PI3Kd inhibition potentiates glucocorticoids in B-ALL by decreased receptor phosphorylation and enhanced gene regulation

Glucocorticoids, including dexamethasone and prednisone, are key components of therapy for B-lymphoblastic leukemia (B-ALL) that work through the glucocorticoid receptor (GR). However, glucocorticoids are not effective for all patients, leading to poor outcomes, and the high doses used in chemotherapy cause many toxicities. We have shown that inhibition of the leukocyte restricted PIK3d is a promising way to specifically enhance glucocorticoids in B-ALL, with the potential to improve outcomes without increasing toxicities. Here, we show that the PI3Kd inhibitor idelalisib potentiates both prednisolone and dexamethasone in both B-ALL cell lines and primary patient specimens, particularly at sub-saturating doses. Potentiation is partially explained by a widespread enhancement of glucocorticoid gene regulation, including effector genes that drive B-ALL cell death. Idelalisib causes a reduction in direct phosphorylation of GR at S203 and S226. Ablation of these phospho-acceptor sites enhances glucocorticoid potency. Phosphorylation of S226 inhibits GR DNA binding, indicating that PI3Kd mediated phosphorylation directly inhibits GR function in part though reduction of DNA binding affinity. Thus, PI3Kd inhibition improves glucocorticoid efficacy in B-ALL in part by decreasing GR phosphorylation and enhancing DNA binding and downstream gene regulation. Identification of this and other contributing mechanisms will help identify patients who can most benefit clinically from this combination therapy. Overall design: We performed RNA-seq (poly A selected) on a BCP-ALL cell line model, NALM-6. Cells were trerated with glucocoroticoids and/or idelalisib for 24 hours. There were five treatment groups: vehicle, idelalisib only (250 nM), low dexamathasone (5 nM), high dexamethasone (50 nM), both low dexamethasone and idelalisib, and both high dexamethasone and idelalisib.