Synergistic Effects of a Novel Water-Soluble Small Molecule, ON 013105, and Rituximab on Mantle Cell Lymphoma In Vitro and In Vivo

Abstract 771

Mantle cell lymphoma (MCL) is a well-defined subtype of B-cell non-Hodgkin's lymphoma characterized by a t(11;14)(q13;q32) chromosomal translocation, and associated with constitutive over-expression of cyclin D1. MCL generally has poor clinical outcome marked by relapse. There is considerable need for novel and more effective agents against MCL. ON 013105 belongs to the styryl benzylsulfones, a novel family of non-ATP competitive kinase inhibitors with potent antitumor activity. Here, we report that ON 013105 induced cell death in a dose-dependent manner in two well-characterized MCL cell lines, Granta 519 and Z138C. In vitro cell death was preceded by the activation of caspases 3 and 9 and cleavage of PARP, indicating induction of apoptosis. In addition, ON 013105-treated cells exhibited reduced expression of cyclin D1 and c-myc. These effects on expression and apoptosis were not evident in cells treated with ON 013101, an inactive (non-cytotoxic) isomer of ON 013105. Since it is common clinical practice to combine Rituximab (RTX) with chemotherapy regimens in treating CD20+ B cell-lymphoma, we studied ON 013105 combined with rituximab, and found ON 013105-induced apoptosis more efficiently than when employed as a single agent. The combination effect on cell death was synergistic in nature. To further study this activity, we focused on Mcl-1, a member of the anti-apoptotic Bcl-2 family known to inhibit apoptosis induced by cytotoxic stimuli through antagonizing pro-apoptotic Bcl-2 family members. We observed a dramatic decrease in Mcl-1 expression in cells treated with ON 013105 (but not with ON 013101) in combination with RTX, compared to ON 013105 alone. We also evaluated the effects of ON 013105 in combination with Doxorubicin or Vincristine and found that both these compounds also significantly enhanced the cytotoxic effects of ON 013105. In vivo pharmacokinetics studies in a mouse model system revealed that plasma concentrations up to 50 μM could be safely achieved by administering ON 013105 at 100 mg/kg via i.v or i.p routes. Significant levels of ON 013100 (30-40% of the peak levels of ON 013105), an active metabolite, were also detected in the circulation, presumably due to the in vivo dephosphorylation of ON 013105 by phosphatase action. ON 013105 was well tolerated in mice, both as a single agent and when used in combination with rituximab, and there were no systemic toxic effects to the host and no loss in body weight. In vivo efficacy studies in mouse xenograft models employing transplanted MCL cells demonstrated that ON 013105 effectively inhibited tumor growth in a dose-dependent manner. ON 013015 at 25 mg/kg (Q2D) and 75mg/kg (Q7D) induced 46% and 80 % reduction of tumor volume, respectively, compared to controls, over 4 weeks of treatment. Moreover, ON 013105 at 25 mg/kg (Q2D) in a combination regimen with RTX (2.5 mg/kg, Q3D) induced over 85% reduction of tumor volume. Though in vivo efficacy studies of ON013015 (25 mg/kg, Q2D) in combination with Doxorubicin (3.5mg/kg, Q7D) or Vincristine (0.3mg/kg, Q2D) showed drastic decrease in tumor growth in mouse models, this effect was accompanied by severe side effects to the host, including mortality. In sum, ON 013105, alone and in combination with RTX may be a potent therapeutic agent against MCL. A Phase I dose escalation trial of ON 013105 as a single agent is underway in patients with relapsed/refractory lymphoma including MCL.