A Novel Celecoxib Derivative, OSU03012, Induces Cytotoxicity in Primary CLL Cells and Transformed B-Cell Lymphoma Via a Caspase and Bcl-2 Independent Mechanism.

Chronic lymphocytic leukemia (CLL) is an incurable adult leukemia characterized by disrupted apoptosis. While the majority of patients with CLL are asymptomatic at diagnosis, most progress and require therapy. Identification of new targets and therapeutic agents is therefore a high priority for the treatment of CLL. Synthetic chemistry yielded derivatives of the COX-2 inhibitor, celecoxib, with increased ability to induce apoptosis in the 1–10 μ M range in prostate cancer cells, a similar proposed mechanism of action, and increased in vivo activity in a murine prostate cancer xenograft model. Based upon these data, a Rapid Access to Intervention Development (RAID) proposal is underway to generate OSU03012 for clinical studies in prostate cancer. In addition, we are examining the biologic effects of these new agents in primary CLL cells and lymphoblastic cell lines, showing a novel mechanism of cell killing independent of caspase activation and bcl-2 over-expression. To determine the in vitro activity against CLL cells, 11 CLL patient PBMCs were incubated in various concentrations of OSU03012. The LC₅₀ at 24 hrs was 7.12μM and decreased to 5.45μM at 72 hrs. We show both early (annexin-V positive) and late (both annexin-V/PI positive) apoptosis concurrent with loss of mitochondrial membrane potential typical of apoptosis. These data suggest OSU03012 is highly cytotoxic toward CLL cells in vitro at doses well below those attainable without toxicity in a murine model. Additionally, we show that OSU03012 mediates apoptosis by activation of the intrinsic, mitochondrial pathway of apoptosis but also activates alternative caspase independent cell death pathways. CLL cells from 8 patients were incubated in 10μM OSU03012 for 24 hrs and assessed for caspase-3 and PARP. Immunoblots reveal a dose dependent increase in active caspase-3 concurrent with a decrease in the pro-form. This occurred concurrently with the appearance of the 85 kD cleaved product of PARP that is a known downstream target of caspase-3. In the same 8 patient lysates we saw no change in the inactive pro-form of caspase-8, but consistent processing of caspase-9. These data suggest that OSU03012 in part utilizes the intrinsic pathway of apoptosis to promote CLL cell death. Incubation of CLL cells with z-VAD-fmk and OSU03012 did not abrogate cell death, but eliminated processing of caspase-9, caspase-3 and PARP, suggesting that this agent also activates caspase independent mechanisms of cell death. Given the caspase dependent and independent pathways utilized by OSU03012, we assessed the dependence of cell death on bcl-2 expression. Here we show that bcl-2 over-expression in the 697 lymphoblastic cell line greatly diminishes the apoptosis observed with fludarabine, but potent apoptosis is equally observed with OSU03012 compared to the empty vector cell line. Furthermore, in the bcl-2 over-expressing cell line, caspase-3 and PARP cleavage was not observed despite equivalent apoptosis supporting further multiple mechanisms of cell killing induced by OSU03012. In summary, OSU03012 is an oral bioavailable therapeutic agent that has potent in vitro activity against primary CLL cells. This cytotoxicity is mediated by both caspase dependent and independent pathways and can overcome bcl-2 over-expression. These data provide support for further investigation of the mechanism of action of OSU03012 in CLL cells and performance of early Phase I studies in CLL as part of the RAID process.