Preclinical Evaluation of a Novel SHIP1 Phosphatase Activator for Inhibition of PI3K Signaling in Malignant B Cells

Purpose: PI3K signaling is a common feature of B-cell neoplasms, including chronic lymphocytic leukemia (CLL) and diffuse large B-cell lymphoma (DLBCL), and PI3K inhibitors have been introduced into the clinic. However, there remains a clear need to develop new strategies to target PI3K signaling. PI3K activity is countered by Src homology domain 2-containing inositol-5'-phosphatase 1 (SHIP1) and, here, we have characterized the activity of a novel SHIP1 activator, AQX-435, in preclinical models of B-cell malignancies.

Experimental design: In vitro activity of AQX-435 was evaluated using primary CLL cells and DLBCL-derived cell lines. In vivo activity of AQX-435, alone or in combination with the Bruton's tyrosine kinase (Btk) inhibitor ibrutinib, was assessed using DLBCL cell line and patient-derived xenograft models.

Results: Pharmacologic activation of SHIP1 using AQX-435 was sufficient to inhibit anti-IgM-induced PI3K-mediated signaling, including induction of Akt phosphorylation and MYC expression, without effects on upstream Syk phosphorylation. AQX-435 also cooperated with the Btk Inhibitor ibrutinib to enhance inhibition of anti-IgM-induced Akt phosphorylation. AQX-435 induced caspase-dependent Apoptosis of CLL cells preferentially as compared with normal B cells, and overcame in vitro survival-promoting effects of microenvironmental stimuli. Finally, AQX-435 reduced Akt phosphorylation and growth of DLBCL in vivo and cooperated with ibrutinib for tumor growth inhibition.

Conclusions: Our results using AQX-435 demonstrate that SHIP1 activation may be an effective novel therapeutic strategy for treatment of B-cell neoplasms, alone or in combination with ibrutinib.