Bortezomib (PS341, Velcade) is a selective, reversible proteosome inhibitor. It is a unique inhibitor of the ubiquitin-proteosome pathway leading to arrest in tumor growth, induction of apoptosis, inhibition of tumor metastasis and angiogenesis. It plays an important role in the activation of NF Kappa B, which is responsible for transcription of inhibitors in apoptosis. It is a novel therapeutic agent that has demonstrated in vitro and in vivo activity in mantle cell lymphoma (MCL). In one phase II study, O’Conner et al reported that bortezomib demonstrated a response rate of 50% in refractory cases of mantle cell lymphoma, where as in small lymphocytic lymphoma there was a 0% response rate. Why certain types of non-Hodgkins lymphoma (NHL) are sensitive to this agent and others resistant is currently not well understood. It is believed that disregulation has been shown to play a role in the development of drug resistance in NHL Our objective was to determine the efficacy of bortezomib in vitro using three subtypes of NHL. Three lymphocytic cell lines were used: Granta-519 (mantle cell lymphoma), Toledo (diffuse large cell lymphoma) and EHEB (chronic B cell leukemia). The cytotoxic effects were determined by the colorimetric MTT assay after 24 hours of treatment with bortezomib (1 – 20 nM). Apoptosis was assessed by measuring Caspase-3 activity after 6 and 16 hours of treatment with bortezomib (4 or 20 nM). Bortezomib demonstrated a dose-dependent cytotoxic effect for all three cell lines, with an IC50 of 3.5 nM, 4.1 nM, and 18.5 nM for Toledo, Granta-519 and EHEB, respectively. Apoptotic studies showed that bortezomib (20 nM) induced a 1.4 to 5.6-fold increase (after 6 hrs) and 5.8 to 16.6-fold increase (after 16 hrs) in apoptosis. The EHEB cell line was most resistant, showing no increase in Caspase-3 activity at the lower dose (4 nM) at either time-point. Granta-519 was most sensitive showing a 5.2-fold increase in apoptosis with the lower dose after 16 hours of treatment. This in vitro study demonstrated a dose and time dependent response of bortezomib in both the mantle cell lymphoma (Granta-519) and the diffuse large cell lymphoma (Toledo) cell lines. The resistance to bortezomib in the chronic B cell leukemia (EHEB) cell line was confirmed in this study as evidenced by the lack of Caspase-3 activity at the 4 nM dose at both 6 and 16 hours. Our future studies will attempt to unmask the reason for this resistance. Perhaps manipulation of the apoptotic pathway through the use of combinations of various biologic response modifiers may be an attractive option in overcoming resistance in certain subtypes of NHL.

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