Pharmacokinetic-pharmacodynamic modelling of the EEG effects of Ro 48-6791, a new short-acting benzodiazepine, in young and elderly subjects

The objectives of this study were to explore, by a modelling approach, in nine young (24-28 yr) and nine elderly (67-81 yr) male subjects, the pharmacokinetics and pharmacodynamics of Po 48-6791, a new water soluble benzodiazepine. A microprocessor-controlled i.v. infusion pump generated linearly increasing arterial plasma concentrations until predetermined EEG and clinical end-points were attained. This concentration was maintained for 15 min and thereafter the infusion was discontinued. Haemodynamic and respiratory variables were monitored continuously. At full reorientation of the subject, a second infusion cycle was started under the same conditions to investigate the reproducibility of the concentration-effect relationship. The plasma concentration-time profiles of Ro 48-6791 were fitted accurately to an open three-compartment model. Plasma concentrations of Po 48-6792, an N-dealkylated metabolite, accumulated during the course of the study. Pharmacokinetic variables of Po 48-6791 were similar for both groups. The largest differences between young and elderly subjects, respectively, were found for clearance (mean 85 (SD 23) vs 71 (15) litre h(-1)) and k(12) (11 (7) vs 7 (3) h(-1)). The concentration-median EEG frequency relationship was described with a sigmoid Emax model. Elderly subjects showed slightly increased drug sensitivity compared with young subjects (EC50 72 (25) and 44 (15) mu g litre(-1) in young and elderly subjects, respectively). The concentration-response data of the second infusion cycle deviated from the fitted curve suggesting either development of acute tolerance to the EEG effects of Ro 48-6791 or a role for drug metabolites. Because of the differences in sensitivity and clearance, lower doses of Po 48-6791 should be administered to elderly compared with young subjects in order to achieve similar effects.