A number of drugs are unstable in an aqueous environment, even when exposed for a short duration, thus requiring packaging, storage, and shipping in a powder or lyophilized state to keep the product stable during its shelf life. Used for parenteral administration, these drugs are commonly known as powder for injection (PI), powder for reconstitution, dry powder injection, or powder for constitution.
Typically, PI drugs are supplied in glass vials with rubber plugs and are mixed or reconstituted with a diluent (usually 5% dextrose solution, normal saline, bacteriostatic water, or sterile water for injection) before administration. An incompletely dissolved product can be hazardous to the patient, thereby making reconstitution a critical performance parameter for lyophilized products.
As mentioned the method most commonly used to obtain lyophilized drugs is lyophilization, this process is done in three steps, under vacuum: freezing, primary and secondary drying. The product to be dried is frozen under atmospheric pressure.
Then, in an initial main or primary drying phase, water, in form of ice, is removed by sublimation; in the secondary drying phase, it is removed by desorption. The reduced moisture content improves the stability and shelf-life of the product; but, in some instances, rehydration can further improve those properties.
Keeping in mind the importance of reconstitution, the knowledge of parameters affecting reconstitution time is critical for lyophilized product development, quality control, and overall product performance.
Intrinsic parameters inherent to an active pharmaceutical ingredient (API) or a formulation (molecular-, particle-level solid-state properties).
Porosity. The porosity of dry powder may have a significant effect on its reconstitution.
Degree of crystallinity. The degree of crystallinity, which is the percentage of crystalline form of any compound in the amorphous matrix.
Formulation factors. The inherently low aqueous solubility of an API in formulation may contribute to incomplete reconstitution. Several formulation interventions are used in such instances, including the use of cosolvents.
Foaming during reconstitution is a serious problem for biopharmaceutical drugs because it may lead to protein denaturation and a consequent loss in their activity.
HIGH PERFORMANCE LYOPHILIZER
The rehydration step can be implemented after the secondary drying phase. During rehydration, low concentrations of moisture are intermittently added to the chamber until the required moisture content of the product has been reached. The length of the rehydration phase is productand moisture content-dependent.
The lyophilizer must be equipped with a set of inlet valves with short closing and stabilization times. Additionally, the chamber must have a moisture sensor with a detection range of 0–100%. Moisture saturation is measured continuously, online, and the residual moisture of the lyophilized product is measured externally using Karl-Fischer, for example.