The mechanism of action of novel therapies for use in Chronic Lymphocytic Leukaemia

Chronic Lymphocytic Leukaemia (CLL) remains incurable and novel treatments are urgently required to combat this disease. The proteasome inhibitor bortezomib induces high levels of apoptosis in vitro in CLL patient samples, but clinical trials have been discouraging. In this study, bortezomib induced apoptosis in all CLL samples tested at nanomolar concentrations. However, incubation of CLL cells with red blood cells (RBCs) reduced the activity of bortezomib. These data imply that RBC uptake may reduce the activity of bortezomib in vivo. TNF-related apoptosis inducing ligand (TRAIL) is an attractive cancer therapy because of its selectivity towards tumour cells. TRAIL and agonistic mAbs to two TRAIL receptors (TRAIL-R1 and TRAIL-R2) are in clinical trials. CLL cells are resistant to TRAIL but can be sensitised by pre-treatment with histone deacetylase inhibitors (HDACi). HDACi sensitised CLL cells to preparations of TRAIL that induce apoptosis through TRAIL-RI but not through TRAIL-R2. In contrast, K562 cells, when pre-treated with HDACi, responded to preparations of TRAIL that induce apoptosis through TRAIL-R2. To confirm this, TRAIL receptor-selective mutants were generated and tested for specificity in cell lines. CLL cells, pre-treated with depsipeptide, responded only to the TRAIL-R1 mutant. These data confirm that CLL cells can be sensitised to TRAIL induced apoptosis primarily through TRAIL-R1 and that clinical trials in CLL should focus on HDACi in combination with preparations of TRAIL that induce apoptosis through TRAIL-R1. Recent publications suggest that internalisation of CD95 and TNFRI are required for induction of apoptosis. TRAIL-internalisation has not been widely studied. In this study, TRAIL-induced DISC formation was shown to occur at the plasma membrane, suggesting that TRAIL signalling is unlike other members of the TNF superfamily (CD95L and TNFa). Inhibiting TRAIL internalisation with hyperosmotic solution did not inhibit apoptosis induction, suggesting that internalisation is not important for TRAIL activity.