Mechanisms by which crystalline mannitol improves the reconstitution time of high concentration lyophilized protein formulations

Lyophilized high concentration protein formulations often have long and variable reconstitution times. The aim is to understand the role of crystalline mannitol in lowering the reconstitution time of these formulations. Novel methods were developed for quantifying the effect of crystalline mannitol on cake attributes influencing reconstitution, specifically, cake wettability, liquid penetration into the cake and cake disintegration. Amorphous and partially crystalline cakes were obtained by varying the freeze-drying conditions, particularly, the freezing rate (slow vs. fast), annealing (annealed vs. unannealed), and primary drying (aggressive vs. conservative). Mannitol crystallinity was quantified using X-ray powder diffractometry. Phase separation of crystalline mannitol from the amorphous, protein rich matrix improved wettability of the cake solids and promoted penetration of the reconstitution fluid into the cake interior. The partially crystalline cakes offered less resistance to crushing in the dry state than the amorphous cakes. Crystalline mannitol provided “weak points” in the freeze-dried cakes, potentially enabling easier cake disintegration upon addition of the reconstitution fluid. There was no evident correlation between the degree of crystallinity and reconstitution time. While crystalline mannitol generally decreased reconstitution time by favorably affecting the cake attributes influencing reconstitution, it did not always reduce reconstitution time.