Stability and partitioning of closantel and rafoxanide in ruminal fluid ofsheep

The stability and the partitioning of closantel and rafoxanide in ruminal f luid (RF) was examined in vitro. Stability was evaluated in two studies in a ruminal fluid-artificial saliva (RF-AS) mixture containing either drug. D rug concentrations were measured in samples collected sequentially from fou r batches of RF-AS fortified with either closantel or rafoxanide in one stu dy and in four separately incubated aliquots of a RF-AS mixture of each dru g in the second study at the start and at various intervals during a 24 h i ncubation period. The viability of the in vitro RF-AS incubation model was validated by the presence of digoxin degradation (T1/2 of 39,1 +/- 13 h) an d by the absence of significant time related differences (P > 0,5) in volum e of gas produced, pH and methylene blue reduction time of the RF-AS drug m ixture. Partitioning of closantel and rafoxanide was determined by measurin g the relative drug concentration of the fluid and particulate phases in RF fortified with either drug at different concentrations. Closantel and rafo xanide were shown to be stable in a PF-AS mixture and were not subjected to any significant biodegradation. An initial marked reduction in drug concen tration measured in the RF-AS mixture during the first 2 h of incubation wa s attributed to the attachment of both drugs onto particulate matter. This was subsequently confirmed in the partitioning study. More than 80 % of clo santel and rafoxanide was shown to be associated with the particulate phase of RF.