Glucosylceramide Synthetase Inhibitors Sensitise B-CLL Cells to Cytotoxic Agents without Reversing P-gp Functional Activity.

Malignant B-cells from a high proportion of chronic lymphocytic leukaemia (B-CLL) patients over express the multidrug resistance (MDR) -1 gene encoded transmembrane efflux pump P-glycoprotein (P-gp). Inhibition of glucosylceramide synthesis has been shown to correlate with the expression and function of P-gp and sensitise cells to cytotoxic agents. We analysed the ability of glucosylceramide synthetase (GCS) inhibitors N-butyl-deoxygalactonojirimycin (OGB-1, 500μM) and N-nonyl-deoxygalactonojirimycin (OGB-2, 100μM) to sensitise B-CLL cells to conventional cytotoxic drugs 2-chlorodeoxyadenosine (CdA), chlorambucil (Chl) and fludarabine (FdR) using the in vitro cytotoxicity MTT assay. The effect on P-gp activity was also analysed using the calcein-AM accumulation assay and the results expressed as multidrug activity factor (MAF), where a MAF of >10 in the presence of a P-gp inhibitor denotes P-gp functional activity. GCS inhibitors were cultured with B-CLL cells for 24-48h before the assays were performed. The P-gp negative cell line CEM-CCRF had no MAF activity with an IC50 for vincristine (a known P-gp substrate) of <1ng/ml. The P-gp over expressing cell line CEM-VLB showed a MAF value of 96.4 with zosuquidar trihydrochloride (Z.3HCL), a specific inhibitor of P-gp, 15.7 with OGB-1 and 45.9 with OGB-2. The IC50 for vincristine was reduced from >10ug/ml to 55.5ng/ml in the presence of OGB-2. In peripheral blood mononuclear cells from 3 normal volunteers (all P-gp +ve), the mean MAF value for Z.3HCL was 23.86 and for OGB-2 was 16.2. In 9/13 B-CLL samples there was P-gp functional activity in the presence of Z.3HCL with a mean MAF value of 22.15 (range 11.27–37.3). P-gp was over expressed in10/13 B-CLL samples. However, when available samples from this cohort were assessed with OGB-1 (n=4) and OGB-2 (n=13) the MAF value was <10. Nevertheless, sensitisation of B-CLL cells was observed by a reduction in the IC50 in the presence of OGB-1 with CdA in 3/4 (to 40% in the presence of cytotoxic drug alone), Chl in 3/4 (39%), FdR in 2/4 (26%) and in the presence of OGB-2 with CdA in 8/13 (42%), Chl in 5/13 (40%) and FdR in 7/13 (34%). Although GCS inhibitors sensitize B-CLL cells to cytotoxic drugs in some B-CLL patients, they do not appear to have any effect on P-gp functional activity.