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  • Do pharmacopoeial substances need to fulfill content specifications of their own monographs?
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Do pharmacopoeial substances need to fulfill content specifications of their own monographs?

Sometimes we are approached by customers, who are finding it strange that some pharmacopoeial reference standards for drug substances do not seem to fulfill the specifications for the monographs they are established for. Fact is, that the pharmacopoeias reserve themselves the right to set up – in exceptional cases – reference standards that do not fulfill ‘their own specifications’. And indeed, they make use of this right in very rare cases. However, what users stumble over somewhat more often is that reference standards are given assay values that lie – on a first glance – below the specifications of the monographs they are used for. This can be explained easily enough though. Example 1: water in molecular structure Take for example a compendial reference standard for Amoxicillin Trihydrate, where the assay value is given ‘as is’ with 86.2 %, and where the monograph is stating in the definition that the content needs to be in the range between 95.0 to 102.0 per cent for the anhydrous substance. The relevant words here are ‘as is’ and ‘for the anhydrous substance’. In Amoxicillin Trihydrate, the minimum theoretical water content is already 12.9 %. So you will find in the HPLC assay test a maximum ‘as is’ value of 87.1 % for the substance to be examined. Let’s assume you will find 86.2 % ‘assay as is’, and a water content of the theoretical 12.9 %. That would mean that of the 100 % of material you have in front of you, 87.1 % are water-free or anhydrous substance. Compared to the 87.1 %, the 86.2 % correspond to an assay value on the anhydrous basis of 99.0 %, so it is well in the range of 95.0 % – 102.0 %. The same would apply for the reference standard with the ‘as is’ assay value of 86.2 %. There will be a water content in the range of 13 % subtracted already, but based ‘on the anhydrous substance’ the content of amoxicillin will very probably lie within the monograph specification. Example 2: no water in molecular structure The same can also apply for cases where no water is present in the molecular structure, but where water is also specified. Take for example a reference standard for Folic Acid (monograph specification 96.0-102.0 % for the anhydrous substance) with an ‘as is’ assay value of 91.3 %, and a compendial water specification of 5.0 to 8.5 %. With an assumed water content of 5.0 % resp. 8.5 %, a 91.3 % assay value on ‘as is’ basis would correspond to 96.1 % resp. 99.8 % content for the anhydrous substance. So here as well the reference standard would fulfill the content specification of the monograph it was set up for. If you have any questions please feel free to contact us at askus@lgcpharma.com. picture: iStock Photo

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