Development of a novel dosage form for intramuscular injection of titrated extract of Centella asiatica in a mixed micellar system

Titrated extract of Centella asiatica (TECA), a drug used in treating systemic scleroderma, is poorly water-soluble. A conventional dosage form for the intramuscular injection of TECA, propylene glycol (PG)-based TECA solution, causes severe pain after intramuscular injection. To improve the solubility of TECA and reduce pain after injection, mixed micellar systems composed of 10% surfactant mixture (Tween 20 and Tween 85) and 90% phosphate-buffered saline, pH 7.0 (PBS) were prepared. As the ratio of Tween 20 to Tween 85 increased from 0:10 to 10:0, the solubility of TECA in the mixed micellar systems increased from 7- to 26-fold compared to that in PBS (pH 7.0). The droplet size of micelles gradually decreased with the increasing ratio of Tween 20 to Tween 85 from 0:10 to 4:6, followed by an abrupt decrease in size above the ratio of 6:4. Furthermore, the micellar systems prepared with Tween 20 and Tween 85 at the ratio of 6:4, 8:2 or 10:0 could solubilize TECA more than 10 mg/ml and the resultant droplet sizes were less than 2 μm. No significant changes were observed in the droplet sizes and asiaticoside contents in these micellar formulations during storage, indicating these systems are stable for at least 60 days. Their osmotic pressures were remarkably lower than those of PG-based TECA solution and similar to that of saline solution, irrespective of dilution ratios. Most importantly, they markedly reduced the number of writhes compared with PG-based TECA solution after injection to mice. All of these results suggest that these three TECA micellar formulations prepared with Tween 20 and Tween 85 improved the solubility of TECA and reduced pain following injection, possibly due to the decrease in osmotic pressure. Thus, these micellar formulations composed of optimum ratios of Tween 20 and Tween 85 may have a potential as dosage forms for the intramuscular injection of a poorly water-soluble TECA.