Chronic lymphocytic leukemia (CLL) has a variable clinical course but once treatment is required the development of drug resistance often ensues. Therefore, delineation of the biological mechanisms of drug resistance and the development of novel therapies designed to overcome this is an important goal of research into this condition. We have recently shown that the transcription factor NF-κB plays a central role in regulating CLL cell survival and this is mediated, at least in part, through the transactivation of a number of anti-apoptotic genes including Bcl-2. In this pre-clinical study we evaluated the cytotoxic effects of the novel parthenolide analog, LC-1 (Leuchemix Inc.), in primary tumor cells derived from 49 CLL patients (20 treated, 29 untreated). LC-1 induced apoptosis, as assessed by the Annexin V assay, in a concentration-dependent manner (0.1 – 10 μM) in all the CLL samples tested with a mean LD50 value (the concentration of drug required to kill 50% of the cells) of 2.9 μM. The induction of apoptosis was preceded by a marked loss of NF-κB activity, as evidenced by electrophoretic mobility shift assay, and a down regulation in Bcl-2 protein expression. Caspase-3 activation was a consistent feature of LC-1-induced apoptosis pointing to the involvement of the intrinsic apoptotic pathway. Importantly, CD38+ samples (> 30% expression) and those derived from patients with unmutated VH genes were more sensitive to LC-1 (LD50 values = 2.3 μM versus 3.4 μM and 2.4 μM versus 3.2 μM; P = 0.0003 and 0.01 respectively) suggesting that these CLL cells have a disproportionate reliance on NF-κB activity to maintain their survival and proliferative advantage. Taken together our data clearly demonstrates that LC-1 preferentially targets CLL cells from poor prognostic subsets. This unique cytotoxicity profile warrants further investigation and supports the use of this agent in early clinical trials for patients with CLL.

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