## Active metabolites: to measure or not to measure [Regulatives / Guidelines]

Hello Elena777,

my recent experience in EU is that they tend to ask for the parent, and only the parent, if this can be measured (and if systemically acting blah blah).
I did not see a dossier recently where they asked additionally into the metabolite.
So, as regards parent versus metabolite they seem in my experience to follow guidelines to the word, but with one exception: You can have a great meeting with 16 questions, all being answered in no time, and you are ready to leave the agency with final handshakes and big smiles, when someone whispers: "Hey, what about back-conversion?" Then all bets are off, especially if you are not 120% sure you have firm control over it. You can be asked to measure everything and do all what that it implies in terms of acquisition of weird reference standards and setting up assays.

There is also some regulatory wobble regarding enantiomers. Sometimes they ask for a chiral assay, and sometimes they don't and their choice seems not to always follow the formal criteria criteria stated in the guideline. I haven't cracked the code yet, but Hötzi has a good story about it from the horse's mouth.

Hötzi, over to you...

This week's list of things I absolutely detest: Corona virus, the which function in R, WIA-WIA interfaces for scanning under Windows 10, the Bee Gees, the smell of my fridge.

Best regards,
ElMaestro