Mannitol - a potential cryoprotectant in frozen and freeze-dried formulations

Abstract

Purpose: To characterize the phase behavior of mannitol and tertiary butyl alcohol (TBA) in TBA-mannitol-water ternary frozen solutions. Methods: Solutions of mannitol (2 and 5% w/w) in TBA-water systems of different compositions (5 to 30% w/w TBA) were prepared and cooled to -40°C. The frozen solutions were thawed to RT. The solutions were characterized, both during cooling and warming, by differential scanning calorimetry (DSC) and X-ray diffractometry (XRD), using both a laboratory source and synchrotron radiation (Argonne National Laboratory). Selected compositions were freeze-dried. Results: At and below the TBA-water eutectic composition (22.5% w/w TBA), mannitol crystallization was completely inhibited while TBA crystallized as a heptahydrate. When these frozen compositions were heated the thermal events observed were (i) two glass transitions, crystallization of mannitol and TBA heptahydrate, (ii) melting of binary eutectics of ice with each TBA dihydrate, TBA heptahydrate and mannitol along with (iii) ice melting. Irrespective of the initial TBA concentration, when freeze-dried with mannitol, the final lyophile consisted of δ-mannitol. Conclusions: While mannitol did not affect the phase behavior of TBA-water systems, TBA completely inhibited mannitol crystallization in the frozen state at compositions ≤ 22.5% w/w TBA. This provides an avenue for mannitol to serve as a cryoprotectants in frozen systems and as a bulking agent in lyophiles.