Antitumor Activity and Mechanism of Action of a Novel Stat3 Inhibitor, WP1066, Against Human B-Cell Non-Hodgkin’s Lymphoma and Multiple Myeloma.

B-cell non-Hodgkin’s lymphomas (B-NHL) have the highest incidence rates among all lymphomas, comprising more than 85% of malignant lymphomas worldwide. Multiple myeloma (MM) is a plasma cell malignancy that affects 14,000 patients per year. Although new therapies have recently been introduced to treat these malignancies, patient survival has not significantly improved, illustrating the need for additional agents that target and suppress this disease. B-cell malignancies over-express c-myc and frequently retain a constitutively activated Stat3, resulting in increased survival gene expression and apoptotic protection. Targeting these transcription factors may reduce apoptotic thresholds and suppress the growth and survival of these malignancies. In this study, we refined and tested the anti-tumor activity of a novel small MW compound discovered by screening a synthetic library for agents that bock Stat3 activation. WP1066, blocked IL-6 mediated Stat3 activation in numerous human B-NHL and MM tumor cells including the Mino, LP, MM-1 and OCI-My5 at low micromolar concentrations. This compound blocks Jak2-depedent cytokine signaling (IL-3, GM-CSF, IL-6) through the rapid (15 min) down-regulation of the upstream tyrosine kinase, Jak2. WP1066 also causes rapid reduction in c-myc in B-cell malignancies but other transcription factors and signaling kinases are not affected by WP1066. The apoptotic effects of WP1066 on B-NHL and MM cells parallel its Stat inhibitory, Jak2 down-regulatory activity. Jak2 down-regulation is not dependent on proteosomal processing but requires activation of a tyrosine phosphatase, suggesting a unique mechanism of WP1066 action. Based on activities described in vitro, we further examined the antitumor activity of WP1066 in a B-NHL xenograft tumor model. Mice were inoculated (i.p.) with 25 × 10⁶ Mino cells (CB17 SCID mice, 4–5 weeks of age) and 13 days later were treated with 10, 20, or 40 mg/kg of WP1066 (i.p.) qd x 5/week for 2 weeks. WP1066 significantly decreased tumor burden and increased the survival of Mino tumor bearing mice treated with 20 or 40 mg/kg of WP1066. These results suggest that WP1066 reduces Stat activation through a unique mechanism and may have therapeutic potential for the treatment of human B-cell non-Hodgkin’s lymphomas and multiple myeloma.