Crystallization-relatedpH changes during freezing of sodium phosphate buffer solutions.

Abstract

As a result of the recent advances in biotechnology, proteins and smaller peptides now comprise a significant portion of the drugs currently under development. Their instability in solution often demands a freeze-dried product. Since protein stability is pH dependent, attention must be given to the potential instability arising from pH shifts during the freezing of buffered formulations. Phosphate buffers are commonly used in freeze-drying. The pH changes during freezing of sodium phosphate buffer solutions have been measured under conditions relevant to pharmaceutical applications, i.e., sample volumes larger than a few microliters experiencing large degrees of supercooling and supersaturation. The effect of initial buffer concentration, initial pH, and the presence of ionic and organic additives on the pH changes, induced by the crystallization of $\rm Na\sb2HPO\sb4\cdot12H\sb2O$ (DSPD), have been investigated. The results are compared to the changes in pH predicted from equilibrium considerations. In general, the higher the initial ion product of DSPD, the larger the extent of salt precipitation and the closer the final pH will be to the equilibrium value of 3.6 at $-$10$\sp\circ$C. The addition of small amounts of NaCl increases the ion product of DSPD, thereby increasing the supersaturation and precipitation of the salt, leading to larger pH changes. The pH at $-$10$\sp\circ$C decreases to a lesser extent when larger quantities of NaCl are used, in spite of an increase in ion product of DSPD. At low ion product of DSPD ($<$10$\sp{-5}$) less NaCl is needed to reduce the extent of the pH change. The presence of KCl decreases the crystallization temperature of DSPD to values as low as $-$18$\sp\circ$C compared to $-$0.5 to $-$4$\sp\circ$C in the absence of KCl. The subsequent pH changes are moderated by precipitation of potassium salts ($\rm KH\sb2PO\sb4$ and KCl). Adding sucrose or mannitol at more than 3 moles of additive per mole of DSPD reduces the pH changes to 0.5 units. Results are explained in terms of the supersaturation of DSPD and the factors that affect salt crystallization.