# PolySorbate-Based Drug Formulations for Brain-Targeted Drug Delivery and Anticancer Therapy
## 2 Basic Characteristics of PSs
### Biosafety Studies of PSs  
Biosafety is an umbrella term that reflects the damage of the material to biological integrity [31]. Any material used for biomedical applications should possess desirable biosafety. To achieve desirable biosafety, materials should be degraded in biological environments without causing any adverse effects after their therapeutic use. Enzymatic degradation is a crucial function of the human body that involves the breakdown or elimination of unnecessary materials. Food additives are generally excreted from the body without causing adverse effects. PSs have been utilized as emulsifiers in a variety of food and cosmetic products [32]. PSs were approved as a food additive in the USA in 1960 and the EU in 1995 [33]. The Joint Food and Agriculture Organization of the United Nations/World Health Organization Expert Committee on Food Additives established an acceptable oral daily intake of PSs (e.g., PS 20, PS 60, PS 65, and PS 80) of 25 mg/kg by adults [34; 35]. Recently, the pediatric safety of PSs in drug formulations has also been investigated [36]. The amount of parenterally administered PSs in formulations was calculated using a progressive pediatric safety factor through an age- and weight-based estimation. For example, the maximum total daily doses of parenterally administered PS 20 and PS 80 for 2-year-old children are 3.37 and 50.6 mg, respectively [36]. Over the past few decades, the cytotoxicity and metabolism of various PSs have been investigated to demonstrate their potential for biomedical applications. Upon parenteral administration, PSs are degraded readily via hydrolysis by esterases in the plasma [37]. The fatty acids generated by the hydrolysis of PSs are expected to be further metabolized via beta-oxidation, leading to the formation of carbon dioxide, which can be exhaled. Polyoxyethylene sorbitan, which is generated by the hydrolysis of PSs, is excreted via urine and feces [38]. An in vivo study showed that PSs were efficiently hydrolyzed by esterase in the digestive tract after oral administration in a rat model [39; 40]. Free fatty acids are then readily absorbed from the gastrointestinal tract and further metabolized to carbon dioxide in exhaled breath, similar to the phenomena observed in normal fatty acid metabolism [39]. The absorption of polyoxyethylene sorbitan into the digestive tract was found to be very low. Approximately 87% of PS 20 and 91% of PS 80 were excreted in the feces [40]. Negligible systemic toxicity was observed when PS 80 (22 g/kg body weight) was orally administered to rats. In addition, liver or kidney dysfunction was not observed [34]. An in vivo rabbit eye model was used to evaluate the irritation effect of aqueous and oily solutions (Miglyol 812) of PS 20 and PS 80. In vitro tests using fibroblast cells demonstrated that cytotoxicity of PSs depended on surfactant concentrations (0.1, 1, or 5%) and types of solvents (water or oil). Treatment with 1% aqueous solutions of PS 20 and PS 80 induced the cell death of approximately 20%. Only 10% of cells were dead when the oily solution of the surfactant was added. The difference in cell viability between aqueous and oily solutions was larger when the concentration of PSs increased from 1 to 5%. In vivo tests showed that aqueous (0.9% NaCl) solutions of PSs were nonirritant,  
Figure 2: Chemical structures of various polysorbates. whereas small changes in conjunctiviva were observed after administration of oily PSs [41]. The in vitro cytotoxicity of PSs against human fibroblast cells has also been studied. The half-maximal inhibitory concentration (IC\({}_{50}\)) values of aqueous solutions of PS 60 and PS 80 were found to be 70.8 and 65.5 mg/mL, respectively [42]. Although PS 20 and PS 80 have been recognized as safe and nonirritant food additives and drug-formulating agents [36], some rare cases of hypersensitivity [43] and hepatotoxicity [44] have been reported, possibly owing to highly dosed PSs as excipients.