Abstract

Rituximab (chimeric anti-CD20 mAb) has been used for the treatment of Non Hodgkin B cell lymphomas (B-NHL), alone or in combination with CHOP. However, a subset of patients does not respond to treatment or develops refractoriness to further treatments. Therefore, there is an urgent need to develop new alternatives to treat these patients. We have reported that treatment of B-NHL cell lines with rituximab inhibits anti-apoptotic survival pathways and down regulates the expression of anti-apoptotic Bcl-2 family proteins (i.e. Bcl-2/Bcl-xl) resulting in sensitization to chemotherapeutic drugs. Further, rituximab-resistant clones showed over-expression of anti-apoptotic gene products (

Jazirehi et al.,
Cancer Research
1
:
1270
–81,
2007
). Therefore, we hypothesize that inhibitors of anti-apoptotic Bcl-2 family may reverse the resistance to apoptotic stimuli. We examined chemical inhibitors that mimic natural ligands of the anti-apoptotic BH3-only proteins. GX15–070 (Gemin X Biotechnologies, Inc., Canada) inhibits Bcl-2 protein-protein interactions resulting in Bak and Bax oligomerization, release of cytochrome C, and activation of caspases (
Shore and Viallet,
Hematology
,
2005
; ASH,
226
–230
). Treatment of B-NHL cell lines (Raji, Ramos, 2F7, DHL-4) with subtoxic concentrations of GX15–070 (20–50 uM) resulted in inhibition of cell proliferation and subsequently induction of apoptosis as determined by TUNEL. There was a time and concentration-dependent effect of GX15–070 on cytostasis and apoptosis. Analysis of cells treated with GX15–070 by western blotting revealed that Bcl-2, Bcl-xl, and Mcl-1 protein expressions were significantly inhibited as compared to controls. The protein inhibition by GX15–070 was not expected and needs further investigation. We also examined the effect of combination treatment of GX15–070 with the chemotherapeutic drug CDDP and there was an additive or synergistic cytotoxic effect. Treatment of rituximab-resistant clones (generated from 2F7, Raji and Ramos) treated with GX15–070 resulted in significant inhibition of cell growth and apoptosis. The cytotoxicity of GX15–070 in the B-NHL cell lines was tumor specific, because treatment of human peripheral blood leukocytes from different donors did not show any cytotoxic effect. Likewise, treatment of nude mice with different concentrations of GX15–070 did not show any detectable toxicity. These findings demonstrate that GX15–070 is cytotoxic to various drug/rituximab-resistant B-NHL cell lines and is not toxic to normal human leukocytes. This study suggests that combination of GX15–070 with subtoxic concentrations of chemotherapeutic drugs may have additive/synergistic effects. The present findings support the potential therapeutic application of GX15–070 in the treatment of patients with B-NHL that are resistant to current therapies.

Author notes

Disclosure: No relevant conflicts of interest to declare.