Silvestrol, a Rocaglate Derivative from the Indonesian Plant Aglaia foveolata, Has Significant Bcl-2- and p53-Independent Anti-Tumor Activity against Chronic Lymphocytic Leukemia Cells.

Chronic Lymphocytic Leukemia (CLL) is an incurable disease with limited therapeutic options. The development of drug resistance through multiple pathways, especially in advanced disease, further restricts these options. Thus, new agents with unique mechanisms of action are crucial to make an impact on patient survival. Silvestrol, a rocaglate derivative with an unusual dioxanyloxy unit, was isolated from Aglaia species using bioassay-guided fractionation. Silvestrol exhibited potent in vitro cytotoxic activity against several tumor cell lines. Silvestrol was further evaluated in vivo in the hollow fiber test and in the murine P-388 leukemia model, in which it demonstrated promising anti-tumor activity with no significant weight loss up to 2.5 mg/kg (3.7 μM, assuming equal distribution) (1). Based on these results, we tested silvestrol against tumor cells obtained from CLL patients. Silvestrol exhibited significant antitumor activity with an estimated LC₅₀ (concentration lethal to 50% of cells relative to untreated control) of 10 nM at 72 hours by MTT assay. In contrast, at this same timepoint using normal human peripheral blood mononuclear cells, an LC₅₀ for silvestrol could not be defined even up to 4.0 μM. Under identical conditions, silvestrol was 50 to 100 fold more potent than the active metabolite of fludarabine, commonly used in the treatment of CLL. To determine the minimum exposure time required for silvestrol to have an effect, cells were incubated for various times in 80 nM silvestrol, then washed and resuspended in media with or without drug and incubated for a total of 72 hours. With only a four hour exposure, an average of 56% cytotoxicity was observed relative to untreated cells, and with a 24 hour exposure, results between samples in which the drug was removed and those incubated continuously were indistinguishable. T cell depletion and concomitant immunodeficiency is a serious risk with therapies currently available for CLL. We therefore tested the relative effects of silvestrol on B and T cells. By MTT assay with selected cells and in whole blood incubations followed by flow cytometry, using blood from both CLL patients and healthy volunteers, silvestrol demonstrated significantly more cytotoxicity toward B cells than T cells. Although some variability was observed between patient samples, silvestrol had activity against all samples tested and there was no detectable difference in average potency against cells from patients with a 17p13 deletion (chromosomal site of p53) relative to those without this risk factor. Furthermore, there was no significant difference in silvestrol-mediated cytotoxicity between lymphoblastic cells with a ten-fold overexpression of Bcl-2 relative to control cells. Together, these data demonstrate that silvestrol has efficacy against CLL cells in vitro and in whole blood, has highly unusual B-cell specificity, and is independent of key CLL resistance mechanisms. Our data strongly support further investigation of silvestrol as an antitumor agent in CLL. Studies are underway to determine the precise mechanism of action of this compound in CLL cells.