Chlortetracycline (CTC) is one of the few antibiotics that can be used without any withdrawal period in chickens laying eggs intended for human consumption. 6-Iso-CTC and 4-epi-6-iso-CTC have recently been identified as the principal metabolites of CTC in eggs. Although not covered by the European Union (EU) maximum residue limit (MRL) for CTC, these compounds, taken together, accumulate in the eggs of birds treated therapeutically with CTC to a mean concentration equivalent to more than twice the EU MRL (200 µg kg–1) in eggs. Plateau concentrations in eggs were achieved after approximately 3 d of medication. Following withdrawal of medication, mean egg concentrations of these compounds were maintained for 48 h, before falling below a level equivalent to the MRL after 5 d. Feeds containing typical sub-therapeutic contamination concentrations of CTC did not produce mean concentrations of 6-iso-CTC plus 4-epi-6-iso-CTC, combined, greater than 200 µg kg–1. It is not known whether these compounds are formed as a result of metabolism or of chemical degradation. However, analysis of ovules pre-lay showed that all of the CTC present in this matrix was in the form of 6-iso-CTC and 4-epi-6-iso-CTC, and not as the parent drug. Although microbiologically inactive, the toxicological properties of 6-iso-CTC and 4-epi-6-iso-CTC are not known.
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