Specific HPLC assay for serum nifedipine.

A method has been devised for the sensitive, specific and reproducible estimation of nifedipine in human serum. The assay is based on reverse-phase HPLC of an ether extract of serum at pH 9 using nisoldipine as internal standard. The two compounds are detected and quantitated by spectrophotometry at 350 nm. The mean recovery of nifedipine added to blank serum (60-250 ng/ml) was 70%, with a minimum detectable level of 10 ng/ml. Replicate determinations of patients' serum nifedipine (90-150 ng/ml) yielded coefficients of variation of 3.3%. Twelve drugs and non-prescription medicaments which could be ingested at the same time as nifedipine were assessed for interference with the assay but none were found to do so. The rates of absorption and peak serum levels of nifedipine were compared in normo- and hypertensive subjects who received sublingually a single 10 mg dose of the drug. Half the subjects displayed mean maximum serum levels of 148 +/- 34 ng/ml while the other half achieved only 48 +/- 15 ng/ml. The times required to reach those peak levels varied from 30 min to greater than 60 min. These results suggest that the rate of absorption of this drug varies widely among individuals. No significant differences were observed between the normo- and hypertensive groups.