In vitro and in vivo binding of phenylbutazone and related drugs to equine feeds and digesta.

In vitro and in vivo studies of phenylbutazone binding to equine ingesta and digesta were undertaken. In vitro binding to chopped hay and powdered pony nuts in buffer solutions at 37 degrees C was found to be time-, concentration- and pH-dependent. Percentage binding generally increased with time, decreased with concentration and varied with buffer pH in an unpredictable manner. Other non-steroidal anti-inflammatory drugs (NSAIDs) also bound to hay, the degree of binding being less for meclofenamate and least for flunixin in comparison with phenylbutazone. Phenylbutazone became bound to digesta collected from eight regions of the gastrointestinal tract when they were spiked with a concentration of 1 mg.10 g-1 digesta, the amounts ranging from 80.0 per cent (duodenum) to 99.6 per cent (stomach). Binding also occurred to equine digesta following the oral administration of phenylbutazone (4.4 mg.kg-1) to three ponies. It was concluded that drug uptake by and release from equine ingesta and digesta were probably adsorptive and desorptive processes. The clinical significance of the findings for the use of NSAIDs in equine medicine was considered.