Long-circulating monensin nanoparticles for the potentiation of immunotoxin and anticancer drugs
Authors: Shaik Madhu Sudhan , Ikediobi Ogechi , Turnage Valerie D , McSween Jelino , Kanikkannan Narayanasamy , Singh Mandip 
Published: July 2003

Abstract
The carboxylic ionophore monensin was formulated into long-circulating nanoparticles with the help of polyethylene glycol/poly (dl-lactide-co-glycolide) diblock copolymers, in an attempt to enhance the cytotoxicity of a ricin-based immunotoxin, anti-My9, and anticancer drugs like adriamycin and tamoxifen. This study looked into various aspects involving the preparation (using a homogenizer and an EmulsiFlex homogenizer-extrusion device) and lyophilization of long-circulating monensin nanoparticles (LMNP) of particle size &lt; 200 nm in diameter. The particle size of LMNP was reduced from 194 nm to 160 nm by passing the nanoparticles through an EmulsiFlex, before freeze-drying. There was a 4.8–83.7% increase in the particle size of LMNP after freeze-drying, which was dependent upon the manufacturing conditions such as use of the EmulsiFlex for size reduction before freeze-drying, the freezing method (rapid/slow) and the concentration of lyoprotectant (mannitol or trehalose) employed for freeze-drying. LMNP freeze-dried with 2.4% of trehalose showed minimal size change (&lt; 9%) after freeze-drying. Further, the freezing method was found to have negligible effect on the particle size of LMNP freeze-dried with trehalose in comparison with mannitol. The entrapment efficiency of monensin in LMNP was found to be 14.2 ± 0.3%. The LMNP were found to be spherical in shape and smooth in surface texture as observed by atomic force microscopy. In-vitro release of monensin from LMNP in phosphate buffered saline (PBS) pH 7.4 or PBS supplemented with 10% human serum indicated that there was an initial rapid release of about 40% in the first 8 h followed by a fairly slow release (about 20%) in the next 88 h. In-vivo studies conducted with Sprague-Dawley rats showed that 20% of monensin remained in circulation 4–8 h after the intravenous administration of LMNP. An in-vitro dye-based cytotoxicity assay (MTS/PMS method) showed that there was 500 times and 5 times potentiation of the cytotoxicity of anti-My9 immunotoxin by LMNP (5 times 10−8 m of monensin) in HL-60 sensitive and resistant human tumour cell lines, respectively. Further, LMNP (5 times 10−8 m of monensin) potentiated the cytotoxicity of adriamycin in MCF 7 and SW 620 cell lines by 100 fold and 10 fold, respectively, and that of tamoxifen by 44 fold in MCF 7 cell line as assessed by crystal violet dye uptake assay. Our results suggest that it is possibleto prepare LMNP possessing appropriate particlesize (&lt; 200 nm), monensin content and in-vitro and in-vivo release characteristics with the help of a homogenizer and an EmulsiFlex homogenizer-extrusion device. LMNP can be freeze-dried with minimal increase in particle size by using a suitable concentration of a lyoprotectant like trehalose. Furthermore, LMNP could potentiate the cytotoxicity of immunotoxin, adriamycin and tamoxifen by 5–500 fold in-vitro, which will be further investigated in-vivo in a suitable animal model.