Cellular Trafficking and Cytotoxicity of Anti-Cd19-Targeted Liposomal Doxorubicin in B Lymphoma Cells

We have previously demonstrated that liposomal doxorubicin (DXR), targeted against the CD 19 receptor of human B lymphoma (Namalwa) cells resulted in selective affinity of SIL[anti-CD19] for CD19+ Namalwa cells in vitro and significantly increased therapeutic efficacy in mice compared to non-targeted liposomal DXR or to free drug (1). In this study we have examined the cellular trafficking of DXR in Namalwa cells for free drug compared to non-targeted liposomal drug (DXR-SL) or immunoliposomal drug targeted via the monoclonal antibody anti-CD19 (DXR-SIL[anti-CD19]). Liposomes were sterically stabilized with lipid derivatives of polyethylene glycol (PEG) and anti-CD 19 was attached to the PEG terminus. Time-dependent studies using flow cytometry revealed that free DXR accumulated rapidly in cells. Drug from DXR-SIL[anti-CD19] accumulated less rapidly in Namalwa cells than free drug but the cellular levels of DXR were several-fold higher than for drug presented in non-targeted DXR-SL. Internalization of SIL[anti-CD 19] into a low pH compartment could be demonstrated using a pH-sensitive probe, HPTS, encapsulated in liposomes. The endocytosis and intracellular fate of DXR-loaded liposomes were also studied by confocal microscopy, subcellular fractionation, and HPLC. At early times (1 h), DXR from targeted preparations appeared mainly at the cell surface with some DXR sequestered within vesicular structures, likely endosomes, in cells. DXR from non-targeted preparations (I)XR-SL) was only found on the cell surface after a one hour incubation. After two hours, drug from the targeted DXR formulation was mostly found within vesicular structures, whereas drug from the non-targeted formulation was still present only at the surface of the cells. The intracellular levels of DXR from DXR-S1L[anti-CD 19) continued to increase with longer incubation periods, and this endocytotic event could be abolished by metabolic inhibitors. Namalwa cells incubated with DXR-SIL[anti-CD 19] for 48 hours appear to demonstrate nuclear accumulation of DXR. This suggests that lysosomal processing of targeted liposomes allows trafficking of DXR from the lysosomal apparatus to its nuclear site of action. The cytotoxicity of DXR-SIL[anti-CD19] was 5-fold higher than that observed for non-targeted controls. The use of the cationic exchange resin, Dowex, to absorb DXR released from liposomes outside the cells demonstrated that a substantial portion of the cytotoxicity of DXR-SL, but not DXR-SIL, was due to uptake of released drug into cells. The targeted formulations were shown to be selectively apoptotic to CD 19 ' cells compared to CD 19 cells.