Yes, paracetamol may hydrolyse into acetate and 4-aminophenol, either during storage or possibly during the dissolution test. You might control this with an appropriate multi-frequency absorbance measurement method (i.e. a chemometric approach), if you want to avoid an HPLC analysis. You may need several frequencies if your excipients interfere, for highest accuracy.

There may in fact be more paracetamol in the suppository than labeled.  manufacturers have +/- 10% cushion in drug amount accuracy.

Thank you Stanton. Would you classify the products of the hydrolysis as impurities, so the presence of impurities is affecting the results?

Also if the absorbance of PEG was excluded from the respective batches by measuring the absorbance of pure PEG suppositories that do not contain Paracetamol, does this mean that the absorbance of the impurities can also be excluded by measuring the absorbance of suppositories that only contain the acetate and 4-aminophenol?

And would fat blooming of the suppositories may a possible sign for instability and degradation?

Thank you Christopher. In my results, there is already an inconsistent content uniformity of Paracetamol in the suppositories. All of my suppositories in PEG 6000 exceed the 95.0% to 105.0% cited in the BP. Does the +/- 10% extend to the BP limit?

I am not sure. In general the ranges are 95% to 120% here in the US. Some of your results are far above that so it is more likely an assay interference now that I think about it more. 

Rachel: Usually you'd term products formed by hydrolysis during storage as "degradation products". "Impurities" frequently designates related compounds left over from synthesis of the API. The language used may vary, worldwide. If the hydrolysis can be demonstrated to have occurred during the dissolution test itself but not over the shelf life of the product, and you validate this, then your method could be set to analyse API and the hydrolysis product and consider both as API. This would be a rare circumstance, though, and not analytically desirable.

You can't just subtract an absorbance value measured in suppositories with a set amount of acetate and 4-aminophenol, because you don't know how much spectrum (so to speak) you should be subtracting -- was there that level of those products, or more, or less? Only a multi-frequency method can (perhaps) disentangle the spectra and quantitate each compound. The same comment goes for subtracting PEG-only suppository spectra -- you'd want to make sure that those controls contained only the amount of PEG as in the active samples (e.g., don't make them the same mass as the actives if the actives are 20% paracetamol).

"Fat blooming" is a physical state change (excipient migration and recrystallization). It's not necessarily a cause of API instability, though it may expose the API to moisture and consequent attack. Certainly it's cosmetically undesirable, and may indicate that the product was exposed to too high a temperature during storage.

Thank you very much, Stanton. The suppositories were hand made and have been stored for a week prior to the dissolution study. I believe that the hydrolysis of the paracetamol was an unintended event that had occurred during storage.

I understand what you mean about the subtraction of values. Unfortunately, both the medicated suppository and non-medicated suppository weighed the same in this experiment.

I think the results are due to hydrolysis of paracetamol, and the best way to check this is performing an HPLC analysis. And Stanton is talking about sustraction of absorbance values, taking a dissolution of blank suppositories (suppositories without paracetamol) as a blank versus the solution of suppositories containing paracetamol, as the PEG could interfere with the determination.

Hello, Luis. Many thanks for clarifying that. I may have misunderstood Stanton's earlier statement.

I have subtracted the absorbance value of PEG from the medicated suppository using a 'blank' or a non-medicated suppository as control. I mean to say that there is not a big difference in weight between the two suppositories (blank and medicated), so the control would have contained more PEG than the medicated suppository. So, this would affect the absorbance reading.

Is it because the degree of hydrolysis is unknown and we are not able to verify the amount of degradation product that exists, it is not practical to subtract the absorbance reading of the degradation products using the same method earlier?

Is the HPLC analysis conducted to analyze the degradation of paracetamol in suppositories?

Thank you, Anwar. I had not considered the pH to be a factor for this discrepancy. In this experiment, the pH of the buffer and dissolution medium used was pH7.2 and the temperature was kept at 37'C during dissolution. I understand that paracetamol is a weak acid (pKa= approximately 3.7, literature value), so in pH 7.2, it would be almost completely ionized. I would be most grateful if you could take the time to further explain how pH affects the UV spectra of paracetamol or refer me to some articles if it's not possible.

Secondly, can I please know what sort of discolouration would be produced due to the oxidation of phenol in paracetamol and how would this affect the absorbance reading?

PEGs also produce absorbance readings at the same wavelength. To resolve this, the absorbance value of 'blanks' is subtracted from the medicated suppository instead, but the method is not perfect as there are some 'blanks' that would contain more PEGs than the medicated suppositories.

Hello Anwar. Many thanks for your explanation. I would love to be able to apply what your explanation in a lab setting, but regrettably I lack the equipment to carry out the experiments right now.

I think it is interesting indeed that you mentioned the peroxide impurities because in my experiment, the suppositories did not contain any antioxidant. They only contain the PEGs as suppository base and the API, which is paracetamol. So I think that the discrepancy in results could also be due to the interference of absorbance readings by impurities.

Thank you very much!

Thank you very much for your help and offer, Anwar.

First, you've to calculate the quantity of Paracetamol per formula according to the certificate of analysis based on the percent assay and the percentage of  loss of drying to get the actual amount of Paracetamol used. Second, you might be added an excess amount of the active by mistake. Third, Paracetamol undergoes polymorphism , from Polymorph A to polymorph B and this may elucidate the high cumulative percent of Paracetamol. .

Dear Rechea

I am totally agree with the statements of Dr. Anwar A Hussain and the procedure whcih he suggested is absolutely right.

Theis is only the way to solve the problem. You will surely get a positive answer.

Dear Ahmed,

Polymorphs are solid state-of-matter crystal forms? But, for an assay you're dealing with a solution. So the specific absorbance (in solution) will be identical, no matter which polymorph dissolved. You may be thinking of mutorotation, but the assay is by absorbance, not optical rotation.

Rachel, paracetamol is stable in the body until metabolized, which processes usually don't involve attack at the phenol group. So it's likely to be comparably stable once in solution in a dissolution test. As to the possible attack by PEG impurities, you might see http://www.spectra-analysis.com/documents/AppNote016Polyethyleneglycol.pdf , with the caveat that the sample had been "vigorously air oxidized" (hot?). Consider that PEG is used in many preparations, and that it may block chemical carcinogenesis, both of which wouldn't be practical if it generated large amounts of peroxides. I do note that sonication (to disperse or dissolve solids in a solution or semisolid) may degrade PEG, see https://en.wikipedia.org/wiki/Polyethylene_glycol . So don't do that!