Excipient hydrolysis and ester formation increase pH in a parenteral solution over aging

Area of Science:

• Pharmaceutical Science
• Drug Development
• Physical Chemistry

Background:

• Increasing prevalence of poorly water-soluble drugs necessitates solubility enhancement strategies.
• Conivaptan hydrochloride, a hydrophobic and alkaline drug, requires careful formulation for injectable solutions.
• Physicochemical stability of drug formulations is critical for ensuring product quality and efficacy.

Purpose of the Study:

• To investigate the physicochemical stability of an injectable conivaptan hydrochloride solution.
• To identify and evaluate the causes of pH increase in the formulation.
• To establish risk mitigation strategies for maintaining drug solubility and preventing precipitation.

Main Methods:

• Formulation of conivaptan hydrochloride in a co-solvent system containing alcohols and lactic acid.
• Monitoring of solution pH over time under specified storage conditions (25°C).
• Risk evaluation involving analysis of lactic acid oligomer hydrolysis and ester formation, including pKa determination using commercial software.

Main Results:

• A gradual increase in solution pH was observed over time.
• The pH increase was attributed to the hydrolysis of lactic acid oligomers and ester formation between lactic acid and alcohols.
• Both hydrolysis and ester formation reached equilibrium by 16 months, allowing for molecular determination of the upper pH limit.

Conclusions:

• Understanding and evaluating chemical reactions between excipients, such as lactic acid oligomer hydrolysis and ester formation, is crucial in pharmaceutical development.
• Proactive risk assessment of formulation components can prevent potential issues like reduced solubility and precipitate formation.
• Molecular determination of pH limits ensures the quality and stability of injectable drug products containing poorly soluble compounds.