MEASUREMENT OF BRIMONIDINE CONCENTRATIONS IN HUMAN PLASMA BY A HIGHLYSENSITIVE GAS-CHROMATOGRAPHY MASS-SPECTROMETRIC ASSAY

Brimonidine is an alpha(2)-adrenergic agonist that is efficacious in l owering intraocular pressure in humans. A highly sensitive and selecti ve gas chromatography/mass spectrometry (GC/MS) assay is described for quantitation of brimonidine in human plasma following ocular installa tion. Brimonidine in 1 mi of plasma was extracted together with tetrad euterated brimonidine (internal standard) and clonidine (carrier) by s olvent extraction. After solvent evaporation, 3,5-bis(trifluoromethyl) benzoyl derivatives were formed and injected onto a GC/MS appartus und er negative chemical ionization conditions. The ions monitored for der ivatized brimonidine and tetradeuterated brimonidine were m/z 691 [M - HBr] and m/z 694 [M - DBr], respectively. Calibration curves were lin ear from 2 to 1000 pg ml(-1) (r(2) = 0.981-0.996). The method was spec ific for brimonidine relative to endogenous compounds in plasma. The i nter-day relative standard deviation for analysis of quality controls was 12% or less, and the inter-day assay accuracy ranged from 97 to 10 4% of nominals. The GC/MS assay showed adequate sensitivity for analys is of human samples from volunteers ocularly dosed with 0.5% brimonidi ne tartrate solution. Overall, the GC/MS assay showed excellent precis ion and accuracy, and a minimum quantifiable concentration of 2 pg ml( -1).