Antech News - June 2002


June • 2002

THERAPEUTIC DRUG MONITORING (CONT'D)

Monitoring: With loading doses: PDCs should be measured one day after loading to verify that the target is reached. If drug concentration is too low, a second smaller loading dose can be given. A general rule of thumb is that a loading dose of 250 mg/kg will increase drug concentrations by ~ 0.5 mg/mL. Drug concentrations should be checked again 1 month later to verify that the target maintenance dose is achieved. Without loading doses: measure drug concentrations 3-4 weeks after starting treatment. At that time (one t½), drug concentrations will be about half of steady state, and so the maintenance dose can be modified, if needed. Bromide and pancreatitis: See above comments for phenobarbital. Bromide in cats: Maintenance dosing at 30 mg/kg/day can be used safely in cats. Steady state is reached at 8 weeks, and t½ is about 10 days. Respiratory disease has been seen in cats receiving bromide. In one case series, 10/17 cats (58%) on bromide had episodes of coughing which resolved in two cats when bromide was discontinued.

THEOPHYLLINE
The brand name sustained release theophylline products are no longer available. Generic preparations may not be equally bioavailable, and so monitoring of trough concentrations is advised to ensure that the therapeutic range is achieved.

Monitoring: Cats: 24 hour pre-medication trough is measured. Dogs: 12 hours pre-medication trough is used. The therapeutic range is 10-20 痢/mL.

DIGOXIN
The elimination t½ is about 30 hours in most dogs and cats. Some animals can have a short t½ which can present problems when monitoring drug concentrations. With long t1/2, a 6-8 hour post-pill sample is fine to check peak digoxin concentrations. With short t1/2, the peak may occur as early as 3 hours post-pill. In these cases, the 6-8 hour post-pill sample will not reflect the peak amount and there is a risk of toxicosis, if dose is subsequently increased.

CYCLOSPORINE
The elimination t½ and may even be only 1-2 hours. A 12 hour dosing interval is still acceptable because of its residual effects.

Monitoring: Measuring cyclosporine can be with RIA or HPLC methods, the former being much less expensive. Whole blood samples must be used here. Target trough concentrations are 400-600 ng/mL, although lower concentrations may be sufficient for some diseases. Experience in treating dogs with perianal fistular indicates that a clinical response begins once their trough cyclosporine concentrations reach 300-400 ng/mL.


Antech clients have requested sensitivity studies and discs for marbofloxacin. The manufacturer (Pfizer) has indicated that sensitivity discs for this antibiotic are in the final phases of quality control and should be released shortly. A soon as they become available, Antech will offer this option.




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