A method has been developed to quantify ritonavir concentrations in human plasma and in mouse serum, liver, and brain using high-performance liquid chromatography. Extraction recoveries for ritonavir and its internal standard averaged greater than 95%. Within-day variability, expressed as a coefficient of variation, averaged 6% over the concentration range 0.5 microg/mL to 15 microg/mL ritonavir, and between-day variability averaged 5.6% over 5 microg/mL to 15 microg/mL ritonavir. The method was applied to quantitation of ritonavir in mouse serum and tissue. Measured values deviated less than 5% from the actual values in mouse serum, liver, and brain samples containing 5 microg/mL ritonavir. The slopes of calibration curves for extracted calf serum, mouse serum, mouse liver and mouse brain standards were nearly identical to the calibration slope of standards which were not extracted. All curves were linear through zero, and r2 was no less than 0.998 for any form of calibration. In addition, there was no chromatographic interference from commonly prescribed medications.
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