Influence of the composition of in-vitro azo-reducing systems on the degradation kinetics of the model compound amaranth

The purpose of this study was to investigate the influence of the composition of in‐vitro azoreducing systems on the degradation kinetics of the model compound amaranth. The degradation kinetics of amaranth were determined under anaerobic conditions both in rat caecal content (ex‐vivo) and in a variety of in‐vitro degradation media derived from rat caecal content. It was observed that the reducing activity was highly dependent on the preparation method and composition of the degradation medium. In pure rat caecal content, the degradation of amaranth was apparent first order (k = 0.044+0.002 min−1), while dilution of the rat caecal content resulted in an apparentzero‐orderdegradation. Both apparentzero‐ and first‐order degradations were also observed in media made up of diluted rat caecal content to which cofactors such as NADP, D‐glucose‐6‐phosphate, glucose‐6‐phosphate dehydrogenase and Bz were added. This study demonstrates that in‐vitro azo‐reducing kinetics are dependent on the composition and mode of preparation of the in‐vitro media used. This has to be taken into account when evaluating the degradability of azo‐aromatic drug delivery systems in‐vitro.