An interim screening limit of detection for naproxen in equine plasma: a review and analysis

Starting in August 2015 Thoroughbred racing in Charles Town, West Virginia experienced a sequence of intermittent low concentration Naproxen identifications from 6.3 to 161 ng/ml of plasma (27.3 to 699 nM). These identifications were ongoing, indicating the horsemen were unaware of their origins. Naproxen is administered orally to horses at substantial doses and is chemically stable in the environment. These identifications are therefore most likely associated with exposure of these horses to environmental traces of Naproxen. Given the low concentrations of these identifications, we were asked to identify a Screening Limit of Detection (SLOD) below which these trace level Naproxen identifications would not be reported. Review of the data set suggested an SLOD of 200 ng/ml, while outlier analysis suggested an ‘extreme’ outlier level at 247 ng/ml, which figure was rounded up to 250 ng/ml Naproxen or 1.09 uM. This proposed SLOD is in good agreement with other US regulatory thresholds for therapeutic medications and this Interim Screening Limit of Detection was presented for review.

Americans with Disabilities Act. Considerations for the practice of occupational medicine

The Americans with Disabilities Act (ADA), although developed in the context of civil rights legislation, is likely to have notable impact on the practice of occupational medicine. The ADA contains provisions limiting the use of preplacement examinations to determinations of the capability to perform the essential functions of the job and of direct threat to the health and safety of the job applicant or others. The Title I employment provisions of the ADA establish definitions and requirements similar to those found in section 504 of the Rehabilitation Act of 1973, as amended; leading cases that have been litigated under the Rehabilitation Act, as amended, are described. The limitations of available scientific and medical information related to determinations of job capability and direct threat and ramifications of the ADA on the practice of occupational medicine are discussed.

Variable-interval responding in the horse: a sensitive method of quantitating effects of centrally acting drugs

An operant conditioning apparatus for studies in equine pharmacology was constructed. Horses interacted with this apparatus by breaking a light beam and were rewarded with 30 ml of oats. Horses readily learned to use this apparatus and were trained to respond on a variable-interval-60 schedule. With this schedule, there was no direct relationship between the rate of light beam breaking and the reward. Horses thus developed their own individual response rates (ie, light-beam breaking rates), and these rates remained stable at between 5 and 35 responses/min for each horse over a period of months. The effects of 2 drugs on this paradigm were tested. Reserpine (5 mg/horse, IV) depressed the response rate in all horses tested. This depression was maximal between 3 and 5 days after treatment and lasted for up to 10 days. After small doses of cocaine (0.01 mg/kg of body weight IV), the response rate of 1 horse was stimulated, whereas 1 mg of cocaine/kg was required for maximal stimulation of response rate in another horse. Larger doses of cocaine inhibited response. Variable-interval response was a sensitive method of measuring drug effects in the horse and allowed accurate quantitation of drug effects that were not detectable by clinical observation.

The pharmacokinetics, pharmacological responses and behavioral effects of acepromazine in the horse

After intravenous (i.v.) injection, acepromazine was distributed widely in the horse (Vd = 6.6 litres/kg) and bound extensively (greater than 99%) plasma proteins. Plasma levels of drug declined with an alpha half-life of 4.2 min, while the beta phase or elimination half-life was 184.8 min. At a dosage level of 0.3 mg/kg acepromazine was detectable in the plasma for 8 h post dosing. The whole blood partitioning of acepromazine was 46% in the plasma phase and 54% in the erythrocyte phase. Penile prolapse was clearly evident at doses from 0.01 mg/kg to 0.4 mg/kg i.v., and the duration and extent of protrusion were dose related. Hematocrit levels were significantly lowered by administration of 0.002 mg/kg i.v. (about 1 mg to a 500 kg horse) and increasing dosages resulted in greater than 20% lowering of the hematocrit from control levels. Pretreatment of horses with acepromazine also reduced the variable interval (VI 60) responding rate in all horses tested. These data show that hematocrit changes are the most sensitive pharmacological responses to acepromazine, followed by changes in penile extension, respiratory rate, VI responding and locomotor responses. Acepromazine is difficult to detect in plasma at normal clinical doses. However, because of its large volume of distribution, its urinary elimination is likely prolonged, and further work on its elimination in equine urine is required.

Pharmacokinetics and behavioral effects of methylphenidate in Thoroughbred horses

In horses given (rapid IV) methylphenidate (Ritalin, alpha-phenyl-2-piperidinacetic acid methyl ester; 0.70 mg/kg), plasma concentrations of the drug decreased rapidly at first, with an apparent alpha half-life of about 19 minutes, and then more slowly, with an apparent beta half-life of about 2.4 hours. These data were well fitted by a 2-compartment open model. In blood, about 40% of the methylphenidate present was in the plasma fraction, and of this, about 80% was plasma-protein bound. If given by subcutaneous or IM injection, plasma concentrations of methylphenidate peaked in about 1 hour and were no longer detectable (cleared) from plasma by 6 hours. Urinary clearance time, however, was longer, and between 12 and 24 hours should be allowed for a dose of methylphenidate to "clear" from urine. Using a variable-interval responding apparatus, methylphenidate was shown to stimulate the responding rate of horses up to 6 times above base line, making it the most potent central stimulant tested in this responding apparatus to date. Peak central stimulation at 30 minutes after IV dosing was seen in horses given 0.4 to 1.0 mg of methylphenidate/kg.

Pharmacology of narcotic analgesics in the horse: selective blockade of narcotic-induced locomotor activity

The locomotor responses of horses given morphine and fentanyl were blocked or lessened by administration of naloxone or acepromazine. Naloxone given at the dosage of 0.015 mg/kg completely blocked the locomotor activity induced in horses given fentanyl (0.020 mg/kg of body weight). The locomotor stimulation produced by morphine given at the dosage of 2.4 mg/kg was reduced by 75% of naloxone (0.020 mg/kg). Acepromazine partially blocked the locomotor responses to fentanyl and morphine. This blockade activity reached its peak about 30 minutes after acepromazine was given (IV) and lasted more than 6 hours. Simultaneous administration of acepromazine and morphine was associated with substantial respiratory depression for more than 4 hours after administration of both drugs. In other experiments, fentanyl did not add to the partial locomotor response observed after large doses of pentazocine were given--this being consistent with the concept that pentazocine possesses both antagonist and agonist actions at the narcotic receptor. Furosemide and phenylbutazone, given at usually used clinical doses, had no effect on the locomotor response to fentanyl, indicating that the usual clinical dosages of neither drug exerted stimulant or depressant actions.