Why is cysteine added as cysteine HCl in PN for preterm infants?

• A. It is more stable in HCl form
• B. It improves solubility
• C. It is less expensive
• D. It reduces oxidation

Answer: (B)

In parenteral nutrition for preterm infants, delivering amino acids in a stable, fully soluble solution is crucial to prevent precipitation with minerals like calcium and phosphate. Cysteine alone has limited aqueous solubility, especially in the slightly acidic to neutral PN solutions used clinically. Forming cysteine as the hydrochloride salt increases its ionization and overall solubility, making it easier to dissolve at the concentrations needed and helping maintain a clear, stable mix. This improves the reliability of delivering the required cysteine dose without risking precipitation. While the salt form can also modestly affect pH, the main reason for using cysteine HCl is to enhance solubility.