Selective crystallization of phosphate buffer components andpH changes during freezing: Implications to protein stability.

Many common pharmaceutical drug products are not stable in aqueous solution and must be freeze-dried and reconstituted. Phosphate buffers have been used to maintain an optimal solution pH while the drug product undergoes the freezing process of lyophilization as well as upon reconstitution of the dry powder. During lyophilization, ice excludes solutes which are concentrated in the residual solution. If the solution becomes supersaturated with respect to a buffer salt, selective precipitation may occur which results in changes in pH and solute composition. pH changes during freezing may be detrimental to the drug product and result in the loss of activity that the lyophilization process was meant to protect. The objective of these studies has been to investigate crystallization-induced pH changes of phosphate buffer solutions during freezing at far-from-equilibrium conditions (similar to commercial freeze-drying) as a function of initial pH, total phosphate concentration, cooling method, and the presence of ionic (NaCl, KCl) and nonionic (sucrose, mannitol, polyethylene glycol) additives. The stability of an oligomeric protein, lactate dehydrogenase, has been investigated as a function of pH and solute composition in phosphate buffer solutions isothermally and after far-from-equilibrium freezing. pH changes during far-from-equilibrium freezing of potassium phosphate buffer solutions are significant, approaching values of 9 which exceed the eutectic pH value of 7.5. Ion activity products of buffer salts (the driving force for crystallization) can be used to predict pH changes. The rate of pH change can be controlled by the cooling method. Ionic and nonionic additives may affect the rate and extent of pH change by changing the properties of the solution and by participating in the inhibition or enhancement of buffer salt nucleation and growth. It has been demonstrated that an oligomeric protein's activity is dependent on the pH, ionic strength, and temperature of the solution during freezing. This work impacts the field of parenteral drug delivery because it proves that events occurring during freezing can affect the outcome of the drug product's activity. Our studies have proven that these events (such as pH changes) are path dependent. It is necessary to have not only a knowledge of additive effects on the drug product but to reproduce processing variables (such as cooling rate and final temperature) to screen formulations for optimal drug stability.Ph.D.Health and Environmental SciencesPharmaceutical sciencesPharmacy sciencesPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130849/2/9811204.pd