Chronic lymphocytic leukemia (CLL) is a common neoplasm which is characterized by the clonal expansion of mature B-cells. CLL cells in circulation are non-proliferating and the systemic accumulation of these cells is primarily due to defects in apoptosis. There is no curative therapy option, and while CLL may run an indolent course for several years in some patients, there is a cohort which develops progressive disease for whom outcome is poor. Current treatment modalities, such as the use of the nucleotide analogue fludarabine, may ameliorate symptoms; however no survival advantage has been demonstrated.

Pyrrolo-1,5-benzoxazepines (PBOX) are novel agents which we have shown induce cell cycle arrest and apoptosis in chronic myeloid leukemia cell lines. In addition we have shown that a potent PBOX compound, PBOX-15, induces apoptosis in CLL cells ex vivo. Recently we have shown that apoptotic-inducing PBOX compounds bind tubulin and cause microtubule depolymerization in breast cancer cells. Microtubule targeting agents act by inducing mitotic arrest and subsequent apoptosis in cancer cells, however this mechanism of action does not account for PBOX-15 induced apoptosis in non-proliferating CLL cells.

In order to further assess the mechanism of PBOX-15 induced apoptosis in CLL, we treated peripheral blood mononuclear cells from CLL patients (n=34) with this agent ex vivo. PBOX-15 induced apoptosis in CLL cells was demonstrated by annexin V binding, PARP cleavage and mitochondrial membrane depolarization. In addition to activity in material from treatment-naïve patients, PBOX-15 induced apoptosis in fludarabine resistant CLL cells (n=5) harboring a 17p deletion resulting in loss of expression of p53. PBOX-15 induced apoptosis in CLL cells was partially inhibited by a specific inhibitor to JNK, and was partially dependant on caspase 8 activation. Anti-apoptotic Bcl-2 proteins are over-expressed in CLL cells and have been implicated in CLL resistance to apoptosis, however no change in Bcl-2 proteins were observed prior to PBOX-15 induced apoptosis. PBOX-15 exhibited selective cytotoxicity against CLL cells when compared with normal donor peripheral blood mononuclear cells (n=5), normal donor B-lymphocytes (n=5) and normal donor bone marrow progenitor cells (n = 5).

In conclusion, PBOX-15 has promise as a novel anticancer agent and may, in addition, provide insights into the molecular mechanisms resulting in the aberrant apoptosis that is characteristic of CLL.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA