As plasma levels of epirubicinol are lower than those of the unchanged drug and the metabolite has an in vitro cytotoxic activity 10% of that of epirubicin, it is unlikely to reach in vivo concentration sufficient for cytotoxicity. No significant toxicity has been reported for the other metabolites. Epirubicinol is the 13(S)-dihydro derivative formed via the reduction of the C-13 keto-group. Both the unchanged drug and epirubicinol can be conjugated with glucuronic acid, creating the 2 glucuronides. This glucuronidation pathway is unique to epirubicin metabolism in humans  as epirubicin is the only anthracycline that serves as a substrate for beta-glucuronidation. This unique pathway might explain the better tolerability of this drug compared with doxorubicin.The 4 aglycones are formed by losing the amino sugar moiety through a hydrolytic process or redox process, creating the doxorubicin and doxorubicinol aglycones and 7-deoxy-doxorubicin and 7-deoxy-doxorubicinol aglycones, respectively. Excretion
Epirubicin and its metabolites are primarily eliminated through biliary excretion. About 11 to 15% of the administered dose is eliminated as an unchanged drug, which makes up 6 to 7% of the excreted compounds, and metabolites. Side effects
The most common side effects of the Epirubicin are alopecia, nausea/vomiting, cardiotoxicity, leukopenia, and stomatitis. Cardiotoxicity is a severe side effect and the exact pathway is still unknown. However, there is good evidence to suggest that cardiotoxicity is caused at least in part by the avid interaction of anthracyclines with iron, resulting in the formation of metal ion complexes.
Daunorubicin
One pathway starts from Daunorubicin, a common byproduct found in fermentation, since it is relatively easily available and already structurally similar to the product (only requiring minor alterations). Firstly, the amine group is protected using trifluoroacetic acid to stop it from further reactions. Next the hydroxyl group needs to be changed from an equatorial position to an axial, this is achieved by firstly oxidizing an intermediate sulfoxy salt to a keto group (losing the optical center) followed by a stereo-specific reduction using sodium borohydride to give the hydroxide group in the axial position. Secondly, the focus shifts to carbon number 13 where it is necessary to add a hydroxide group which is achieved by bromination followed by a reaction with an alkali salt of formic acid and water to give the final product. There is an older variant of this pathway which involves first splitting the Daunorubicin, into daunomycin one and daunosamine methyl ether, using methanol. Analogous reactions are performed to get the two hydroxyl groups onto their positions and the rings are then recombined and the protecting groups released. The drawbacks are more chemicals are used and daunomycin one and daunosamine need to be separated first. 13-daunorubicinol
The second pathway startsfrom 13-daunorubicinol (hydroxyl group on carbon 13 instead of Daunorubicins keto group). Firstly, the amine group is protected using trifluoroacetic acid then both the hydroxyl groups at positions 4 and 13 are oxidized simultaneously to keto groups again using DMSO, but a different alkylating agent.