Kazuhiko Sasagawa

Enhanced Tumor Delivery and Antitumor Activity of Palmitoyl Rhizoxin Using Stable Lipid Emulsions in Mice

AbstractPurpose. A highly lipophilic antitumor agent, 13-O-palmitoyl-rhizoxin (RS-1541), was incorporated into lipid emulsions of various sizes consisting of triglyceride ODO and surfactant HCO-60. Pharmacokinetics, toxicities, and antitumor activities were evaluated after intravenous administration to mice bearing subcutaneously inoculated M5076 sarcoma cells. Methods. The levels of RS-1541 in the plasma and tissues including tumor, were determined by HPLC. The maximum tolerated dose (MTD) was estimated by toxic death and change in body weight. The decrease in tumor diameter was measured for antitumor activity. Results. There existed large variations in pharmacokinetics of RS-1541, depending on the size of emulsion particles. Compared with a colloidal solution (reference solution), the small (110nm) and medium (230nm) size emulsions showed high concentrations of RS-1541 in the tumor, while the large emulsions (350nm–630nm) exhibited low concentrations. The MTD of RS-1541 was reduced, when incorporated in the emulsions larger than 220nm in size. At MTD, each size of emulsions (70nm–380nm) effectively retarded the tumor growth and increased survival time. The maximum effect was achieved for the 220 nm emulsions. Conclusions. When particle size is properly selected, these emulsions could be promising and effective as an injectable carrier for lipophilic antitumor agents in order to enhance the tumor delivery and efficacies while reducing toxicities.

Lipid emulsions of palmitoylrhizoxin : Effects of composition on lipolysis and biodistribution

Four types of lipid emulsion for highly lipophilic antitumour agent RS-1541 (13-O-palmitoylrhizoxin) with mean particle diameters of 200-260 nm were prepared using soybean oil (SOY) or dioctanoyldecanoylglycerol (ODO) for the oil phase and lecithin (LEC) or polyoxyethylene-(60)-hydrogenated castor oil (HCO-60) for surfactants. The lipolysis rate of HCO-60-emulsified emulsions by lipoprotein lipase was much slower than that of LEC-emulsified emulsions. Particle sizes of emulsions incubated in plasma with the lipase for six hours were 75%, 79%, 101%, and 93% of initial values for SOY/LEC, ODO/LEC, SOY/HCO-60, and ODO/HCO-60 emulsions, respectively, showing an apparent size decrease for LEC-emulsified emulsions. In rats, uptake clearance values of SOY/LEC and ODO/LEC emulsions of RS-1541 in the reticuloendothelial system (RES) were 81.2 and 135.3 mL h(-1), respectively, and AUC values were 4.0 and 1.3 microg h mL(-1), respectively. In contrast, RES uptake clearances of HCO-60 emulsions of RS-1541 were considerably lower (4.2 mL h(-1) for SOY/HCO-60; 2.2 mL h(-1) for ODO/HCO-60), resulting in high AUC values (35.4 microg h mL(-1) for SOY/ HCO-60; 63.9 microg h mL(-1) for ODO/HCO-60). The concentrations of RS-1541 in tumour tissues after an intravenous administration of ODO/HCO-60 emulsions of RS-1541 to mice bearing solid tumour M5076 sarcoma were about ten times higher than those after the administration of SOY/LEC emulsions. These results indicate that HCO-60 emulsions, compared with conventional LEC emulsions, are more stable to lipoprotein lipase and show low uptakes by RES organs, long circulations in the plasma, and high distributions in tumours. Thus, these sterically stabilized emulsions could show potential as effective carriers for highly lipophilic antitumour agents to enhance the drug delivery in tumours.

Tumor Selective Effect of RS-1541 (Palmitoyl-Rhizoxin) in M5076 Sarcoma and Host Tissues in Vivo

RS-1541 is a 13-O-palmitoyl derivative of rhizoxin, an inhibitor of tubulin polymerization. After intravenous administration of RS-1541 to mice bearing M5076 sarcoma, the maximal inhibitory effect of RS-1541 on DNA synthesis in the tumor was observed 24 h after administration, in agreement with the Cmax of rhizoxin produced from RS-1541, but not with the Cmax of RS-1541. The inhibitory effect after RS-1541 was much higher than that after rhizoxin itself. In the spleen, thymus and bone marrow, DNA synthesis was strongly inhibited by rhizoxin but not by RS-1541. After administration of RS-1541, no significant amounts of rhizoxin were detected in the tissues, except for the tumor. In acute toxicity tests, RS-1541 appeared to be less toxic than rhizoxin. These results indicate that RS-1541 possesses a high tumor-selective effect compared with rhizoxin, because of the selective production of rhizoxin in the tumor after administration of RS-1541.