Impurities in drug products

[santech]

Hello,
I have a question on related impurities. What is the probable limit of Isotretinoin allowed in a tretinoin drug product. USP allows a limit of 5% of Isotretinoin in tretinoin drug substance.
As per the guidelines of Impurities in new drug products,enantiomeric impurities or impurities of the drug substance is not considered as impurities. Moreover tretinoin is considered to be more toxic when compared to isotretinoin.so in this case tretinoin's level is very much controlled in an oral dosage form of Isotretinoin. But how about the level of isotretinoin in tretinoin topical dosage form? is a level more than 10% allowed? Or how to control isotretinoin in tretinoin based products?This post relates to a combination product containing hydroquinone, flucinolone acetonide and tretinoin. Please reply

[krickos]

Hi

and EP do not allow more than 0,5 of said impurity (1,0% in total impurities) for the drug substance. So already here FDA potentially can call the USP drug substance monograph outdated.

As this seems like a risk/benefit drug that I have not worked with I can not give any further advice. But I know that the brittish pharmacopia has limits for a solution and a Gel.

[InfraredRoses]

Yes, no single impurity should be over 0.5%

[santech]

Yes, no single impurity should be over 0.5%

But why has USP specified 5.0%?