Pharmacologic Inhibition Of Protein Phosphatase-2a, With Novel Inhibitor Lb-100, Achieves Durable Immune-Mediated Antitumor Activity When Combined With Pd1 Blockade

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase. Mounting evidences suggest that inhibition of PP2A could enhance cancer immunity. LB-100 is a novel small molecule inhibitor of PP2A that is well tolerated in a Phase I trial. We hypothesized that inhibition of PP2A may enhance effect of checkpoint inhibition. Mice were inoculated subcutaneously with CT26.CL25 murine colon cancer cells. When tumors were 50-100 mm3, mice were randomized into treatment groups: control (PBS), anti-PD1, LB-100 and anti-PD1/LB-100 combination. Where indicated, mice received CD8-depleting antibodies. Tumors were harvested after two treatments and FACS analysis performed to evaluate tumor infiltrating lymphocytes. CD3 or naïve CD4 cells were harvested from splenocytes. CD3 cells were activated for 3 hours before staining for phospho-protein. For differentiation experiments, naïve CD4 cells were cultured with TGF-b or IL-4 for induction of regulatory or Th2 CD4 T cells respectively. FACS analysis was performed on matured lymphocytes and cytokine secretion was measured using ELISA. While single-agent treatment with LB-100 or anti-PD1 was insufficient to decrease tumor burden, combination therapy led to complete regression of tumor in >50% of animals. This therapeutic effect was negated by temporary CD8 T cell ablation. TILS analysis demonstrated an increase in CD8 and a decrease in regulatory CD4 T cells. CD8 cells had enhanced IFN-g and TNF production. In vitro, LB-100 increased mTORC1 signaling as measured by pS6 phosphorylation, resulting in decreased differentiation of naïve CD4 T-cells into T-regulatory and Th2 lineage in their respective skewing conditions. In addition, secretion of Th1 cytokines, including IFN-g, TNF and IL2 were significantly increased in both Th1 and Th2 conditions. These data indicate that pharmacologic inhibition of PP2A by LB-100 can achieves durable immune-mediated antitumor activity when combined with PD1 blockade and highlights the potential of PP2A inhibition to enhance cancer immunotherapy.