The European Agency for the Evaluation of Medicinal Products Veterinary Medicines Evaluation Unit

a) Thiamphenicol chemical structure differs from that of chloramphenicol in having a sulpho-group instead of a nitro-group; consequently the two compounds have different metabolic pathways. In fact thiamphenicol, unlike chloramphenicol, is not an optimal substrate for the hepatic microsomal enzyme glucuronyl transferase and therefore more than 95% of the administered dose is excreted unchanged in laboratory animals; when tested in vitro on pig hepatocytes, approximately 30% of thiamphenicol undergoes glucuronidation. Only 15-20% of chloramphenicol is excreted as parent compound. Moreover, some arylamine is found in the urine of chloramphenicol-treated animals, deriving from the reduction of the nitro-group. Of course, no arylamine is present after treatment with thiamphenicol.