Question
Posted on: August 29 2017Discoloration gel with lidocaïneHCl, tetracaïneHCl, Adrenalinetartraat in HPMC gel
For emergencies we prepare an analgeary gel for children with following composition: lidocaïneHCl 4g, tetracaïneHCl 500mg, adrenaline tartrate 181.9 mg in HPMC-gel ad 100g. When prepared, this gel is clear, but when stored (shelf life 1 month) This gives a light brown discoloration.
After some search work I have found that Adrenalinetartraat is only stable in acidic environment. When adding lidocaïneHCl the pH rises and the adrenaline can no longer be stable. I once measured and the pH of our gel is 7. Would a replacement of lidocaïneHCl by lidocaine base remedy this problem? Or does tetracaïneHCl give the same stability problems in adrenaline? In this case, I could suggest to the emergency doctor to only work with bases, eg. Lidocaine and Benzocaine, in combination with Adrenalinetartraat. Therefore, these bases do not give any incompatibility with Carbopol, which allows us to use Carbomeergel as a base instead of HMPC gel. Please advise about this.
Answer
I was surprised by the value of the pH of your gel. But after consulting some references it seems to me not abnormal. The use of lidocaine base is not the right solution. You should keep it at the hydrochloride salts and Consequently the HPMC gel.
In any case, the pH of gel should become more acidic. I would use nametabisulfite for this because it also acts as an Antixoxidans. According to the amount of epinephrine the quantity would be 100 mg. But I don't know if this will be enough in the presence of your locale anesthetics. You should reach a pH between 4 to 5.
The gel is kept in the dark and in the refrigerator