The solubility of sirolimus drug in different organic solvents, viz., acetone, chloroform, methanol, ethanol, and dichloromethane, has been measured for the temperature range of (295 to 345) K by a gravimetric method. The relation between solubility and crystal size formation has also been studied. It was found that the higher the solubility of material, the larger the crystal size and vice versa. The stability of crystals has been measured, and it was found that smaller crystals have a moderate level of stability when compared with that of larger crystals. From the observations of the present study, it may be suggested that for cardiovascular applications of sirolimus drug one can use ethanol and methanol as a solvent for deriving crystals of less than 400 nm size.

Sirolimus, generally used in organ transplantation, is derived from bacterium Streptomyces hygroscopicus. Mass transfer controlled ultrasonic antisolvent method was used for determining the precipitation kinetics of sirolimus. The effect of temperature was determined on the particles size, percentage recovery, critical radius of nucleus, mass transfer coefficient, etc. for sirolimus dissolved in methanol and antisolvent water using ultrasonic treatment. The study was done using classical nucleation theory, which can also be applied to precipitation processes. Experiments were carried out at various temperatures; viz: 45, 50, 60 and 70 °C and the percentage recoveries of sirolimus were found to be 90.74, 91.5, 92.64 and 93.61%, respectively, for initial amount of 8 mg dissolved in 1 mL of solvent and further introduced into 12 mL of HPLC water. The final average diameters of crystals observed for the temperatures were 1371, 1287, 1063 and 863 nm, respectively. The systems were found to be mass transfer controlling and that the mass diffusivities were found to be about 3.97×10 , 3.01×10 -9 and 1.92×10 -9 m 2 /s, respectively.

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