Purpose: To study spatial heterogeneity in phase composition when mannitol is co-lyophilized with non-crystallizing lyoprotectant, such as sucrose or trehalose. To study the influence of formulation composition and processing conditions on the extent of mannitol hemihydrate (MHH) formation in the final lyophile.
Methods: We used synchrotron X-ray diffractometry (XRD) to spatially map and thereby comprehensively characterize mannitol phase composition in unperturbed lyophiles. Low temperature thermal analysis and XRD was used to study phase behavior of frozen systems.
Result: When colyophilized with sucrose, trehalose or lysozyme as a second solute, mannitol crystallized partially as MHH (mannitol hemihydrate). The MHH content, based on the intensity of characteristic MHH peak (d-spacing 4.92 Å), was highest in the middle region of lyophile. This heterogeneity, studied in detail in presence of sucrose, occurred irrespective of cosolute content. Annealing the frozen solution at -30°C for 2 h essentially eliminated the heterogeneity, accompanied by an overall increase in MHH content. From differential scanning calorimetry it was evident that annealing caused mannitol crystallization while XRD revealed the crystallizing phase to be MHH.
Conclusion: The intra-vial heterogeneity and total MHH content in the final lyophile is a complex interplay of formulation composition and processing conditions. Graphical Abstract Figure depicting spatial heterogeneity in mannitol hemihydrate content, when mannitol is lyophilized with a cosolute, such as sucrose, trealose or lysozyme.
Keywords: annealing; freeze-drying; intra-vial heterogeneity; mannitol; mannitol hemihydrate; synchrotron XRD.