Cellulose/Mg stearate solubility

[jdlh199]

I have two solid samples I am using for method development:

1. The active ingredient (analyte) we are trying to quantitate
2. A mixture sample containing the active ingredient (5%), silicified microcrystalline cellulose (92.5%), and magnesium stearate (2.5%).

We developed a simple HPLC method for the active ingredient using a C18 column, UV detection at 280nm, and an isocratic mobile phase consisting of Acetonitrile/20mM PO4 pH7 35:65. Calibration and peak shape is perfect!

The active ingredient is completely soluble in the mobile phase, however the mixture sample is not.

Can I simply dissolve the mixture sample in the mobile phase, centrifuge, filter, and then run it? Is this good enough? Or should I expect some of the soluble active ingredient to be left behind in the precipitate?

Any suggestions on how to get the cellulose/Mg stearate into solution?

[Noser222]

You don't need the entire formulation to be in solution, but you have to test the recovery of the active as part of method validation. For example, prepare formulations with 70%, 100%, and 130% of your specified concentration and then measure your recovery against a reference standard.

[jdlh199]

OK, but let's presume there is no method validation and it's non-GLP work. And lets presume I have no blank matrix, I have only the reference standard, which is 100% active ingredient, and I have an unknown sample which should contain 5% active ingredient.

If I were to run it as normal and just filter it before analysis, and got a result of 4%, can I trust that data, or is the other 1% mixed with the precipitate?

Or is there any way to get the other components into solution?

[Bruce Hamilton]

I would suggest just trying to extract all of the active ingredient, and leaving behind the excipients. The less junk you inject onto the column, the better - even if the detector doesn't see it.

I'd try for a simple single quantitative dissolution into an larger volume of an organic solvent like acetonitrile ( or methanol ), using ultrasonics or another form of agitation, and perhaps some warmth, to assist dissolution. I'd then centrifuge ( filter would be less preferred ), and mix a portion of the supernatant extract with the mobile phase buffer.

You can then decant off the supernatant, add more solvent, and re-extract. There may be about 10-25% residue from the first extraction - depending on solvent/sample ratio, but you can do a few repeat extractions to demonstrate recovery. Three sequential extractions often get about 99+% recovery of analytes on good methods.

Alternatively, you can also prepare test mixtures with the excipients to ensure none of your active remains behind. Unfortunately, spiking your formulated sample won't confirm that.

Good luck,

Bruce Hamilton

[Dan]

OK, so method validation is not required. However, a little method development is needed.

Bruce gave you good ideas along those lines. Let me add a few suggestions.

As Bruce said, don't try to dissolve your cellulose and Mg stearate. It is better to leave them behind as undissolved so they don't give you column/chromatographic problems.

One problem you may have from the cellelose and Mg stearate is that they may trap your active and keep it from going into solution, even if your active is very soluble in the mobile phase. So, you need the agitation in your sample prep. as Bruce suggested.

It may work OK to use the mobile phase and some sonication/shaking. You may need to try for different agitation times. You want a long enough time to get all product components to disintegrate and then get your active into solution. Don't sonicate for very long if your active is heat sensitive.

I like to try the simple approach first. Drop your product into an erlenmeyer flask and then add in a given amount of mobile phase, agitate, centrifuge or filter and inject. Having additional steps can add chances for error while not really providing additional benefit. The method development part is in determining the right size of flask, volume of added solvent, agitation type and agitation time. If you centrifuge, there are parameters there to consider. If you filter, you just need to choose the right type of filter.

Regards,
Dan