Potentiation of antitumor immunotoxins by liposomal monensin

The cytotoxicity of specific ricin A-chain immunotoxins is greatly enhanced in vitro by the carboxylic ionophore monensin. However, the highly lipophilic nature of monensin, which is reflected in its poor solubility and short half-life, has restricted its use in in vivo animal studies.

The purpose of this study was to assess the ability of monensin incorporated in unilamellar vesicles (liposomes) to potentiate antitumor immunotoxins in vitro and in vivo.

Monensin was incorporated into liposomes and used in combination with specific immunotoxins against human tumor cell lines in vitro and in vivo. Inhibition of [3H]leucine incorporation was used to evaluate the cytotoxic action of immunotoxin with or without monensin in vitro on the following human tumor cell lines: H-MESO-1 malignant mesothelioma, LS174T colorectal carcinoma, and U373, U87, and MG-1 glioblastomas. For the in vivo studies of immunotoxins and liposomal monensin, BALB/c nu/nu mice were inoculated intraperitoneally with H-MESO-1 cells.

Liposomal monensin potentiated the cytotoxic action of cell-specific anti-human transferrin receptor immunotoxin on H-MESO-1 target cells at a molar concentration of monensin that was 160-fold lower than the concentration of monensin in buffer that produced the same effect (0.3 nM versus 0.05 microM). Moreover, immunotoxin plus 0.1 microM liposomal monensin was fivefold more toxic for H-MESO-1 cells and 1000-fold and 2200-fold more toxic for human glioblastoma U373 and U87 cells, respectively, than immunotoxin plus 0.1 microM free monensin in buffer. Liposomal monensin produced similar effects when it was combined with different specific immunotoxins and other target cell lines (i.e., LS174T, U87, and CEM). Immunotoxin specificity was preserved with liposomal monensin, as shown by the absence of effect with non-cell-binding immunotoxins or on antigen-negative cell lines. In mice, liposomal monensin in combination with specific immunotoxin substantially prolonged survival, and three (21%) of 14 mice bearing H-MESO-1 xenografts treated with the liposomes showed no evidence of tumor at day 160 after treatment. Treatment with control immunotoxin plus liposomal monensin was ineffective.

These findings suggest that encapsulation of monensin into liposomes increased the capacity of monensin to enhance the potency of cell-specific immunotoxin in vitro and in vivo.