The present study focuses on the development of an alternative 'thermally gentle' strategy such as freeze-drying to obtain not only solubility enhanced but also physically stabilised amorphous solid dispersions of acetazolamide, which melt with decomposition (M.P.~260°C). The solid dispersions were prepared by freeze-drying an aqueous dispersion of acetazolamide containing a lyoprotectant as sugar alcohol (mannitol) in 1:0.5, 1:1 and 1:2 proportions by weight. All the proportions of solid dispersions reported a marked increase in solubility characteristics compared to those of pure drug; with outstanding performance by 1:1 ratio of about 6 folds rise in saturation solubility and 90% drug release in about initial 30 minutes. This could be attributed to the formation amorphous molecular dispersions, cosolvent effect of mannitol on dispersed acetazolamide as well as its local solubilisation effect at the diffusion layer. During stability study also, 1:1 ratio of solid dispersions reported an insignificant change in solubility characteristics subjected to an unchanged amorphous nature. Such physical stability could be attributed to decreased molecular mobility of the drug molecules in amorphous carrier because of weaker drug-carrier interactions. Thus, it was demonstrated that freeze-drying is an effective method of forming dissolution-enhanced, amorphous solid dispersions of thermally degradable APIs.

Abstract. An amorphous phase produced by micronization up to the molecular or colloidal level of a poorly soluble drug having low lipophilicity can distinctly enhance its solubility characteristics. However, though dispersing the molten mass of a poorly water-soluble drug within polymeric matrix has been found to be most effective in formation of molecular dispersions, the drug molecules which melt at high temperature also accompanied by decomposition, such as acetazolamide, are difficult to formulate as molecular dispersions. Hence, a method is proposed to obtain molecular dispersions of acetazolamide with poloxamer-237 by spray congealing under optimal heat treatment. Uniform molecular and/or colloidal dispersions of the drug were achieved with instantaneous solvent evaporation by mixing a drug solution with molten mass of the plasticizer matrix. Immobilization of dispersed drug molecules was effected subsequently through rapid solidification by spray congealing. Initial characterization of 1:1, 1:1.5, and 1:2 ratios of solid dispersions and devitrification study of an optimized (1:2) ratio ensured efficacy of the proposed method in formation of physically stabilized amorphous systems without thermal degradation and hence resulted in more than ninefold rise in solubility and more than 90% dissolution within initial 10 min. With 1:2 ratio, molecular dispersions could be achieved by initial solvent evaporation stage, which when subjected to spray congealing produced physically stable amorphous systems, without signs of thermal degradation. This study also proposes an opportunity for selection of those polymers with which the drug is immiscible in their fluid state, yet obtaining molecular dispersions.

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