PI3K-Delta Inhibitors Induce Primary Monocyte Cytotoxicity but Do Not Alter Monocyte Differentiation

It is well appreciated that an altered tumor microenvironment is a hallmark of cancer, including in chronic lymphocytic leukemia (CLL) where monocyte-derived cells known as "nurse-like cells" (NLC) in the lymph node microenvironment support CLL cell viability, survival, and resistance to drug-induced apoptosis. CLL directed therapy may include conventional chemotherapy, immunotherapy, targeted kinase inhibitors, and/or Bcl2 inhibitors. Of these options, kinase inhibitors such as PI3K-delta inhibitors are an effective therapy that have been found to disrupt NLC-CLL cell interaction in vitro and induce initial lymphocytosis thought to be related to this disrupted cellular interaction. We previously demonstrated that the PI3K-delta inhibitors idelalisib and umbralisib (TGR-1202) induce cytotoxicity, apoptosis, and inhibition of pAKT in primary CLL cells. We now investigate the effect of idelalisib and umbralisib on monocytes by evaluating cytotoxicity and cytokine-induced differentiation. We isolated monocytes from blood obtained from normal donors using negative selection (RosetteSep monocyte). For cytotoxicity experiments, primary purified monocytes were incubated ± M-CSF (10 ng/mL) ± PI3K-delta inhibitor (at 1.25 to 20 μM) for three days, and cell viability was measured with the MTS reagent after three hours. For differentiation experiments, primary purified monocytes were incubated first with M-CSF (10 ng/mL) for three days, then washed and incubated ± IL-10 (20 ng/mL) ± PI3K-delta inhibitor (10 μM) for three days. To judge monocyte differentiation, we used flow cytometry to measure expression of CD14, CD206, CD163, CD124, CD80, and CD86. We also visually assessed monocyte appearance using phase microscopy with these conditions. Both idelalisib and umbralisib induced concentration dependent cytotoxicity (N = 3), but cytotoxicity was higher with idelalisib (62% cytotoxicity at 1.25 μM) compared to umbralisib (24% cytotoxicity at 1.25 μM). The addition of M-CSF reduced PI3K-delta inhibitor-induced cytotoxicity of primary monocytes. For example, 1.25 μM idelalisib induced 62% cytotoxicity without M-CSF, compared with 5% cytotoxicity with M-CSF. Staged cytokine incubation of monocytes with M-CSF and IL-10 induced the expression of M2 differentiation markers (CD163 and CD124) and reduced expression of the M1 differentiation marker (CD86). Addition of idelalisib or umbralisib did not significantly affect monocyte surface marker expression compared to no drug (N = 4). Even though idelalisib and umbralisib were cytotoxic for monocytes, these drugs did not affect morphologic changes induced by IL-10 (marked spreading and shape change). Thus, PI3K-delta inhibitors do not appear to affect cytokine-induced monocyte differentiation, but they do induce monocyte cytotoxicity (idelalisib > umbralisib). The addition of M-CSF, which contributes to M2 differentiation, blocks these effects. Together, our results suggest that depleting monocyte or NLC number may contribute to the anti-CLL therapeutic effects of PI3K-delta inhibitors. In addition, a higher level of monocyte cytotoxicity with idelalisib compared to umbralisib could potentially contribute to the higher infectious side effect profile with idelalisib seen in clinical use, as monocyte derived cells are a key component of the innate immune system. The anti-monocyte effect of PI3K-delta inhibitors could provide rationale for using these agents in other malignancies in which monocyte-derived cells are an integral part of the tumor microenvironment.