Preparation of an amorphous sodium furosemide salt improves solubility and dissolution rate and leads to a faster Tmax after oral dosing to rats

Abstract

Amorphous forms of furosemide sodium salt and furosemide free acid were prepared by spray drying. For the preparation of the amorphous free acid, methanol was utilised as the solvent, whereas the amorphous sodium salt was formed from a sodium hydroxide-containing aqueous solvent in equimolar amounts of NaOH and furosemide. Information about the structural differences between the two amorphous forms was obtained by Fourier Transform Infrared Spectroscopy (FTIR), and glass transition temperature (Tg) was determined using Differential Scanning Calorimetry (DSC). The stability and devitrification tendency of the two amorphous forms were investigated by X-ray Powder Diffraction (XRPD). The apparent solubility of the two amorphous forms and the crystalline free acid form of furosemide in various gastric and intestinal stimulated media was determined. Moreover, the dissolution characteristics of the two amorphous forms and of crystalline free acid were investigated.FTIR confirmed molecular differences between the amorphous free acid and salt. The amorphous salt showed a Tg of 101.2°C, whereas the Tg for the amorphous free acid was found to be 61.8°C. The amorphous free acid was physically stable for 4days at 22°C and 33% relative humidity (RH), while the amorphous salt exhibited physical stability for 291days at the same storage conditions. When storing the amorphous forms at 40°C and 75% RH both forms converted to crystalline forms after 2days.The apparent solubility of the amorphous salt form was higher than that of both amorphous and crystalline free acid in all media studied. All three forms of furosemide exhibited a greater solubility in the presence of biorelevant media as compared to buffer, however, an overall trend for a further increase in solubility in relation to an increase in media surfactant concentration was not seen. The amorphous salt demonstrated an 8- and 20-fold higher intrinsic dissolution rate (IDR) when compared to amorphous and crystalline free acid, respectively.The promising properties of the amorphous salt in vitro were further evaluated in an in vivo study, where solid dosage forms of the amorphous salt, amorphous and crystalline free acid and a solution of furosemide were administered orally to rats. The amorphous salt exhibited a significantly faster Tmax compared to the solution and amorphous and crystalline free acid. Cmax for the solution was significantly higher compared to the three furosemide forms. No significant difference was found in AUC and absolute bioavailability for the solution, crystalline free acid and the two amorphous forms of furosemide. It can be concluded that the higher IDR and higher apparent solubility of the amorphous salt resulted in a faster Tmax compared to the amorphous and crystalline free acid.