Investigation of tryptophan-related yellowing in parenteral amino acid solution: Development of a stability-indicating method and assessment of degradation products in pharmaceutical formulations.

Parenteral amino acid solutions containing tryptophan tend to develop a yellow colouration upon storage. Hence, the aim of the present study was to find out whether tryptophan degradation products are the reason for the yellowing. The degree of discolouration and tryptophan degradation was examined by visual examination and UV/Vis measurements with respect to oxygen presence, pH value, and duration of steam sterilization. LC-UV analyses of autoclaved tryptophan solutions indicated eight degradation products, namely R,R/R,S 2-amino-3-(oxoindolin-3-yl)propanoic acid, R,R/R,S 2-amino-3-hydroxy-2-oxoindolin-3-yl)propanoic acids, cis/trans 3a-hydroxy-1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole-2-carboxylic acid, N´-formylkynurenine, and kynurenine. The proposed degradation products were confirmed by spiking of synthesized degradation products and LC-UV/MS analyses. The LC-UV analysis method was optimized and validated according to the ICH guideline Q2 (R1). Tryptophan stability in commercially available parenteral amino acid formulations was evaluated over a storing period of 12 months in two common types of primary packaging after autoclave procedure.