Disposition of Stanozolol in Plasma After Intra-articular Administration in the Horse

The purpose of this study was to provide data on the disposition of stanozolol after bilateral intra-articular injection in the tarsal joints, to discover the length of time for which the drug can be detected in plasma after administration. Fourteen horses were included in the study. After aseptically preparing the injection site, 1 mL of an aqueous suspension containing 5 mg of stanozolol was injected into both the right and left tarsal joints of 12 horses; the two remaining animals were not treated and were used as a control group. Five milliliters of blood was collected immediately before stanozolol administration (t0) and at 1, 2, 4, 6, 12, 18, 24, 36, 48, 60, 72, 84, 96, 108, 120, 132, 144, 156, and 168 hours after injection. The plasma concentration was determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS) after solid phase extraction. The maximum plasma concentration was 1.7 ng/mL (range, 0.5-3.0 ng/mL), measured at 6 hours (range, 4-12 hours). The plasma elimination half life varied between 4 and 12 hours, whereas the plasma clearance per fraction of dose absorbed was in the 257.85-820.88 L/h range. The results of the present study make a preliminary contribution toward understanding the elimination profile of intra-articularly administered stanozolol in the horse. The drug passes rapidly into the systemic circulation, is eliminated rapidly, and is detected in plasma for no more than 36 hours after local administration.