Metabolic fate of anabolic agents in treated animals and residue levels in their meat.

Anabolic agents applied in animal production can be classified as sex hormones, as far as one aspect of their biological activity is concerned. In respect to their chemical nature the agents can be divided into three sub-groups: (a) steroids natural to the body (endogenous steroids); (b) steroids foreign to the body; (c) other compounds foreign to the body. After exogenous application (oral or parenteral) metabolism of compounds in group (a), e.g. 17beta-estradiol or testosterone, follows the pathways for the identical endogenous hormones; i.e. enzymatic transformation of the biologically active molecule into less active compounds. Excretion seems to occur predominantly via the feces, followed by the excretion via the urine. Steroids foreign to the body (group [b]) may be simple esters of endogenous steroids (estradiol benzoate, testosterone propionate) or compounds with basically modified steroidal structures, like trenbolone acetate (estra-4, 9, 11-trien-17beta-OH-3one-Ac). After enzymatic cleavage of an ester in the animal, metabolism of natural steroids again follows the endogenous pathways. Other steroids are mainly excreted without changes of the original steroid structure. It has been demonstrated, that compounds applied as esters have a delayed elimination. Anabolic agents of nonsteroidal structure, (group [c]) like stilbene derivatives and zeranol (6-[6, 10-dihydroxy-undecyl] beta-resorcylicacid-mu-lactone) are not, or only to a minor percentage, catabolized in the body. It can be generalized so far, that the fraction of the hormone eliminated in the urine, is in the conjugated from, while the fraction appearing in the feces is in the free form. This is due to enterohepatic circulation, which occurs with both, the intact or catabolized compound. The elimination rate of anabolic agents not only depends on the absorption after oral or parenteral application, which is strongly related to the type of formula---or cleavage of possible esters, but also on the metabolic clearance rate, which is a function of several factors, for example binding to plasma proteins or tissue structures. Differences in the elimination rate between compounds could be attributed to these phenomena. Tissue residue formation not only depends on the elimination rate, but also on other factors like mode of application, formula and withdrawal time. The possibility for their detection is related to the method applied. In cases of adequate treatments of the animals, and with the exception of the implantation site (see below), positive hormone values in tissues (qualitative and quantitative) could only be obtained, when radioimmunoassay techniques with a sensitivity in the low nanogram range were applied...