Chromatography Forum

Hi all,
Thank you for very enlightening posts. I don't work in pharma but I am asking on pure curiosity.
Why do you need to make a mass balance, or why is it 'good to have a mass balance' after forced degradation? My point is that to measure the amounts of major degradants and amount left. If extinction coefficients are unknown for some degradants, it will be trivial to calculate each, as said before.
I wonder, where does the remainder mass expected to go? Isn't it a better option to calculate the remainder? (ie. 92% active ingredient, %6 total of three major degradants (ext.coeff. known) and remainder %2 are other degradants)
I apologize if I misinterpret something very obvious.

Actually the ELSD is better in the sense that it gives a roughly equimolar response on an overall mass basis. The problem with ELSD is that it is not sensitive enough (at least with the current state of the art).

The CLND does give a very consistent response factor but only when calculated per mass of nitrogen - not the overall mass of the molecule. So if you don't know the structure (which is often the case) it may not be the best solution.

Indeed, good point Adam...

foren, two points to the mass balance.
- the idea in forced degradation is to degrade a certain percentage of the api and then identify the big pieces. This gives some idea what to look for in longer-term stability testing.
- you want to avoid the possibility that you are "blind" to a significant amount of degradant which might, for example, be completely retained on the column.

Adam, I empathize with your fustration around the mass balance issue. I do agree with your strategy of using low wavelength.

I only had one case where I was not able to achieve mass balance, in which case, some of the degradation products did not have a chromophore. I had to use alternative technique (light scattering detector). In all other cases, I was able to achieve mass balance quite easily. In fact, it appears that 98-102% is quite loose.

I recently attended a conference where a government regulatory personnel confirms that they expect 98-102% for mass balance. A guidance document is in the works and will be available shortly by that group. I believe the reason for the 98-102% (ie. potency + TRS) is to allow for method variability, including variability relating to unknown RRF.

Really... well that's somewhat depressing but not at all surprising.

It seems the FDA sees its purpose as making life miserable for the pharmaceutical companies; with very little thought given to what actually protects the health of the public (which is supposed to be their ultimate purpose).

First up, I agree whole heartedly that this is one of the best discussions with had on this board for quite some time (mainly due to the fact that some of my current work is trying to deal with this particular problem).

The crux of the problem, as I see it, is that if you have a drop in potency (re. assay value) of 10 - 20% of you API and only determine 2-5% %area/area impurities the question is inevitably going to be asked by regulators / patient lobbies: what's happening to the "missing" 5-15 % of your drug molecule?

Further I agree that some definitive and useful guidance, on how to deal with this issue should be addressed in depth by the regulators.

PS. For anyone not aware, there is a rather useful paper on how to determine relative UV response factors by use of the CLND (in helping to solve mass balance problems): M. A. Nussbaum et. al. J. Pharm. Biomed. Anal. 27 (2002) 983-993.