Salt formation during freeze-drying - An approach to enhance indomethacin dissolution

Purpose: (i) Prepare a freeze-dried injectable indomethacin (IMC) dosage form. (ii) Convert IMC to its tris salt during freeze-drying so as to facilitate rapid dissolution (reconstitution). (iii) Modulate salt crystallinity by annealing the frozen solution. Methods: Aqueous IMC solutions buffered with tris were freeze dried, with or without annealing the frozen solutions. The lyophiles were characterized by X-ray diffractometry, differential scanning calorimetry and infra-red spectroscopy and also subjected to water sorption and dissolution studies. Results: Based on IR spectroscopy, the final lyophile was confirmed to contain the IMC tris salt. In the absence of annealing, the lyophile was X-ray amorphous with a glass transition temperature of 19°C. Annealing the frozen solutions caused a substantial increase in lyophile crystallinity. Interestingly, both the amorphous and partially crystalline lyophiles dissolved "instantaneously" and completely in the dissolution medium. In contrast, the crystalline IMC as well as its physical mixture with tris exhibited much slower dissolution with ∼ 50% drug dissolved in 30 min. Conclusion: In situ IMC tris salt formation resulted in an elegant lyophile with a very short reconstitution time. Tris served two roles - as a buffer in the prelyophilization solution and as the counterion for the salt in the final lyophile. This approach for solubility enhancement could be extended to other acidic drugs wherein salt formation was observed during freeze-drying.