Metabolism of tilmicosin in rabbit primary hepatocytes and liver microsomes

Tilmicosin (TLM), a macrolide antibiotic currently used for treatment of respiratory diseases in large animals (Micotil®, Elanco), has recently obtained the MRL extension for rabbits. To compare its metabolic pattern with that reported in cattle, swine, chicken and sheep, its biotransformation was studied in vitro using liver microsomes and hepatocyte primary cultures obtained from rabbit liver. HPLC analysis of hepatocyte extracted medium collected after 0, 2, 4, 8 and 24 hrs of incubation showed several peaks eluting before and after the parent compound; however, their low concentrations did not allow further structural identification. No metabolites were detected in microsome incubation. TLM was then incubated with liver microsomes from rifampicin induced rabbits, according to metabolic profiles obtained and the evidence that macrolides are mainly metabolised by P4503A subfamily as well. The abundance of the metabolites produced allowed LC/MSn analysis; the masses obtained at m/z 869, m/z 855, m/z 871 and m/z 885 matched the structures of the parent compound and of demethylated, reduced and oxidised TLM, respectively. No striking differences appear to exist in the biotransformation pathways of TLM between rabbits and the other food producing species.