Influence of the active pharmaceutical ingredient concentration on the physical state of mannitol-implications in freeze-drying

Purpose. The aim of this study was to investigate the effect of the concentration of the active pharmaceutical ingredient on the physical state of mannitol in frozen aqueous systems. Methods. A human monoclonal antibody was used as the model protein. Mannitol and sucrose were used as the bulking agent and the lyoprotectant, respectively. The thermal behavior of frozen mannitol-sucrose solutions during and after annealing, in the absence and presence of the protein, were characterized by low-temperature powder X-ray diffractometry and differential scanning calorimetry. The influence of the protein on the crystallization behavior of mannitol was also evaluated. Results. The excipient concentration had a pronounced effect on the glass transition temperature of maximally freeze-concentrated amorphous phase (T'_g). At fixed excipient compositions, the protein had no effect on the T'_g if the protein concentration was ≤20 mg/ml. However, at higher protein concentrations, there was a marked increase in T'_g as a function of protein concentration. The inhibitory effect of the protein on mannitol crystallization was concentration dependent and was directly evident from X-ray diffractometry experiments. Annealing facilitated both mannitol nucleation and crystal growth even in the presence of the protein. Conclusions. The ratio of mannitol to sucrose and the protein concentration have an impact on the T' _g and may therefore influence the primary drying temperature. The protein inhibits both the nucleation and growth of mannitol crystals and this effect seems to be concentration dependent. The presence of the protein and the protein concentration dictate the processing conditions, i.e., annealing time, annealing temperature, and primary drying temperature.