the authors show that pharmacological inhibition of PP2A with a clinically-relevant inhibitor enhances response to immune checkpoint blockade in pre-clinical models of cancer, resulting in long lasting immunity.

Purpose: To determine the MTD and to assess the safety, tolerability, and potential activity of LB-100, a first-in-class small-molecule inhibitor of protein phosphatase 2A (PP2A) in adult patients with progressive solid tumors. Experimental Design: LB-100 was administered intravenously daily for 3 days in 21-day cycles in a 3 + 3 dose escalation design. Results: There were 29 patient entries over 7 dose escalations. One patient stopped treatment after one dose because of an acute infection and was reenrolled after recovery; each course was analyzed as a separate patient entry. Two patients had dose-limiting toxicity (reversible increases in serum creatinine or calculated serum creatinine clearance) at the 3.1 mg/m2 level. Probable or possible study drug-related grade 3 adverse events occurred in 6 (20.7%) patients [anemia (n = 2), decreased creatinine clearance, dyspnea, hyponatremia, and lymphopenia]. Ten (50%) of 20 response-evaluable patients had stable disease for four or more cycles. One patient with pancreatic adenocarcinoma had a partial response noted after 10 cycles, which was maintained for five additional cycles. The other patients achieving stable disease had one of the following: fibrosarcoma, chondrosarcoma, thymoma, atypical carcinoid of lung, or ovarian, testicular, breast (n = 2), and prostate cancer. The recommended phase II dose of LB-100 is 2.33 mg/m2 daily for 3 days every 3 weeks. Conclusions: The safety, tolerability, preliminary evidence of antitumor activity, and novel mechanism of action of LB-100 support its continued development alone and in combination with other therapies. Clin Cancer Res; 23(13); 3277–84. ©2016 AACR.

LB-100 is a novel, first-in-class, small molecule inhibitor of protein phosphatase 2A (PP2A) recently shown in a Phase I trial to be well-tolerated at doses associated with stabilization of progressive solid tumors (Chung. Clin Cancer Res. 2017). PP2A has been implicated in mediating Akt signaling downstream of CTLA-4 (Parry. MolCell Biol. 2005). An in vivo pooled short hairpin RNA screen identified PP2A as a key regulator in suppressing T-cell proliferation in the tumor microenvironment (Zhou. Nature. 2014) and to be essential for regulatory-T-cell (Treg) function (Apostolidis. Nat Immunol. 2014). We hypothesized that pharmacologic inhibition of PP2A could enhance cancer immunity. We assessed the effect of LB-100 on T-cells in human allogenic mixed lymphocyte reactions, in which CD8+ or CD4+ T cells were co-cultured with monocyte-derived dendritic cells. We found a dose dependent increase in T cell proliferation in CD8+ and CD4+ cells and an increase in IFNγ secretion in CD4+ T cells. We investigated the effect of LB-100 plus anti-PD-1 antibody on CD4+ T cells in the same assay. The combination enhanced proliferation and IFNγ production compared to anti-PD-1 alone. For in vivo syngeneic studies, BALB/c mice were implanted subcutaneously in the right flank with CT26 colon carcinoma cells (5 x10^5 cells). 12 days after implantation, mice with tumors between 30-100 mm3 were randomized into four treatment groups (placebo, LB-100 - 0.16 mg/kg, anti-PD1- 10 mg/kg, and combination). Treatment was given every 2 days up to 28 days. 10 days after treatment, there was a significant decrease in tumor growth in the combination group compared to placebo or anti-PD-1 alone. 7/11 (63.6%) of mice treated with the combination and none in the other treatment groups achieved complete remission (CR). FACS analysis of the tumor infiltrating lymphocytes (TIL) after 10 days of treatment demonstrated enhanced IFNγ production in CD8+ T cells in the combination group compared to either treatment alone. There was also a marked decrease in FoxP3+ Treg cells in LB-100 treated group compared to placebo (3% vs 12% of CD4+ T cells, p Citation Format: Winson S. Ho, Herui Wang, John S. Kovach, Rongze Lu, Zhengping Zhuang. Protein phosphatase 2A inhibition,with a novel small molecule inhibitor, LB-100, achieves durable immune-mediated antitumor activity when combined with PD1 blockade in a preclinical model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-193. doi:10.1158/1538-7445.AM2017-LB-193

Background Protein phosphatase (PP2A) is a multifunctional protein involved in regulation of cell cycle, DNA-damage response, and apoptosis. LB-100, a novel small molecule inhibitor of PP2A, inhibits the growth of a broad spectrum of leukemic and solid tumor cell lines. In addition, LB100 potentiates the effectiveness of cytotoxic drugs (cisplatin, docetaxel, doxorubicin, temozolomide) and radiation without significant increases in toxicity. The predominant mechanisms responsible for potentiation are inhibition of mitotic exit and homologous recombination repair. Methods This is an open label, first-time-in-human, multicenter, phase 1 study of LB-100 in patients with advanced cancer refractory to standard therapies. The first part of the study determines the maximum tolerated dose (MTD) of LB-100 as a single agent when given intravenously days 1-3 every 21 days. Utilizing a standard 3+3 design, patients (pts) are evaluated for dose limiting toxicities (DLT) through 2 cycles. Once the single agent MTD is determined, the dose will be reduced by 2 dose levels (DL) and combined with docetaxel given on day 2. Escalation will continue until the MTD of the combination is determined. Plasma sampling for pharmacokinetics of LB-100 will be collected on days 1 and 3 of cycle 1 in the MTD confirmation cohort. Results The starting dose of LB-100 was 0.25 mg/m2 and 21 pts have enrolled in part 1 of the study through six dose levels. At DL6, pts received 2.33 mg/m2 and no DLTs have been observed. One pt with metastatic colon cancer at DL6 had a grade 2 creatinine after 2 doses that resolved with hydration. This was related to LB-100 and the study was amended to increase the volume and infusion time. One pt on DL3 with stage 4 pancreas cancer had stable disease through 15 cycles of treatment and another pt on DL5 with metastatic thymoma remains on treatment through 8 cycles. Stable disease for 4-6 cycles was also observed in breast, ovarian, carcinoid and testicular cancer patients. Conclusions Rb and p53 mutations are common in malignancies leading to chromosomal instability and overexpression of the mitotic checkpoint gene Mad2. PP2A inhibition results in synthetic lethality of cancer cells overexpressing Mad2 which may be a biomarker for LB100 responsiveness. Through 6 DLs, LB-100 has been well tolerated without any DLTs and early activity has been observed with stabilization of disease in a wide variety of cancers. Correlative studies for biomarkers of response are planned. Clinical trial identifier: NCTO1837667 Citation Format: Vincent Chung, Donald Richards, Fadi Braiteh, John S. Kovach, Aaron Scott Mansfield. A phase 1 study of a novel inhibitor of protein phosphatase 2A alone or in combination with docetaxel. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A175.

TPS2602 Background: Protein phosphatase (PP2A) is a multifunctional protein involved in regulation of cell cycle, DNA-damage response, and apoptosis. In pre-clinical studies, LB-100, a novel small ...