Synergistic Targeting of the Regulatory and Catalytic Subunits of PI3Kδ in Mature B-cell Malignancies

Purpose: Aberrant activation of the B-cell receptor (BCR) is implicated in the pathogenesis of mature B-cell tumors, a concept validated in part by the clinical success of inhibitors of the BCR-related kinases BTK (Bruton's tyrosine kinase) and PI3Kd. These inhibitors have limitations, including the paucity of complete responses, acquired resistance, and toxicity. Here, we examined the mechanism by which the cyclic-AMP/PDE4 signaling axis suppresses PI3K, toward identifying a novel mechanism-based combinatorial strategy to attack BCR-dependency in mature B-cell malignancies. Experimental Design: We used in vitro and in vivo diffuse large B-cell lymphoma (DLBCL) cell lines and primary chronic lymphocytic leukemia (CLL) samples to preclinically evaluate the effects of the combination of the FDA-approved phosphodiesterase 4 (PDE4) inhibitor roflumilast and idelalisib on cell survival and tumor growth. Genetic models of gain-and loss-of-function were used to map multiple signaling intermediaries downstream of the BCR. Results: Roflumilast elevates the intracellular levels of cyclic-AMP and synergizes with idelalisib in suppressing tumor growth and PI3K activity. Mechanistically, we show that roflumilast suppresses PI3K by inhibiting BCR-mediated activation of the P85 regulatory subunit, distinguishing itself from idelalisib, an ATP-competitive inhibitor of the catalytic P110 subunit. Using genetic models, we linked the PDE4-regulated modulation of P85 activation to the oncogenic kinase SYK. Conclusions: These data demonstrate that roflumilast and idelalisib suppress PI3K by distinct mechanisms, explaining the basis for their synergism, and suggest that the repurposing of PDE4 inhibitors to treat BCR-dependent malignancies is warranted. (C) 2017 AACR.