Pharmacokinetic-pharmacodynamic modeling of midazolam effects on the human central nervous system

The effect of midazolam on α-activity of the EEG and latency of the P-100 of the visual evoked response (VER) was studied in six healthy subjects. Drug concentration was related to effect with the Emax model that was used with either a threshold drug concentration or a sigmoid exponent. An effect compartment was included in the pharmacokinetic-pharmacodynamic model. Four subjects showed hysteresis, and mean values of half-lives-keo ranged from 0.26 to 0.60 hour. Mean values of EC50 ranged from 42.0 to 48.1 ng/ml. Goodness of fit did not differ significantly between the sigmoid Emax model and the threshold Emax model. The sigmoid exponent estimated was 3.7 ± 1.8 (EEG, mean ± SD) and 2.9 ± 1.4 (VER); the threshold concentration was estimated at 15.7 ± 11.1 ng/ml (EEG) and 11.3 ± 7.0 ng/ml (VER). We conclude that the Emax model adequately describes the relationship between midazolam concentration and effect and that the sigmoid exponent can be substituted by a threshold drug concentration, with a comparable fit of the model to the data. Clinical Pharmacology and Therapeutics (1988) 44, 14–22; doi:10.1038/clpt.1988.106