Quality by design in formulation and process development for a freeze-dried, small molecule parenteral product: a case study

Abstract

A case study has been developed to illustrate one way of incorporating a Quality by Design approach into formulation and process development for a small molecule, freeze-dried parenteral product. Sodium ethacrynate was chosen as the model compound. Principal degradation products of sodium ethacrynate result from hydrolysis of the unsaturated ketone in aqueous solution, and dimer formation from a Diels–Alder condensation in the freeze-dried solid state. When the drug crystallizes in a frozen solution, the eutectic melting temperature is above −5°C. Crystallization in the frozen system is affected by pH in the range of pH 6–8 and buffer concentration in the range of 5–50 mM, where higher pH and lower buffer concentration favor crystallization. Physical state of the drug is critical to solid state stability, given the relative instability of amorphous drug. Stability was shown to vary considerably over the ranges of pH and buffer concentration examined, and vial-to-vial variability in degree of crystallinity is a potential concern. The formulation design space was constructed in terms of pH and drug concentration, and assuming a constant 5 mM concentration of buffer. The process design space is constructed to take into account limitations on the process imposed by the product and by equipment capability.

Keywords::

• Sodium ethacrynate
• lyophilization
• design space
• risk assessment
• formulation development

Notes

¹In general, a bulking agent would be included in this list, but we chose, based on preliminary data, not to include a bulking agent in this formulation.

²Whether a given factor should be a product characteristic or a quality attribute is not always obvious. We chose to list residual moisture as a product characteristic because of the potential role of residual moisture in influencing product stability.