2015 ACVIM Forum Research Abstract Program

Validated liquid chromatography-mass spectrophotometry (LC-MS/MS) assays were developed for the detection of PBZ and its metabolite oxyphenbutazone (OXPBZ) in equine serum and urine (Maxxam Analytics) and tissues (Canadian Food Inspection Agency). The limits of quantification (LOQ) for the assays were 1 ng/mL for PBZ and 2 ng/mL for OXPBZ residues in serum and urine and 0.5 ng/g for PBZ and OXPBZ in all equine tissues. These LOQs are well below the EU's Minimum Required Performance Limit (MRPL) for analytical methods for phenylbutazone of 5 ng/g. Utilizing horses destined for slaughter and a high dose PBZ treatment regimen (8.8 mg/kg/day for 4 days), we conducted a pilot study to determine the sampling time points for a full drug depletion study. As we predicted, pilot study results indicated that blood concentrations of PBZ and OXPBZ depleted rapidly and did not reflect tissue concentrations. Both PBZ and OXPBZ depleted rapidly from equine muscle, being readily detectable only at the first slaughter time point of 7 days post-PBZ administration. In the full depletion study, using 20 horses slaughtered in groups of 5, we confirmed that even with a very high dose treatment regimen, PBZ depleted very rapidly from equine muscle and that liver is the tissue from which PBZ residues deplete at the slowest rate. While OXPBZ concentrations exceeded PBZ concentrations in urine and were still detected 42 days post-PBZ administration, no residues of OXPBZ were detectable in muscle, kidney, or liver from 21 days post-PBZ administration. Our results indicate PBZ should be the regulatory marker compound and liver should be the target tissue for regulatory enforcement. In conclusion, even after a high dose PBZ treatment regimen, PBZ and OXPBZ deplete rapidly from equine muscle and a lifetime ban on its use in horses intended for human food is not justified. If the medication history of a horse is questionable, pre-slaughter testing of urine or postslaughter testing of liver will prevent PBZ contaminated horse meat from entering the human food chain. Previous work by the authors showed that erythromycin-associated hyperthermia is caused by impaired sweating. The effect of rifampin on erythromycin-induced anhidrosis is unknown. This study compared terbutaline-induced sweating in erythromycintreated foals with sweating responses in foals given rifampin alone or in combination with erythromycin. In separate experimental sessions, 12 pony-cross foals (nine female, three male), 1 month old at enrollment, were each treated for 5 days with erythromycin base (25 mg/kg orally, three times daily), rifampin (10 mg/kg orally, twice daily), or a combination of the 2 medications according to a duplicated 6 9 3 counterbalanced measures design. Quantitative intradermal terbutaline sweat tests were performed on each of three successive days before treatment (baseline) and on days 1, 2, 5, 9, 24, and 39 after treatment began. There was significant (P < 0.0005) suppression of sweating in foals given erythromycin with or without rifampin but no effect (P = 0.800) of rifampin alone. Compared with terbutalineinduced sweating at baseline, values were significantly reduced (P < 0.0005) in foals given erythromycin with or without rifampin at all evaluation days except d 39. The addition of rifampin treatment to the standard erythromycin regimen did not change overall sweating responses to intradermal terbutaline (P = 0.071). Results show that, despite evidence that rifampin red