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| June • 2002 | ||
| THERAPEUTIC DRUG MONITORING (CONT'D) | ||
| PHENOBARBITAL | ||
The t½ is 32-75 hours in most dogs, although some patients can have a shorter t½ such as E24 hours or even E12 hours. Peak and trough concentrations should be measured initially, or when there is difficulty controlling seizures. Calculation of the t½, which allows more accurate determination of the proper dosing interval, can be made as follows: |
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If the t½ of phenobarbital is <24 hours, then dosing every 8 hours may improve seizure control. |
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| Monitoring Recommendations: At 2 weeks, perform peak (4-5 hours) and trough (just prior to next dose)
concentrations to determine t½ and optimal dosing interval. At 8-12 weeks, perform trough measurement to detect
induction. At 6 month intervals, measured trough concentrations, although this may not be an essential time as data
show peak and trough concentrations at steady state are not appreciably different. Anytime a patient experiences
"breakthrough" seizures, measure peak and trough drug concentrations. Phenobarbital and Pancreatitis:There are anecdotal reports of pancreatitis in dogs receiving phenobarbital or bromide, but there is no documentation of a cause-and-effect relationship. When present, pancreatitis may be secondary to polyphagia. |
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| BROMIDE | ||
The t½ is about 24 days. Steady state concentrations are not reached for at least 2-3 months.
The loading dose is designed to establish steady state concentrations immediately rather than in 2-3 months needed with maintenance therapy. To achieve ~1 mg/mL PDC, a loading dose of 450 mg/kg split and administered over 5 days is needed. The maintenance dose (~ 30 mg/kg to maintain a PDC of ~ 1mg/mL) needs to be given along with the loading dose. Higher doses are needed to quickly achieve and maintain higher target concentrations (e.g. 600 mg/kg loading and 40 mg/kg maintenance to achieve and maintain ~ 1.5 mg/mL). |
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| Maintenance: Most animals achieve or maintain bromide concentrations of 1 mg/mL on a maintenance dose of 30 mg/kg/day. | ||
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