Question
Posted on: November 21 2016Affusine, a cream with fusidic acid, breaks after adding metronidazole
R/Hydrocortisone, metronidazole, Affusine cream (fusidic acid, cetyl alcohol, glycerol, paraffin, polysorbate, HCl, water)
Is there a pH problem between metronidazole and fusidic acid? Is it easy to solve by adding extra w/o emulsifier like emulsify?
Answer
This is not an easy question. After all, we have to do with two molecules with a complex chemical structure.
Metronidazole has a pKa = 2.38 and fusidic acid 5,35.
Maybe there is an interaction between both molecules but breaking the cream would suggest to me that the formed compound has W/O Emulgatorische properties. The Affusine cream is an O/W emulsion cream so I opt earlier for adding extra polysorbate.
I hope I have just been reasoned??