EP Impurity B is not USP Related Compound B: the letters do not always match
· Compendial naming · CATO technical documentation
A customer asked us to confirm that a standard we list as Clarithromycin EP Impurity A was the same material as the USP Related Compound A they had been quoting. The names look interchangeable. They are not, and the difference is the kind that invalidates a system-suitability run.
What the letters actually mean
An EP impurity letter and a USP related-compound letter are each assigned **inside their own monograph**. Neither body follows the other's numbering, and neither is obliged to keep the same set of impurities. So Impurity B in one pharmacopoeia and Related Compound B in the other are two independent labels that may — or may not — point at the same molecule.
A worked example, with CAS numbers
| Material | Pharmacopoeia naming | CAS |
|---|---|---|
| Clarithromycin impurity E (BP/EP) | clarithromycin impurity E | 81103-14-2 |
| The same molecule, USP naming | Clarithromycin Related Compound A | 81103-14-2 |
| A different molecule often assumed to be "the A" | Clarithromycin Related Compound A (USP catalogue) | 81103-14-2 |
Our own catalogue item C4X-100213 | Clarithromycin EP Impurity A | 124412-58-4 |
The third and fourth rows are the trap: the EP A and the USP Related Compound A are different substances. Only the CAS number settles it.
Other pairs we have checked and found mis-matched include Erythromycin (EP Impurity N, CAS 3225-82-9, against USP Related Compound N, CAS 992-62-1) and Azithromycin (EP Impurity D, CAS 612069-26-8, against USP Related Compound F, CAS 612069-28-0 — a single digit apart).
Three checks before you file a standard under a compendial name
- Match on CAS, not on the letter. If the certificate does not print a CAS number, ask for the
identity evidence behind the name.
- Do not accept a vendor's
(USP)tag as proof. We have found supplier pages that echo the EP
letter back as a USP name, printing the EP CAS next to it — internally consistent, and wrong. Accept a USP name only from the USP catalogue record, the USP monograph text, or a supplier entry that prints the USP CAS.
- Keep the two names in your method file. Quote the standard by catalogue number plus CAS, and
record both compendial names with the source you took them from. When the monograph is revised, you can see immediately whether your standard still matches.
What CATO publishes
Where a compound is listed in both pharmacopoeias with a different letter, we record the correspondence only when we can show the same CAS number or a character-identical molecular formula. Where the evidence is not there, we say nothing — an unverified name is worse than a missing one, because it is the kind of error that survives all the way to a release decision.
*CATO supplies impurity, metabolite and isotope-labelled reference standards with a certificate of analysis issued per lot. To check a specific pair, send the CAS number or the catalogue number — we will confirm the correspondence, or tell you plainly that we cannot.*
Written by CATO. Where a claim depends on an external source we say which one; where it depends on our own catalogue we name the catalogue number so you can check it. Spotted an error? Email chao.lu@cato-chem.com — corrections are published in place.