Stealth monensin immunoliposomes as potentiator of immunotoxins in vitro

Stealth monensin liposomes (SML) were prepared using dipalmitoyl phosphatidylcholine, cholesterol, distearoyl glycerophosphoethanolamine coupled to polyethylene glycol, stearylamine, and N-succinimidyl pyridodithiopropionate linked to stearyl amine, in the molar ratio of 10:5:1.4:1.4:1.5. SML was conjugated to the anti-MY9 antibody by a disulfide linkage to form stealth monensin immunoliposomes (SMIL) by an already established procedure. The encapsulation concentrations of monensin in SML and SMIL were 10(-7) and 4.9x10(-8) M, respectively. More than 20% of monensin remained in circulation after 24 h in BALB/c mice. The ability of SML and SMIL to potentiate the effect of anti-MY9 immunotoxin (anti-MY9-IT) was tested against human leukemia HL-60 sensitive and resistant tumor cells in vitro. SML and SMIL potentiated the activity of anti-MY9-IT by 10-20 times against HL-60 sensitive tumor cell lines. However, greater potentiation of anti-MY9-IT was observed in combination with SML and SMIL against HL-60 resistant tumor cells, found to be 200 and 500 times, respectively. The potentiation of anti-MY9-IT by SMIL was more than two-fold compared with SML against both HL-60 sensitive and resistant tumor cells. Transmission electron microscopy studies conducted with HL-60 resistant cells incubated with anti-MY9-IT and monensin liposomes showed significant dilation of the golgi, which was reversible after re-incubation in fresh medium. Our studies show that SML and SMIL can be successfully used to potentiate the activity of ricin based anti-MY9-IT in vitro, and further in vivo studies will demonstrate the usefulness of this approach.