Differential Regulation of T Cells By PI3K Delta Inhibitors in a CLL Murine Model

In the microenvironment of chronic lymphocytic leukemia (CLL), T cells are typically dysfunctional. Idelalisib, a PI3Kδ inhibitor, is approved for R/R CLL but trials in treatment-naïve patients were discontinued due to immune-mediated severe adverse events (SAE). Duvelisib is a PI3Kγ/δ inhibitor also in clinical development with a similarly high incidence of immune-mediated SAEs. By contrast, umbralisib (TGR-1202) is a next-generation PI3Kδ inhibitor associated with lower rates of SAEs in clinical trials, even with long-term followup, which has also demonstrated activity against CK1e. Previously, we demonstrated in normal human T cells that ex vivo treatment with umbralisib relatively preserved Treg number (CD4+ CD25hi CD127lo FoxP3+) and function (Treg co-culture supression assay) compared to treatment with idelalisib or duvelisib. FoxP3 mRNA levels and IL-10 secretion from T cells also remained closer to normal after umbralisib treatment compared to idelalisib or duvelisib.

The purpose of the current study was to compare the effects of umbralisib, duvelisib, and idelalisib on T cells in a CLL mouse model and analyze immune-mediated adverse events following oral administration. We hypothesized that umbralisib may preserve the number and function of the regulatory T cell (Treg) population, translating to decreased immune-mediated side effects after treatment. Leukemic euTCL1 splenocytes were adoptively transferred into wildtype mice to induce CLL disease and treated via oral administration with vehicle, umbralisib, idelalisib or duvelisib. Tcon:Treg and Teff:Treg ratios in periphery and spleen measured by flow cytometry were found to be spared in the umbralisib-treated group and decreased on duvelisib and idealisib groups. Expression of functional Treg markers TGFB-1, CD39, CD103, PD-1 and CTLA-4 was also spared in the umbralisib group but affected in the other two. To assess immune-mediated toxicity, GI tract and liver tissues were collected and H&E stained. Pathology analysis was performed and immune-mediated toxicity was scored according to the following parameters: inflammatory cell infiltrate, kuppfer cell hyperplasia, microvesicular steatosis, cell degeneration (liver) and denuded mucosa, chronic inflammation, acute inflammation (GI tract). Overall toxicity grade was significantly lower in umbralisib-treated group compared to duvelisib-treated group in both liver (p=0.0009) and GI tract (p=0.0025). Toxicity grade in these tissues negatively correlated with total Treg count in spleen (R-squared~0.7). Immunohistochemistry staining of FoxP3+ Tregs in liver and GI tract was concordant with the assessment of Treg count in spleen by flow cytometry, as the number of FoxP3+ cells was closer to normal in umbralisib-treated mice compared to duvelisib-treated mice. Next, we investigated whether co-inhibition of CK1e by umbralisib may be involved in the differential regulation of T cells. Combination of a selective CK1e inhibitor, SR-4471, with duvelisib, prevented the reduction of total Treg number and functional markers in ex vivo culture of murine euTCL1 T cells, mimicking the effect of umbralisib. We have found that canonical Wnt signaling is inhibited dose-dependantly in euTCL1 T cells treated with umbralisib, demonstrated by lower levels of B-catenin and downstream TCF-1/7. These data determine Tregs to be a major player involved in immune-mediated toxicity characteristic of the PI3K inhibitor class of drugs. Umbralisib may differentially regulate CLL T cells through complimentary inhibition of both PI3Kδ and CK1e, potentially preserving Treg number and function to provide protection from immune-mediated SAEs.