In-vitro stability and metabolism of a tacrine-silibinin codrug

A tacrine-silibinin codrug showed promising results in pharmacological and toxicity testing, superior to an equimolar mixture of tacrine and silibinin. The aim of this study was to get more information about its stability, possible degradation products, metabolites, and especially its active principle in vitro and in vivo.

The stability of the codrug was analysed under in-vitro assay conditions. Additionally, its metabolism was investigated using pooled human liver microsomes. Metabolites were identified via liquid chromatography-high resolution electrospray ionization mass spectrometry. Furthermore, the influence of one of the main cleavage products, tacrine hemi succinamide, on viability and mitochondria of hepatic stellate cells was analysed.

The codrug remained stable in culture medium (Dulbecco's modified Eagle's medium) over an incubation period of 24 h, whereas exposition to microsomal enzymes led to rapid cleavage of the ester bond to form silibinin and a tacrine hemi succinamide. In addition, glucuronidated metabolites of both silibinin and the codrug were detected. For the tacrine hemi succinamide, no effects were observed with regard to cell viability and mitochondrial impairment.

This study helps understand and interpret previous results concerning the effects and the absence of toxicity of the tacrine-silibinin codrug and supplies important information for further identification of the active principles of the codrug in vivo.